Expression of heterologous O-antigen in Yersinia pestis KIM does not affect virulence by the intravenous route

Identification and characterization of P2-like bacteriophages of Yersinia pestis

Yersinia pestis is the cause of plague, historically known as the "Black Death". Marmota himalayana in the Qinghai-Tibet Plateau (QTP) natural plague focus is the primary host in China. Although several phages originating from Y. pestis have been characterized. This is the first report of isolation of P2-like phages of Y. pestis from M. himalayana. In this study, the isolation and characterization of three P2-like phages of Y. pestis were reported, which were named as vB_YpM_22, vB_YpM_46 and vB_YpM_50. Comparative genome analysis revealed that vB_YpM_22, vB_YpM_46 and vB_YpM_50 are members of the nonlambdoid P2 family, and are highly similar and collinear with enterobacteriophage P2, plague diagnostic phage L-413C and enterobacteriophage fiAA91-ss. The role of LPS core structure of Y. pestis in the phages' receptor was pinpointed. The findings of this study contribute an advance in our current knowledge of Y. pestis phages and will also play a key role in understanding the evolution of Y. pestis phages.

Proteus mirabilis Targets Atherosclerosis Plaques in Human Coronary Arteries via DC-SIGN (CD209)

Bacterial DNAs are constantly detected in atherosclerotic plaques (APs), suggesting that a combination of chronic infection and inflammation may have roles in AP formation. A series of studies suggested that certain Gram-negative bacteria were able to interact with dendritic cell-specific intercellular adhesion molecule-3-grabbing non-integrin [DC-SIGN; cluster of differentiation (CD) 209] or langerin (CD207), thereby resulting in deposition of CD209s at infection sites. We wondered if Proteus mirabilis (a member of Proteobacteria family) could interact with APs through CD209/CD207. In this study, we first demonstrated that CD209/CD207 were also receptors for P. mirabilis that mediated adherence and phagocytosis by macrophages. P. mirabilis interacted with fresh and CD209s/CD207-expressing APs cut from human coronary arteries, rather than in healthy and smooth arteries. These interactions were inhibited by addition of a ligand-mimic oligosaccharide and the coverage of the ligand, as well as by anti-CD209 antibody. Finally, the hearts from an atherosclerotic mouse model contained higher numbers of P. mirabilis than that of control mice during infection-challenging. We therefore concluded that the P. mirabilis interacts with APs in human coronary arteries via CD209s/CD207. It may be possible to slow down the progress of atherosclerosis by blocking the interactions between CD209s/CD207 and certain atherosclerosis-involved bacteria with ligand-mimic oligosaccharides.

Murine SIGNR1 (CD209b) Contributes to the Clearance of Uropathogenic Escherichia coli During Urinary Tract Infections

Uropathogenic Escherichia coli (UPEC), a Gram-negative bacterial pathogen, is a major causative agent of urinary tract infections (UTIs). However, the molecular mechanisms of how UPEC causes infections have not been determined. Recent studies indicated that certain enteric Gram-negative bacteria interact with and hijack innate immune receptors DC-SIGN (CD209a) and SIGNR1 (CD209b), often expressed by antigen-presenting cells (APCs), such as macrophages, leading to dissemination and infection. It was not known whether UPEC could utilize DC-SIGN receptors to promote its infection and dissemination similarly to the enteric pathogens. The results of this study reveal that UPEC interacts with CD209-expressing macrophages and transfectants. This interaction is inhibited by anti-CD209 antibody, indicating that CD209s are receptors for UPEC. Additionally, in contrast to the results of previous studies, mice lacking SIGNR1 are more susceptible to infection of this uropathogen, leading to prolonged bacterial persistence. Overall, the results of our study indicate that the innate immune receptor CD209s participate in the clearance of UPEC during UTIs.

Salmonella enterica Serovar Typhimurium Interacts with CD209 Receptors To Promote Host Dissemination and Infection

Salmonella enterica serovar Typhimurium, a Gram-negative bacterium, can cause infectious diseases ranging from gastroenteritis to systemic dissemination and infection. However, the molecular mechanisms underlying this bacterial dissemination have yet to be elucidated. A study indicated that using the lipopolysaccharide (LPS) core as a ligand, S Typhimurium was able to bind human dendritic cell-specific intercellular adhesion molecule-3-grabbing nonintegrin (hCD209a), an HIV receptor that promotes viral dissemination by hijacking antigen-presenting cells (APCs). In this study, we showed that S Typhimurium interacted with CD209s, leading to the invasion of APCs and potentially the dissemination to regional lymph nodes, spleen, and liver in mice. Shielding of the exposed LPS core through the expression of O-antigen reduces dissemination and infection. Thus, we propose that similar to HIV, S Typhimurium may also utilize APCs via interactions with CD209s as a way to disseminate to the lymph nodes, spleen, and liver to initiate host infection.

Yersinia pestis Interacts With SIGNR1 (CD209b) for Promoting Host Dissemination and Infection

Yersinia pestis, a Gram-negative bacterium and the etiologic agent of plague, has evolved from Yersinia pseudotuberculosis, a cause of a mild enteric disease. However, the molecular and biological mechanisms of how Y. pseudotuberculosis evolved to such a remarkably virulent pathogen, Y. pestis, are not clear. The ability to initiate a rapid bacterial dissemination is a characteristic hallmark of Y. pestis infection. A distinguishing characteristic between the two Yersinia species is that Y. pseudotuberculosis strains possess an O-antigen of lipopolysaccharide (LPS) while Y. pestis has lost the O-antigen during evolution and therefore exposes its core LPS. In this study, we showed that Y. pestis utilizes its core LPS to interact with SIGNR1 (CD209b), a C-type lectin receptor on antigen presenting cells (APCs), leading to bacterial dissemination to lymph nodes, spleen and liver, and the initiation of a systemic infection. We therefore propose that the loss of O-antigen represents a critical step in the evolution of Y. pseudotuberculosis into Y. pestis in terms of hijacking APCs, promoting bacterial dissemination and causing the plague.

Yersinia pseudotuberculosis Exploits CD209 Receptors for Promoting Host Dissemination and Infection

Yersinia pseudotuberculosis is a Gram-negative enteropathogen and causes gastrointestinal infections. It disseminates from gut to mesenteric lymph nodes (MLNs), spleen, and liver of infected humans and animals. Although the molecular mechanisms for dissemination and infection are unclear, many Gram-negative enteropathogens presumably invade the small intestine via Peyer's patches to initiate dissemination. In this study, we demonstrate that Y. pseudotuberculosis utilizes its lipopolysaccharide (LPS) core to interact with CD209 receptors, leading to invasion of human dendritic cells (DCs) and murine macrophages. These Y. pseudotuberculosis-CD209 interactions result in bacterial dissemination to MLNs, spleens, and livers of both wild-type and Peyer's patch-deficient mice. The blocking of the Y. pseudotuberculosis-CD209 interactions by expression of O-antigen and with oligosaccharides reduces infectivity. Based on the well-documented studies in which HIV-CD209 interaction leads to viral dissemination, we therefore propose an infection route for Y. pseudotuberculosis where this pathogen, after penetrating the intestinal mucosal membrane, hijacks the Y. pseudotuberculosis-CD209 interaction antigen-presenting cells to reach their target destinations, MLNs, spleens, and livers.

High-throughput analysis of Yersinia pseudotuberculosis gene essentiality in optimised in vitro conditions, and implications for the speciation of Yersinia pestis

Background

Yersinia pseudotuberculosis is a zoonotic pathogen, causing mild gastrointestinal infection in humans. From this comparatively benign pathogenic species emerged the highly virulent plague bacillus, Yersinia pestis, which has experienced significant genetic divergence in a relatively short time span. Much of our knowledge of Yersinia spp. evolution stems from genomic comparison and gene expression studies. Here we apply transposon-directed insertion site sequencing (TraDIS) to describe the essential gene set of Y. pseudotuberculosis IP32953 in optimised in vitro growth conditions, and contrast these with the published essential genes of Y. pestis.

Results

The essential genes of an organism are the core genetic elements required for basic survival processes in a given growth condition, and are therefore attractive targets for antimicrobials. One such gene we identified is yptb3665, which encodes a peptide deformylase, and here we report for the first time, the sensitivity of Y. pseudotuberculosis to actinonin, a deformylase inhibitor. Comparison of the essential genes of Y. pseudotuberculosis with those of Y. pestis revealed the genes whose importance are shared by both species, as well as genes that were differentially required for growth. In particular, we find that the two species uniquely rely upon different iron acquisition and respiratory metabolic pathways under similar in vitro conditions.

Conclusions

The discovery of uniquely essential genes between the closely related Yersinia spp. represent some of the fundamental, species-defining points of divergence that arose during the evolution of Y. pestis from its ancestor. Furthermore, the shared essential genes represent ideal candidates for the development of novel antimicrobials against both species.

Electronic supplementary material

The online version of this article (10.1186/s12866-018-1189-5) contains supplementary material, which is available to authorized users.

Immunisation of two rodent species with new live-attenuated mutants of Yersinia pestis CO92 induces protective long-term humoral- and cell-mediated immunity against pneumonic plague

We showed recently that the live-attenuated Δlpp ΔmsbB Δail and Δlpp ΔmsbB::ailL2 mutants of Yersinia pestis CO92 provided short-term protection to mice against developing subsequent lethal pneumonic plague. These mutants were either deleted for genes encoding Braun lipoprotein (Lpp), an acetyltransferase (MsbB) and the attachment invasion locus (Ail) (Δlpp ΔmsbB Δail) or contained a modified version of the ail gene with diminished virulence (Δlpp ΔmsbB::ailL2). Here, long-term immune responses were first examined after intramuscular immunisation of mice with the above-mentioned mutants, as well as the newly constructed Δlpp ΔmsbB Δpla mutant, deleted for the plasminogen-activator protease (pla) gene instead of ail. Y. pestis-specific IgG levels peaked between day 35 and 56 in the mutant-immunised mice and were sustained until the last tested day 112. Splenic memory B cells peaked earlier (day 42) before declining in the Δlpp ΔmsbB::ailL2 mutant-immunised mice while being sustained for 63 days in the Δlpp ΔmsbB Δail and Δlpp ΔmsbB Δpla mutant-immunised mice. Splenic CD4⁺ T cells increased in all immunised mice by day 42 with differential cytokine production among the immunised groups. On day 120, immunised mice were exposed intranasally to wild-type (WT) CO92, and 80–100% survived pneumonic challenge. Mice immunised with the above-mentioned three mutants had increased innate as well as CD4⁺ responses immediately after WT CO92 exposure, and coupled with sustained antibody production, indicated the role of both arms of the immune response in protection. Likewise, rats vaccinated with either Δlpp ΔmsbB Δail or the Δlpp ΔmsbB Δpla mutant also developed long-term humoral and cell-mediated immune responses to provide 100% protection against developing pneumonic plague. On the basis of the attenuated phenotype, the Δlpp ΔmsbB Δail mutant was recently excluded from the Centers for Disease Control and Prevention select agent list.

Intramuscular Immunization of Mice with a Live-Attenuated Triple Mutant of Yersinia pestis CO92 Induces Robust Humoral and Cell-Mediated Immunity To Completely Protect Animals against Pneumonic Plague

Earlier, we showed that the Δlpp ΔmsbB Δail triple mutant of Yersinia pestis CO92 with deleted genes encoding Braun lipoprotein (Lpp), an acyltransferase (MsbB), and the attachment invasion locus (Ail), respectively, was avirulent in a mouse model of pneumonic plague. In this study, we further evaluated the immunogenic potential of the Δlpp ΔmsbB Δail triple mutant and its derivative by different routes of vaccination. Mice were immunized via the subcutaneous (s.c.) or the intramuscular (i.m.) route with two doses (2 × 10(6) CFU/dose) of the above-mentioned triple mutant with 100% survivability of the animals. Upon subsequent pneumonic challenge with 70 to 92 50% lethal doses (LD(50)) of wild-type (WT) strain CO92, all of the mice survived when immunization occurred by the i.m. route. Since Ail has virulence and immunogenic potential, a mutated version of Ail devoid of its virulence properties was created, and the genetically modified ail replaced the native ail gene on the chromosome of the Δlpp ΔmsbB double mutant, creating a Δlpp ΔmsbB::ailL2 vaccine strain. This newly generated mutant was attenuated similarly to the Δlpp ΔmsbB Δail triple mutant when administered by the i.m. route and provided 100% protection to animals against subsequent pneumonic challenge. Not only were the two above-mentioned mutants cleared rapidly from the initial i.m. site of injection in animals with no histopathological lesions, the immunized mice did not exhibit any disease symptoms during immunization or after subsequent exposure to WT CO92. These two mutants triggered balanced Th1- and Th2-based antibody responses and cell-mediated immunity. A substantial increase in interleukin-17 (IL-17) from the T cells of vaccinated mice, a cytokine of the Th17 cells, further augmented their vaccine potential. Thus, the Δlpp ΔmsbB Δail and Δlpp ΔmsbB::ailL2 mutants represent excellent vaccine candidates for plague, with the latter mutant still retaining Ail immunogenicity but with a much diminished virulence potential.

Host Langerin (CD207) is a receptor for Yersinia pestis phagocytosis and promotes dissemination

Yersinia pestis is a Gram-negative bacterium that causes plague. After Y. pestis overcomes the skin barrier, it encounters antigen-presenting cells (APCs), such as Langerhans and dendritic cells. They transport the bacteria from the skin to the lymph nodes. However, the molecular mechanisms involved in bacterial transmission are unclear. Langerhans cells (LCs) express Langerin (CD207), a calcium-dependent (C-type) lectin. Furthermore, Y. pestis possesses exposed core oligosaccharides. In this study, we show that Y. pestis invades LCs and Langerin-expressing transfectants. However, when the bacterial core oligosaccharides are shielded or truncated, Y. pestis propensity to invade Langerhans and Langerin-expressing cells decreases. Moreover, the interaction of Y. pestis with Langerin-expressing transfectants is inhibited by purified Langerin, a DC-SIGN (DC-specific intercellular adhesion molecule 3 grabbing nonintegrin)-like molecule, an anti-CD207 antibody, purified core oligosaccharides and several oligosaccharides. Furthermore, covering core oligosaccharides reduces the mortality associated with murine infection by adversely affecting the transmission of Y. pestis to lymph nodes. These results demonstrate that direct interaction of core oligosaccharides with Langerin facilitates the invasion of LCs by Y. pestis. Therefore, Langerin-mediated binding of Y. pestis to APCs may promote its dissemination and infection.

Klebsiella pneumoniae O antigen loss alters the outer membrane protein composition and the selective packaging of proteins into secreted outer membrane vesicles

Klebsiella pneumoniae is a nosocomial pathogen which naturally secretes lipopolysaccharide (LPS) and cell envelope associated proteins into the environment through the production of outer membrane vesicles (OMVs). The loss of the LPS O antigen has been demonstrated in other bacterial species to significantly alter the composition of OMVs. Therefore, this study aimed to comprehensively analyze the impact of O antigen loss on the sub-proteomes of both the outer membrane and secreted OMVs from K. pneumoniae. As determined by LC-MS/MS, OMVs were highly enriched with outer membrane proteins involved in cell wall, membrane, and envelope biogenesis as compared to the source cellular outer membrane. Deletion of wbbO, the enzyme responsible for O antigen attachment to LPS, decreased but did not eliminate this enrichment effect. Additionally, loss of O antigen resulted in OMVs with increased numbers of proteins involved in post-translational modification, protein turnover, and chaperones as compared to secreted vesicles from the wild type. This alteration of OMV composition may be a compensatory mechanism to deal with envelope stress. This comprehensive analysis confirms the highly distinct protein composition of OMVs as compared to their source membrane, and provides evidence for a selective sorting mechanism that involves LPS polysaccharides. These data support the hypothesis that modifications to LPS alters both the mechanics of protein sorting and the contents of secreted OMVs and significantly impacts the protein composition of the outer membrane.

Combinational deletion of three membrane protein-encoding genes highly attenuates yersinia pestis while retaining immunogenicity in a mouse model of pneumonic plague

Previously, we showed that deletion of genes encoding Braun lipoprotein (Lpp) and MsbB attenuated Yersinia pestis CO92 in mouse and rat models of bubonic and pneumonic plague. While Lpp activates Toll-like receptor 2, the MsbB acyltransferase modifies lipopolysaccharide. Here, we deleted the ail gene (encoding the attachment-invasion locus) from wild-type (WT) strain CO92 or its lpp single and Δlpp ΔmsbB double mutants. While the Δail single mutant was minimally attenuated compared to the WT bacterium in a mouse model of pneumonic plague, the Δlpp Δail double mutant and the Δlpp ΔmsbB Δail triple mutant were increasingly attenuated, with the latter being unable to kill mice at a 50% lethal dose (LD50) equivalent to 6,800 LD50s of WT CO92. The mutant-infected animals developed balanced TH1- and TH2-based immune responses based on antibody isotyping. The triple mutant was cleared from mouse organs rapidly, with concurrent decreases in the production of various cytokines and histopathological lesions. When surviving animals infected with increasing doses of the triple mutant were subsequently challenged on day 24 with the bioluminescent WT CO92 strain (20 to 28 LD50s), 40 to 70% of the mice survived, with efficient clearing of the invading pathogen, as visualized in real time by in vivo imaging. The rapid clearance of the triple mutant, compared to that of WT CO92, from animals was related to the decreased adherence and invasion of human-derived HeLa and A549 alveolar epithelial cells and to its inability to survive intracellularly in these cells as well as in MH-S murine alveolar and primary human macrophages. An early burst of cytokine production in macrophages elicited by the triple mutant compared to WT CO92 and the mutant's sensitivity to the bactericidal effect of human serum would further augment bacterial clearance. Together, deletion of the ail gene from the Δlpp ΔmsbB double mutant severely attenuated Y. pestis CO92 to evoke pneumonic plague in a mouse model while retaining the required immunogenicity needed for subsequent protection against infection.

Deletion of the Braun lipoprotein-encoding gene and altering the function of lipopolysaccharide attenuate the plague bacterium

Braun (murein) lipoprotein (Lpp) and lipopolysaccharide (LPS) are major components of the outer membranes of Enterobacteriaceae family members that are capable of triggering inflammatory immune responses by activating Toll-like receptors 2 and 4, respectively. Expanding on earlier studies that demonstrated a role played by Lpp in Yersinia pestis virulence in mouse models of bubonic and pneumonic plague, we characterized an msbB in-frame deletion mutant incapable of producing an acyltransferase that is responsible for the addition of lauric acid to the lipid A moiety of LPS, as well as a Δlpp ΔmsbB double mutant of the highly virulent Y. pestis CO92 strain. Although the ΔmsbB single mutant was minimally attenuated, the Δlpp single mutant and the Δlpp ΔmsbB double mutant were significantly more attenuated than the isogenic wild-type (WT) bacterium in bubonic and pneumonic animal models (mouse and rat) of plague. These data correlated with greatly reduced survivability of the aforementioned mutants in murine macrophages. Furthermore, the Δlpp ΔmsbB double mutant was grossly compromised in its ability to disseminate to distal organs in mice and in evoking cytokines/chemokines in infected animal tissues. Importantly, mice that survived challenge with the Δlpp ΔmsbB double mutant, but not the Δlpp or ΔmsbB single mutant, in a pneumonic plague model were significantly protected against a subsequent lethal WT CO92 rechallenge. These data were substantiated by the fact that the Δlpp ΔmsbB double mutant maintained an immunogenicity comparable to that of the WT strain and induced long-lasting T-cell responses against heat-killed WT CO92 antigens. Taken together, the data indicate that deletion of the msbB gene augmented the attenuation of the Δlpp mutant by crippling the spread of the double mutant to the peripheral organs of animals and by inducing cytokine/chemokine responses. Thus, the Δlpp ΔmsbB double mutant could provide a new live-attenuated background vaccine candidate strain, and this should be explored in the future.

Identification of the lipopolysaccharide core of Yersinia pestis and Yersinia pseudotuberculosis as the receptor for bacteriophage φA1122

φA1122 is a T7-related bacteriophage infecting most isolates of Yersinia pestis, the etiologic agent of plague, and used by the CDC in the identification of Y. pestis. φA1122 infects Y. pestis grown both at 20 °C and at 37 °C. Wild-type Yersinia pseudotuberculosis strains are also infected but only when grown at 37 °C. Since Y. pestis expresses rough lipopolysaccharide (LPS) missing the O-polysaccharide (O-PS) and expression of Y. pseudotuberculosis O-PS is largely suppressed at temperatures above 30 °C, it has been assumed that the phage receptor is rough LPS. We present here several lines of evidence to support this. First, a rough derivative of Y. pseudotuberculosis was also φA1122 sensitive when grown at 22 °C. Second, periodate treatment of bacteria, but not proteinase K treatment, inhibited the phage binding. Third, spontaneous φA1122 receptor mutants of Y. pestis and rough Y. pseudotuberculosis could not be isolated, indicating that the receptor was essential for bacterial growth under the applied experimental conditions. Fourth, heterologous expression of the Yersinia enterocolitica O:3 LPS outer core hexasaccharide in both Y. pestis and rough Y. pseudotuberculosis effectively blocked the phage adsorption. Fifth, a gradual truncation of the core oligosaccharide into the Hep/Glc (L-glycero-D-manno-heptose/D-glucopyranose)-Kdo/Ko (3-deoxy-D-manno-oct-2-ulopyranosonic acid/D-glycero-D-talo-oct-2-ulopyranosonic acid) region in a series of LPS mutants was accompanied by a decrease in phage adsorption, and finally, a waaA mutant expressing only lipid A, i.e., also missing the Kdo/Ko region, was fully φA1122 resistant. Our data thus conclusively demonstrated that the φA1122 receptor is the Hep/Glc-Kdo/Ko region of the LPS core, a common structure in Y. pestis and Y. pseudotuberculosis.

Plasminogen activator Pla of Yersinia pestis utilizes murine DEC-205 (CD205) as a receptor to promote dissemination

Yersinia pestis, a Gram-negative bacterium that causes bubonic and pneumonic plague, is able to rapidly disseminate to other parts of its mammalian hosts. Y. pestis expresses plasminogen activator (PLA) on its surface, which has been suggested to play a role in bacterial dissemination. It has been speculated that Y. pestis hijacks antigen-presenting cells, such as macrophages (MPhis) and dendritic cells, to be delivered to lymph nodes to initiate dissemination and infection. Both alveolar MPhis and pulmonary dendritic cells express a C-type lectin receptor, DEC-205 (CD205), which mediates antigen uptake and presentation. However, no ligand has been identified for DEC-205. In this study, we show that the invasion of alveolar MPhisby Y. pestis depends both in vitro and in vivo on the expression of PLA. DEC-205-expressing MPhis and transfectants, but not their negative counterparts, phagocytosed PLA-expressing Y. pestis and Escherichia coli K12 more efficiently than PLA-negative controls. The interactions between PLA-expressing bacteria and DEC-205-expressing transfectants or alveolar MPhis could be inhibited by an anti-DEC-205 antibody. Importantly, the blockage of the PLA-DEC-205 interaction reduced the dissemination of Y. pestis in mice. In conclusion, murine DEC-205 is a receptor for PLA of Y. pestis, and this host-pathogen interaction appears to play a key role in promoting bacterial dissemination.

Human dendritic cell-specific intercellular adhesion molecule-grabbing nonintegrin (CD209) is a receptor for Yersinia pestis that promotes phagocytosis by dendritic cells

Yersinia pestis is the etiologic agent of bubonic and pneumonic plagues. It is speculated that Y. pestis hijacks antigen-presenting cells (APCs), such as dendritic cells (DCs) and alveolar macrophages, in order to be delivered to lymph nodes. However, how APCs initially capture the bacterium remains uncharacterized. It is well known that HIV-1 uses human DC-specific intercellular adhesion molecule-grabbing nonintegrin (DC-SIGN) (CD209) receptor, expressed by APCs, to be captured and delivered to target cell, such as CD4+ lymphocytes. Several gram-negative bacteria utilize their core lipopolysaccharides (LPS) as ligands to interact with the human DC-SIGN. Therefore, it is possible that Y. pestis, whose core LPS is naturally exposed, might exploit DC-SIGN to invade APCs. We demonstrate in this study that Y. pestis directly interacts with DC-SIGN and invades both DCs and alveolar macrophages. In contrast, when engineered to cover the core LPS, Y. pestis loses its ability to invade DCs, alveolar macrophages, and DC-SIGN-expressing transfectants. The interaction between Y. pestis and human DCs can be reduced by a combination treatment with anti-CD209 and anti-CD207 antibodies. This study shows that human DC-SIGN is a receptor for Y. pestis that promotes phagocytosis by DCs in vitro.

Capsular antigen fraction 1 and Pla modulate the susceptibility of Yersinia pestis to pulmonary antimicrobial peptides such as cathelicidin

Inhaled Yersinia pestis produces a severe primary pneumonia known as pneumonic plague, which is contagious and highly lethal to humans and animals. In this study, we first determined the susceptibility of Y. pestis KIM6 to antimicrobial molecules of the airways. We found that (i) rat bronchoalveolar lavage fluid (rBALF) effectively killed KIM6 cells growing at 37 degrees C; (ii) the antibacterial components of rBALF were small peptides (<10 kDa) that included two cationic antimicrobial peptides (CAMPs), the rat cathelicidin rCRAMP, and beta-defensin RBD-1; (iii) the human cathelicidin LL-37 killed KIM6 cells as well as rBALF did; and (iv) the bactericidal property of LL-37 was synergistically amplified by human beta-defensin 1, another constitutively expressed pulmonary CAMP. Second, the effects of three major surface proteins of Y. pestis, namely, the capsular antigen fraction 1 (F1), the pH 6 antigen (Psa fimbriae), and the outer membrane protease Pla, on the bactericidal effect of the antimicrobial rBALF peptides was determined with corresponding deletion mutants. We showed that (i) a Y. pestis psa mutant was only slightly more susceptible to rBALF than the parental KIM6 strain, (ii) a caf (F1 gene) mutant and a caf psa mutant were resistant to rBALF or LL-37, (iii) a caf pla mutant was as susceptible to the effect of rBALF or LL-37 as KIM6 was (caf+ pla+), and (iv) only the single caf mutant (pla+), but not KIM6 or the caf pla double mutant, degraded LL-37. The activity of Pla toward LL-37 was confirmed with pla mutants carrying a single-residue substitution affecting plasminogen cleavage. Taken together, our data indicated that Pla might act as a virulence factor not only by processing plasminogen but also by inactivating CAMPs, particularly when F1 is not expressed.

Molecular and physiological insights into plague transmission, virulence and etiology

Plague is caused by Yersinia pestis, which evolved from the enteric pathogen Y. pseudotuberculosis, which normally causes a chronic and relatively mild disease. Y. pestis is not only able to parasitize the flea but also highly virulent to rodents and humans, causing epidemics of a systemic and often fatal disease. Y. pestis could be used as a bio-weapon and for bio-terrorism. It uses a number of strategies that allow the pathogen to change its lifestyle rapidly to survive in fleas and to grow in the mammalian hosts. Extensive studies reviewed here give an overall picture of the determinants responsible for plague pathogenesis in mammalians and the transmission by fleas. The availability of multiple genomic sequences and more extensive use of genomics and proteomics technologies should allow a comprehensive dissection of the complex of host-adaptation and virulence in Y. pestis.

Pseudogene accumulation might promote the adaptive microevolution of Yersinia pestis

Plague is a natural focus-based disease, and for better understanding of this disease it is crucial to determine the molecular mechanisms of its pathogen, Yersinia pestis, for adapting to different foci. Gene inactivation, loss and acquisition are the main mechanisms that contribute to a pathogen's fitness. Determination of the whole-genome sequences of three Y. pestis strains, CO92, KIM and 91001, provided a good opportunity to probe into its genome in minute detail. Many genetic variations were found between the three strains. The present work focused on adaptive microevolutionary analysis of Y. pestis from different natural plague foci in China based on pseudogene profiles. Twenty-four mutations that led to inactivation in the corresponding genes were analysed, and a PCR-based screening method was employed to investigate the distribution of these mutations among Y. pestis isolates from different foci and also among seven strains of Yersinia pseudotuberculosis. It was found that Y. pestis isolates from the same focus had identical mutation profiles, and 260 isolates of Y. pestis were divided into eight genotypes, while Y. pseudotuberculosis harboured wild-type alleles for all the mutations. The isolates of three known biovars were grouped into distinct branches in the phylogenetic tree, which supports the proposition that biovars mediaevalis and orientalis directly arose from biovar antiqua individually. The constructed phylogenetic tree suggests that the isolates from focus B should be the oldest lineage of Y. pestis in China except for isolates from foci L and M, which might be a special lineage of Y. pestis and originated differently to the others.

Evaluation of the role of constitutive isocitrate lyase activity in Yersinia pestis infection of the flea vector and mammalian host

Yersinia pestis, unlike the closely related Yersinia pseudotuberculosis, constitutively produces isocitrate lyase (ICL). Here we show that the Y. pestis aceA homologue encodes ICL and is required for growth on acetate but not for flea infection or virulence in mice. Thus, deregulation of the glyoxylate pathway does not underlie the recent adaptation of Y. pestis to arthropod-borne transmission.

DNA microarray analysis of genome dynamics in Yersinia pestis: insights into bacterial genome microevolution and niche adaptation

Genomics research provides an unprecedented opportunity for us to probe into the pathogenicity and evolution of the world's most deadly pathogenic bacterium, Yersinia pestis, in minute detail. In our present work, extensive microarray analysis in conjunction with PCR validation revealed that there are considerable genome dynamics, due to gene acquisition and loss, in natural populations of Y. pestis. We established a genomotyping system to group homologous isolates of Y. pestis, based on profiling or gene acquisition and loss in their genomes, and then drew an outline of parallel microevolution of the Y. pestis genome. The acquisition of a number of genomic islands and plasmids most likely induced Y. pestis to evolve rapidly from Yersinia pseudotuberculosis to a new, deadly pathogen. Horizontal gene acquisition also plays a key role in the dramatic evolutionary segregation of Y. pestis lineages (biovars and genomovars). In contrast to selective genome expansion by gene acquisition, genome reduction occurs in Y. pestis through the loss of DNA regions. We also theorized about the links between niche adaptation and genome microevolution. The transmission, colonization, and expansion of Y. pestis in the natural foci of endemic plague are parallel and directional and involve gradual adaptation to the complex of interactions between the environment, the hosts, and the pathogen itself. These adaptations are based on the natural selections against the accumulation of genetic changes within genome. Our data strongly support that the modern plague originated from Yunnan Province in China, due to the arising of biovar orientalis from biovar antiqua rather than mediaevalis.

The biosynthesis and biological role of lipopolysaccharide O-antigens of pathogenic Yersiniae

Lipopolysaccharide (LPS) is the major component of the outer leaflet of the outer membrane of Gram-negative bacteria. The LPS molecule is composed of two biosynthetic entities: the lipid A--core and the O-polysaccharide (O-antigen). Most biological effects of LPS are due to the lipid A part, however, there is an increasing body of evidence indicating that O-antigen (O-ag) plays an important role in effective colonization of host tissues, resistance to complement-mediated killing and in the resistance to cationic antimicrobial peptides that are key elements of the innate immune system. In this review, we will discuss: (i) the work done on the genetics and biosynthesis of the O-ags in the genus Yersinia; (ii) the role of O-ag in virulence of these bacteria; (iii) the work done on regulation of the O-ag gene cluster expression and; (iv) the impact that the O-ag expression has on other bacterial surface and membrane components.