Biogenesis of the fraction 1 capsule and analysis of the ultrastructure of Yersinia pestis

Bacterial capsules: Occurrence, mechanism, and function

In environments characterized by extended multi-stress conditions, pathogens develop a variety of immune escape mechanisms to enhance their ability to infect the host. The capsules, polymers that bacteria secrete near their cell wall, participates in numerous bacterial life processes and plays a crucial role in resisting host immune attacks and adapting to their niche. Here, we discuss the relationship between capsules and bacterial virulence, summarizing the molecular mechanisms of capsular regulation and pathogenesis to provide new insights into the research on the pathogenesis of pathogenic bacteria.

A Novel Loop-Mediated Isothermal Amplification Assay for Rapid Detection of Yersinia pestis

Rapid detection of Yersinia pestis, the causative agent of plague, is essential during field investigations to enable prompt control measures for prevention of the spread of the disease. Affordable, efficient, reliable, and simple detection assays are extremely useful, particularly in plague-endemic regions with limited resources. We developed a loop-mediated isothermal amplification (LAMP) assay that detects Y. pestis within 30 min by simply incubating at 65°C on a dry bath heater. The assay targeted the caf1A gene that is situated on the pMT1 plasmid using six specific primers. Y. pestis presence is visually detected based on the color change in the reactions. For comparison of the assay performance, a real-time LAMP with fluorescent dye detection was conducted on a real-time PCR instrument using the same six primers. Sensitivity assessment showed that the limit of detection (LOD) was 0.2 and 0.03 pg when performed on the dry bath heater and on the real-time PCR instrument, respectively. The assay was 100% specific, having no cross-reactivity with closely related Yersinia spp. and other bacterial species. We tested the LAMP assay on field-collected fleas and showed that it successfully detected Y. pestis with identical results to that of a previously published pentaplex real-time PCR assay. These findings suggest that the relatively inexpensive and simpler LAMP assay could be used to support field investigations, yielding comparable results to more expensive and complex PCR assays.

Unraveling the molecular determinants of the anti-phagocytic protein cloak of plague bacteria

The pathogenic bacterium Yersina pestis is protected from macrophage engulfment by a capsule like antigen, F1, formed of long polymers of the monomer protein, Caf1. However, despite the importance of this pathogen, the mechanism of protection was not understood. Here we demonstrate how F1 protects the bacteria from phagocytosis. First, we show that Escherichia coli expressing F1 showed greatly reduced adherence to macrophages. Furthermore, the few cells that did adhere remained on the macrophage surface and were not engulfed. We then inserted, by mutation, an “RGDS” integrin binding motif into Caf1. This did not change the number of cells adhering to macrophages but increased the fraction of adherent cells that were engulfed. Therefore, F1 protects in two separate ways, reducing cell adhesion, possibly by acting as a polymer brush, and hiding innate receptor binding sites needed for engulfment. F1 is very robust and we show that E. coli expressing weakened mutant polymers are engulfed like the RGDS mutant. This suggests that innate attachment sites on the native cell surface are exposed if F1 is weakened. Single-molecule force spectroscopy (SMFS) experiments revealed that wild-type F1 displays a very high mechanical stability of 400 pN. However, the mechanical resistance of the destabilised mutants, that were fully engulfed, was only 20% weaker. By only marginally exceeding the mechanical force applied to the Caf1 polymer during phagocytosis it may be that the exceptional tensile strength evolved to resist the forces applied at this stage of engulfment.

Macrophages, a type of white blood cell, form an important element of our immune defence. They interrogate other cells’ surfaces for molecular clues and ingest those presenting a threat in a process known as phagocytosis. Not surprisingly, pathogenic bacteria have developed ways to evade this fate. The plague bacterium, Yersinia pestis, produces the long polymeric F1 coat protein which enables it to avoid ingestion, but the mechanism was unclear. We show that equipping Escherichia coli cells with an F1 coat protected them from phagocytosis by two separate mechanisms, reducing contact with the macrophage surface and hiding the signals that tell the macrophages they are targets. F1 is also a very stable protein polymer and using single molecule force spectroscopy we showed it also has a very high resistance to pulling forces. Surprisingly, mutations which reduced this by only 20% caused adherent bacteria to be fully ingested, indicating that cells are subject to significant forces prior to recognition and ingestion. Thus, F1 has evolved three notable properties (i) physical; creation of a hydrated polymer brush to inhibit surface interactions, (ii) chemical; absence of molecular recognition clues needed for engulfment and (iii) mechanical; strength that maintains the camouflage layer during surface stretching.

Plague Prevention and Therapy: Perspectives on Current and Future Strategies

Plague, caused by the bacterial pathogen Yersinia pestis, is a vector-borne disease that has caused millions of human deaths over several centuries. Presently, human plague infections continue throughout the world. Transmission from one host to another relies mainly on infected flea bites, which can cause enlarged lymph nodes called buboes, followed by septicemic dissemination of the pathogen. Additionally, droplet inhalation after close contact with infected mammals can result in primary pneumonic plague. Here, we review research advances in the areas of vaccines and therapeutics for plague in context of Y. pestis virulence factors and disease pathogenesis. Plague continues to be both a public health threat and a biodefense concern and we highlight research that is important for infection mitigation and disease treatment.

Tn-Seq Analysis Identifies Genes Important for Yersinia pestis Adherence during Primary Pneumonic Plague

Colonization of the lung by Yersinia pestis is a critical first step in establishing infection during primary pneumonic plague, a disease characterized by high lethality. However, the mechanisms by which Y. pestis adheres in the lung after inhalation remain elusive. Here, we used Tn-seq to identify Y. pestis genes important for adherence early during primary pneumonic plague. Our mutant enrichment strategy resulted in the identification of genes important for regulation and assembly of genes and proteins rather than adhesin genes themselves. These results reveal that there may be multiple Y. pestis adhesins or redundancy among adhesins. Identifying the adhesins regulated by the genes identified in our enrichment screen may reveal novel therapeutic targets for preventing Y. pestis adherence and the subsequent development of pneumonic plague.

Following inhalation, Yersinia pestis rapidly colonizes the lung to establish infection during primary pneumonic plague. Although several adhesins have been identified in Yersinia spp., the factors mediating early Y. pestis adherence in the lung remain unknown. To identify genes important for Y. pestis adherence during primary pneumonic plague, we used transposon insertion sequencing (Tn-seq). Wild-type and capsule mutant (Δcaf1) Y. pestis transposon mutant libraries were serially passaged in vivo to enrich for nonadherent mutants in the lung using a mouse model of primary pneumonic plague. Sequencing of the passaged libraries revealed six mutants that were significantly enriched in both the wild-type and Δcaf1Y. pestis backgrounds. The enriched mutants had insertions in genes that encode transcriptional regulators, chaperones, an endoribonuclease, and YPO3903, a hypothetical protein. Using single-strain infections and a transcriptional analysis, we identified a significant role for YPO3903 in Y. pestis adherence in the lung and showed that YPO3903 regulated transcript levels of psaA, which encodes a fimbria previously implicated in Y. pestis adherence in vitro. Deletion of psaA had a minor effect on Y. pestis adherence in the lung, suggesting that YPO3903 regulates other adhesins in addition to psaA. By enriching for mutations in genes that regulate the expression or assembly of multiple genes or proteins, we obtained screen results indicating that there may be not just one dominant adhesin but rather several factors that contribute to early Y. pestis adherence during primary pneumonic plague.

IMPORTANCE Colonization of the lung by Yersinia pestis is a critical first step in establishing infection during primary pneumonic plague, a disease characterized by high lethality. However, the mechanisms by which Y. pestis adheres in the lung after inhalation remain elusive. Here, we used Tn-seq to identify Y. pestis genes important for adherence early during primary pneumonic plague. Our mutant enrichment strategy resulted in the identification of genes important for regulation and assembly of genes and proteins rather than adhesin genes themselves. These results reveal that there may be multiple Y. pestis adhesins or redundancy among adhesins. Identifying the adhesins regulated by the genes identified in our enrichment screen may reveal novel therapeutic targets for preventing Y. pestis adherence and the subsequent development of pneumonic plague.

Functional expression of the entire adhesiome of Salmonella enterica serotype Typhimurium

Adhesins are crucial virulence factors of pathogenic bacteria involved in colonization, transmission and pathogenesis. Many bacterial genomes contain the information for a surprisingly large number of diverse adhesive structures. One prominent example is the invasive and facultative intracellular pathogen Salmonella enterica with an adhesiome of up to 20 adhesins. Such large repertoire of adhesins contributes to colonization of a broad range of host species and may allow adaptation to various environments within the host, as well as in non-host environments. For S. enterica, only few members of the adhesiome are functionally expressed under laboratory conditions, and accordingly the structural and functional understanding of the majority of adhesins is sparse. We have devised a simple and versatile approach to functionally express all adhesins of S. enterica serotype Typhimurium, either within Salmonella or within heterologous hosts such as Escherichia coli. We demonstrate the surface expression of various so far cryptic adhesins and show ultrastructural features using atomic force microscopy and transmission electron microscopy. In summary, we report for the first time the expression of the entire adhesiome of S. enterica serotype Typhimurium.

Functional analysis of Escherichia coli Yad fimbriae reveals their potential role in environmental persistence

Initial adhesion of bacterial cells to surfaces or host tissues is a key step in colonisation and biofilm formation processes, and is mediated by cell surface appendages. It was previously demonstrated that Escherichia coli K-12 possesses an arsenal of silenced chaperone-usher fimbriae that were functional when constitutively expressed. Among them, production of prevalent Yad fimbriae induces adhesion to abiotic surfaces. Functional characterisation of Yad fimbriae were undertook, and YadN was identified as the most abundant and potential major pilin, and YadC as the potential tip-protein of Yad fimbriae. It was showed that Yad production participates to binding of E. coli K-12 to human eukaryotic cells (Caco-2) and inhibits macrophage phagocytosis, but also enhances E. coli K-12 binding to xylose, a major component of the plant cell wall, through its tip-lectin YadC. Consistently, it was demonstrated that Yad production provides E. coli with a competitive advantage in colonising corn seed rhizospheres. The latter phenotype is correlated with induction of Yad expression at temperatures below 37°C, and under anaerobic conditions, through a complex regulatory network. Taken together, these results suggest that Yad fimbriae are versatile adhesins that beyond potential capacities to modulate host-pathogen interactions might contribute to E. coli environmental persistence.

Adhesive properties of YapV and paralogous autotransporter proteins of Yersinia pestis

Yersinia pestis is the causative agent of plague. This bacterium evolved from an ancestral enteroinvasive Yersinia pseudotuberculosis strain by gene loss and acquisition of new genes, allowing it to use fleas as transmission vectors. Infection frequently leads to a rapidly lethal outcome in humans, a variety of rodents, and cats. This study focuses on the Y. pestis KIM yapV gene and its product, recognized as an autotransporter protein by its typical sequence, outer membrane localization, and amino-terminal surface exposure. Comparison of Yersinia genomes revealed that DNA encoding YapV or each of three individual paralogous proteins (YapK, YapJ, and YapX) was present as a gene or pseudogene in a strain-specific manner and only in Y. pestis and Y. pseudotuberculosis. YapV acted as an adhesin for alveolar epithelial cells and specific extracellular matrix (ECM) proteins, as shown with recombinant Escherichia coli, Y. pestis, or purified passenger domains. Like YapV, YapK and YapJ demonstrated adhesive properties, suggesting that their previously related in vivo activity is due to their capacity to modulate binding properties of Y. pestis in its hosts, in conjunction with other adhesins. A differential host-specific type of binding to ECM proteins by YapV, YapK, and YapJ suggested that these proteins participate in broadening the host range of Y. pestis. A phylogenic tree including 36 Y. pestis strains highlighted an association between the gene profile for the four paralogous proteins and the geographic location of the corresponding isolated strains, suggesting an evolutionary adaption of Y. pestis to specific local animal hosts or reservoirs.

A Yersinia pestis tat mutant is attenuated in bubonic and small-aerosol pneumonic challenge models of infection but not as attenuated by intranasal challenge

Bacterial proteins destined for the Tat pathway are folded before crossing the inner membrane and are typically identified by an N-terminal signal peptide containing a twin arginine motif. Translocation by the Tat pathway is dependent on the products of genes which encode proteins possessing the binding site of the signal peptide and mediating the actual translocation event. In the fully virulent CO92 strain of Yersinia pestis, the tatA gene was deleted. The mutant was assayed for loss of virulence through various in vitro and in vivo assays. Deletion of the tatA gene resulted in several consequences for the mutant as compared to wild-type. Cell morphology of the mutant bacteria was altered and demonstrated a more elongated form. In addition, while cultures of the mutant strain were able to produce a biofilm, we observed a loss of adhesion of the mutant biofilm structure compared to the biofilm produced by the wild-type strain. Immuno-electron microscopy revealed a partial disruption of the F1 antigen on the surface of the mutant. The virulence of the ΔtatA mutant was assessed in various murine models of plague. The mutant was severely attenuated in the bubonic model with full virulence restored by complementation with the native gene. After small-particle aerosol challenge in a pneumonic model of infection, the mutant was also shown to be attenuated. In contrast, when mice were challenged intranasally with the mutant, very little difference in the LD₅₀ was observed between wild-type and mutant strains. However, an increased time-to-death and delay in bacterial dissemination was observed in mice infected with the ΔtatA mutant as compared to the parent strain. Collectively, these findings demonstrate an essential role for the Tat pathway in the virulence of Y. pestis in bubonic and small-aerosol pneumonic infection but less important role for intranasal challenge.

A negative effect of Campylobacter capsule on bacterial interaction with an analogue of a host cell receptor

BACKGROUND

Campylobacter jejuni (C. jejuni) is the leading causative agent of bacterial gastrointestinal infections. The rise of antibiotic resistant forms of this pathogen necessitates the development of novel intervention strategies. One approach is the design of drugs preventing bacterial attachment to host cells. Although some putative C. jejuni adhesins have been identified, the molecular mechanisms of their interaction with host cells and their role in pathogenesis remain to be elucidated. C. jejuni adhesion may also be modulated by a bacterial capsule. However, the role of this structure in adhesion was not clear due to conflicting results published by different research groups. The aim of this study was to clarify the role of capsule in bacterial interaction with host cells by using an in vitro model of adhesion and an analogue of a host cell receptor.

RESULTS

In this study, we developed an in vitro bacterial adhesion assay, which was validated using various tests, including competitive inhibition studies, exoglycosydase treatment and site-directed mutagenesis. We demonstrate that PEB3 is one of the cell surface glycoproteins required for bacterial interaction with an analogue of a host cell receptor. In contrast, JlpA glycoprotein adhesin is not required for such interaction. We demonstrate that the production of capsule reduces bacterial attachment, and that the genes involved in capsule and PEB3 adhesin biosynthesis are differentially regulated.

CONCLUSIONS

In this study we report an in vitro model for the investigation of bacterial interaction with analogs of host cell receptors. The results suggest an interfering effect of capsule on bacterial attachment. In addition, using a liquid culture, we demonstrate differential expression of a gene involved in capsule production (kpsM) and a gene encoding a glycoprotein adhesin (peb3). Further studies are required in order to establish if these genes are also differentially regulated during the infection process. The results will assist in better understanding of the mechanism of pathogenesis of C. jejuni in general and the role of capsule in the process in particular.

Direct neutralization of type III effector translocation by the variable region of a monoclonal antibody to Yersinia pestis LcrV

Plague is an acute infection caused by the Gram-negative bacterium Yersinia pestis. Antibodies that are protective against plague target LcrV, an essential virulence protein and component of a type III secretion system of Y. pestis. Secreted LcrV localizes to the tips of type III needles on the bacterial surface, and its function is necessary for the translocation of Yersinia outer proteins (Yops) into the cytosol of host cells infected by Y. pestis. Translocated Yops counteract macrophage functions, for example, by inhibiting phagocytosis (YopE) or inducing cytotoxicity (YopJ). Although LcrV is the best-characterized protective antigen of Y. pestis, the mechanism of protection by anti-LcrV antibodies is not fully understood. Antibodies bind to LcrV at needle tips, neutralize Yop translocation, and promote opsonophagocytosis of Y. pestis by macrophages in vitro. However, it is not clear if anti-LcrV antibodies neutralize Yop translocation directly or if they do so indirectly, by promoting opsonophagocytosis. To determine if the protective IgG1 monoclonal antibody (MAb) 7.3 is directly neutralizing, an IgG2a subclass variant, a deglycosylated variant, F(ab')2, and Fab were tested for the ability to inhibit the translocation of Yops into Y. pestis-infected macrophages in vitro. Macrophage cytotoxicity and cellular fractionation assays show that the Fc of MAb 7.3 is not required for the neutralization of YopJ or YopE translocation. In addition, the use of Fc receptor-deficient macrophages, and the use of cytochalasin D to inhibit actin polymerization, confirmed that opsonophagocytosis is not required for MAb 7.3 to neutralize translocation. These data indicate that the binding of the variable region of MAb 7.3 to LcrV is sufficient to directly neutralize Yop translocation.

Structural basis for the specific recognition of dual receptors by the homopolymeric pH 6 antigen (Psa) fimbriae of Yersinia pestis

The pH 6 antigen (Psa) of Yersinia pestis consists of fimbriae that bind to two receptors: β1-linked galactosyl residues in glycosphingolipids and the phosphocholine group in phospholipids. Despite the ubiquitous presence of either moiety on the surface of many mammalian cells, Y. pestis appears to prefer interacting with certain types of human cells, such as macrophages and alveolar epithelial cells of the lung. The molecular mechanism of this apparent selectivity is not clear. Site-directed mutagenesis of the consensus choline-binding motif in the sequence of PsaA, the subunit of the Psa fimbrial homopolymer, identified residues that abolish galactosylceramide binding, phosphatidylcholine binding, or both. The crystal structure of PsaA in complex with both galactose and phosphocholine reveals separate receptor binding sites that share a common structural motif, thus suggesting a potential interaction between the two sites. Mutagenesis of this shared structural motif identified Tyr126, which is part of the choline-binding consensus sequence but is found in direct contact with the galactose in the structure of PsaA, important for both receptor binding. Thus, this structure depicts a fimbrial subunit that forms a polymeric adhesin with a unique arrangement of dual receptor binding sites. These findings move the field forward by providing insights into unique types of multiple receptor-ligand interactions and should steer research into the synthesis of dual receptor inhibitor molecules to slow down the rapid progression of plague.

Surface organelles assembled by secretion systems of Gram-negative bacteria: diversity in structure and function

Gram-negative bacteria express a wide variety of organelles on their cell surface. These surface structures may be the end products of secretion systems, such as the hair-like fibers assembled by the chaperone/usher (CU) and type IV pilus pathways, which generally function in adhesion to surfaces and bacterial-bacterial and bacterial-host interactions. Alternatively, the surface organelles may be integral components of the secretion machinery itself, such as the needle complex and pilus extensions formed by the type III and type IV secretion systems, which function in the delivery of bacterial effectors inside host cells. Bacterial surface structures perform functions critical for pathogenesis and have evolved to withstand forces exerted by the external environment and cope with defenses mounted by the host immune system. Given their essential roles in pathogenesis and exposed nature, bacterial surface structures also make attractive targets for therapeutic intervention. This review will describe the structure and function of surface organelles assembled by four different Gram-negative bacterial secretion systems: the CU pathway, the type IV pilus pathway, and the type III and type IV secretion systems.

Roles of chaperone/usher pathways of Yersinia pestis in a murine model of plague and adhesion to host cells

Yersinia pestis and many other Gram-negative pathogenic bacteria use the chaperone/usher (CU) pathway to assemble virulence-associated surface fibers termed pili or fimbriae. Y. pestis has two well-characterized CU pathways: the caf genes coding for the F1 capsule and the psa genes coding for the pH 6 antigen. The Y. pestis genome contains additional CU pathways that are capable of assembling pilus fibers, but the roles of these pathways in the pathogenesis of plague are not understood. We constructed deletion mutations in the usher genes for six of the additional Y. pestis CU pathways. The wild-type (WT) and usher deletion strains were compared in the murine bubonic (subcutaneous) and pneumonic (intranasal) plague infection models. Y. pestis strains containing deletions in CU pathways y0348-0352, y1858-1862, and y1869-1873 were attenuated for virulence compared to the WT strain by the intranasal, but not subcutaneous, routes of infection, suggesting specific roles for these pathways during pneumonic plague. We examined binding of the Y. pestis WT and usher deletion strains to A549 human lung epithelial cells, HEp-2 human cervical epithelial cells, and primary human and murine macrophages. Y. pestis CU pathways y0348-0352 and y1858-1862 were found to contribute to adhesion to all host cells tested, whereas pathway y1869-1873 was specific for binding to macrophages. The correlation between the virulence attenuation and host cell binding phenotypes of the usher deletion mutants identifies three of the additional CU pathways of Y. pestis as mediating interactions with host cells that are important for the pathogenesis of plague.

Vulnerabilities in Yersinia pestis caf operon are unveiled by a Salmonella vector

During infection, Yersinia pestis uses its F1 capsule to enhance survival and cause virulence to mammalian host. Since F1 is produced in large quantities and secreted into the host tissues, it also serves as a major immune target. To hold this detrimental effect under proper control, Y. pestis expresses the caf operon (encoding the F1 capsule) in a temperature-dependent manner. However, additional properties of the caf operon limit its expression. By overexpressing the caf operon in wild-type Salmonella enterica serovar Typhimurium under a potent promoter, virulence of Salmonella was greatly attenuated both in vitro and in vivo. In contrast, expression of the caf operon under the regulation of its native promoter exhibited negligible impairment of Salmonellae virulence. In-depth investigation revealed all individual genes in the caf operon attenuated Salmonella when overexpressed. The deleterious effects of caf operon and the caf individual genes were further confirmed when they were overexpressed in Y. pestis KIM6+. This study suggests that by using a weak inducible promoter, the detrimental effects of the caf operon are minimally manifested in Y. pestis. Thus, through tight regulation of the caf operon, Y. pestis precisely balances its capsular anti-phagocytic properties with the detrimental effects of caf during interaction with mammalian host.

[Immunobiological properties of Yersinia pestis antigens]

The present review contains information concerning immunobiological properties of plague microbe antigens. All of the identified antigens are evaluated in relation to pathogenicity of Yersinia pestis namely a resistance to phagocytosis, toxicity, adhesiveness etc. as well as persistence ability and adaptation to variable environment. In addition, the role of antigens in immunogenicity of living plague microbe for experimental animals is considered. The data concerning mechanisms of antigenic contribution to the development of adaptive immunity are presented.

Trimeric form of intracellular ATP synthase subunit β of Aggregatibacter actinomycetemcomitans binds human interleukin-1β

Bacterial biofilms resist host defenses and antibiotics partly because of their decreased metabolism. Some bacteria use proinflammatory cytokines, such as interleukin (IL)-1β, as cues to promote biofilm formation and to alter virulence. Although one potential bacterial IL-1β receptor has been identified, current knowledge of the bacterial IL-1β sensing mechanism is limited. In chronic biofilm infection, periodontitis, Aggregatibacter actinomycetemcomitans requires tight adherence (tad)-locus to form biofilms, and tissue destroying active lesions contain more IL-1β than inactive ones. The effect of IL-1β on the metabolic activity of A. actinomycetemcomitans biofilm was tested using alamarBlue™. The binding of IL-1β to A. actinomycetemcomitans cells was investigated using transmission electron microscopy and flow cytometry. To identify the proteins which interacted with IL-1β, different protein fractions from A. actinomycetemcomitans were run in native-PAGE and blotted using biotinylated IL-1β and avidin-HRP, and identified using mass spectroscopy. We show that although IL-1β slightly increases the biofilm formation of A. actinomycetemcomitans, it reduces the metabolic activity of the biofilm. A similar reduction was observed with all tad-locus mutants except the secretin mutant, although all tested mutant strains as well as wild type strains bound IL-1β. Our results suggest that IL-1β might be transported into the A. actinomycetemcomitans cells, and the trimeric form of intracellular ATP synthase subunit β interacted with IL-1β, possibly explaining the decreased metabolic activity. Because ATP synthase is highly conserved, it might universally enhance biofilm resistance to host defense by binding IL-1β during inflammation.

Characterization of an F1 deletion mutant of Yersinia pestis CO92, pathogenic role of F1 antigen in bubonic and pneumonic plague, and evaluation of sensitivity and specificity of F1 antigen capture-based dipsticks

We evaluated two commercial F1 antigen capture-based immunochromatographic dipsticks, Yersinia Pestis (F1) Smart II and Plague BioThreat Alert test strips, in detecting plague bacilli by using whole-blood samples from mice experimentally infected with Yersinia pestis CO92. To assess the specificities of these dipsticks, an in-frame F1-deficient mutant of CO92 (Δcaf) was generated by homologous recombination and used as a negative control. Based on genetic, antigenic/immunologic, and electron microscopic analyses, the Δcaf mutant was devoid of a capsule. The growth rate of the Δcaf mutant generally was similar to that of the wild-type (WT) bacterium at both 26 and 37 °C, although the mutant's growth dropped slightly during the late phase at 37 °C. The Δcaf mutant was as virulent as WT CO92 in the pneumonic plague mouse model; however, it was attenuated in developing bubonic plague. Both dipsticks had similar sensitivities, requiring a minimum of 0.5 μg/ml of purified F1 antigen or 1 × 10(5) to 5 × 10(5) CFU/ml of WT CO92 for positive results, while the blood samples were negative for up to 1 × 10(8) CFU/ml of the Δcaf mutant. Our studies demonstrated the diagnostic potential of two plague dipsticks in detecting capsular-positive strains of Y. pestis in bubonic and pneumonic plague.

The dependence of the Yersinia pestis capsule on pathogenesis is influenced by the mouse background

Yersinia pestis is a highly pathogenic Gram-negative organism and the causative agent of bubonic and pneumonic plague. Y. pestis is capable of causing major epidemics; thus, there is a need for vaccine targets and a greater understanding of the role of these targets in pathogenesis. Two prime Y. pestis vaccine candidates are the usher-chaperone fimbriae Psa and Caf. Herein we report that Y. pestis requires, in a nonredundant manner, both PsaA and Caf1 to achieve its full pathogenic ability in both pneumonic and bubonic plague in C57BL/6J mice. Deletion of psaA leads to a decrease in the organ bacterial burden and to a significant increase in the 50% lethal dose (LD₅₀) after subcutaneous infection. Deletion of caf1 also leads to a significant decrease in the organ bacterial burden but more importantly leads to a significantly greater increase in the LD₅₀ than was observed for the ΔpsaA mutant strain after subcutaneous infection of C57BL/6J mice. Furthermore, the degree of attenuation of the Δcaf1 mutant strain is mouse background dependent, as the Δcaf1 mutant strain was attenuated to a lesser degree in BALB/cJ mice by the subcutaneous route than in C57BL/6J mice. This observation that the degree of requirement for Caf1 is dependent on the mouse background indicates that the virulence of Y. pestis is dependent on the genetic makeup of its host and provides further support for the hypothesis that PsaA and Caf1 have different targets.

Contributions of chaperone/usher systems to cell binding, biofilm formation and Yersinia pestis virulence

Yersinia pestis genome sequencing projects have revealed six intact uncharacterized chaperone/usher systems with the potential to play roles in plague pathogenesis. We cloned each locus and expressed them in the Δfim Escherichia coli strain AAEC185 to test the assembled Y. pestis surface structures for various activities. Expression of each chaperone/usher locus gave rise to specific novel fibrillar structures on the surface of E. coli. One locus, y0561-0563, was able to mediate attachment to human epithelial cells (HEp-2) and human macrophages (THP-1) but not mouse macrophages (RAW264.7), while several loci were able to facilitate E. coli biofilm formation. When each chaperone/usher locus was deleted in Y. pestis, only deletion of the previously described pH 6 antigen (Psa) chaperone/usher system resulted in decreased adhesion and biofilm formation. Quantitative RT-PCR (qRT-PCR) revealed low expression levels for each novel chaperone/usher system in vitro as well as in mouse tissues following intravenous infection. However, a Y. pestis mutant in the chaperone/usher locus y1858-1862 was attenuated for virulence in mice via the intravenous route of infection, suggesting that expression of this locus is, at some stage, sufficient to affect the outcome of a plague infection. qRT-PCR experiments also indicated that expression of the chaperone/usher-dependent capsule locus, caf1, was influenced by oxygen availability and that the well-described chaperone/usher-dependent pilus, Psa, was strongly induced in minimal medium even at 28 °C rather than 37 °C, a temperature previously believed to be required for Psa expression. These data indicate several potential roles for the novel chaperone/usher systems of Y. pestis in pathogenesis and infection-related functions such as cell adhesion and biofilm formation.

Comparative Global Gene Expression Profiles of Wild-Type Yersinia pestis CO92 and Its Braun Lipoprotein Mutant at Flea and Human Body Temperatures

Braun/murein lipoprotein (Lpp) is involved in inflammatory responses and septic shock. We previously characterized a Deltalpp mutant of Yersinia pestis CO92 and found that this mutant was defective in surviving in macrophages and was attenuated in a mouse inhalation model of plague when compared to the highly virulent wild-type (WT) bacterium. We performed global transcriptional profiling of WT Y. pestis and its Deltalpp mutant using microarrays. The organisms were cultured at 26 and 37 degrees Celsius to simulate the flea vector and mammalian host environments, respectively. Our data revealed vastly different effects of lpp mutation on the transcriptomes of Y. pestis grown at 37 versus 26 degrees C. While the absence of Lpp resulted mainly in the downregulation of metabolic genes at 26 degrees C, the Y. pestis Deltalpp mutant cultured at 37 degrees C exhibited profound alterations in stress response and virulence genes, compared to WT bacteria. We investigated one of the stress-related genes (htrA) downregulated in the Deltalpp mutant relative to WT Y. pestis. Indeed, complementation of the Deltalpp mutant with the htrA gene restored intracellular survival of the Y. pestis Deltalpp mutant. Our results support a role for Lpp in Y. pestis adaptation to the host environment, possibly via transcriptional activation of htrA.

A solvent-exposed patch in chaperone-bound YopE is required for translocation by the type III secretion system

Most effector proteins of bacterial type III secretion (T3S) systems require chaperone proteins for translocation into host cells. Such effectors are bound by chaperones in a conserved and characteristic manner, with the chaperone-binding (Cb) region of the effector wound around the chaperone in a highly extended conformation. This conformation has been suggested to serve as a translocation signal in promoting the association between the chaperone-effector complex and a bacterial component required for translocation. We sought to test a prediction of this model by identifying a potential association site for the Yersinia pseudotuberculosis chaperone-effector pair SycE-YopE. We identified a set of residues in the YopE Cb region that are required for translocation but are dispensable for expression, SycE binding, secretion into the extrabacterial milieu, and stability in mammalian cells. These residues form a solvent-exposed patch on the surface of the chaperone-bound Cb region, and thus their effect on translocation is consistent with the structure of the chaperone-bound Cb region serving as a signal for translocation.

Adhesive organelles of Gram-negative pathogens assembled with the classical chaperone/usher machinery: structure and function from a clinical standpoint

This review summarizes current knowledge on the structure, function, assembly and biomedical applications of the superfamily of adhesive fimbrial organelles exposed on the surface of Gram-negative pathogens with the classical chaperone/usher machinery. High-resolution three-dimensional (3D) structure studies of the minifibers assembling with the FGL (having a long F1-G1 loop) and FGS (having a short F1-G1 loop) chaperones show that they exploit the same principle of donor-strand complementation for polymerization of subunits. The 3D structure of adhesive subunits bound to host-cell receptors and the final architecture of adhesive fimbrial organelles reveal two functional families of the organelles, respectively, possessing polyadhesive and monoadhesive binding. The FGL and FGS chaperone-assembled polyadhesins are encoded exclusively by the gene clusters of the γ3- and κ-monophyletic groups, respectively, while gene clusters belonging to the γ1-, γ2-, γ4-, and π-fimbrial clades exclusively encode FGS chaperone-assembled monoadhesins. Novel approaches are suggested for a rational design of antimicrobials inhibiting the organelle assembly or inhibiting their binding to host-cell receptors. Vaccines are currently under development based on the recombinant subunits of adhesins.

Humans and evolutionary and ecological forces shaped the phylogeography of recently emerged diseases

The development of human civilizations and global commerce has led to the emergence and worldwide circulation of many infectious diseases. Anthrax, plague and tularaemia are three zoonotic diseases that have been intensely studied through genome characterization of the causative species and phylogeographical analyses. A few highly fit genotypes in each species represent the causative agents for most of the observed disease cases. Together, mutational and selective forces create highly adapted pathogens, but this must be coupled with ecological opportunities for global expansion. This Review describes the distributions of the bacteria that cause anthrax, plague and tularaemia and investigates the forces that created clonal structures in these species.

Yersinia pestis IS1541 transposition provides for escape from plague immunity

Yersinia pestis is perhaps the most feared infectious agent due to its ability to cause epidemic outbreaks of plague disease in animals and humans with high mortality. Plague infections elicit strong humoral immune responses against the capsular antigen (fraction 1 [F1]) of Y. pestis, and F1-specific antibodies provide protective immunity. Here we asked whether Y. pestis generates mutations that enable bacterial escape from protective immunity and isolated a variant with an IS1541 insertion in caf1A encoding the F1 outer membrane usher. The caf1A::IS1541 insertion prevented assembly of F1 pili and provided escape from plague immunity via F1-specific antibodies without a reduction in virulence in mouse models of bubonic or pneumonic plague. F1-specific antibodies interfere with Y. pestis type III transport of effector proteins into host cells, an inhibitory effect that was overcome by the caf1A::IS1541 insertion. These findings suggest a model in which IS1541 insertion into caf1A provides for reversible changes in envelope structure, enabling Y. pestis to escape from adaptive immune responses and plague immunity.

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.

Vaccination of mice with a Yop translocon complex elicits antibodies that are protective against infection with F1- Yersinia pestis

Yersinia pestis, the bacterial agent of plague, secretes several proteins important for pathogenesis or host protection. The F1 protein forms a capsule on the bacterial cell surface and is a well-characterized protective antigen but is not essential for virulence. A type III secretion system that is essential for virulence exports Yop proteins, which function as antiphagocytic or anti-inflammatory factors. Yop effectors (e.g., YopE) are delivered across the host cell plasma membrane by a translocon, composed of YopB and YopD. Complexes of YopB, YopD, and YopE (BDE) secreted by Yersinia pseudotuberculosis were purified by affinity chromatography and used as immunogens to determine if antibodies to the translocon could provide protection against Y. pestis in mice. Mice vaccinated with BDE generated high-titer immunoglobulin G antibodies specific for BDE, as shown by enzyme-linked immunosorbent assay and immunoblotting, and were protected against lethal intravenous challenge with F1(-) but not F1(+) Y. pestis. Mice passively immunized with anti-BDE serum were protected from lethal challenge with F1(-) Y. pestis. The YopB protein or a complex of YopB and YopD (BD) was purified and determined by vaccination to be immunogenic in mice. Mice actively vaccinated with BD or passively vaccinated with anti-BD serum were protected against lethal challenge with F1(-) Y. pestis. These results indicate that anti-translocon antibodies can be used as immunotherapy to treat infections by F1(-) Y. pestis.

Caspase-1 activation in macrophages infected with Yersinia pestis KIM requires the type III secretion system effector YopJ

Pathogenic Yersinia species utilize a type III secretion system (T3SS) to translocate effectors called Yersinia outer proteins (Yops) into infected host cells. Previous studies demonstrated a role for effector Yops in the inhibition of caspase-1-mediated cell death and secretion of interleukin-1beta (IL-1beta) in naïve macrophages infected with Yersinia enterocolitica. Naïve murine macrophages were infected with a panel of different Yersinia pestis and Yersinia pseudotuberculosis strains to determine whether Yops of these species inhibit caspase-1 activation. Cell death was measured by release of lactate dehydrogenase (LDH), and enzyme-linked immunosorbent assay for secreted IL-1beta was used to measure caspase-1 activation. Surprisingly, isolates derived from the Y. pestis KIM strain (e.g., KIM5) displayed an unusual ability to activate caspase-1 and kill infected macrophages compared to other Y. pestis and Y. pseudotuberculosis strains tested. Secretion of IL-1beta following KIM5 infection was reduced in caspase-1-deficient macrophages compared to wild-type macrophages. However, release of LDH was not reduced in caspase-1-deficient macrophages, indicating that cell death occurred independently of caspase-1. Analysis of KIM-derived strains defective for production of functional effector or translocator Yops indicated that translocation of catalytically active YopJ into macrophages was required for caspase-1 activation and cell death. Release of LDH and secretion of IL-1beta were not reduced when actin polymerization was inhibited in KIM5-infected macrophages, indicating that extracellular bacteria translocating YopJ could trigger cell death and caspase-1 activation. This study uncovered a novel role for YopJ in the activation of caspase-1 in macrophages.