Evaluation of the Role of the opgGH Operon in Yersinia pseudotuberculosis and Its Deletion during the Emergence of Yersinia pestis

The CpxAR signaling system confers a fitness advantage for flea gut colonization by the plague bacillus

The adaptation of Yersinia pestis, the flea-borne plague agent, to fluctuating environmental conditions is essential for the successful colonization of the flea vector. A previous comparative transcriptomic analysis showed that the Cpx pathway of Y. pestis is up-regulated in infected fleas. The CpxAR two-component system is a component of the envelope stress response and is critical for maintaining the integrity of the cell. Here, a phenotypic screening revealed a survival defect of the cpxAR mutant to oxidative stress and copper. The measured copper concentration in the digestive tract contents of fed fleas increased fourfold during the digestive process. By direct analysis of phosphorylation of CpxR by a Phos-Tag gel approach, we demonstrated that biologically relevant concentrations of copper triggered the system. Then, a competitive challenge highlighted the role of the CpxAR system in bacterial fitness during flea infection. Lastly, an in vitro sequential exposure to copper and then H2O2 to mimic the flea suggests a model in which, within the insect digestive tract, the CpxAR system would be triggered by copper, establishing an oxidative stress response.

IMPORTANCE

The bacterium Yersinia pestis is the agent of flea-borne plague. Our knowledge of the mechanisms used by the plague bacillus to infect the flea vector is limited. The up-regulation of the envelope stress response under the control of the Cpx signaling pathway was previously shown in a transcriptomic study. Here, our in vivo and in vitro approaches suggest a model in which Y. pestis uses the CpxAR phosphorelay system to sense and respond to the copper present in the flea gut, thereby optimizing the flea gut colonization. In other words, the system is essential for bacterial fitness in the flea.

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.

Lytic transglycosylases mitigate periplasmic crowding by degrading soluble cell wall turnover products

The peptidoglycan cell wall is a predominant structure of bacteria, determining cell shape and supporting survival in diverse conditions. Peptidoglycan is dynamic and requires regulated synthesis of new material, remodeling, and turnover - or autolysis - of old material. Despite exploitation of peptidoglycan synthesis as an antibiotic target, we lack a fundamental understanding of how peptidoglycan synthesis and autolysis intersect to maintain the cell wall. Here, we uncover a critical physiological role for a widely misunderstood class of autolytic enzymes, lytic transglycosylases (LTGs). We demonstrate that LTG activity is essential to survival by contributing to periplasmic processes upstream and independent of peptidoglycan recycling. Defects accumulate in Vibrio cholerae LTG mutants due to generally inadequate LTG activity, rather than absence of specific enzymes, and essential LTG activities are likely independent of protein-protein interactions, as heterologous expression of a non-native LTG rescues growth of a conditional LTG-null mutant. Lastly, we demonstrate that soluble, uncrosslinked, endopeptidase-dependent peptidoglycan chains, also detected in the wild-type, are enriched in LTG mutants, and that LTG mutants are hypersusceptible to the production of diverse periplasmic polymers. Collectively, our results suggest that LTGs prevent toxic crowding of the periplasm with synthesis-derived peptidoglycan polymers and, contrary to prevailing models, that this autolytic function can be temporally separate from peptidoglycan synthesis.

Conditional filamentation as an adaptive trait of bacteria and its ecological significance in soils

Bacteria can regulate cell morphology in response to environmental conditions, altering their physiological and metabolic characteristics to improve survival. Conditional filamentation, in which cells suspend division while continuing lateral growth, is a strategy with a range of adaptive benefits. Here, we review the causes and consequences of conditional filamentation with respect to bacterial physiology, ecology and evolution. We describe four major benefits from conditional filamentation: stress tolerance, surface colonization, gradient spanning and the facilitation of biotic interactions. Adopting a filamentous growth habit involves fitness trade-offs which are also examined. We focus on the role of conditional filamentation in soil habitats, where filamentous morphotypes are highly prevalent and where environmental heterogeneity can benefit a conditional response. To illustrate the use of information presented in our review, we tested the conditions regulating filamentation by the forest soil isolate Paraburkholderia elongata 5N^T . Filamentation by P. elongata was induced at elevated phosphate concentrations, and was associated with the accumulation of intracellular polyphosphate, highlighting the role of filamentation in a phosphate-solubilizing bacterium. Conditional filamentation enables bacteria to optimize their growth and metabolism in environments that are highly variable, a trait that can impact succession, symbioses, and biogeochemistry in soil environments.

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.

Absence of osmoregulated periplasmic glucan confers antimicrobial resistance and increases virulence in Escherichia coli

Clarifying the molecular mechanisms by which bacteria acquire virulence traits is important toward understanding the bacterial virulence system. In the present study, we utilized a bacterial evolution method in a silkworm-infection model and revealed that deletion of the opgGH operon encoding synthases for osmoregulated periplasmic glucan (OPG) increased the virulence of non-pathogenic laboratory strain of Escherichia coli against silkworms. The opgGH knockout mutant exhibited resistance to the host antimicrobial peptides and antibiotics. Compared with the parent strain, the opgGH knockout mutant produced greater amounts of colanic acid, which is involved in E. coli resistance to antibiotics. RNA sequence analysis revealed that the opgGH knockout altered the expression of various genes, including the evgS/evgA two-component system that functions in antibiotic resistance. In both a colanic acid-negative background and evgS-null background, the opgGH knockout increased E. coli resistance to antibiotics and increased the silkworm killing activity of E. coli In the null background of the envZ/ompR two-component system, which genetically interacts with opgGH, the opgGH knockout increased the antibiotic resistance and the virulence in silkworms. These findings suggest that the absence of OPG confers antimicrobial resistance and virulence of E. coli in a colanic acid-, evgS/evgA-, and envZ/ompR- independent manner.IMPORTANCEThe gene mutation types that increase bacterial virulence of Escherichia coli remain unclear, in part due to the limited number of methods available for isolating bacterial mutants with increased virulence. We utilized a bacterial evolution method in the silkworm infection model, in which silkworms were infected with mutagenized bacteria and highly virulent bacterial mutants were isolated from dead silkworms. We revealed that knockout of OPG synthases increases E. coli virulence against silkworms. The OPG-knockout mutants were resistant to host antimicrobial peptides as well as antibiotics. Our findings not only suggest a novel mechanism for virulence acquisition in E. coli, but also support the usefulness of utilizing the bacterial experimental evolution method in the silkworm infection model.

Pentaplex real-time PCR for differential detection of Yersinia pestis and Y. pseudotuberculosis and application for testing fleas collected during plague epizootics

Upon acquiring two unique plasmids (pMT1 and pPCP1) and genome rearrangement during the evolution from Yersinia pseudotuberculosis, the plague causative agent Y. pestis is closely related to Y. pseudotuberculosis genetically but became highly virulent. We developed a pentaplex real-time PCR assay that not only detects both Yersinia species but also differentiates Y. pestis strains regarding their plasmid profiles. The five targets used were Y. pestis-specific ypo2088, caf1, and pst located on the chromosome, plasmids pMT1 and pPCP1, respectively; Y. pseudotuberculosis-specific chromosomal gene opgG; and 18S ribosomal RNA gene as an internal control for flea DNA. All targets showed 100% specificity and high sensitivity with limits of detection ranging from 1 fg to 100 fg, with Y. pestis-specific pst as the most sensitive target. Using the assay, Y. pestis strains were differentiated 100% by their known plasmid profiles. Testing Y. pestis and Y. pseudotuberculosis-spiked flea DNA showed there is no interference from flea DNA on the amplification of targeted genes. Finally, we applied the assay for testing 102 fleas collected from prairie dog burrows where prairie dog die-off was reported months before flea collection. All flea DNA was amplified by 18S rRNA; no Y. pseudotuberculosis was detected; one flea was positive for all Y. pestis-specific targets, confirming local Y. pestis transmission. Our results indicated the assay is sensitive and specific for the detection and differentiation of Y. pestis and Y. pseudotuberculosis. The assay can be used in field investigations for the rapid identification of the plague causative agent.

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.

Osmoregulated Periplasmic Glucans Transmit External Signals Through Rcs Phosphorelay Pathway in Yersinia enterocolitica

Fast response to environmental changes plays a key role in the transmission and pathogenesis of Yersinia enterocolitica. Osmoregulated periplasmic glucans (OPGs) are known to be involved in environmental perception of several Enterobacteriaceae pathogens; however, the biological function of OPGs in Y. enterocolitica is still unclear. In this study, we investigated the role of OPGs in Y. enterocolitica by deleting the opgGH operon encoding enzymes responsible for OPGs biosynthesis. Complete loss of OPGs in the ΔopgGH mutant resulted in decreased motility, c-di-GMP production, biofilm formation and smaller cell size, whereas the overproduction of OPGs through restoration of opgGH expression promoted c-di-GMP/biofilm production and increased antibiotic resistance of Y. enterocolitica. Gene expression analysis revealed that opgGH deletion reduced transcription of flhDC, ftsAZ, hmsT and hmsHFRS genes regulated by the Rcs phosphorelay system, whereas additional deletion of rcs family genes (rcsF, rcsC, or rcsB) reversed this effect and restored motility and c-di-GMP/biofilm production but further reduced cell size. Furthermore, disruption of the Rcs phosphorelay increased the motility and promoted the induction of biofilm and c-di-GMP production regulated by OPGs through upregulating the expression of flhDC, hmsHFRS, and hmsT. However, deletion of genes encoding the EnvZ/OmpR phosphorelay downregulated the flhDC, hmsHFRS and hmsT expression, leading to the decreased motility and prevented the induction of biofilm and c-di-GMP production regulated by OPGs. These results indicated that Rcs phosphorelay had the effect on OPGs-mediated functional responses in Y. enterocolitica. Our findings disclose part of the biological role of OPGs and the underlying molecular mechanisms associated with Rcs system in the regulation of the pathogenic phenotype in Y. enterocolitica.

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.

Unraveling neutrophil- Yersinia interactions during tissue infection

The human and animal pathogens Yersinia pestis, which causes bubonic and pneumonic plague, and Yersinia pseudotuberculosis and Yersinia enterocolitica, which cause gastroenteritis, share a type 3 secretion system which injects effector proteins, Yops, into host cells. This system is critical for virulence of all three pathogens in tissue infection. Neutrophils are rapidly recruited to infected sites and all three pathogens frequently interact with and inject Yops into these cells during tissue infection. Host receptors, serum factors, and bacterial adhesins appear to collaborate to promote neutrophil- Yersinia interactions in tissues. The ability of neutrophils to control infection is mixed depending on the stage of infection and points to the efficiency of Yops and other bacterial factors to mitigate bactericidal effects of neutrophils. Yersinia in close proximity to neutrophils has higher levels of expression from yop promoters, and neutrophils in close proximity to Yersinia express higher levels of pro-survival genes than migrating neutrophils. In infected tissues, YopM increases neutrophil survival and YopH targets a SKAP2/SLP-76 signal transduction pathway. Yet the full impact of these and other Yops and other Yersinia factors on neutrophils in infected tissues has yet to be understood.

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.

Osmoregulated Periplasmic Glucans

Among all the systems developed by enterobacteria to face osmotic stress, only osmoregulated periplasmic glucans (OPGs) were found to be modulated during osmotic fluxes. First detected in 1973 by E.P. Kennedy's group in a study of phospholipid turnover in Escherichia coli, OPGs have been shown across alpha, beta, and gamma subdivisions of the proteobacteria. Discovery of OPG-like compounds in the epsilon subdivision strongly suggested that the presence of periplasmic glucans is essential for almost all proteobacteria. This article offers an overview of the different classes of OPGs. Then, the biosynthesis of OPGs and their regulation in E. coli and other species are discussed. Finally, the biological role of OPGs is developed. Beyond structural function, OPGs are involved in pathogenicity, in particular, by playing a role in signal transduction pathways. Recently, OPG synthesis proteins have been suggested to control cell division and growth rate.