Role of the Yersinia pestis plasminogen activator in the incidence of distinct septicemic and bubonic forms of flea-borne plague

Yersinia pestis intracellular parasitism of macrophages from hosts exhibiting high and low severity of plague

BACKGROUND

Yersinia pestis causes severe disease in natural rodent hosts, but mild to inapparent disease in certain rodent predators such as dogs. Y. pestis initiates infection in susceptible hosts by parasitizing and multiplying intracellularly in local macrophages prior to systemic dissemination. Thus, we hypothesize that Y. pestis disease severity may depend on the degree to which intracellular Y. pestis overcomes the initial host macrophage imposed stress.

METHODOLOGY/PRINCIPAL FINDINGS

To test this hypothesis, the progression of in vitro infection by Y. pestis KIM62053.1+ of mouse splenic and RAW264.7 tissue culture macrophages and dog peripheral blood-derived and DH82 tissue culture macrophages was studied using microscopy and various parameters of infection. The study showed that during the early stage of infection, intracellular Y. pestis assumed filamentous cellular morphology with multiple copies of the genome per bacterium in both mouse and dog macrophages. Later, in mouse macrophages, the infection elicited spacious vacuolar extension of Yersinia containing vacuoles (YCV), and the filamentous Y. pestis reverted to coccobacillary morphology with genomic equivalents approximately equaling colony forming units. In contrast, Y. pestis infected dog macrophages did not show noticeable extension of YCV, and intracellular Y. pestis retained the filamentous cellular morphology for the entire experiment in DH82 cells or were killed by blood-derived macrophages. In addition, during the later stage of infection, Y. pestis infected mouse macrophages exhibited cell lysis whereas dog macrophages did not.

CONCLUSION/SIGNIFICANCE

Overall, these results support our hypothesis that Y. pestis in mouse macrophages can overcome the initial intracellular stress necessary for subsequent systemic infection. However, in dogs, failure of Y. pestis to overcome macrophage imposed stress may result in mild or in apparent disease in dogs.

Structural basis for activation of an integral membrane protease by lipopolysaccharide

Omptins constitute a unique family of outer membrane proteases that are widespread in Enterobacteriaceae. The plasminogen activator (Pla) of Yersinia pestis is an omptin family member that is very important for development of both bubonic and pneumonic plague. The physiological function of Pla is to cleave (activate) human plasminogen to form the plasma protease plasmin. Uniquely, lipopolysaccharide (LPS) is essential for the catalytic activity of all omptins, including Pla. Why omptins require LPS for enzymatic activity is unknown. Here, we report the co-crystal structure of LPS-free Pla in complex with the activation loop peptide of human plasminogen, its natural substrate. The structure shows that in the absence of LPS, the peptide substrate binds deep within the active site groove and displaces the nucleophilic water molecule, providing an explanation for the dependence of omptins on LPS for enzymatic activity.

Identification of chromosomal genes in Yersinia pestis that influence type III secretion and delivery of Yops into target cells

Pathogenic Yersinia species possess a type III secretion system, which is required for the delivery of effector Yop proteins into target cells during infection. Genes encoding the type III secretion machinery, its substrates, and several regulatory proteins all reside on a 70-Kb virulence plasmid. Genes encoded in the chromosome of yersiniae are thought to play important roles in bacterial perception of host environments and in the coordinated activation of the type III secretion pathway. Here, we investigate the contribution of chromosomal genes to the complex regulatory process controlling type III secretion in Yersinia pestis. Using transposon mutagenesis, we identified five chromosomal genes required for expression or secretion of Yops in laboratory media. Four out of the five chromosomal mutants were defective to various extents at injecting Yops into tissue culture cells. Interestingly, we found one mutant that was not able to secrete in vitro but was fully competent for injecting Yops into host cells, suggesting independent mechanisms for activation of the secretion apparatus. When tested in a mouse model of plague disease, three mutants were avirulent, whereas two strains were severely attenuated. Together these results demonstrate the importance of Y. pestis chromosomal genes in the proper function of type III secretion and in the pathogenesis of plague.

Yersinia--flea interactions and the evolution of the arthropod-borne transmission route of plague

Yersinia pestis, the causative agent of plague, is unique among the enteric group of Gram-negative bacteria in relying on a blood-feeding insect for transmission. The Yersinia-flea interactions that enable plague transmission cycles have had profound historical consequences as manifested by human plague pandemics. The arthropod-borne transmission route was a radical ecologic change from the food-borne and water-borne transmission route of Yersinia pseudotuberculosis, from which Y. pestis diverged only within the last 20000 years. Thus, the interactions of Y. pestis with its flea vector that lead to colonization and successful transmission are the result of a recent evolutionary adaptation that required relatively few genetic changes. These changes from the Y. pseudotuberculosis progenitor included loss of insecticidal activity, increased resistance to antibacterial factors in the flea midgut, and extending Yersinia biofilm-forming ability to the flea host environment.

The Yersinia pestis Rcs phosphorelay inhibits biofilm formation by repressing transcription of the diguanylate cyclase gene hmsT

Yersinia pestis, which causes bubonic plague, forms biofilms in fleas, its insect vectors, as a means to enhance transmission. Biofilm development is positively regulated by hmsT, encoding a diguanylate cyclase that synthesizes the bacterial second messenger cyclic-di-GMP. Biofilm development is negatively regulated by the Rcs phosphorelay signal transduction system. In this study, we show that Rcs-negative regulation is accomplished by repressing transcription of hmsT.

An additional step in the transmission of Yersinia pestis?

Plague, caused by the bacterium Yersinia pestis, is a mammalian vector-borne disease, transmitted by fleas that serve as the vector between rodent hosts. For many pathogens, including Y. pestis, there are strong evolutionary pressures that lead to a reduction in 'useless genes', with only those retained that reflect function in the specific environment inhabited by the pathogen. Genetic traits critical for survival and transmission between two environments, the rodent and the flea, are conserved in epizootic/epidemic plague strains. However, there are genes that remain conserved for which no function in the flea-rodent cycle has yet been observed, indicating an additional environment may exist in the transmission cycle of plague. Here, we present evidence for highly conserved genes that suggests a role in the persistence of Y. pestis after death of its host. Furthermore, maintenance of these genes points to Y. pestis traversing a post-mortem path between, and possibly within, epizootic periods and offering insight into mechanisms that may allow Y. pestis an alternative route of transmission in the natural environment.

Tumor necrosis factor alpha modulates the dynamics of the plasminogen-mediated early interaction between Bifidobacterium animalis subsp. lactis and human enterocytes

The capacity to intervene with the host plasminogen system has recently been considered an important component in the interaction process between Bifidobacterium animalis subsp. lactis and the human host. However, its significance in the bifidobacterial microecology within the human gastrointestinal tract is still an open question. Here we demonstrate that human plasminogen favors the B. animalis subsp. lactis BI07 adhesion to HT29 cells. Prompting the HT29 cell capacity to activate plasminogen, tumor necrosis factor alpha (TNF-α) modulated the plasminogen-mediated bacterium-enterocyte interaction, reducing the bacterial adhesion to the enterocytes and enhancing migration to the luminal compartment.

Blood-brain barrier invasion by Cryptococcus neoformans is enhanced by functional interactions with plasmin

Cryptococcus neoformans can invade the central nervous system through diverse mechanisms. We examined a possible role for host plasma proteases in the neurotropic behaviour of this blood-borne fungal pathogen. Plasminogen is a plasma-enriched zymogen that can passively coat the surface of blood-borne pathogens and, upon conversion to the serine protease plasmin, facilitate pathogen dissemination by degrading vascular barriers. In this study, plasminogen-to-plasmin conversion on killed and viable hypoencapsulated strains of C. neoformans required the addition of plasminogen activator (PA), but this conversion occurred in the absence of supplemented PA when viable strains were cultured with brain microvascular endothelial cells (BMEC). Plasmin-coated C. neoformans showed an enhanced invasive ability in Matrigel invasion assays that was significantly augmented in the presence of BMEC. The invasive effect of plasmin required viable pathogen and correlated with rapid declines in BMEC barrier function. Plasmin-enhanced invasion was inhibited by aprotinin, carboxypeptidase B, the lysine analogue epsilon-aminocaproic acid, and by capsule development. C. neoformans caused plasminogen-independent declines in BMEC barrier function that were associated with pathogen-induced host damage; however, such declines were significantly delayed and less extensive than those observed with plasmin-coated pathogen. BMEC adhesion and damage by hypoencapsulated C. neoformans were diminished by capsule induction but unaltered by plasminogen and/or PA. We conclude that hypoencapsulated C. neoformans can invade BMEC by a plasmin-dependent mechanism, in vitro, and that small, or minimal, surface capsule expression during the blood-borne phase of cryptococcosis may promote virulence by means of plasmin(ogen) acquisition.

Role of the Yersinia pestis Ail protein in preventing a protective polymorphonuclear leukocyte response during bubonic plague

The ability of Yersinia pestis to forestall the mammalian innate immune response is a fundamental aspect of plague pathogenesis. In this study, we examined the effect of Ail, a 17-kDa outer membrane protein that protects Y. pestis against complement-mediated lysis, on bubonic plague pathogenesis in mice and rats. The Y. pestis ail mutant was attenuated for virulence in both rodent models. The attenuation was greater in rats than in mice, which correlates with the ability of normal rat serum, but not mouse serum, to kill ail-negative Y. pestis in vitro. Intradermal infection with the ail mutant resulted in an atypical, subacute form of bubonic plague associated with extensive recruitment of polymorphonuclear leukocytes (PMN or neutrophils) to the site of infection in the draining lymph node and the formation of large purulent abscesses that contained the bacteria. Systemic spread and mortality were greatly attenuated, however, and a productive adaptive immune response was generated after high-dose challenge, as evidenced by high serum antibody levels against Y. pestis F1 antigen. The Y. pestis Ail protein is an important bubonic plague virulence factor that inhibits the innate immune response, in particular the recruitment of a protective PMN response to the infected lymph node.

Exploration of the host haemostatic system by group A streptococcus: implications in searching for novel antimicrobial therapies

The haemostatic system is heavily involved in the host response to infection. A number of host haemostatic factors, notably plasminogen and fibrinogen have been reported to bind and interact with various bacterial proteins. This review summarises the roles of host haemostatic factors such as plasminogen, factor V and fibrinogen in host defence against group A streptococcus infection and discusses the potential of targeting the host haemostatic system for therapeutic intervention against infectious diseases.

Progress on plague vaccine development

Yersinia pestis (YP), the gram-negative plague bacterium, has shaped human history unlike any other pathogen known to mankind. YP (transmitted by the bite of an infected flea) diverged only recently from the related enteric pathogen Yersinia pseudotuberculosis but causes radically different diseases. Three forms of plague exist in humans: bubonic (swollen lymph nodes or bubos), septicemic (spread of YP through the lymphatics or bloodstream from the bubos to other organs), and contagious, pneumonic plague which can be communicated via YP-charged respiratory droplets resulting in person-person transmission and rapid death if left untreated (50-90% mortality). Despite the potential threat of weaponized YP being employed in bioterrorism and YP infections remaining prevalent in endemic regions of the world where rodent populations are high (including the four corner regions of the USA), an efficacious vaccine that confers immunoprotection has yet to be developed. This review article will describe the current vaccine candidates being evaluated in various model systems and provide an overall summary on the progress of this important endeavor.

Tn5AraOut mutagenesis for the identification of Yersinia pestis genes involved in resistance towards cationic antimicrobial peptides

Bacterial pathogens display a variety of protection mechanisms against the inhibitory and lethal effects of host cationic antimicrobial peptides (CAMPs). To identify Yersinia pestis genes involved in CAMP resistance, libraries of DSY101 (KIM6 caf1 pla psa) minitransposon Tn5AraOut mutants were selected at 37°C for resistance to the model CAMPs polymyxin B or protamine. This approach targeted genes that needed to be repressed (null mutations) or induced (upstream P(BAD) insertions) for the detection of CAMP resistance, and predictably for improved pathogen fitness in mammalian hosts. Ten mutants demonstrated increased resistance to polymyxin B or protamine, with the mapped mutations pointing towards genes suspected to participate in modifying membrane components, genes encoding transport proteins or enzymes, or the regulator of a ferrous iron uptake system (feoC). Not all the mutants were resistant to both CAMPs used for selection. None of the polymyxin B- and only some protamine-resistant mutants, including the feoC mutant, showed increased resistance to rat bronchoalveolar lavage fluid (rBALF) known to contain cathelicidin and β-defensin 1. Thus, findings on bacterial resistance to polymyxin B or protamine don't always apply to CAMPs of the mammalian innate immune system, such as the ones in rBALF.

Regulatory effects of cAMP receptor protein (CRP) on porin genes and its own gene in Yersinia pestis

Background

The cAMP receptor protein (CRP) is a global bacterial regulator that controls many target genes. The CRP-cAMP complex regulates the ompR-envZ operon in E. coli directly, involving both positive and negative regulations of multiple target promoters; further, it controls the production of porins indirectly through its direct action on ompR-envZ. Auto-regulation of CRP has also been established in E. coli. However, the regulation of porin genes and its own gene by CRP remains unclear in Y. pestis.

Results

Y. pestis employs a distinct mechanism indicating that CRP has no regulatory effect on the ompR-envZ operon; however, it stimulates ompC and ompF directly, while repressing ompX. No transcriptional regulatory association between CRP and its own gene can be detected in Y. pestis, which is also in contrast to the fact that CRP acts as both repressor and activator for its own gene in E. coli. It is likely that Y. pestis OmpR and CRP respectively sense different signals (medium osmolarity, and cellular cAMP levels) to regulate porin genes independently.

Conclusion

Although the CRP of Y. pestis shows a very high homology to that of E. coli, and the consensus DNA sequence recognized by CRP is shared by the two bacteria, the Y. pestis CRP can recognize the promoters of ompC, F, and X directly rather than that of its own gene, which is different from the relevant regulatory circuit of E. coli. Data presented here indicate a remarkable remodeling of the CRP-mediated regulation of porin genes and of its own one between these two bacteria.

Molecular adaptation of a plant-bacterium outer membrane protease towards plague virulence factor Pla

Background

Omptins are a family of outer membrane proteases that have spread by horizontal gene transfer in Gram-negative bacteria that infect vertebrates or plants. Despite structural similarity, the molecular functions of omptins differ in a manner that reflects the life style of their host bacteria. To simulate the molecular adaptation of omptins, we applied site-specific mutagenesis to make Epo of the plant pathogenic Erwinia pyrifoliae exhibit virulence-associated functions of its close homolog, the plasminogen activator Pla of Yersinia pestis. We addressed three virulence-associated functions exhibited by Pla, i.e., proteolytic activation of plasminogen, proteolytic degradation of serine protease inhibitors, and invasion into human cells.

Results

Pla and Epo expressed in Escherichia coli are both functional endopeptidases and cleave human serine protease inhibitors, but Epo failed to activate plasminogen and to mediate invasion into a human endothelial-like cell line. Swapping of ten amino acid residues at two surface loops of Pla and Epo introduced plasminogen activation capacity in Epo and inactivated the function in Pla. We also compared the structure of Pla and the modeled structure of Epo to analyze the structural variations that could rationalize the different proteolytic activities. Epo-expressing bacteria managed to invade human cells only after all extramembranous residues that differ between Pla and Epo and the first transmembrane β-strand had been changed.

Conclusions

We describe molecular adaptation of a protease from an environmental setting towards a virulence factor detrimental for humans. Our results stress the evolvability of bacterial β-barrel surface structures and the environment as a source of progenitor virulence molecules of human pathogens.

Cpa, the outer membrane protease of Cronobacter sakazakii, activates plasminogen and mediates resistance to serum bactericidal activity

Cronobacter spp. are emerging neonatal pathogens in humans, associated with outbreaks of meningitis and sepsis. To cause disease, they must survive in blood and invade the central nervous system by penetrating the blood-brain barrier. C. sakazakii BAA-894 possesses an ~131-kb plasmid (pESA3) that encodes an outer membrane protease (Cpa) that has significant identity to proteins that belong to the Pla subfamily of omptins. Members of this subfamily of proteins degrade a number of serum proteins, including circulating complement, providing protection from the complement-dependent serum killing. Moreover, proteins of the Pla subfamily can cause uncontrolled plasmin activity by converting plasminogen to plasmin and inactivating the plasmin inhibitor α2-antiplasmin (α2-AP). These reactions enhance the spread and invasion of bacteria in the host. In this study, we found that an isogenic cpa mutant showed reduced resistance to serum in comparison to its parent C. sakazakii BAA-894 strain. Overexpression of Cpa in C. sakazakii or Escherichia coli DH5α showed that Cpa proteolytically cleaved complement components C3, C3a, and C4b. Furthermore, a strain of C. sakazakii overexpressing Cpa caused a rapid activation of plasminogen and inactivation of α2-AP. These results strongly suggest that Cpa may be an important virulence factor involved in serum resistance, as well as in the spread and invasion of C. sakazakii.

Characterization of pPCP1 Plasmids in Yersinia pestis Strains Isolated from the Former Soviet Union

Complete sequences of 9.5-kb pPCP1 plasmids in three Yersinia pestis strains from the former Soviet Union (FSU) were determined and compared with those of pPCP1 plasmids in three well-characterized, non-FSU Y. pestis strains (KIM, CO92, and 91001). Two of the FSU plasmids were from strains C2614 and C2944, isolated from plague foci in Russia, and one plasmid was from strain C790 from Kyrgyzstan. Sequence analyses identified four sequence types among the six plasmids. The pPCP1 plasmids in the FSU strains were most genetically related to the pPCP1 plasmid in the KIM strain and least related to the pPCP1 plasmid in Y. pestis 91001. The FSU strains generally had larger pPCP1 plasmid copy numbers compared to strain CO92. Expression of the plasmid's pla gene was significantly (P ≤ .05) higher in strain C2944 than in strain CO92. Given pla's role in Y. pestis virulence, this difference may have important implications for the strain's virulence.

Role of the Yersinia pestis yersiniabactin iron acquisition system in the incidence of flea-borne plague

Plague is a flea-borne zoonosis caused by the bacterium Yersinia pestis. Y. pestis mutants lacking the yersiniabactin (Ybt) siderophore-based iron transport system are avirulent when inoculated intradermally but fully virulent when inoculated intravenously in mice. Presumably, Ybt is required to provide sufficient iron at the peripheral injection site, suggesting that Ybt would be an essential virulence factor for flea-borne plague. Here, using a flea-to-mouse transmission model, we show that a Y. pestis strain lacking the Ybt system causes fatal plague at low incidence when transmitted by fleas. Bacteriology and histology analyses revealed that a Ybt-negative strain caused only primary septicemic plague and atypical bubonic plague instead of the typical bubonic form of disease. The results provide new evidence that primary septicemic plague is a distinct clinical entity and suggest that unusual forms of plague may be caused by atypical Y. pestis strains.

An active site water network in the plasminogen activator pla from Yersinia pestis

The plasminogen activator Pla from Yersinia pestis is an outer membrane protease (omptin) that is important for the virulence of plague. Here, we present the high-resolution crystal structure of wild-type, enzymatically active Pla at 1.9 A. The structure shows a water molecule located between active site residues D84 and H208, which likely corresponds to the nucleophilic water. A number of other water molecules are present in the active site, linking residues important for enzymatic activity. The R211 sidechain in loop L4 is close to the nucleophilic water and possibly involved in the stabilization of the oxyanion intermediate. Subtle conformational changes of H208 result from the binding of lipopolysaccharide to the outside of the barrel, explaining the unusual dependence of omptins on lipopolysaccharide for activity. The Pla structure suggests a model for the interaction with plasminogen substrate and provides a more detailed understanding of the catalytic mechanism of omptin proteases.

Thrombin-activatable fibrinolysis inhibitor is degraded by Salmonella enterica and Yersinia pestis

BACKGROUND

 Pathogenic bacteria modulate the host coagulation system to evade immune responses or to facilitate dissemination through extravascular tissues. In particular, the important bacterial pathogens Salmonella enterica and Yersinia pestis intervene with the plasminogen/fibrinolytic system. Thrombin-activatable fibrinolysis inhibitor (TAFI) has anti-fibrinolytic properties as the active enzyme (TAFIa) removes C-terminal lysine residues from fibrin, thereby attenuating accelerated plasmin formation.

RESULTS

 Here, we demonstrate inactivation and cleavage of TAFI by homologous surface proteases, the omptins Pla of Y. pestis and PgtE of S. enterica. We show that omptin-expressing bacteria decrease TAFI activatability by thrombin-thrombomodulin and that the anti-fibrinolytic potential of TAFIa was reduced by recombinant Escherichia coli expressing Pla or PgtE. The functional impairment resulted from C-terminal cleavage of TAFI by the omptins.

CONCLUSIONS

 Our results indicate that TAFI is degraded directly by the omptins PgtE of S. enterica and Pla of Y. pestis. This may contribute to the ability of PgtE and Pla to damage tissue barriers, such as fibrin, and thereby to enhance spread of S. enterica and Y. pestis during infection.

Corruption of innate immunity by bacterial proteases

The innate immune system of the human body has developed numerous mechanisms to control endogenous and exogenous bacteria and thus prevent infections by these microorganisms. These mechanisms range from physical barriers such as the skin or mucosal epithelium to a sophisticated array of molecules and cells that function to suppress or prevent bacterial infection. Many bacteria express a variety of proteases, ranging from non-specific and powerful enzymes that degrade many proteins involved in innate immunity to proteases that are extremely precise and specific in their mode of action. Here we have assembled a comprehensive picture of how bacterial proteases affect the host's innate immune system to gain advantage and cause infection. This picture is far from being complete since the numbers of mechanisms utilized are as astonishing as they are diverse, ranging from degradation of molecules vital to innate immune mechanisms to subversion of the mechanisms to allow the bacterium to hide from the system or take advantage of it. It is vital that such mechanisms are elucidated to allow strategies to be developed to aid the innate immune system in controlling bacterial infections.

Outer membrane protein X (Ail) contributes to Yersinia pestis virulence in pneumonic plague and its activity is dependent on the lipopolysaccharide core length

Yersinia pestis, the causative agent of plague, is one of the most virulent microorganisms known. The outer membrane protein X (OmpX) in Y. pestis KIM is required for efficient bacterial adherence to and internalization by cultured HEp-2 cells and confers resistance to human serum. Here, we tested the contribution of OmpX to disease progression in the fully virulent Y. pestis CO92 strain by engineering a deletion mutant and comparing its ability in mediating pneumonic plague to that of the wild type in two animal models. The deletion of OmpX delayed the time to death up to 48 h in a mouse model and completely attenuated virulence in a rat model of disease. All rats challenged with 1 × 10(8) CFU of the ompX mutant survived, compared to the 50% lethal dose (LD50) of 1.2 × 10(3) CFU for the wild-type strain. Because murine serum is not bactericidal for the ompX mutant, the mechanism underlying the delay in time to death in mice was attributed to loss of adhesion/internalization properties but not serum resistance. The rat model, which is most similar to humans, highlighted the critical role of serum resistance in disease. To resolve conflicting evidence for the role of Y. pestis lipopolysaccharide (LPS) and OmpX in serum resistance, ompX was cloned into Escherichia coli D21 and three isogenic derivatives engineered to have progressively truncated LPS core saccharides. OmpX-mediated serum resistance, adhesiveness, and invasiveness, although dependent on LPS core length, displayed these functions in E. coli, independently of other Yersinia proteins and/or LPS. Also, autoaggregation was required for efficient OmpX-mediated adhesiveness and internalization but not serum resistance.

The omptins of Yersinia pestis and Salmonella enterica cleave the reactive center loop of plasminogen activator inhibitor 1

Plasminogen activator inhibitor 1 (PAI-1) is a serine protease inhibitor (serpin) and a key molecule that regulates fibrinolysis by inactivating human plasminogen activators. Here we show that two important human pathogens, the plague bacterium Yersinia pestis and the enteropathogen Salmonella enterica serovar Typhimurium, inactivate PAI-1 by cleaving the R346-M347 bait peptide bond in the reactive center loop. No cleavage of PAI-1 was detected with Yersinia pseudotuberculosis, an oral/fecal pathogen from which Y. pestis has evolved, or with Escherichia coli. The cleavage and inactivation of PAI-1 were mediated by the outer membrane proteases plasminogen activator Pla of Y. pestis and PgtE protease of S. enterica, which belong to the omptin family of transmembrane endopeptidases identified in Gram-negative bacteria. Cleavage of PAI-1 was also detected with the omptins Epo of Erwinia pyrifoliae and Kop of Klebsiella pneumoniae, which both belong to the same omptin subfamily as Pla and PgtE, whereas no cleavage of PAI-1 was detected with omptins of Shigella flexneri or E. coli or the Yersinia chromosomal omptins, which belong to other omptin subfamilies. The results reveal a novel serpinolytic mechanism by which enterobacterial species expressing omptins of the Pla subfamily bypass normal control of host proteolysis.

Three Yersinia pestis adhesins facilitate Yop delivery to eukaryotic cells and contribute to plague virulence

To establish a successful infection, Yersinia pestis requires the delivery of cytotoxic Yops to host cells. Yops inhibit phagocytosis, block cytokine responses, and induce apoptosis of macrophages. The Y. pestis adhesin Ail facilitates Yop translocation and is required for full virulence in mice. To determine the contributions of other adhesins to Yop delivery, we deleted five known adhesins of Y. pestis. In addition to Ail, plasminogen activator (Pla) and pH 6 antigen (Psa) could mediate Yop translocation to host cells. The contribution of each adhesin to binding and Yop delivery was dependent upon the growth conditions. When cells were pregrown at 28°C and pH 7, the order of importance for adhesins in cell binding and cytotoxicity was Ail > Pla > Psa. Y. pestis grown at 37°C and pH 7 had equal contributions from Ail and Pla but an undetectable role for Psa. At 37°C and pH 6, both Ail and Psa contributed to binding and Yop delivery, while Pla contributed minimally. Pla-mediated Yop translocation was independent of protease activity. Of the three single mutants, the Δail mutant was the most defective in mouse virulence. The expression level of ail was also the highest of the three adhesins in infected mouse tissues. Compared to an ail mutant, additional deletion of psaA (encoding Psa) led to a 130,000-fold increase in the 50% lethal dose for mice relative to that of the KIM5 parental strain. Our results indicate that in addition to Ail, Pla and Psa can serve as environmentally specific adhesins to facilitate Yop secretion, a critical virulence function of Y. pestis.

Delineation and analysis of chromosomal regions specifying Yersinia pestis

Yersinia pestis, the causative agent of plague, has recently diverged from the less virulent enteropathogen Yersinia pseudotuberculosis. Its emergence has been characterized by massive genetic loss and inactivation and limited gene acquisition. The acquired genes include two plasmids, a filamentous phage, and a few chromosomal loci. The aim of this study was to characterize the chromosomal regions acquired by Y. pestis. Following in silico comparative analysis and PCR screening of 98 strains of Y. pseudotuberculosis and Y. pestis, we found that eight chromosomal loci (six regions [R1pe to R6pe] and two coding sequences [CDS1pe and CDS2pe]) specified Y. pestis. Signatures of integration by site specific or homologous recombination were identified for most of them. These acquisitions and the loss of ancestral DNA sequences were concentrated in a chromosomal region opposite to the origin of replication. The specific regions were acquired very early during Y. pestis evolution and were retained during its microevolution, suggesting that they might bring some selective advantages. Only one region (R3pe), predicted to carry a lambdoid prophage, is most likely no longer functional because of mutations. With the exception of R1pe and R2pe, which have the potential to encode a restriction/modification and a sugar transport system, respectively, no functions could be predicted for the other Y. pestis-specific loci. To determine the role of the eight chromosomal loci in the physiology and pathogenicity of the plague bacillus, each of them was individually deleted from the bacterial chromosome. None of the deletants exhibited defects during growth in vitro. Using the Xenopsylla cheopis flea model, all deletants retained the capacity to produce a stable and persistent infection and to block fleas. Similarly, none of the deletants caused any acute flea toxicity. In the mouse model of infection, all deletants were fully virulent upon subcutaneous or aerosol infections. Therefore, our results suggest that acquisition of new chromosomal materials has not been of major importance in the dramatic change of life cycle that has accompanied the emergence of Y. pestis.

Bacteria-induced phagocyte secondary necrosis as a pathogenicity mechanism

Triggering of phagocyte apoptosis is a major virulence mechanism used by some successful bacterial pathogens. A central issue in the apoptotic death context is that fully developed apoptosis results in necrotic cell autolysis (secondary necrosis) with release of harmful cell components. In multicellular animals, this occurs when apoptosing cells are not removed by scavengers, mainly macrophages. Secondary necrotic lysis of neutrophils and macrophages may occur in infection when extensive phagocyte apoptosis is induced by bacterial cytotoxins and removal of apoptosing phagocytes is defective because the apoptotic process exceeds the available scavenging capacity or targets macrophages directly. Induction of phagocyte secondary necrosis is an important pathogenic mechanism, as it combines the pathogen evasion from phagocyte antimicrobial activities and the release of highly cytotoxic molecules, particularly of neutrophil origin, such as neutrophil elastase. This pathogenicity mechanism therefore promotes the unrestricted multiplication of the pathogen and contributes directly to the pathology of several necrotizing infections, where extensive apoptosis and necrosis of macrophages and neutrophils are present. Here, examples of necrotizing infectious diseases, where phagocyte secondary necrosis is implicated, are reviewed.

ppGpp conjures bacterial virulence

Like for all microbes, the goal of every pathogen is to survive and replicate. However, to overcome the formidable defenses of their hosts, pathogens are also endowed with traits commonly associated with virulence, such as surface attachment, cell or tissue invasion, and transmission. Numerous pathogens couple their specific virulence pathways with more general adaptations, like stress resistance, by integrating dedicated regulators with global signaling networks. In particular, many of nature's most dreaded bacteria rely on nucleotide alarmones to cue metabolic disturbances and coordinate survival and virulence programs. Here we discuss how components of the stringent response contribute to the virulence of a wide variety of pathogenic bacteria.

Effective, broad spectrum control of virulent bacterial infections using cationic DNA liposome complexes combined with bacterial antigens

Protection against virulent pathogens that cause acute, fatal disease is often hampered by development of microbial resistance to traditional chemotherapeutics. Further, most successful pathogens possess an array of immune evasion strategies to avoid detection and elimination by the host. Development of novel, immunomodulatory prophylaxes that target the host immune system, rather than the invading microbe, could serve as effective alternatives to traditional chemotherapies. Here we describe the development and mechanism of a novel pan-anti-bacterial prophylaxis. Using cationic liposome non-coding DNA complexes (CLDC) mixed with crude F. tularensis membrane protein fractions (MPF), we demonstrate control of virulent F. tularensis infection in vitro and in vivo. CLDC+MPF inhibited bacterial replication in primary human and murine macrophages in vitro. Control of infection in macrophages was mediated by both reactive nitrogen species (RNS) and reactive oxygen species (ROS) in mouse cells, and ROS in human cells. Importantly, mice treated with CLDC+MPF 3 days prior to challenge survived lethal intranasal infection with virulent F. tularensis. Similarly to in vitro observations, in vivo protection was dependent on the presence of RNS and ROS. Lastly, CLDC+MPF was also effective at controlling infections with Yersinia pestis, Burkholderia pseudomallei and Brucella abortus. Thus, CLDC+MPF represents a novel prophylaxis to protect against multiple, highly virulent pathogens.

Conventional treatment of bacterial infections typically includes administration of antibiotics. However, many pathogens have developed spontaneous resistance to commonly used antibiotics. Development of new compounds that stimulate the host immune system to directly kill bacteria by mechanisms different from those utilized by antibiotics may serve as effective alternatives to antibiotic therapy. In this report, we describe a novel compound capable of controlling infections mediated by different, unrelated bacteria via the induction of host derived reactive oxygen and reactive nitrogen species. This compound is comprised of cationic liposome DNA complexes (CLDC) and crude membrane preparations (MPF) obtained from attenuated Francisella tularensis Live Vaccine Strain (LVS). Pretreatment of primary mouse or human cells limited replication of virulent F. tularensis, Burkholderia pseudomallei, Yersinia pestis and Brucella abortus in vitro. CLDC+MPF was also effective for controlling lethal pulmonary infections with virulent F. tularensis. Thus, CLDC+MPF represents a novel antimicrobial for treatment of lethal, acute, bacterial infections.

Temperature-induced changes in the lipopolysaccharide of Yersinia pestis affect plasminogen activation by the pla surface protease

The Pla surface protease of Yersinia pestis activates human plasminogen and is a central virulence factor in bubonic and pneumonic plague. Pla is a transmembrane beta-barrel protein and member of the omptin family of outer membrane proteases which require bound lipopolysaccharide (LPS) to be proteolytically active. Plasminogen activation and autoprocessing of Pla were dramatically higher in Y. pestis cells grown at 37 degrees C than in cells grown at 20 degrees C; the difference in enzymatic activity by far exceeded the increase in the cellular content of the Pla protein. Y. pestis modifies its LPS structure in response to growth temperature. We purified His(6)-Pla under denaturing conditions and compared various LPS types for their capacity to enhance plasmin formation by His(6)-Pla solubilized in detergent. Reactivation of His(6)-Pla was higher with Y. pestis LPSs isolated from bacteria grown at 37 degrees C than with LPSs from cells grown at 25 degrees C. Lack of O antigens and the presence of the outer core region as well as a lowered level of acylation in LPS were found to enhance the Pla-LPS interaction. Genetic substitution of arginine 138, which is part of a three-dimensional protein motif for binding to lipid A phosphates, decreased both the enzymatic activity of His(6)-Pla and the amount of Pla in Y. pestis cells, suggesting the importance of the Pla-lipid A phosphate interaction. The temperature-induced changes in LPS are known to help Y. pestis to avoid innate immune responses, and our results strongly suggest that they also potentiate Pla-mediated proteolysis.

Transit through the flea vector induces a pretransmission innate immunity resistance phenotype in Yersinia pestis

Yersinia pestis, the agent of plague, is transmitted to mammals by infected fleas. Y. pestis exhibits a distinct life stage in the flea, where it grows in the form of a cohesive biofilm that promotes transmission. After transmission, the temperature shift to 37 degrees C induces many known virulence factors of Y. pestis that confer resistance to innate immunity. These factors are not produced in the low-temperature environment of the flea, however, suggesting that Y. pestis is vulnerable to the initial encounter with innate immune cells at the flea bite site. In this study, we used whole-genome microarrays to compare the Y. pestis in vivo transcriptome in infective fleas to in vitro transcriptomes in temperature-matched biofilm and planktonic cultures, and to the previously characterized in vivo gene expression profile in the rat bubo. In addition to genes involved in metabolic adaptation to the flea gut and biofilm formation, several genes with known or predicted roles in resistance to innate immunity and pathogenicity in the mammal were upregulated in the flea. Y. pestis from infected fleas were more resistant to phagocytosis by macrophages than in vitro-grown bacteria, in part attributable to a cluster of insecticidal-like toxin genes that were highly expressed only in the flea. Our results suggest that transit through the flea vector induces a phenotype that enhances survival and dissemination of Y. pestis after transmission to the mammalian host.

Plague in the 21st Century: Global Public Health Challenges and Goals

Yersinia pestis, the Gram-negative bacterial agent of plague, is a zoonotic pathogen that primarily infects wild rodents and is transmitted by fleas. Y. pestis is one of the most invasive and virulent bacterial pathogens and has caused devastating pandemics, including the Black Death of 14th century Europe. The last plague pandemic began in Asia in the last half of the 19th century and lingered well into the 20th century, causing tens of millions of deaths as it spread across the world.

Deletion of Braun lipoprotein gene (lpp) attenuates Yersinia pestis KIM/D27 strain: role of Lpp in modulating host immune response, NF-kappaB activation and cell death

The pathogenic species of yersiniae potently blocks immune responses in host cells by using the type III secretion apparatus and its effector proteins. In this study, we characterized potential mechanisms associated with the Braun lipoprotein (Lpp) that contributed to a further attenuation of a pigmentation locus-minus Yersinia pestis KIM/D27 mutant strain and its ability to generate immune responses in mice. The lpp gene encodes one of the major outer membrane lipoproteins that is involved in inflammatory responses and septic shock. We found that sera and splenocytes from Deltalpp mutant-immunized mice, when transferred to naïve animals, provided protection to the latter against challenge with a lethal dose of the Y. pestis parental strain. Further, the Deltalpp mutant promoted ex vivo a significantly higher interleukin (IL)-2 and interferon-gamma production from T cells of immunized mice, when compared to those from animals infected with the sub-lethal dose of the parental Y. pestis KIM/D27 strain. Likewise, murine primary macrophages infected with the mutant, when compared to those infected with the parental strain in vitro, produced significantly higher IL-12 levels. Importantly, increased nuclear factor-kappa B activation and decreased apoptosis were noted in splenocytes and primary macrophages of mice challenged with the Deltalpp mutant, when compared to those in animals infected with the parental Y. pestis KIM/D27 strain. Finally, significantly higher levels of antibodies specific for the parental Y. pestis antigens were developed in mice first immunized with the Deltalpp mutant and then challenged with the parental strain, compared to the antibody levels in animals that were immunized and then infected with the parental KIM/D27 strain. To our knowledge, this is the first report of a mechanistic basis for attenuation and immunological responses associated with deletion of the lpp gene from the Y. pestis KIM/D27 strain.

Innate immune response during Yersinia infection: critical modulation of cell death mechanisms through phagocyte activation

Yersinia pestis, the etiological agent of plague, is one of the most deadly pathogens on our planet. This organism shares important attributes with its ancestral progenitor, Yersinia pseudotuberculosis, including a 70-kb virulence plasmid, lymphotropism during growth in the mammalian host, and killing of host macrophages. Infections with both organisms are biphasic, where bacterial replication occurs initially with little inflammation, followed by phagocyte influx, inflammatory cytokine production, and tissue necrosis. During infection, plasmid-encoded attributes facilitate bacterial-induced macrophage death, which results from two distinct processes and corresponds to the inflammatory crescendo observed in vivo: Naïve cells die by apoptosis (noninflammatory), and later in infection, activated macrophages die by pyroptosis (inflammatory). The significance of this redirected cell death for the host is underscored by the importance of phagocyte activation for immunity to Yersinia and the protective role of pyroptosis during host responses to anthrax lethal toxin and infections with Francisella, Legionella, Pseudomonas, and Salmonella. The similarities of Y. pestis and Y. pseudotuberculosis, including conserved, plasmid-encoded functions inducing at least two distinct mechanisms of cell death, indicate that comparative studies are revealing about their critical pathogenic mechanism(s) and host innate immune responses during infection. Validation of this idea and evidence of similar interactions with the host immune system are provided by Y. pseudotuberculosis-priming, cross-protective immunity against Y. pestis. Despite these insights, additional studies indicate much remains to be understood concerning effective host responses against Yersinia, including chromosomally encoded attributes that also contribute to bacterial evasion and modulation of innate and adaptive immune responses.

Polyphosphate and omptins: novel bacterial procoagulant agents

Derangement of the blood clotting system contributes strongly to multiple organ failure in severe sepsis. In this review, we examine two microbial modulators of the clotting system: polyphosphates and omptins. Polyphosphates are linear polymers of inorganic phosphate that are abundant in the acidocalcisomes of prokaryotes and unicellular organisms as well as in the dense granules of human platelets. Polyphosphates modulate haemostasis by: (1) triggering clotting via the contact pathway; (2) accelerating the activation of coagulation factor V (a key cofactor in blood clotting) and (3) causing fibrin to form clots whose fibrils are thicker and more resistant to fibrinolysis. While polyphosphates are found in all prokaryotes, omptins have a more limited distribution among certain Gram-negative species. Omptins are outer membrane aspartyl proteases which were recently found to proteolytically inactivate tissue factor pathway inhibitor (TFPI), the main inhibitor of the initiation phase of blood clotting. Omptin activity against TFPI requires lipopolysaccharide without O-antigen (rough LPS) such as is found on the surface of Yersinia pestis, the etiologic agent of plague. Interestingly, expression of Pla, the Yersinia pestis omptin, has a demonstrated virulence role in converting plasminogen into the fibrinolytic enzyme plasmin, which would seemingly antagonize any procoagulant effect of TFPI inactivation. However, since the rate of TFPI inactivation is much higher than the rate of plasminogen activation, we suggest that Pla may have a dual function in supporting the bubonic form of plague which is unique to Yersinia pestis.

Direct and negative regulation of the sycO-ypkA-ypoJ operon by cyclic AMP receptor protein (CRP) in Yersinia pestis

Background

Pathogenic yersiniae, including Y. pestis, share a type III secretion system (T3SS) that is composed of a secretion machinery, a set of translocation proteins, a control system, and six Yop effector proteins including YpkA and YopJ. The cyclic AMP receptor protein (CRP), a global regulator, was recently found to regulate the laterally acquired genes (pla and pst) in Y. pestis. The regulation of T3SS components by CRP is unknown.

Results

The sycO, ypkA and yopJ genes constitute a single operon in Y. pestis. CRP specifically binds to the promoter-proximate region of sycO, and represses the expression of the sycO-ypkA-yopJ operon. A single CRP-dependent promoter is employed for the sycO-ypkA-yopJ operon, but two CRP binding sites (site 1 and site 2) are detected within the promoter region. A CRP box homologue is found in site 1 other than site 2. The determination of CRP-binding sites, transcription start site and core promoter element (-10 and -35 regions) promotes us to depict the structural organization of CRP-dependent promoter, giving a map of CRP-promoter DNA interaction for sycO-ypkA-yopJ.

Conclusion

The sycO-ypkA-yopJ operon is under the direct and negative regulation of CRP in Y. pestis. The sycO-ypkA-yopJ promoter-proximate regions are extremely conserved in Y. pestis, Y. pseudotuberculosis and Y. enterocolitica. Therefore, data presented here can be generally applied to the above three pathogenic yersiniae.

The role of relA and spoT in Yersinia pestis KIM5 pathogenicity

The ppGpp molecule is part of a highly conserved regulatory system for mediating the growth response to various environmental conditions. This mechanism may represent a common strategy whereby pathogens such as Yersinia pestis, the causative agent of plague, regulate the virulence gene programs required for invasion, survival and persistence within host cells to match the capacity for growth. The products of the relA and spoT genes carry out ppGpp synthesis. To investigate the role of ppGpp on growth, protein synthesis, gene expression and virulence, we constructed a ΔrelA ΔspoT Y. pestis mutant. The mutant was no longer able to synthesize ppGpp in response to amino acid or carbon starvation, as expected. We also found that it exhibited several novel phenotypes, including a reduced growth rate and autoaggregation at 26°C. In addition, there was a reduction in the level of secretion of key virulence proteins and the mutant was>1,000-fold less virulent than its wild-type parent strain. Mice vaccinated subcutaneously (s.c.) with 2.5×10⁴ CFU of the ΔrelA ΔspoT mutant developed high anti-Y. pestis serum IgG titers, were completely protected against s.c. challenge with 1.5×10⁵ CFU of virulent Y. pestis and partially protected (60% survival) against pulmonary challenge with 2.0×10⁴ CFU of virulent Y. pestis. Our results indicate that ppGpp represents an important virulence determinant in Y. pestis and the ΔrelA ΔspoT mutant strain is a promising vaccine candidate to provide protection against plague.

Invited review: Breaking barriers--attack on innate immune defences by omptin surface proteases of enterobacterial pathogens

The omptin family of Gram-negative bacterial transmembrane aspartic proteases comprises surface proteins with a highly conserved beta-barrel fold but differing biological functions. The omptins OmpT of Escherichia coli, PgtE of Salmonella enterica, and Pla of Yersinia pestis differ in their substrate specificity as well as in control of their expression. Their functional differences are in accordance with the differing pathogenesis of the infections caused by E. coli, Salmonella, and Y. pestis, which suggests that the omptins have adapted to the life-styles of their host species. The omptins Pla and PgtE attack on innate immunity by affecting the plasminogen/plasmin, complement, coagulation, fibrinolysis, and matrix metalloproteinase systems, by inactivating antimicrobial peptides, and by enhancing bacterial adhesiveness and invasiveness. Although the mechanistic details of the functions of Pla and PgtE differ, the outcome is the same: enhanced spread and multiplication of Y. pestis and S. enterica in the host. The omptin OmpT is basically a housekeeping protease but it also degrades cationic antimicrobial peptides and may enhance colonization of E. coli at uroepithelia. The catalytic residues in the omptin molecules are spatially conserved, and the differing polypeptide substrate specificities are dictated by minor sequence variations at regions surrounding the catalytic cleft. For enzymatic activity, omptins require association with lipopolysaccharide on the outer membrane. Modification of lipopolysaccharide by in vivo conditions or by bacterial gene loss has an impact on omptin function. Creation of bacterial surface proteolysis is thus a coordinated function involving several surface structures.

Involvement of the post-transcriptional regulator Hfq in Yersinia pestis virulence

Background

Yersinia pestis is the causative agent of plague, which is transmitted primarily between fleas and mammals and is spread to humans through the bite of an infected flea or contact with afflicted animals. Hfq is proposed to be a global post-transcriptional regulator that acts by mediating interactions between many regulatory small RNAs (sRNAs) and their mRNA targets. Sequence comparisons revealed that Y. pestis appears to produce a functional homologue of E. coli Hfq.

Methodology and Principal Findings

Phenotype comparisons using in vitro assays demonstrated that Y. pestis Hfq was involved in resistance to H₂O₂, heat and polymyxin B and contributed to growth under nutrient-limiting conditions. The role of Hfq in Y. pestis virulence was also assessed using macrophage and mouse infection models, and the gene expression affected by Hfq was determined using microarray-based transcriptome and real time PCR analysis. The macrophage infection assay showed that the Y. pestis hfq deletion strain did not have any significant difference in its ability to associate with J774A.1 macrophage cells. However, hfq deletion appeared to significantly impair the ability of Y. pestis to resist phagocytosis and survive within macrophages at the initial stage of infection. Furthermore, the hfq deletion strain was highly attenuated in mice after subcutaneous or intravenous injection. Transcriptome analysis supported the results concerning the attenuated phenotype of the hfq mutant and showed that the deletion of the hfq gene resulted in significant alterations in mRNA abundance of 243 genes in more than 13 functional classes, about 23% of which are known or hypothesized to be involved in stress resistance and virulence.

Conclusions and Significance

Our results indicate that Hfq is a key regulator involved in Y. pestis stress resistance, intracellular survival and pathogenesis. It appears that Hfq acts by controlling the expression of many virulence- and stress-associated genes, probably in conjunction with small noncoding RNAs.

Deletion of Braun lipoprotein gene (lpp) and curing of plasmid pPCP1 dramatically alter the virulence of Yersinia pestis CO92 in a mouse model of pneumonic plague

Deletion of the murein (Braun) lipoprotein gene, lpp, attenuates the Yersinia pestis CO92 strain in mouse models of bubonic and pneumonic plague. In this report, we characterized the virulence of strains from which the plasminogen activating protease (pla)-encoding pPCP1 plasmid was cured from either the wild-type (WT) or the Deltalpp mutant strain of Y. pestis CO92 in the mouse model of pneumonic infection. We noted a significantly increased survival rate in mice infected with the Y. pestis pPCP(-)/Deltalpp mutant strain up to a dose of 5000 LD(50). Additionally, mice challenged with the pPCP(-)/Deltalpp strain had substantially less tissue injury and a strong decrease in the levels of most cytokines and chemokines in tissue homogenates and sera when compared with the WT-infected group. Importantly, the Y. pestis pPCP(-)/Deltalpp mutant strain was detectable in high numbers in the livers and spleens of some of the infected mice. In the lungs of pPCP(-)/Deltalpp mutant-challenged animals, however, bacterial numbers dropped at 48 h after infection when compared with tissue homogenates from 1 h post-infection. Similarly, we noted that this mutant was unable to survive within murine macrophages in an in vitro assay, whereas survivability of the pPCP(-) mutant within the macrophage environment was similar to that of the WT. Taken together, our data indicated that a significant and possibly synergistic attenuation in bacterial virulence occurred in a mouse model of pneumonic plague when both the lpp gene and the virulence plasmid pPCP1 encoding the pla gene were deleted from Y. pestis.

Omptin proteins: an expanding family of outer membrane proteases in Gram-negativeEnterobacteriaceae(Review)

The Escherichia coli K-12 outer membrane protein OmpT is a prototype of a unique family of bacterial endopeptidases known as the omptins. This family includes OmpT and OmpP of E. coli, SopA of Shigella flexneri, PgtE of Salmonella enterica, and Pla of Yersinia pestis. Despite their sequence similarities, the omptins vary in their reported functions. The OmpT protease is characterized by narrow cleavage specificity defined by the extracellular loops of the beta-barrel protruding above the lipid bilayer. It employs a distinct proteolytic mechanism that involves a histidine and an aspartate residue. Most of the omptin proteins have been implicated in bacterial pathogenesis. As a result, the omptins are potential targets for antimicrobial drug and vaccine development. This review summarizes recent developments in omptins structure and function, emphasizes their role in pathogenesis, proposes evolutionary relation among the existing omptins, and offers possible directions for future research.

Gr1+ cells control growth of YopM-negative yersinia pestis during systemic plague

YopM, a protein toxin of Yersinia pestis, is necessary for virulence in a mouse model of systemic plague. We previously reported YopM-dependent natural killer (NK) cell depletion from blood and spleen samples of infected mice. However, in this study we found that infection with Y. pestis KIM5 (YopM(+)) caused depletion of NK cells in the spleen, but not in the liver, and antibody-mediated ablation of NK cells had no effect on bacterial growth. There was no YopM-associated effect on the percentage of dendritic cells (DCs) or polymorphonuclear leukocytes (PMNs) in the early stage of infection; however, there was a YopM-associated effect on PMN integrity and on the influx of monocytes into the spleen. Ablation of Gr1(+) cells caused loss of the growth defect of YopM(-) Y. pestis in both the liver and spleen. In contrast, ablation of macrophages/DCs inhibited growth of both parent and mutant bacteria, accompanied by significantly fewer lesion sites in the liver. These results point toward PMNs and inflammatory monocytes as major cell types that control growth of YopM(-) Y. pestis. Infection with fully virulent Y. pestis CO92 and a YopM(-) derivative by intradermal and intranasal routes showed that the absence of YopM significantly increased the 50% lethal dose only in the intradermal model, suggesting a role for YopM in bubonic plague, in which acute inflammation occurs soon after infection.

The single substitution I259T, conserved in the plasminogen activator Pla of pandemic Yersinia pestis branches, enhances fibrinolytic activity

The outer membrane plasminogen activator Pla of Yersinia pestis is a central virulence factor in plague. The primary structure of the Pla beta-barrel is conserved in Y. pestis biovars Antiqua, Medievalis, and Orientalis, which are associated with pandemics of plague. The Pla molecule of the ancestral Y. pestis lineages Microtus and Angola carries the single amino acid change T259I located in surface loop 5 of the beta-barrel. Recombinant Y. pestis KIM D34 or Escherichia coli XL1 expressing Pla T259I was impaired in fibrinolysis and in plasminogen activation. Lack of detectable generation of the catalytic light chain of plasmin and inactivation of plasmin enzymatic activity by the Pla T259I construct indicated that Microtus Pla cleaved the plasminogen molecule more unspecifically than did common Pla. The isoform pattern of the Pla T259I molecule was different from that of the common Pla molecule. Microtus Pla was more efficient than wild-type Pla in alpha(2)-antiplasmin inactivation. Pla of Y. pestis and PgtE of Salmonella enterica have evolved from the same omptin ancestor, and their comparison showed that PgtE was poor in plasminogen activation but exhibited efficient antiprotease inactivation. The substitution (259)IIDKT/TIDKN in PgtE, constructed to mimic the L5 region in Pla, altered proteolysis in favor of plasmin formation, whereas the reverse substitution (259)TIDKN/IIDKT in Pla altered proteolysis in favor of alpha(2)-antiplasmin inactivation. The results suggest that Microtus Pla represents an ancestral form of Pla that has evolved into a more efficient plasminogen activator in the pandemic Y. pestis lineages.

Protection against Yersinia pseudotuberculosis infection conferred by a Lactococcus lactis mucosal delivery vector secreting LcrV

Herein, we sought to evaluate the potential of a recombinant Lactococcus lactis strain secreting the Yersinia pseudotuberculosis low-calcium response V (LcrV) antigen for mucosal vaccination against Yersinia infections. We showed that the recombinant strain induced specific systemic and mucosal antibody and cellular immune responses after intranasal immunization and protected mice against both oral and systemic Y. pseudotuberculosis infections. This constitutes the first proof of principle for a novel anti-Yersinia mucosal vaccination strategy using recombinant lactic acid bacteria.

Intracellular pathogenic bacteria and fungi--a case of convergent evolution?

The bacterium Yersinia pestis and the fungus Cryptococcus neoformans are the causative agents of human plague and cryptococcosis, respectively. Both microorganisms are facultatively intracellular pathogens. A comparison of their pathogenic strategies reveals similar tactics for intracellular survival in Y. pestis and C. neoformans despite their genetic unrelatedness. Both organisms can survive in environments where they are vulnerable to predation by amoeboid protozoal hosts. Here, we propose that the overall similarities in their pathogenic strategies are an example of convergent evolution that has solved the problem of intracellular survival.

The Yersinia pestis caf1M1A1 fimbrial capsule operon promotes transmission by flea bite in a mouse model of bubonic plague

Plague is a zoonosis transmitted by fleas and caused by the gram-negative bacterium Yersinia pestis. During infection, the plasmidic caf1M1A1 operon that encodes the Y. pestis F1 protein capsule is highly expressed, and anti-F1 antibodies are protective. Surprisingly, the capsule is not required for virulence after injection of cultured bacteria, even though it is an antiphagocytic factor and capsule-deficient Y. pestis strains are rarely isolated. We found that a caf-negative Y. pestis mutant was not impaired in either flea colonization or virulence in mice after intradermal inoculation of cultured bacteria. In contrast, absence of the caf operon decreased bubonic plague incidence after a flea bite. Successful development of plague in mice infected by flea bite with the caf-negative mutant required a higher number of infective bites per challenge. In addition, the mutant displayed a highly autoaggregative phenotype in infected liver and spleen. The results suggest that acquisition of the caf locus via horizontal transfer by an ancestral Y. pestis strain increased transmissibility and the potential for epidemic spread. In addition, our data support a model in which atypical caf-negative strains could emerge during climatic conditions that favor a high flea burden. Human infection with such strains would not be diagnosed by the standard clinical tests that detect F1 antibody or antigen, suggesting that more comprehensive surveillance for atypical Y. pestis strains in plague foci may be necessary. The results also highlight the importance of studying Y. pestis pathogenesis in the natural context of arthropod-borne transmission.