Role of the Yersinia pestis hemin storage (hms) locus in the transmission of plague by fleas

Pathogenicity and virulence of Yersinia

The genus Yersinia includes human, animal, insect, and plant pathogens as well as many symbionts and harmless bacteria. Within this genus are Yersinia enterocolitica and the Yersinia pseudotuberculosis complex, with four human pathogenic species that are highly related at the genomic level including the causative agent of plague, Yersinia pestis. Extensive laboratory, field work, and clinical research have been conducted to understand the underlying pathogenesis and zoonotic transmission of these pathogens. There are presently more than 500 whole genome sequences from which an evolutionary footprint can be developed that details shared and unique virulence properties. Whereas the virulence of Y. pestis now seems in apparent homoeostasis within its flea transmission cycle, substantial evolutionary changes that affect transmission and disease severity continue to ndergo apparent selective pressure within the other Yersiniae that cause intestinal diseases. In this review, we will summarize the present understanding of the virulence and pathogenesis of Yersinia, highlighting shared mechanisms of virulence and the differences that determine the infection niche and disease severity.

Role of the Yersinia pestis phospholipase D (Ymt) in the initial aggregation step of biofilm formation in the flea

Transmission of Yersinia pestis by fleas depends on the formation of condensed bacterial aggregates embedded within a gel-like matrix that localizes to the proventricular valve in the flea foregut and interferes with normal blood feeding. This is essentially a bacterial biofilm phenomenon, which at its end stage requires the production of a Y. pestis exopolysaccharide that bridges the bacteria together in a cohesive, dense biofilm that completely blocks the proventriculus. However, bacterial aggregates are evident within an hour after a flea ingests Y. pestis, and the bacterial exopolysaccharide is not required for this process. In this study, we characterized the biochemical composition of the initial aggregates and demonstrated that the yersinia murine toxin (Ymt), a Y. pestis phospholipase D, greatly enhances rapid aggregation following infected mouse blood meals. The matrix of the bacterial aggregates is complex, containing large amounts of protein and lipid (particularly cholesterol) derived from the flea's blood meal. A similar incidence of proventricular aggregation occurred after fleas ingested whole blood or serum containing Y. pestis, and intact, viable bacteria were not required. The initial aggregation of Y. pestis in the flea gut is likely due to a spontaneous physical process termed depletion aggregation that occurs commonly in environments with high concentrations of polymers or other macromolecules and particles such as bacteria. The initial aggregation sets up subsequent binding aggregation mediated by the bacterially produced exopolysaccharide and mature biofilm that results in proventricular blockage and efficient flea-borne transmission.

IMPORTANCE

Yersinia pestis, the bacterial agent of plague, is maintained in nature in mammal-flea-mammal transmission cycles. After a flea feeds on a mammal with septicemic plague, the bacteria rapidly coalesce in the flea's digestive tract to form dense aggregates enveloped in a viscous matrix that often localizes to the foregut. This represents the initial stage of biofilm development that potentiates transmission of Y. pestis when the flea later bites a new host. The rapid aggregation likely occurs via a depletion-aggregation mechanism, a non-canonical first step of bacterial biofilm development. We found that the biofilm matrix is largely composed of host blood proteins and lipids, particularly cholesterol, and that the enzymatic activity of a Y. pestis phospholipase D (Ymt) enhances the initial aggregation. Y. pestis transmitted by flea bite is likely associated with this host-derived matrix, which may initially shield the bacteria from recognition by the host's intradermal innate immune response.

Yersinia pestis can infect the Pawlowsky glands of human body lice and be transmitted by louse bite

Yersinia pestis, the causative agent of plague, is a highly lethal vector-borne pathogen responsible for killing large portions of Europe's population during the Black Death of the Middle Ages. In the wild, Y. pestis cycles between fleas and rodents; occasionally spilling over into humans bitten by infectious fleas. For this reason, fleas and the rats harboring them have been considered the main epidemiological drivers of previous plague pandemics. Human ectoparasites, such as the body louse (Pediculus humanus humanus), have largely been discounted due to their reputation as inefficient vectors of plague bacilli. Using a membrane-feeder adapted strain of body lice, we show that the digestive tract of some body lice become chronically infected with Y. pestis at bacteremia as low as 1 × 105 CFU/ml, and these lice routinely defecate Y. pestis. At higher bacteremia (≥1 × 107 CFU/ml), a subset of the lice develop an infection within the Pawlowsky glands (PGs), a pair of putative accessory salivary glands in the louse head. Lice that developed PG infection transmitted Y. pestis more consistently than those with bacteria only in the digestive tract. These glands are thought to secrete lubricant onto the mouthparts, and we hypothesize that when infected, their secretions contaminate the mouthparts prior to feeding, resulting in bite-based transmission of Y. pestis. The body louse's high level of susceptibility to infection by gram-negative bacteria and their potential to transmit plague bacilli by multiple mechanisms supports the hypothesis that they may have played a role in previous human plague pandemics and local outbreaks.

Co-formulation of the rF1V plague vaccine with depot-formulated cytokines enhances immunogenicity and efficacy to elicit protective responses against aerosol challenge in mice

This study evaluated a depot-formulated cytokine-based adjuvant to improve the efficacy of the recombinant F1V (rF1V) plague vaccine and examined the protective response following aerosol challenge in a murine model. The results of this study showed that co-formulation of the Alhydrogel-adsorbed rF1V plague fusion vaccine with the depot-formulated cytokines recombinant human interleukin 2 (rhuIL-2) and/or recombinant murine granulocyte macrophage colony-stimulating factor (rmGM-CSF) significantly enhances immunogenicity and significant protection at lower antigen doses against a lethal aerosol challenge. These results provide additional support for the co-application of the depot-formulated IL-2 and/or GM-CSF cytokines to enhance vaccine efficacy.

Reciprocal regulation of NagC and quorum sensing systems and their roles in hmsHFRS expression and biofilm formation in Yersinia pseudotuberculosis

Biofilm formation by Yersinia pseudotuberculosis is regulated by quorum sensing (QS) and dependent on the haemin storage locus hms , required for the extracellular polysaccharide poly-N -acetylglucosamine (poly-GlcNAc) production. In Escherichia coli NagC regulates both GlcNAc biosynthesis and metabolism with GlcNAc acting as a signal that co-ordinates these and other activities. However, the contribution of NagC and GlcNAc to biofilm development in Y. pseudotuberculosis is not known. Here we show that a Y. pseudotuberculosis nagC mutant is impaired for biofilm production on abiotic (glass) and biotic (Caenorhabitis elegans ) surfaces. Genetic complementation restored poly-GlcNAc production and biofilm formation on C. elegans . Using lux -based promoter fusions, hmsHFRS expression was found to be nagC dependent. Given that NagC and QS both regulate aggregation and biofilm formation, we investigated their regulatory relationship using lux -based promoter fusions. These revealed that (i) nagC is negatively autoregulated, but expression can be partially restored in the nagC mutant by exogenous GlcNAc, (ii) NagC negatively regulates the ytbI and ypsI QS genes and (iii) nagC expression is reduced in the ytbI , ypsI and ypsR mutants but not the ytbR mutant. These data establish the existence of a reciprocal regulatory relationship between NagC and QS, which in the case of the luxRI pair ytbRI , is also GlcNAc-dependent. NagC and GlcNAc are therefore components of a regulatory system involving QS that modulates biofilm formation and aggregation.

CsrA-Mediated Translational Activation of the hmsE mRNA Enhances HmsD-Dependent C-di-GMP-Enabled Biofilm Production in Yersinia pestis

The plague bacterium, Yersinia pestis, forms a biofilm-mediated blockage in the flea foregut that enhances its transmission by fleabite. Biofilm formation is positively controlled by cyclic di-GMP (c-di-GMP), which is synthesized by the diguanylate cyclases (DGC), HmsD and HmsT. While HmsD primarily promotes biofilm-mediated blockage of fleas, HmsT plays a more minor role in this process. HmsD is a component of the HmsCDE tripartite signaling system. HmsC and HmsE posttranslationally inhibit or activate HmsD, respectively. HmsT-dependent c-di-GMP levels and biofilm formation are positively regulated by the RNA-binding protein CsrA. In this study we determined whether CsrA positively regulates HmsD-dependent biofilm formation through interactions with the hmsE mRNA. Gel mobility shift assays determined that CsrA binds specifically to the hmsE transcript. RNase T1 footprint assays identified a single CsrA binding site and CsrA-induced structural changes in the hmsE leader region. Translational activation of the hmsE mRNA was confirmed in vivo using plasmid-encoded inducible translational fusion reporters and by HmsE protein expression studies. Furthermore, mutation of the CsrA binding site in the hmsE transcript significantly reduced HmsD-dependent biofilm formation. These results suggest that CsrA binding leads to structural changes in the hmsE mRNA that enhance its translation to enable increased HmsD-dependent biofilm formation. Given the requisite function of HmsD in biofilm-mediated flea blockage, this CsrA-dependent increase in HmsD activity underscores that complex and conditionally defined modulation of c-di-GMP synthesis within the flea gut is required for Y. pestis transmission. IMPORTANCE Mutations enhancing c-di-GMP biosynthesis drove the evolution of Y. pestis to flea-borne transmissibility. c-di-GMP-dependent biofilm-mediated blockage of the flea foregut enables regurgitative transmission of Y. pestis by fleabite. The Y. pestis diguanylate cyclases (DGC), HmsT and HmsD, which synthesize c-di-GMP, play significant roles in transmission. Several regulatory proteins involved in environmental sensing, as well as signal transduction and response regulation, tightly control DGC function. An example is CsrA, a global posttranscriptional regulator that modulates carbon metabolism and biofilm formation. CsrA integrates alternative carbon usage metabolism cues to activate c-di-GMP biosynthesis through HmsT. Here, we demonstrated that CsrA additionally activates hmsE translation to promote c-di-GMP biosynthesis through HmsD. This emphasizes that a highly evolved regulatory network controls c-di-GMP synthesis and Y. pestis transmission.

A Widefield Light Microscopy-Based Approach Provides Further Insights into the Colonization of the Flea Proventriculus by Yersinia pestis

Yersinia pestis (the agent of flea-borne plague) must obstruct the flea's proventriculus to maintain transmission to a mammalian host. To this end, Y. pestis must consolidate a mass that entrapped Y. pestis within the proventriculus very early after its ingestion. We developed a semiautomated fluorescent image analysis method and used it to monitor and compare colonization of the flea proventriculus by a fully competent flea-blocking Y. pestis strain, a partially competent strain, and a noncompetent strain. Our data suggested that flea blockage results primarily from the replication of Y. pestis trapped in the anterior half of the proventriculus. However, consolidation of the bacteria-entrapping mass and colonization of the entire proventricular lumen increased the likelihood of flea blockage. The data also showed that consolidation of the bacterial mass is not a prerequisite for colonization of the proventriculus but allowed Y. pestis to maintain itself in a large flea population for an extended period of time. Taken as the whole, the data suggest that a strategy targeting bacterial mass consolidation could significantly reduce the likelihood of Y. pestis being transmitted by fleas (due to gut blockage), but also the possibility of using fleas as a long-term reservoir. IMPORTANCE Yersinia pestis (the causative agent of plague) is one of the deadliest bacterial pathogens. It circulates primarily among rodent populations and their fleas. Better knowledge of the mechanisms leading to the flea-borne transmission of Y. pestis is likely to generate strategies for controlling or even eradicating this bacillus. It is known that Y. pestis obstructs the flea's foregut so that the insect starves, frantically bites its mammalian host, and regurgitates Y. pestis at the bite site. Here, we developed a semiautomated fluorescent image analysis method and used it to document and compare foregut colonization and disease progression in fleas infected with a fully competent flea-blocking Y. pestis strain, a partially competent strain, and a noncompetent strain. Overall, our data provided new insights into Y. pestis' obstruction of the proventriculus for transmission but also the ecology of plague.

A frameshift in Yersinia pestis rcsD alters canonical Rcs signalling to preserve flea-mammal plague transmission cycles

Multiple genetic changes in the enteric pathogen Yersinia pseudotuberculosis have driven the emergence of Yesinia pestis, the arthropod-borne, etiological agent of plague. These include developing the capacity for biofilm-dependent blockage of the flea foregut to enable transmission by flea bite. Previously, we showed that pseudogenization of rcsA, encoding a component of the Rcs signalling pathway, is an important evolutionary step facilitating Y. pestis flea-borne transmission. Additionally, rcsD, another important gene in the Rcs system, harbours a frameshift mutation. Here, we demonstrated that this rcsD mutation resulted in production of a small protein composing the C-terminal RcsD histidine-phosphotransferase domain (designated RcsD-Hpt) and full-length RcsD. Genetic analysis revealed that the rcsD frameshift mutation followed the emergence of rcsA pseudogenization. It further altered the canonical Rcs phosphorylation signal cascade, fine-tuning biofilm production to be conducive with retention of the pgm locus in modern lineages of Y. pestis. Taken together, our findings suggest that a frameshift mutation in rcsD is an important evolutionary step that fine-tuned biofilm production to ensure perpetuation of flea-mammal plague transmission cycles.

Yersinia pestis Δail Mutants Are Not Susceptible to Human Complement Bactericidal Activity in the Flea

Ail confers serum resistance in humans and is a critical virulence factor of Y. pestis, the causative agent of plague. Here, the contribution of Ail for Y. pestis survival in the flea vector was examined. Rat or human but not mouse sera were bactericidal against a Y. pestis Δail mutant at 28°C in vitro. Complement components deposited rapidly on the Y. pestis surface as measured by immunofluorescent microscopy. Ail reduced the amount of active C3b on the Y. pestis surface. Human sera retained bactericidal activity against a Y. pestis Δail mutant in the presence of mouse sera. However, in the flea vector, the serum protective properties of Ail were not required. Flea colonization studies using murine sera and Y. pestis KIM6+ wild type, a Δail mutant, and the Δail/ail+ control showed no differences in bacterial prevalence or numbers during the early stage of flea colonization. Similarly, flea studies with human blood showed Ail was not required for serum resistance. Finally, a variant of Ail (AilF100V E108_S109insS) from a human serum-sensitive Y. pestis subsp. microtus bv. Caucasica 1146 conferred resistance to human complement when expressed in the Y. pestis KIM6+ Δail mutant. This indicated that Ail activity was somehow blocked, most likely by lipooligosaccharide, in this serum sensitive strain. IMPORTANCE This work contributes to our understanding of how highly virulent Y. pestis evolved from its innocuous enteric predecessor. Among identified virulence factors is the attachment invasion locus protein, Ail, that is required to protect Y. pestis from serum complement in all mammals tested except mice. Murine sera is not bactericidal. In this study, we asked, is bactericidal sera from humans active in Y. pestis colonized fleas? We found it was not. The importance of this observation is that it identifies a protective niche for the growth of serum sensitive and nonsensitive Y. pestis strains.

Variability of <i>pgm</i>‑Region Genes in <i>Yersinia pestis</i> Strains from the Caspian Sandy and Adjacent Plague Foci

The aim of the study was to compare the nucleotide sequences of pgm‑region genes in Yersinia pestis strains isolated on the territory of the Caspian sandy and adjacent plague foci in 1925–2015. Materials and methods. 65 Y. pestis strains from the Caspian sandy and adjacent plague foci were used in the work. DNA isolation was performed using the PureLink Genomic DNA Mini Kit. Whole genome sequencing was conducted in Ion S5 XL System (Thermo Fischer Scientific). Data processing was carried out using Ion Torrent Suite software package 3.4.2 and NewblerGS Assembler 2.6. To compare the obtained sequences with the NCBI GenBank database, the Blast algorithm was used. The phylogenetic analysis was performed according to the data of whole genome SNP analysis based on 1183 identified SNPs. The search for marker SNPs was performed using the Snippy 4.6 program. The phylogenetic tree was constructed using the Maximum Likelihood algorithm, the GTR nucleotide substitution model. Results and discussion. The nucleotide sequences of pgm‑region genes of 65 Y. pestis strains from the Caspian sandy and adjacent plague foci have been assessed. Single nucleotide substitutions have been identified in Y. pestis strains from the Caspian sandy and Kobystan plain-foothill foci in the hmsR, astB, ybtS, ypo1944, ypo1943, ypo1936 genes, as well as a deletion of 5 bp in the ypo1945 gene, which is characteristic of strains of one of the phylogenetic lines of Y. pestis from the foci of Caucasus and Transcaucasia, isolated in 1968–2001. The data obtained can be used to differentiate Y. pestis strains from the Caspian sandy focus, as well as to establish the directions of microevolution of the plague pathogen in this region and adjacent foci.

Yersinia pestis and Plague: some knowns and unknowns

Since its first identification in 1894 during the third pandemic in Hong Kong, there has been significant progress of understanding the lifestyle of Yersinia pestis, the pathogen that is responsible for plague. Although we now have some understanding of the pathogen's physiology, genetics, genomics, evolution, gene regulation, pathogenesis and immunity, there are many unknown aspects of the pathogen and its disease development. Here, we focus on some of the knowns and unknowns relating to Y. pestis and plague. We notably focus on some key Y. pestis physiological and virulence traits that are important for its mammal-flea-mammal life cycle but also its emergence from the enteropathogen Yersinia pseudotuberculosis. Some aspects of the genetic diversity of Y. pestis, the distribution and ecology of plague as well as the medical countermeasures to protect our population are also provided. Lastly, we present some biosafety and biosecurity information related to Y. pestis and plague.

A Role for Early-Phase Transmission in the Enzootic Maintenance of Plague

Yersinia pestis, the bacterial agent of plague, is enzootic in many parts of the world within wild rodent populations and is transmitted by different flea vectors. The ecology of plague is complex, with rodent hosts exhibiting varying susceptibilities to overt disease and their fleas exhibiting varying levels of vector competence. A long-standing question in plague ecology concerns the conditions that lead to occasional epizootics among susceptible rodents. Many factors are involved, but a major one is the transmission efficiency of the flea vector. In this study, using Oropsylla montana (a ground squirrel flea that is a major plague vector in the western United States), we comparatively quantified the efficiency of the two basic modes of flea-borne transmission. Transmission efficiency by the early-phase mechanism was strongly affected by the host blood source. Subsequent biofilm-dependent transmission by blocked fleas was less influenced by host blood and was more efficient. Mathematical modeling predicted that early-phase transmission could drive an epizootic only among highly susceptible rodents with certain blood characteristics, but that transmission by blocked O. montana could do so in more resistant hosts irrespective of their blood characteristics. The models further suggested that for most wild rodents, exposure to sublethal doses of Y. pestis transmitted during the early phase may restrain rapid epizootic spread by increasing the number of immune, resistant individuals in the population.

The magnitude of the germinal center B cell and T follicular helper cell response predicts long-lasting antibody titers to plague vaccination

The development of a safe and effective vaccine against Yersinia pestis, the causative organism for plague disease, remains an important global health priority. Studies have demonstrated effective immune-based protection against plague challenge that is induced by plague antigen subunit vaccination in an aqueous alhydrogel formulation; however, whether these candidate vaccines in this formulation and presentation, induce long-lasting immunological memory in the form of durable cellular and antibody recall responses has not been fully demonstrated. In this study, we analyzed germinal center T follicular helper and germinal center B cell responses following F1V and F1 + V plague subunit immunization of mice with vaccines formulated in various adjuvants. Our data demonstrate that recombinant plague protein immunization formulated with IL-2/GM-CSF cytokines bound to alhydrogel adjuvant drive an increase in the magnitude of the germinal center T follicular helper and germinal center B cell responses following primary immunization, compared to vaccines formulated with Alhydrogel adjuvant alone. In contrast, plague protein subunit immunization combined with CpG ODN bound to alhydrogel increased the magnitude and duration of the germinal center Tfh and B cell responses following booster immunization. Importantly, enhanced germinal center Tfh and B cell responses correlated with long-lasting and high F1V-specific antibody titers and more robust antibody recall responses to F1V re-exposure. These findings indicate that vaccine formulations that drive enhancement of the germinal center Tfh and B cell responses are critical for inducing durable plague-specific humoral immunity.

Cpx-signalling facilitates Hms-dependent biofilm formation by Yersinia pseudotuberculosis

Bacteria often reside in sessile communities called biofilms, where they adhere to a variety of surfaces and exist as aggregates in a viscous polymeric matrix. Biofilms are resistant to antimicrobial treatments, and are a major contributor to the persistence and chronicity of many bacterial infections. Herein, we determined that the CpxA-CpxR two-component system influenced the ability of enteropathogenic Yersinia pseudotuberculosis to develop biofilms. Mutant bacteria that accumulated the active CpxR~P isoform failed to form biofilms on plastic or on the surface of the Caenorhabditis elegans nematode. A failure to form biofilms on the worm surface prompted their survival when grown on the lawns of Y. pseudotuberculosis. Exopolysaccharide production by the hms loci is the major driver of biofilms formed by Yersinia. We used a number of molecular genetic approaches to demonstrate that active CpxR~P binds directly to the promoter regulatory elements of the hms loci to activate the repressors of hms expression and to repress the activators of hms expression. Consequently, active Cpx-signalling culminated in a loss of exopolysaccharide production. Hence, the development of Y. pseudotuberculosis biofilms on multiple surfaces is controlled by the Cpx-signalling, and at least in part this occurs through repressive effects on the Hms-dependent exopolysaccharide production.

What do we know about osmoadaptation of Yersinia pestis?

The plague agent Yersinia pestis mainly spreads among mammalian hosts and their associated fleas. Production of a successful mammal-flea-mammal life cycle implies that Y. pestis senses and responds to distinct cues in both host and vector. Among these cues, osmolarity is a fundamental parameter. The plague bacillus lives in a tightly regulated environment in the mammalian host, while osmolarity fluctuates in the flea gut (300-550 mOsM). Here, we review the mechanisms that enable Y. pestis to perceive fluctuations in osmolarity, as well as genomic plasticity and physiological adaptation of the bacterium to this stress.

Yersiniabactin contributes to overcoming zinc restriction during Yersinia pestis infection of mammalian and insect hosts

Yersinia pestis causes human plague and colonizes both a mammalian host and a flea vector during its transmission cycle. A key barrier to bacterial infection is the host's ability to actively sequester key biometals (e.g., iron, zinc, and manganese) required for bacterial growth. This is referred to as nutritional immunity. Mechanisms to overcome nutritional immunity are essential virulence factors for bacterial pathogens. Y. pestis produces an iron-scavenging siderophore called yersiniabactin (Ybt) that is required to overcome iron-mediated nutritional immunity and cause lethal infection. Recently, Ybt has been shown to bind to zinc, and in the absence of the zinc transporter ZnuABC, Ybt improves Y. pestis growth in zinc-limited medium. These data suggest that, in addition to iron acquisition, Ybt may also contribute to overcoming zinc-mediated nutritional immunity. To test this hypothesis, we used a mouse model defective in iron-mediated nutritional immunity to demonstrate that Ybt contributes to virulence in an iron-independent manner. Furthermore, using a combination of bacterial mutants and mice defective in zinc-mediated nutritional immunity, we identified calprotectin as the primary barrier for Y. pestis to acquire zinc during infection and that Y. pestis uses Ybt to compete with calprotectin for zinc. Finally, we discovered that Y. pestis encounters zinc limitation within the flea midgut, and Ybt contributes to overcoming this limitation. Together, these results demonstrate that Ybt is a bona fide zinc acquisition mechanism used by Y. pestis to surmount zinc limitation during the infection of both the mammalian and insect hosts.

CsrA Enhances Cyclic-di-GMP Biosynthesis and Yersinia pestis Biofilm Blockage of the Flea Foregut by Alleviating Hfq-Dependent Repression of the hmsT mRNA

Plague-causing Yersinia pestis is transmitted through regurgitation when it forms a biofilm-mediated blockage in the foregut of its flea vector. This biofilm is composed of an extracellular polysaccharide substance (EPS) produced when cyclic-di-GMP (c-di-GMP) levels are elevated. The Y. pestis diguanylate cyclase enzymes HmsD and HmsT synthesize c-di-GMP. HmsD is required for biofilm blockage formation but contributes minimally to in vitro biofilms. HmsT, however, is necessary for in vitro biofilms and contributes to intermediate rates of biofilm blockage. C-di-GMP synthesis is regulated at the transcriptional and posttranscriptional levels. In this, the global RNA chaperone, Hfq, posttranscriptionally represses hmsT mRNA translation. How c-di-GMP levels and biofilm blockage formation is modulated by nutritional stimuli encountered in the flea gut is unknown. Here, the RNA-binding regulator protein CsrA, which controls c-di-GMP-mediated biofilm formation and central carbon metabolism responses in many Gammaproteobacteria, was assessed for its role in Y. pestis biofilm formation. We determined that CsrA was required for markedly greater c-di-GMP and EPS levels when Y. pestis was cultivated on alternative sugars implicated in flea biofilm blockage metabolism. Our assays, composed of mobility shifts, quantification of mRNA translation, stability, and abundance, and epistasis analyses of a csrA hfq double mutant strain substantiated that CsrA represses hfq mRNA translation, thereby alleviating Hfq-dependent repression of hmsT mRNA translation. Additionally, a csrA mutant exhibited intermediately reduced biofilm blockage rates, resembling an hmsT mutant. Hence, we reveal CsrA-mediated control of c-di-GMP synthesis in Y. pestis as a tiered, posttranscriptional regulatory process that enhances biofilm blockage-mediated transmission from fleas.

Interplay between Yersinia pestis and its flea vector in lipoate metabolism

To thrive, vector-borne pathogens must survive in the vector's gut. How these pathogens successfully exploit this environment in time and space has not been extensively characterized. Using Yersinia pestis (the plague bacillus) and its flea vector, we developed a bioluminescence-based approach and employed it to investigate the mechanisms of pathogenesis at an unprecedented level of detail. Remarkably, lipoylation of metabolic enzymes, via the biosynthesis and salvage of lipoate, increases the Y. pestis transmission rate by fleas. Interestingly, the salvage pathway's lipoate/octanoate ligase LplA enhances the first step in lipoate biosynthesis during foregut colonization but not during midgut colonization. Lastly, Y. pestis primarily uses lipoate provided by digestive proteolysis (presumably as lipoyl peptides) rather than free lipoate in blood, which is quickly depleted by the vector. Thus, spatial and temporal factors dictate the bacterium's lipoylation strategies during an infection, and replenishment of lipoate by digestive proteolysis in the vector might constitute an Achilles' heel that is exploited by pathogens.

Molecular and Genetic Mechanisms That Mediate Transmission of Yersinia pestis by Fleas

The ability to cause plague in mammals represents only half of the life history of Yersinia pestis. It is also able to colonize and produce a transmissible infection in the digestive tract of the flea, its insect host. Parallel to studies of the molecular mechanisms by which Y. pestis is able to overcome the immune response of its mammalian hosts, disseminate, and produce septicemia, studies of Y. pestis-flea interactions have led to the identification and characterization of important factors that lead to transmission by flea bite. Y. pestis adapts to the unique conditions in the flea gut by altering its metabolic physiology in ways that promote biofilm development, a common strategy by which bacteria cope with a nutrient-limited environment. Biofilm localization to the flea foregut disrupts normal fluid dynamics of blood feeding, resulting in regurgitative transmission. Many of the important genes, regulatory pathways, and molecules required for this process have been identified and are reviewed here.

A horizontally acquired expansin gene increases virulence of the emerging plant pathogen Erwinia tracheiphila

Erwinia tracheiphila is a bacterial plant pathogen that causes a fatal wilt infection in some cucurbit crop plants. Wilt symptoms are thought to be caused by systemic bacterial colonization through xylem that impedes sap flow. However, the genetic determinants of within-plant movement are unknown for this pathogen species. Here, we find that E. tracheiphila has horizontally acquired an operon with a microbial expansin (exlx) gene adjacent to a glycoside hydrolase family 5 (gh5) gene. Plant inoculation experiments with deletion mutants in the individual genes (Δexlx and Δgh5) and the full operon (Δexlx-gh5) resulted in decreased severity of wilt symptoms, decreased mortality rate, and impaired systemic colonization compared to the Wt strain. Co-inoculation experiments with Wt and Δexlx-gh5 rescued the movement defect of the mutant strain, suggesting that expansin and GH5 function extracellularly. Together, these results show that expansin-GH5 contributes to systemic movement through xylem, leading to rapid wilt symptom development and higher rates of plant death. The presence of expansin genes in diverse species of bacterial and fungal wilt-inducing pathogens suggests that microbial expansin proteins may be an under-appreciated virulence factor for many pathogen species.

Comparison of the transmission efficiency and plague progression dynamics associated with two mechanisms by which fleas transmit Yersinia pestis

Yersinia pestis can be transmitted by fleas during the first week after an infectious blood meal, termed early-phase or mass transmission, and again after Y. pestis forms a cohesive biofilm in the flea foregut that blocks normal blood feeding. We compared the transmission efficiency and the progression of infection after transmission by Oropsylla montana fleas at both stages. Fleas were allowed to feed on mice three days after an infectious blood meal to evaluate early-phase transmission, or after they had developed complete proventricular blockage. Transmission was variable and rather inefficient by both modes, and the odds of early-phase transmission was positively associated with the number of infected fleas that fed. Disease progression in individual mice bitten by fleas infected with a bioluminescent strain of Y. pestis was tracked. An early prominent focus of infection at the intradermal flea bite site and dissemination to the draining lymph node(s) soon thereafter were common features, but unlike what has been observed in intradermal injection models, this did not invariably lead to further systemic spread and terminal disease. Several of these mice resolved the infection without progression to terminal sepsis and developed an immune response to Y. pestis, particularly those that received an intermediate number of early-phase flea bites. Furthermore, two distinct types of terminal disease were noted: the stereotypical rapid onset terminal disease within four days, or a prolonged onset preceded by an extended, fluctuating infection of the lymph nodes before eventual systemic dissemination. For both modes of transmission, bubonic plague rather than primary septicemic plague was the predominant disease outcome. The results will help to inform mathematical models of flea-borne plague dynamics used to predict the relative contribution of the two transmission modes to epizootic outbreaks that erupt periodically from the normal enzootic background state.

Yersinia pestis can be transmitted by fleas within a few days after taking a blood meal from a highly bacteremic host, termed early-phase or mass transmission; and again after it forms a dense biofilm in the foregut of its vector that can eventually block blood feeding. The relative importance of the two transmission modes in the ecology of plague is a matter of current debate, but estimates of transmission rate, efficiency, and other parameters are limited. We compared transmission and disease progression dynamics in mice bitten by groups of fleas three days after their infectious blood meal (early-phase or mass transmission mode) and in mice bitten by individual blocked fleas. In general, a higher percentage of transmissions by blocked fleas led to terminal disease, whereas early-phase transmissions more often led to survival and an immune response, which are nonproductive infections in the sense that the bacteremia required to continue the Y. pestis life cycle did not develop and these animals would be removed from the pool of susceptibles in the host population. The data will be useful in mathematical models of plague dynamics in wild rodent populations.

Influence of Temperature on Development of Yersinia pestis Foregut Blockage in Xenopsylla cheopis (Siphonaptera: Pulicidae) and Oropsylla montana (Siphonaptera: Ceratophyllidae)

Plague, caused by the flea-transmitted bacterial pathogen Yersinia pestis, is primarily a disease of wild rodents distributed in temperate and tropical zones worldwide. The ability of Y. pestis to develop a biofilm blockage that obstructs the flea foregut proventriculus facilitates its efficient transmission through regurgitation into the host bite site during flea blood sucking. While it is known that temperature influences transmission, it is not well-known if blockage dynamics are similarly in accord with temperature. Here, we determine the influence of the biologically relevant temperatures, 10 and 21°C, on blockage development in flea species, Xenopsylla cheopis (Rothschild) and Oropsylla montana (Baker), respectively, characterized by geographical distribution as cosmopolitan, tropical or endemic, temperate. We find that both species exhibit delayed development of blockage at 10°C. In Y. pestis infected X. cheopis, this is accompanied by significantly lower survival rates and slightly decreased blockage rates, even though these fleas maintain similar rates of persistent infection as at 21°C. Conversely, irrespective of infection status, O. montana withstand 21 and 10°C similarly well and show significant infection rate increases and slightly greater blocking rates at 10 versus 21°C, emphasizing that cooler temperatures are favorable for Y. pestis transmission from this species. These findings assert that temperature is a relevant parameter to consider in assessing flea transmission efficiency in distinct flea species residing in diverse geographical regions that host endemic plague foci. This is important to predict behavioral dynamics of plague regarding epizootic outbreaks and enzootic maintenance and improve timeous implementation of flea control programs.

A Trimeric Autotransporter Enhances Biofilm Cohesiveness in Yersinia pseudotuberculosis but Not in Yersinia pestis

Cohesion of biofilms made by Yersinia pestis and Yersinia pseudotuberculosis has been attributed solely to an extracellular polysaccharide matrix encoded by the hms genes (Hms-dependent extracellular matrix [Hms-ECM]). However, mutations in the Y. pseudotuberculosis BarA/UvrY/CsrB regulatory cascade enhance biofilm stability without dramatically increasing Hms-ECM production. We found that treatment with proteinase K enzyme effectively destabilized Y. pseudotuberculosis csrB mutant biofilms, suggesting that cell-cell interactions might be mediated by protein adhesins or extracellular matrix proteins. We identified an uncharacterized trimeric autotransporter lipoprotein (YPTB2394), repressed by csrB, which has been referred to as YadE. Biofilms made by a ΔyadE mutant strain were extremely sensitive to mechanical disruption. Overexpression of yadE in wild-type Y. pseudotuberculosis increased biofilm cohesion, similar to biofilms made by csrB or uvrY mutants. We found that the Rcs signaling cascade, which represses Hms-ECM production, activated expression of yadE The yadE gene appears to be functional in Y. pseudotuberculosis but is a pseudogene in modern Y. pestis strains. Expression of functional yadE in Y. pestis KIM6+ weakened biofilms made by these bacteria. This suggests that although the YadE autotransporter protein increases Y. pseudotuberculosis biofilm stability, it may be incompatible with the Hms-ECM production that is essential for Y. pestis biofilm production in fleas. Inactivation of yadE in Y. pestis may be another instance of selective gene loss in the evolution of flea-borne transmission by this species.IMPORTANCE The evolution of Yersinia pestis from its Y. pseudotuberculosis ancestor involved gene acquisition and gene losses, leading to differences in biofilm production. Characterizing the unique biofilm features of both species may provide better understanding of how each adapts to its specific niches. This study identifies a trimeric autotransporter, YadE, that promotes biofilm stability of Y. pseudotuberculosis but which has been inactivated in Y. pestis, perhaps because it is not compatible with the Hms polysaccharide that is crucial for biofilms inside fleas. We also reveal that the Rcs signaling cascade, which represses Hms expression, activates YadE in Y. pseudotuberculosis The ability of Y. pseudotuberculosis to use polysaccharide or YadE protein for cell-cell adhesion may help it produce biofilms in different environments.

Transcriptomic profiling of the digestive tract of the rat flea, Xenopsylla cheopis, following blood feeding and infection with Yersinia pestis

Yersinia pestis, the causative agent of plague, is a highly lethal pathogen transmitted by the bite of infected fleas. Once ingested by a flea, Y. pestis establish a replicative niche in the gut and produce a biofilm that promotes foregut colonization and transmission. The rat flea Xenopsylla cheopis is an important vector to several zoonotic bacterial pathogens including Y. pestis. Some fleas naturally clear themselves of infection; however, the physiological and immunological mechanisms by which this occurs are largely uncharacterized. To address this, RNA was extracted, sequenced, and distinct transcript profiles were assembled de novo from X. cheopis digestive tracts isolated from fleas that were either: 1) not fed for 5 days; 2) fed sterile blood; or 3) fed blood containing ~5x10⁸ CFU/ml Y. pestis KIM6+. Analysis and comparison of the transcript profiles resulted in identification of 23 annotated (and 11 unknown or uncharacterized) digestive tract transcripts that comprise the early transcriptional response of the rat flea gut to infection with Y. pestis. The data indicate that production of antimicrobial peptides regulated by the immune-deficiency pathway (IMD) is the primary flea immune response to infection with Y. pestis. The remaining infection-responsive transcripts, not obviously associated with the immune response, were involved in at least one of 3 physiological themes: 1) alterations to chemosensation and gut peristalsis; 2) modification of digestion and metabolism; and 3) production of chitin-binding proteins (peritrophins). Despite producing several peritrophin transcripts shortly after feeding, including a subset that were infection-responsive, no thick peritrophic membrane was detectable by histochemistry or electron microscopy of rat flea guts for the first 24 hours following blood-feeding. Here we discuss the physiological implications of rat flea infection-responsive transcripts, the function of X. cheopis peritrophins, and the mechanisms by which Y. pestis may be cleared from the flea gut.

The goal of this study was to characterize the transcriptional response of the digestive tract of the rat flea, Xenopsylla cheopis, to infection with Yersinia pestis, the causative agent of plague. This flea is generally considered the most prevalent and efficient vector of Y. pestis. Because most pathogens transmitted by fleas, including Y. pestis, reside in the insect digestive tract prior to transmission, the transcriptional program induced in the gut epithelium likely influences bacterial colonization of the flea. To determine the specific response to infection, RNA profiles were generated from fleas that were either unfed, fed sterile blood, or fed blood containing Y. pestis. Comparative analyses of the transcriptomes resulted in identification of 34 infection-responsive transcripts. The functions of these differentially regulated genes indicate that infection of fleas with Y. pestis induces a limited immune response and potentially alters the insect’s behavior, metabolism, and other aspects of its physiology. Based on these data, we describe potential mechanisms fleas use to eliminate bacteria and the corresponding strategies Y. pestis uses to resist elimination. These findings may be helpful for developing targeted strategies to make fleas resistant to microbial infection and thereby reduce the incidence of diseases they spread.

Biovar-related differences apparent in the flea foregut colonization phenotype of distinct Yersinia pestis strains do not impact transmission efficiency

Background

Yersinia pestis is the flea-transmitted etiological agent of bubonic plague. Sylvatic plague consists of complex tripartite interactions between diverse flea and wild rodent species, and pathogen strains. Transmission by flea bite occurs primarily by the Y. pestis biofilm-mediated foregut blockage and regurgitation mechanism, which has been largely detailed by studies in the model interaction between Y. pestis KIM6+ and Xenopsylla cheopis. Here, we test if pathogen-specific traits influence this interaction by determining the dynamics of foregut blockage development in X. cheopis fleas among extant avirulent pCD1-Y. pestis strains, KIM6+ and CO92, belonging to distinct biovars, and a non-circulating mutant CO92 strain (CO92gly), restored for glycerol fermentation; a key biochemical difference between the two biovars.

Methods

Separate flea cohorts infected with distinct strains were evaluated for (i) blockage development, bacterial burdens and flea foregut blockage pathology, and (ii) for the number of bacteria transmitted by regurgitation during membrane feeding. Strain burdens per flea was determined for fleas co-infected with CO92 and KIM6+ strains at a ratio of 1:1.

Results

Strains KIM6+ and CO92 developed foregut blockage at similar rates and peak temporal incidences, but the CO92gly strain showed significantly greater blockage rates that peak earlier post-infection. The KIM6+ strain, however, exhibited a distinctive foregut pathology wherein bacterial colonization extended the length of the esophagus up to the feeding mouthparts in ~65% of blocked fleas; in contrast to 32% and 26%, respectively, in fleas blocked with CO92 and CO92gly. The proximity of KIM6+ to the flea mouthparts in blocked fleas did not result in higher regurgitative transmission efficiencies as all strains transmitted variable numbers of Y. pestis, albeit slightly lower for CO92gly. During competitive co-infection, strains KIM6+ and CO92 were equally fit maintaining equivalent infection proportions in fleas over time.

Conclusions

We demonstrate that disparate foregut blockage pathologies exhibited by distinct extant Y. pestis strain genotypes do not influence transmission efficiency from X. cheopis fleas. In fact, distinct extant Y. pestis genotypes maintain equivalently effective blockage and transmission efficiencies which is likely advantageous to maintaining continued successful plague spread and establishment of new plague foci.

Putative Horizontally Acquired Genes, Highly Transcribed during Yersinia pestis Flea Infection, Are Induced by Hyperosmotic Stress and Function in Aromatic Amino Acid Metabolism

Distinct gene repertoires are expressed during Y. pestis infection of its flea and mammalian hosts. The functions of many of these genes remain predicted or unknown, necessitating their characterization, as this may provide a better understanding of Y. pestis specialized biological adaptations to the discrete environments of its two hosts. This study provides functional context to adjacently clustered horizontally acquired genes predominantly expressed in the flea host by deciphering their fundamental processes with regard to (i) transcriptional organization, (ii) transcription activation signals, and (iii) biochemical function. Our data support a role for these genes in osmoadaptation and aromatic amino acid metabolism, highlighting these as preferential processes by which Y. pestis gene expression is modulated during flea infection.

While alternating between insects and mammals during its life cycle, Yersinia pestis, the flea-transmitted bacterium that causes plague, regulates its gene expression appropriately to adapt to these two physiologically disparate host environments. In fleas competent to transmit Y. pestis, low-GC-content genes y3555, y3551, and y3550 are highly transcribed, suggesting that these genes have a highly prioritized role in flea infection. Here, we demonstrate that y3555, y3551, and y3550 are transcribed as part of a single polycistronic mRNA comprising the y3555, y3554, y3553, y355x, y3551, and y3550 genes. Additionally, y355x-y3551-y3550 compose another operon, while y3550 can be also transcribed as a monocistronic mRNA. The expression of these genes is induced by hyperosmotic salinity stress, which serves as an explicit environmental stimulus that initiates transcriptional activity from the predicted y3550 promoter. Y3555 has homology to pyridoxal 5′-phosphate (PLP)-dependent aromatic aminotransferases, while Y3550 and Y3551 are homologous to the Rid protein superfamily (YjgF/YER057c/UK114) members that forestall damage caused by reactive intermediates formed during PLP-dependent enzymatic activity. We demonstrate that y3551 specifically encodes an archetypal RidA protein with 2-aminoacrylate deaminase activity but Y3550 lacks Rid deaminase function. Heterologous expression of y3555 generates a critical aspartate requirement in a Salmonella entericaaspC mutant, while its in vitro expression, and specifically its heterologous coexpression with y3550, enhances the growth rate of an Escherichia coli ΔaspC ΔtyrB mutant in a defined minimal amino acid-supplemented medium. Our data suggest that the y3555, y3551, and y3550 genes operate cooperatively to optimize aromatic amino acid metabolism and are induced under conditions of hyperosmotic salinity stress.

IMPORTANCE Distinct gene repertoires are expressed during Y. pestis infection of its flea and mammalian hosts. The functions of many of these genes remain predicted or unknown, necessitating their characterization, as this may provide a better understanding of Y. pestis specialized biological adaptations to the discrete environments of its two hosts. This study provides functional context to adjacently clustered horizontally acquired genes predominantly expressed in the flea host by deciphering their fundamental processes with regard to (i) transcriptional organization, (ii) transcription activation signals, and (iii) biochemical function. Our data support a role for these genes in osmoadaptation and aromatic amino acid metabolism, highlighting these as preferential processes by which Y. pestis gene expression is modulated during flea infection.

A refined model of how Yersinia pestis produces a transmissible infection in its flea vector

In flea-borne plague, blockage of the flea's foregut by Yersinia pestis hastens transmission to the mammalian host. Based on microscopy observations, we first suggest that flea blockage results from primary infection of the foregut and not from midgut colonization. In this model, flea infection is characterized by the recurrent production of a mass that fills the lumen of the proventriculus and encompasses a large number of Y. pestis. This recurrence phase ends when the proventricular cast is hard enough to block blood ingestion. We further showed that ymt (known to be essential for flea infection) is crucial for cast production, whereas the hmsHFRS operon (known to be essential for the formation of the biofilm that blocks the gut) is needed for cast consolidation. By screening a library of mutants (each lacking a locus previously known to be upregulated in the flea gut) for biofilm formation, we found that rpiA is important for flea blockage but not for colonization of the midgut. This locus may initially be required to resist toxic compounds within the proventricular cast. However, once the bacterium has adapted to the flea, rpiA helps to form the biofilm that consolidates the proventricular cast. Lastly, we used genetic techniques to demonstrate that ribose-5-phosphate isomerase activity (due to the recent gain of a second copy of rpiA (y2892)) accentuated blockage but not midgut colonization. It is noteworthy that rpiA is an ancestral gene, hmsHFRS and rpiA2 were acquired by the recent ancestor of Y. pestis, and ymt was acquired by Y. pestis itself. Our present results (i) highlight the physiopathological and molecular mechanisms leading to flea blockage, (ii) show that the role of a gene like rpiA changes in space and in time during an infection, and (iii) emphasize that evolution is a gradual process punctuated by sudden jumps.

Co-infection Assay to Determine Yersinia pestis Competitive Fitness in Fleas

Co-infection refers to the simultaneous infection of a host by multiple pathogenic organisms. Experimental co-infection studies using a mutant and its isogenic wild type have proven to be profoundly sensitive to analysis of pathogen factor mutation-associated fitness effects in in vivo models of infectious disease. Here we discuss the use of such co-infection experiments in studying the interaction between Yersinia pestis and its flea vector to more sensitively determine the critical bacterial determinants for Y. pestis survival, adaptation, and transmission from fleas. This chapter comprises two main sections, the first detailing how to infect fleas with mutant and wild type Y. pestis strains, and secondly how to process infected fleas and specifically quantify distinct Y. pestis strain burdens per flea. The Y. pestis competitive fitness co-infection model in fleas is insightful in evaluating the consequence of a mutation which may not be obvious in single-strain flea infections where there is less selective pressure.

Protection and Safety Evaluation of Live Constructions Derived from the Pgm- and pPCP1- Yersinia pestis Strain

Based on a live attenuated Yersinia pestis KIM10(pCD1Ap) strain (Pgm-, pPCP1-), we attempted to engineer its lipid A species to achieve improvement of immunogenicity and safety. A mutant strain designated as YPS19(pCD1Ap), mainly synthesizing the hexa-acylated lipid A, and another mutant strain designated as YPS20(pCD1Ap), synthesizing 1-dephosphalated hexa-acylated lipid A (detoxified lipid A), presented relatively low virulence in comparison to KIM10(pCD1Ap) by intramuscular (i.m.) or subcutaneous (s.c.) administration. The i.m. administration with either the KIM10(pCD1Ap) or YPS19(pCD1Ap) strain afforded significant protection against bubonic and pneumonic plague compared to the s.c. administration, while administration with completely attenuated YPS20(pCD1Ap) strain failed to afford significant protection. Antibody analysis showed that i.m. administration induced balanced Th1 and Th2 responses but s.c. administration stimulated Th2-biased responses. Safety evaluation showed that YPS19(pCD1Ap) was relatively safer than its parent KIM10(pCD1Ap) in Hfe-/- mice manifesting iron overload in tissues, which also did not impair its protection. Therefore, the immune activity of hexa-acylated lipid A can be harnessed for rationally designing bacteria-derived vaccines.

Evolutionary selection of biofilm-mediated extended phenotypes in Yersinia pestis in response to a fluctuating environment

Yersinia pestis is transmitted from fleas to rodents when the bacterium develops an extensive biofilm in the foregut of a flea, starving it into a feeding frenzy, or, alternatively, during a brief period directly after feeding on a bacteremic host. These two transmission modes are in a trade-off regulated by the amount of biofilm produced by the bacterium. Here by investigating 446 global isolated Y. pestis genomes, including 78 newly sequenced isolates sampled over 40 years from a plague focus in China, we provide evidence for strong selection pressures on the RNA polymerase ω-subunit encoding gene rpoZ. We demonstrate that rpoZ variants have an increased rate of biofilm production in vitro, and that they evolve in the ecosystem during colder and drier periods. Our results support the notion that the bacterium is constantly adapting-through extended phenotype changes in the fleas-in response to climate-driven changes in the niche.

Nutrient depletion may trigger the Yersinia pestis OmpR-EnvZ regulatory system to promote flea-borne plague transmission

The flea's lumen gut is a poorly documented environment where the agent of flea-borne plague, Yersinia pestis, must replicate to produce a transmissible infection. Here, we report that both the acidic pH and osmolarity of the lumen's contents display simple harmonic oscillations with different periods. Since an acidic pH and osmolarity are two of three known stimuli of the OmpR-EnvZ two-component system in bacteria, we investigated the role and function of this Y. pestis system in fleas. By monitoring the in vivo expression pattern of three OmpR-EnvZ-regulated genes, we concluded that the flea gut environment triggers OmpR-EnvZ. This activation was not, however, correlated with changes in pH and osmolarity but matched the pattern of nutrient depletion (the third known stimulus for OmpR-EnvZ). Lastly, we found that the OmpR-EnvZ and the OmpF porin are needed to produce the biofilm that ultimately obstructs the flea's gut and thus hastens the flea-borne transmission of plague. Taken as a whole, our data suggest that the flea gut is a complex, fluctuating environment in which Y. pestis senses nutrient depletion via OmpR-EnvZ. Once activated, the latter triggers a molecular program (including at least OmpF) that produces the biofilm required for efficient plague transmission.

Epidemiology of Plague: Problems with the Use of Mathematical Epidemiological Models in Plague Research and the Question of Transmission by Human Fleas and Lice

This article addresses the recent use of mathematical epidemiological SIR or SEIR models in plague research. This use of S(E)IR models is highly problematic, but the problems are not presented and considered. Serious problems show in that such models are used to "prove" that historical plague was a (1) Filoviridae disease and (2) a bacterial disease caused by Yersinia pestis which was transmitted by human fleas and lice. (3) They also support early-phase transmission (by fleas). They purportedly consistently disprove (4) the conventional view that plague is/was a rat-and-rat-flea-borne disease. For these reasons, the focus is on methodological problems and on empirical testing by modern medical, entomological, and historical epidemiological data. An important or predominant vectorial role in plague epidemics for human fleas and lice requires that several necessary conditions are satisfied, which are generally not considered by advocates of the human ectoparasite hypothesis of plague transmission: (1) the prevalence and levels of human plague bacteraemia (human plague cases as sources of infection of feeding human ectoparasites); (2) the general size of blood meals ingested by human fleas and lice; (3) the consequent number of ingested plague bacteria; (4) the lethal dose of bacteria for 50% of a normal sample of infected human beings, LD50; and (5) efficient mechanism of transmission by lice and by fleas. The factual answers to these crucial questions can be ascertained and shown to invalidate the human ectoparasite hypothesis. The view of the standard works on plague has been corroborated, that bubonic plague, historical and modern, is/was a rat-and-rat-flea-borne disease caused by Yersinia pestis. These conclusions are concordant with and corroborate recent studies which, by laboratory experiments, invalidated the early-transmission hypothesis as a mechanism of transmission of LDs to humans in plague epidemics and removed this solution to the problem of transmission by human fleas.

Virulence potential of Corynebacterium striatum towards Caenorhabditis elegans

Corynebacterium striatum strains have been increasingly reported as etiological agents of nosocomial infections and outbreaks in industrialized and developing countries. However, there are few studies focused on the virulence potential of C. striatum. A growing body of research supports the use of Caenorhabditis elegans as a model host for investigating the virulence potential of pathogenic bacteria, including corynebacteria. In the present study, chemotaxis behaviour, mortality, and morphological changes were investigated in nematodes infected by four C. striatum strains isolated from different clinical sites, and with different MDR profiles and PFGE types. The results showed chemotaxis of nematodes towards C. striatum. Nematode death (> 60%) was detected from the first day post-infection with all strains tested, but at different levels, independent of biofilm formation on catheter surfaces and differences in growth temperature between nematodes (20 °C) and mammals (37 °C). C. striatum 2369/II multidrug-resistant (MDR; from tracheal aspirate of a patient undergoing endotracheal intubation) and 1961/III multidrug-sensitive (MDS; urine) strains led to 100% mortality in worms. Survival of nematodes was observed until 4 days post-infection with the C. striatum 1954/IV MDS strain isolated from a surgical wound (13%) and 1987/I MDR strain isolated from a patient with a lower respiratory tract infection (39%). The Dar phenotype was observed post-infection with all MDS and MDR strains except 1954/IV. All strains showed the capacity for bagging formation. Star formation was observed only with strains that led to 100% nematode mortality. In conclusion, C. striatum was found to exert virulence for C. elegans. Variations in nematode morphological changes and levels of mortality indicate differences in the virulence potential of C. striatum independent of clinical isolation site, capacity for biofilm formation, and MDR and PFGE profiles.

To block or not to block: The adaptive manipulation of plague transmission

The ability of the agent of plague, Yersinia pestis, to form a biofilm blocking the gut of the flea has been considered to be a key evolutionary step in maintaining flea‐borne transmission. However, blockage decreases dramatically the life expectancy of fleas, challenging the adaptive nature of blockage. Here, we develop an epidemiological model of plague that accounts for its different transmission routes, as well as the within‐host competition taking place between bacteria within the flea vector. We use this theoretical framework to identify the environmental conditions promoting the evolution of blockage. We also show that blockage is favored at the onset of an epidemic, and that the frequencies of bacterial strains exhibiting different strategies of blockage can fluctuate in seasonal environments. This analysis quantifies the contribution of different transmission routes in plague and makes testable predictions on the adaptive nature of blockage.

Differential Gene Expression Patterns of Yersinia pestis and Yersinia pseudotuberculosis during Infection and Biofilm Formation in the Flea Digestive Tract

Yersinia pestis, the etiologic agent of plague, emerged as a fleaborne pathogen only within the last 6,000 years. Just five simple genetic changes in the Yersinia pseudotuberculosis progenitor, which served to eliminate toxicity to fleas and to enhance survival and biofilm formation in the flea digestive tract, were key to the transition to the arthropodborne transmission route. To gain a deeper understanding of the genetic basis for the development of a transmissible biofilm infection in the flea foregut, we evaluated additional gene differences and performed in vivo transcriptional profiling of Y. pestis, a Y. pseudotuberculosis wild-type strain (unable to form biofilm in the flea foregut), and a Y. pseudotuberculosis mutant strain (able to produce foregut-blocking biofilm in fleas) recovered from fleas 1 day and 14 days after an infectious blood meal. Surprisingly, the Y. pseudotuberculosis mutations that increased c-di-GMP levels and enabled biofilm development in the flea did not change the expression levels of the hms genes responsible for the synthesis and export of the extracellular polysaccharide matrix required for mature biofilm formation. The Y. pseudotuberculosis mutant uniquely expressed much higher levels of Yersinia type VI secretion system 4 (T6SS-4) in the flea, and this locus was required for flea blockage by Y. pseudotuberculosis but not for blockage by Y. pestis. Significant differences between the two species in expression of several metabolism genes, the Psa fimbrial genes, quorum sensing-related genes, transcription regulation genes, and stress response genes were evident during flea infection. IMPORTANCE Y. pestis emerged as a highly virulent, arthropod-transmitted pathogen on the basis of relatively few and discrete genetic changes from Y. pseudotuberculosis. Parallel comparisons of the in vitro and in vivo transcriptomes of Y. pestis and two Y. pseudotuberculosis variants that produce a nontransmissible infection and a transmissible infection of the flea vector, respectively, provided insights into how Y. pestis has adapted to life in its flea vector and point to evolutionary changes in the regulation of metabolic and biofilm development pathways in these two closely related species.

Intravital Confocal Microscopy of Dermal Innate Immune Responses to Flea-Transmitted Yersinia pestis

The technique known as intravital microscopy (IVM), when used in conjunction with transgenic mice expressing fluorescent proteins in various cell populations, is a powerful tool with the potential to provide new insights into host-pathogen interactions in infectious disease pathogenesis in vivo. Yersinia pestis, the causative agent of plague, is typically deposited in a host's skin during feeding of an infected flea. IVM has been used to characterize the innate immune response to Y. pestis in the skin and identify differences between the responses to needle-inoculated and flea-transmitted bacteria that would have been difficult, if not impossible, to detect by other means. Here we describe techniques used to image the neutrophil response to flea-transmitted Y. pestis in the dermis of live mice using conventional confocal microscopy.

Ail provides multiple mechanisms of serum resistance to Yersinia pestis

Ail, a multifunctional outer membrane protein of Yersinia pestis, confers cell binding, Yop delivery and serum resistance activities. Resistance to complement proteins in serum is critical for the survival of Y. pestis during the septicemic stage of plague infections. Bacteria employ a variety of tactics to evade the complement system, including recruitment of complement regulatory factors, such as factor H, C4b-binding protein (C4BP) and vitronectin (Vn). Y. pestis Ail interacts with the regulatory factors Vn and C4BP, and Ail homologs from Y. enterocolitica and Y. pseudotuberculosis recruit factor H. Using co-sedimentation assays, we demonstrate that two surface-exposed amino acids, F80 and F130, are required for the interaction of Y. pestis Ail with Vn, factor H and C4BP. However, although Ail-F80A/F130A fails to interact with these complement regulatory proteins, it still confers 10,000-fold more serum resistance than a Δail strain and prevents C9 polymerization, potentially by directly interfering with MAC assembly. Using site-directed mutagenesis, we further defined this additional mechanism of complement evasion conferred by Ail. Finally, we find that at Y. pestis concentrations reflective of early-stage septicemic plague, Ail weakly recruits Vn and fails to recruit factor H, suggesting that this alternative mechanism of serum resistance may be essential during plague infection.

BfvR, an AraC-Family Regulator, Controls Biofilm Formation and pH6 Antigen Production in Opposite Ways in Yersinia pestis Biovar Microtus

Biofilm formation is critical for blocking flea foregut and hence for transmission of Y. pestis by flea biting. In this study, we identified the regulatory role of the AraC-family transcriptional regulator BfvR (YPO1737 in strain CO92) in biofilm formation and virulence of Yersinia pestis biovar Microtus. Crystal violet staining, Caenorhabditis elegans biofilm assay, colony morphology assay, intracellular c-di-GMP concentration determination, and BALB/c mice challenge were employed to reveal that BfvR enhanced Y. pestis biofilm formation while repressed its virulence in mice. Further molecular biological assays demonstrated that BfvR directly stimulated the expression of hmsHFRS, waaAE-coaD, and hmsCDE, which, in turn, affected the production of exopolysaccharide, LPS, and c-di-GMP, respectively. In addition, BfvR directly and indirectly repressed psaABC and psaEF transcription, respectively. We concluded that the modulation of biofilm- and virulence-related genes by BfvR led to increased biofilm formation and reduced virulence of Y. pestis biovar Microtus.

Yersinia pseudotuberculosis BarA-UvrY Two-Component Regulatory System Represses Biofilms via CsrB

The formation of biofilms by Yersinia pseudotuberculosis (Yptb) and Y. pestis requires the hmsHFRS genes, which direct production of a polysaccharide extracellular matrix (Hms-ECM). Despite possessing identical hmsHFRS sequences, Yptb produces much less Hms-ECM than Y. pestis. The regulatory influences that control Yptb Hms-ECM production and biofilm formation are not fully understood. In this study, negative regulators of biofilm production in Yptb were identified. Inactivation of the BarA/UvrY two-component system or the CsrB regulatory RNA increased binding of Congo Red dye, which correlates with extracellular polysaccharide production. These mutants also produced biofilms that were substantially more cohesive than the wild type strain. Disruption of uvrY was not sufficient for Yptb to cause proventricular blockage during infection of Xenopsylla cheopis fleas. However, this strain was less acutely toxic toward fleas than wild type Yptb. Flow cytometry measurements of lectin binding indicated that Yptb BarA/UvrY/CsrB mutants may produce higher levels of other carbohydrates in addition to poly-GlcNAc Hms-ECM. In an effort to characterize the relevant downstream targets of the BarA/UvrY system, we conducted a proteomic analysis to identify proteins with lower abundance in the csrB::Tn5 mutant strain. Urease subunit proteins were less abundant and urease enzymatic activity was lower, which likely reduced toxicity toward fleas. Loss of CsrB impacted expression of several potential regulatory proteins that may influence biofilms, including the RcsB regulator. Overexpression of CsrB did not alter the Congo-red binding phenotype of an rcsB::Tn5 mutant, suggesting that the effect of CsrB on biofilms may require RcsB. These results underscore the regulatory and compositional differences between Yptb and Y. pestis biofilms. By activating CsrB expression, the Yptb BarA/UvrY two-component system has pleiotropic effects that impact biofilm production and stability.

"Fleaing" the Plague: Adaptations of Yersinia pestis to Its Insect Vector That Lead to Transmission

Interest in arthropod-borne pathogens focuses primarily on how they cause disease in humans. How they produce a transmissible infection in their arthropod host is just as critical to their life cycle, however. Yersinia pestis adopts a unique life stage in the digestive tract of its flea vector, characterized by rapid formation of a bacterial biofilm that is enveloped in a complex extracellular polymeric substance. Localization and adherence of the biofilm to the flea foregut is essential for transmission. Here, we review the molecular and genetic mechanisms of these processes and present a comparative evaluation and updated model of two related transmission mechanisms.

A Single Amino Acid Change in the Response Regulator PhoP, Acquired during Yersinia pestis Evolution, Affects PhoP Target Gene Transcription and Polymyxin B Susceptibility

Yersinia pestis, the causative agent of plague, evolved from the closely related pathogen Yersinia pseudotuberculosis During its emergence, Y. pestis is believed to have acquired its unique pathogenic characteristics through numerous gene gains/losses, genomic rearrangements, and single nucleotide polymorphism (SNP) changes. One such SNP creates a single amino acid variation in the DNA binding domain of PhoP, the response regulator in the PhoP/PhoQ two-component system. Y. pseudotuberculosis and the basal human-avirulent strains of Y. pestis harbor glycines at position 215 of PhoP, whereas the modern human-virulent strains (e.g., KIM and CO92) harbor serines at this residue. Since PhoP plays multiple roles in the adaptation of Y. pestis to stressful host conditions, we tested whether this amino acid substitution affects PhoP activity or the ability of Y. pestis to survive in host environments. Compared to the parental KIM6+ strain carrying the modern allele of phoP (phoP-S215), a derivative carrying the basal allele (phoP-G215) exhibited slightly defective growth under a low-Mg2+ condition and decreased transcription of a PhoP target gene, ugd, as well as an ∼8-fold increase in the susceptibility to the antimicrobial peptide polymyxin B. The phoP-G215 strain showed no apparent defect in flea colonization, although a phoP-null mutant showed decreased flea infectivity in competition experiments. Our results suggest that the amino acid variation at position 215 of PhoP causes subtle changes in the PhoP activity and raise the possibility that the change in this residue have contributed to the evolution of increased virulence in Y. pestisIMPORTANCEY. pestis acquired a single nucleotide polymorphism (SNP) in phoP when the highly human-virulent strains diverged from less virulent basal strains, resulting in an amino acid substitution in the DNA binding domain of the PhoP response regulator. We show that Y. pestis carrying the modern phoP allele has an increased ability to induce the PhoP-regulated ugd gene and resist antimicrobial peptides compared to an isogenic strain carrying the basal allele. Given the important roles PhoP plays in host adaptation, the results raise an intriguing possibility that this amino acid substitution contributed to the evolution of increased virulence in Y. pestis Additionally, we present the first evidence that phoP confers a survival fitness advantage to Y. pestis inside the flea midgut.

Yersinia pestis Targets the Host Endosome Recycling Pathway during the Biogenesis of the Yersinia-Containing Vacuole To Avoid Killing by Macrophages

Yersinia pestis has evolved many strategies to evade the innate immune system. One of these strategies is the ability to survive within macrophages. Upon phagocytosis, Y. pestis prevents phagolysosome maturation and establishes a modified compartment termed the Yersinia-containing vacuole (YCV). Y. pestis actively inhibits the acidification of this compartment, and eventually, the YCV transitions from a tight-fitting vacuole into a spacious replicative vacuole. The mechanisms to generate the YCV have not been defined. However, we hypothesized that YCV biogenesis requires Y. pestis interactions with specific host factors to subvert normal vesicular trafficking. In order to identify these factors, we performed a genome-wide RNA interference (RNAi) screen to identify host factors required for Y. pestis survival in macrophages. This screen revealed that 71 host proteins are required for intracellular survival of Y. pestis. Of particular interest was the enrichment for genes involved in endosome recycling. Moreover, we demonstrated that Y. pestis actively recruits Rab4a and Rab11b to the YCV in a type three secretion system-independent manner, indicating remodeling of the YCV by Y. pestis to resemble a recycling endosome. While recruitment of Rab4a was necessary to inhibit YCV acidification and lysosomal fusion early during infection, Rab11b appeared to contribute to later stages of YCV biogenesis. We also discovered that Y. pestis disrupts global host endocytic recycling in macrophages, possibly through sequestration of Rab11b, and this process is required for bacterial replication. These data provide the first evidence that Y. pestis targets the host endocytic recycling pathway to avoid phagolysosomal maturation and generate the YCV.

Yersinia pestis can infect and survive within macrophages. However, the mechanisms that the bacterium use to subvert killing by these phagocytes have not been defined. To provide a better understanding of these mechanisms, we used an RNAi approach to identify host factors required for intracellular Y. pestis survival. This approach revealed that the host endocytic recycling pathway is essential for Y. pestis to avoid clearance by the macrophage. We further demonstrate that Y. pestis remodels the phagosome to resemble a recycling endosome, allowing the bacterium to avoid the normal phagolysosomal maturation pathway. Moreover, we show that infection with Y. pestis disrupts normal recycling in the macrophage and that disruption is required for bacterial replication. These findings provide the first evidence that Y. pestis targets the host endocytic recycling pathway in order to evade killing by macrophages.

Infectious blood source alters early foregut infection and regurgitative transmission of Yersinia pestis by rodent fleas

Fleas can transmit Yersinia pestis by two mechanisms, early-phase transmission (EPT) and biofilm-dependent transmission (BDT). Transmission efficiency varies among flea species and the results from different studies have not always been consistent. One complicating variable is the species of rodent blood used for the infectious blood meal. To gain insight into the mechanism of EPT and the effect that host blood has on it, fleas were fed bacteremic mouse, rat, guinea pig, or gerbil blood; and the location and characteristics of the infection in the digestive tract and transmissibility of Y. pestis were assessed 1 to 3 days after infection. Surprisingly, 10–28% of two rodent flea species fed bacteremic rat or guinea pig blood refluxed a portion of the infected blood meal into the esophagus within 24 h of feeding. We term this phenomenon post-infection esophageal reflux (PIER). In contrast, PIER was rarely observed in rodent fleas fed bacteremic mouse or gerbil blood. PIER correlated with the accumulation of a dense mixed aggregate of Y. pestis, red blood cell stroma, and oxyhemoglobin crystals that filled the proventriculus. At their next feeding, fleas with PIER were 3–25 times more likely to appear partially blocked, with fresh blood retained within the esophagus, than were fleas without PIER. Three days after feeding on bacteremic rat blood, groups of Oropsylla montana transmitted significantly more CFU than did groups infected using mouse blood, and this enhanced transmission was biofilm-dependent. Our data support a model in which EPT results from regurgitation of Y. pestis from a partially obstructed flea foregut and that EPT and BDT can sometimes temporally overlap. The relative insolubility of the hemoglobin of rats and Sciurids and the slower digestion of their blood appears to promote regurgitative transmission, which may be one reason why these rodents are particularly prominent in plague ecology.

Yersinia pestis, the bacterial agent of plague, is transmitted by fleas that feed on blood from rodents that carry this disease. The conclusions from studies comparing how efficiently fleas transmit plague after becoming infected have been inconsistent, possibly because a variety of rodent blood sources have been used. To investigate this, we infected three different flea species with Y. pestis using four different types of rodent blood and compared how well they could transmit three days later. The two rodent flea species that transmitted efficiently tended to reflux bacteria and blood into their esophagus when rat or guinea pig blood was used for the infections, but not when mouse or gerbil blood was used. This reflux phenomenon appears to be related to the solubility of the hemoglobin molecule of different rodent species. In contrast, cat fleas, inefficient transmitters, never refluxed their infected blood meal into the esophagus. Rodent fleas that were infected using reflux-inducing rat blood transmitted more Y. pestis than those that fed on infected mouse blood. These findings improve our understanding of how fleas transmit Y. pestis soon after becoming infected and suggest a reason why certain rodents figure more prominently in plague ecology than others.

Cyclic-di-GMP regulation of virulence in bacterial pathogens

Signaling pathways allow bacteria to adapt to changing environments. For pathogenic bacteria, signaling pathways allow for timely expression of virulence factors and the repression of antivirulence factors within the mammalian host. As the bacteria exit the mammalian host, signaling pathways enable the expression of factors promoting survival in the environment and/or nonmammalian hosts. One such signaling pathway uses the dinucleotide cyclic-di-GMP (c-di-GMP), and many bacterial genomes encode numerous proteins that are responsible for synthesizing and degrading c-di-GMP. Once made, c-di-GMP binds to individual protein and RNA receptors to allosterically alter the macromolecule function to drive phenotypic changes. Each bacterial genome encodes unique sets of genes for c-di-GMP signaling and virulence factors so the regulation by c-di-GMP is organism specific. Recent works have pointed to evidence that c-di-GMP regulates virulence in different bacterial pathogens of mammalian hosts. In this review, we discuss the criteria for determining the contribution of signaling nucleotides to pathogenesis using a well-characterized signaling nucleotide, cyclic AMP (cAMP), in Pseudomonas aeruginosa. Using these criteria, we review the roles of c-di-GMP in mediating virulence and highlight common themes that exist among eight diverse pathogens that cause different diseases through different routes of infection and transmission. WIREs RNA 2018, 9:e1454. doi: 10.1002/wrna.1454 This article is categorized under: RNA in Disease and Development > RNA in Disease.

Trade routes and plague transmission in pre-industrial Europe

Numerous historical works have mentioned that trade routes were to blame for the spread of plague in European history, yet this relationship has never been tested by quantitative evidence. Here, we resolve the hypothetical role of trade routes through statistical analysis on the geo-referenced major trade routes in the early modern period and the 6,656 geo-referenced plague outbreak records in AD1347-1760. Ordinary Least Square (OLS) estimation results show that major trade routes played a dominant role in spreading plague in pre-industrial Europe. Furthermore, the negative correlation between plague outbreaks and their distance from major trade ports indicates the absence of a permanent plague focus in the inland areas of Europe. Major trade routes decided the major plague outbreak hotspots, while navigable rivers determined the geographic pattern of sporadic plague cases. A case study in Germany indicates that plague penetrated further into Europe through the local trade route network. Based on our findings, we propose the mechanism of plague transmission in historical Europe, which is imperative in demonstrating how pandemics were spread in recent human history.

Genomic changes associated with the evolutionary transition of an insect gut symbiont into a blood-borne pathogen

The genus Bartonella comprises facultative intracellular bacteria with a unique lifestyle. After transmission by blood-sucking arthropods they colonize the erythrocytes of mammalian hosts causing acute and chronic infectious diseases. Although the pathogen–host interaction is well understood, little is known about the evolutionary origin of the infection strategy manifested by Bartonella species. Here we analyzed six genomes of Bartonella apis, a honey bee gut symbiont that to date represents the closest relative of pathogenic Bartonella species. Comparative genomics revealed that B. apis encodes a large set of vertically inherited genes for amino acid and cofactor biosynthesis and nitrogen metabolism. Most pathogenic bartonellae have lost these ancestral functions, but acquired specific virulence factors and expanded a vertically inherited gene family for harvesting cofactors from the blood. However, the deeply rooted pathogen Bartonella tamiae has retained many of the ancestral genome characteristics reflecting an evolutionary intermediate state toward a host-restricted intraerythrocytic lifestyle. Our findings suggest that the ancestor of the pathogen Bartonella was a gut symbiont of insects and that the adaptation to blood-feeding insects facilitated colonization of the mammalian bloodstream. This study highlights the importance of comparative genomics among pathogens and non-pathogenic relatives to understand disease emergence within an evolutionary-ecological framework.

Comparative Ability of Oropsylla montana and Xenopsylla cheopis Fleas to Transmit Yersinia pestis by Two Different Mechanisms

Background

Transmission of Yersinia pestis by flea bite can occur by two mechanisms. After taking a blood meal from a bacteremic mammal, fleas have the potential to transmit the very next time they feed. This early-phase transmission resembles mechanical transmission in some respects, but the mechanism is unknown. Thereafter, transmission occurs after Yersinia pestis forms a biofilm in the proventricular valve in the flea foregut. The biofilm can impede and sometimes completely block the ingestion of blood, resulting in regurgitative transmission of bacteria into the bite site. In this study, we compared the relative efficiency of the two modes of transmission for Xenopsylla cheopis, a flea known to become completely blocked at a high rate, and Oropsylla montana, a flea that has been considered to rarely develop proventricular blockage.

Methodology/Principal findings

Fleas that took an infectious blood meal containing Y. pestis were maintained and monitored for four weeks for infection and proventricular blockage. The number of Y. pestis transmitted by groups of fleas by the two modes of transmission was also determined. O. montana readily developed complete proventricular blockage, and large numbers of Y. pestis were transmitted by that mechanism both by it and by X. cheopis, a flea known to block at a high rate. In contrast, few bacteria were transmitted in the early phase by either species.

Conclusions

A model system incorporating standardized experimental conditions and viability controls was developed to more reliably compare the infection, proventricular blockage and transmission dynamics of different flea vectors, and was used to resolve a long-standing uncertainty concerning the vector competence of O. montana. Both X. cheopis and O. montana are fully capable of transmitting Y. pestis by the proventricular biofilm-dependent mechanism.

The ecology of plague is complex and its epidemiology is enigmatic. Many different flea species are able to transmit Yersinia pestis, the plague bacillus, and they can transmit in two different ways. Early-phase transmission can occur during the first week after a flea has fed on a diseased animal. Thereafter, transmission occurs only as bacterial growth in the flea foregut interferes with and eventually blocks blood feeding. Comparisons of the relative ability of different flea vectors to transmit have been problematic, and contradictory results have been reported for the ability of the ground squirrel flea Oropsylla montana to transmit beyond the early phase. Our results show that O. montana readily develops foregut blockage, and transmission by that mechanism was as good as or better than observed for Xenopsylla cheopis, a flea known to block at a high rate. In contrast, very few bacteria were transmitted in the early phase by either of these fleas compared to later times after infection, suggesting that early-phase transmission is pertinent only to highly susceptible animals. Improved characterization of the transmission patterns of different flea vectors will aid in modeling plague incidence in its various natural settings.

'Add, stir and reduce': Yersinia spp. as model bacteria for pathogen evolution

Pathogenic species in the Yersinia genus have historically been targets for research aimed at understanding how bacteria evolve into mammalian pathogens. The advent of large-scale population genomic studies has greatly accelerated the progress in this field, and Yersinia pestis, Yersinia pseudotuberculosis and Yersinia enterocolitica have once again acted as model organisms to help shape our understanding of the evolutionary processes involved in pathogenesis. In this Review, we highlight the gene gain, gene loss and genome rearrangement events that have been identified by genomic studies in pathogenic Yersinia species, and we discuss how these findings are changing our understanding of pathogen evolution. Finally, as these traits are also found in the genomes of other species in the Enterobacteriaceae, we suggest that they provide a blueprint for the evolution of enteropathogenic bacteria.