Functional mapping of YadA- and Ail-mediated binding of human factor H to Yersinia enterocolitica serotype O:3

Contributions of Yersinia pestis outer membrane protein Ail to plague pathogenesis

PURPOSE OF REVIEW

Pathogenic Yersinia have been a productive model system for studying bacterial pathogenesis. Hallmark contributions of Yersinia research to medical microbiology are legion and include: (i) the first identification of the role of plasmids in virulence, (ii) the important mechanism of iron acquisition from the host, (iii) the first identification of bacterial surface proteins required for host cell invasion, (iv) the archetypical type III secretion system, and (v) elucidation of the role of genomic reduction in the evolutionary trajectory from a fairly innocuous pathogen to a highly virulent species.

RECENT FINDINGS

The outer membrane (OM) protein Ail (attachment invasion locus) was identified over 30 years ago as an invasin-like protein. Recent work on Ail continues to provide insights into Gram-negative pathogenesis. This review is a synopsis of the role of Ail in invasion, serum resistance, OM stability, thermosensing, and vaccine development.

SUMMARY

Ail is shown to be an essential virulence factor with multiple roles in pathogenesis. The recent adaptation of Yersinia pestis to high virulence, which included genomic reduction to eliminate redundant protein functions, is a model to understand the emergence of new bacterial pathogens.

The inverse autotransporters of Yersinia ruckeri, YrInv and YrIlm, contribute to biofilm formation and virulence

Yersinia ruckeri causes enteric redmouth disease (ERM) that mainly affects salmonid fishes and leads to significant economic losses in the aquaculture industry. An increasing number of outbreaks and the lack of effective vaccines against some serotypes necessitates novel measures to control ERM. Importantly, Y. ruckeri survives in the environment for long periods, presumably by forming biofilms. How the pathogen forms biofilms and which molecular factors are involved in this process, remains unclear. Yersinia ruckeri produces two surface-exposed adhesins, belonging to the inverse autotransporters (IATs), called Y. ruckeri invasin (YrInv) and Y. ruckeri invasin-like molecule (YrIlm). Here, we investigated whether YrInv and YrIlm play a role in biofilm formation and virulence. Functional assays revealed that YrInv and YrIlm promote biofilm formation on different abiotic substrates. Confocal microscopy revealed that they are involved in microcolony interaction and formation, respectively. The effect of both IATs on biofilm formation correlated with the presence of different biopolymers in the biofilm matrix, including extracellular DNA, RNA and proteins. Moreover, YrInv and YrIlm contributed to virulence in the Galleria mellonella infection model. Taken together, we propose that both IATs are possible targets for the development of novel diagnostic and preventative strategies to control ERM.

Interaction with the host: the role of fibronectin and extracellular matrix proteins in the adhesion of Gram-negative bacteria

The capacity of pathogenic microorganisms to adhere to host cells and avoid clearance by the host immune system is the initial and most decisive step leading to infections. Bacteria have developed different strategies to attach to diverse host surface structures. One important strategy is the adhesion to extracellular matrix (ECM) proteins (e.g., collagen, fibronectin, laminin) that are highly abundant in connective tissue and basement membranes. Gram-negative bacteria express variable outer membrane proteins (adhesins) to attach to the host and to initiate the process of infection. Understanding the underlying molecular mechanisms of bacterial adhesion is a prerequisite for targeting this interaction by “anti-ligands” to prevent colonization or infection of the host. Future development of such “anti-ligands” (specifically interfering with bacteria-host matrix interactions) might result in the development of a new class of anti-infective drugs for the therapy of infections caused by multidrug-resistant Gram-negative bacteria. This review summarizes our current knowledge about the manifold interactions of adhesins expressed by Gram-negative bacteria with ECM proteins and the use of this information for the generation of novel therapeutic antivirulence strategies.

Native display of a huge homotrimeric protein fiber on the cell surface after precise domain deletion

AtaA, a trimeric autotransporter adhesin from Acinetobacter sp. Tol 5, exhibits nonspecific, high adhesiveness to abiotic surfaces. For identification of the functional domains of AtaA, precise design of domain-deletion mutants is necessary so as not to cause undesirable structural distortion. Here, we designed and constructed three types of AtaA mutants from which the same domain, FGG1, was deleted. The first mutant was designed to preserve the periodicity of hydrophobic residues in the coiled-coil segments sandwiching the deleted region. After the deletion, the protein was properly displayed on the cell surface and had the same adhesive function as the wild type. Transmission electron microscopy (TEM) imaging and circular dichroism (CD) spectroscopy showed that its isolated passenger domain had the same fiber structure as in the AtaA wild type. In contrast, a mutant designed to disturb the coiled-coil periodicity at the deletion site failed to reach the cell surface. Although secretion occurred for the mutant designed with a flexible connector between the coiled coils, the cells exhibited a decrease in adhesiveness. Furthermore, TEM imaging of the mutant fibers showed bending at the fiber tip and changes in their CD spectrum indicated a decrease in secondary structure content. Thus, we succeeded to natively display the huge homotrimeric fiber structure of AtaA on the cell surface after precise deletion of a domain, maintaining the proper folding state and adhesive function by preserving its coiled-coil periodicity. This strategy enables us to construct various domain-deletion mutants of AtaA without structural distortion for complete functional mapping.

Unraveling neutrophil- Yersinia interactions during tissue infection

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

Biogenesis and function of the autotransporter adhesins YadA, intimin and invasin

Bacteria often express numerous virulence factors. These virulence factors make them successful pathogens, by e.g. mediating attachment to host cells and thereby facilitating persistence or invasion, or by contributing to the evasion of the host immune system to allow proliferation and spread within the host and in the environment. The site of first contact of Gram negative bacteria with the host is the bacterial outer membrane (OM). Consisting of an asymmetrical lipid bilayer with phospholipids forming the inner, and lipopolysaccharides forming the outer leaflet, the OM harbors numerous integral membrane proteins that are almost exclusively β-barrel proteins. One distinct family of OM β-barrel proteins strongly linked to bacterial virulence are the autotransporter (AT) proteins. During the last years huge progress has been made to better understand the mechanisms underlying the insertion of AT proteins into the OM and also AT function for interaction with the host. This review shortly summarizes our current knowledge about outer membrane protein (OMP) and more specifically AT biogenesis and function. We focused on the AT proteins that we haved studied in most detail: i.e. the Yersinia adhesin A (YadA) and invasin of Yersinia enterocolitica (Ye) as well as its homolog intimin (Int) expressed by enteropathogenic Escherichia coli. In addition, this review provides a short outlook about how we could possibly use this knowledge to fight infection.

Molecular Mechanisms of Bartonella and Mammalian Erythrocyte Interactions: A Review

Bartonellosis is an infectious disease caused by Bartonella species that are distributed worldwide with animal and public health impact varying according to Bartonella species, infection phase, immunological characteristics, and geographical region. Bartonella is widely present in various mammals including cats, rodents, ruminants, and humans. At least 13 Bartonella species or subspecies are zoonotic. Each species has few reservoir animals in which it is often asymptomatic. Bartonella infection may lead to various clinical symptoms in humans. As described in the B.tribocorum-rat model, when Bartonella was seeded into the blood stream, they could escape immunity, adhered to and invaded host erythrocytes. They then replicated and persisted in the infected erythrocytes for several weeks. This review summarizes the current knowledge of how Bartonella prevent phagocytosis and complement activation, what pathogenesis factors are involved in erythrocyte adhesion and invasion, and how Bartonella could replicate and persist in mammalian erythrocytes. Current advances in research will help us to decipher molecular mechanisms of interactions between Bartonella and mammalian erythrocytes and may help in the development of biological strategies for the prevention and control of bartonellosis.

Ail provides multiple mechanisms of serum resistance to Yersinia pestis

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

Immunology of Yersinia pestis Infection

As a pathogen of plague, Yersinia pestis caused three massive pandemics in history that killed hundreds of millions of people. Yersinia pestis is highly invasive, causing severe septicemia which, if untreated, is usually fatal to its host. To survive in the host and maintain a persistent infection, Yersinia pestis uses several stratagems to evade the innate and the adaptive immune responses. For example, infections with this organism are biphasic, involving an initial "noninflammatory" phase where bacterial replication occurs initially with little inflammation and following by extensive phagocyte influx, inflammatory cytokine production, and considerable tissue destruction, which is called "proinflammatory" phase. In contrast, the host also utilizes its immune system to eliminate the invading bacteria. Neutrophil and macrophage are the first defense against Yersinia pestis invading through phagocytosis and killing. Other innate immune cells also play different roles, such as dendritic cells which help to generate more T helper cells. After several days post infection, the adaptive immune response begins to provide organism-specific protection and has a long-lasting immunological memory. Thus, with the cooperation and collaboration of innate and acquired immunity, the bacterium may be eliminated from the host. The research of Yersinia pestis and host immune systems provides an important topic to understand pathogen-host interaction and consequently develop effective countermeasures.

Vitronectin Binds to a Specific Stretch within the Head Region of Yersinia Adhesin A and Thereby Modulates Yersinia enterocolitica Host Interaction

Complement resistance is an important virulence trait of Yersinia enterocolitica (Ye). The predominant virulence factor expressed by Ye is Yersinia adhesin A (YadA), which enables bacterial attachment to host cells and extracellular matrix and additionally allows the acquisition of soluble serum factors. The serum glycoprotein vitronectin (Vn) acts as an inhibitory regulator of the terminal complement complex by inhibiting the lytic pore formation. Here, we show YadA-mediated direct interaction of Ye with Vn and investigated the role of this Vn binding during mouse infection in vivo. Using different Yersinia strains, we identified a short stretch in the YadA head domain of Ye O:9 E40, similar to the 'uptake region' of Y. pseudotuberculosis YPIII YadA, as crucial for efficient Vn binding. Using recombinant fragments of Vn, we found the C-terminal part of Vn, including heparin-binding domain 3, to be responsible for binding to YadA. Moreover, we found that Vn bound to the bacterial surface is still functionally active and thus inhibits C5b-9 formation. In a mouse infection model, we demonstrate that Vn reduces complement-mediated killing of Ye O:9 E40 and, thus, improved bacterial survival. Taken together, these findings show that YadA-mediated Vn binding influences Ye pathogenesis.

Contribution of the outer membrane protein OmpW in Escherichia coli to complement resistance from binding to factor H

The serum complement system is essential for innate immune defense against invading pathogenic bacteria. Some of the 8-stranded β-barrel outer membrane proteins confer bacterial resistance to the innate host immunity. We have previously demonstrated that OmpW, also an 8-stranded β-barrel protein that was identified a decade ago, protects bacteria against host phagocytosis. In this study, we investigated the complement resistance of OmpW. Our results indicate that the upregulation of OmpW is associated with increased survival when bacteria are exposed to normal human sera (NHS). Mutant bacteria lacking OmpW in NHS exhibited significantly lower survival rates in comparison to wild-type and ompW complemented bacteria. Furthermore, the bacterial survival significantly decreased in NHS that was supplemented with EGTA-Mg(2+) compared to that in NHS supplemented with EDTA. These results suggest that OmpW confer resistance to alternative complement pathway-mediated killing. Moreover, the binding of OmpW to factor H, a major inhibitor of alternative pathway, was found, indicating that OmpW recruitment of factor H is a mechanism for bacterial evasion of complement attack.

Yersinia virulence factors - a sophisticated arsenal for combating host defences

The human pathogens Yersinia pseudotuberculosis and Yersinia enterocolitica cause enterocolitis, while Yersinia pestis is responsible for pneumonic, bubonic, and septicaemic plague. All three share an infection strategy that relies on a virulence factor arsenal to enable them to enter, adhere to, and colonise the host while evading host defences to avoid untimely clearance. Their arsenal includes a number of adhesins that allow the invading pathogens to establish a foothold in the host and to adhere to specific tissues later during infection. When the host innate immune system has been activated, all three pathogens produce a structure analogous to a hypodermic needle. In conjunction with the translocon, which forms a pore in the host membrane, the channel that is formed enables the transfer of six ‘effector’ proteins into the host cell cytoplasm. These proteins mimic host cell proteins but are more efficient than their native counterparts at modifying the host cell cytoskeleton, triggering the host cell suicide response. Such a sophisticated arsenal ensures that yersiniae maintain the upper hand despite the best efforts of the host to counteract the infecting pathogen.

Yersinia adhesins: An arsenal for infection

The Yersiniae are a group of Gram-negative coccobacilli inhabiting a wide range of habitats. The genus harbors three recognized human pathogens: Y. enterocolitica and Y. pseudotuberculosis, which both cause gastrointestinal disease, and Y. pestis, the causative agent of plague. These three organisms have served as models for a number of aspects of infection biology, including adhesion, immune evasion, evolution of pathogenic traits, and retracing the course of ancient pandemics. The virulence of the pathogenic Yersiniae is heavily dependent on a number of adhesin molecules. Some of these, such as the Yersinia adhesin A and invasin of the enteropathogenic species, and the pH 6 antigen of Y. pestis, have been extensively studied. However, genomic sequencing has uncovered a host of other adhesins present in these organisms, the functions of which are only starting to be investigated. Here, we review the current state of knowledge on the adhesin molecules present in the Yersiniae, and their functions and putative roles in the infection process.

Role of β1 integrins and bacterial adhesins for Yop injection into leukocytes in Yersinia enterocolitica systemic mouse infection

Injection of Yersinia outer proteins (Yops) into host cells by a type III secretion system is an important immune evasion mechanism of Yersinia enterocolitica (Ye). In this process Ye invasin (Inv) binds directly while Yersinia adhesin A (YadA) binds indirectly via extracellular matrix (ECM) proteins to β1 integrins on host cells. Although leukocytes turned out to be an important target of Yop injection by Ye, it was unclear which Ye adhesins and which leukocyte receptors are required for Yop injection. To explain this, we investigated the role of YadA, Inv and β1 integrins for Yop injection into leukocytes and their impact on the course of systemic Ye infection in mice. Ex vivo infection experiments revealed that adhesion of Ye via Inv or YadA is sufficient to promote Yop injection into leukocytes as revealed by a β-lactamase reporter assay. Serum factors inhibit YadA- but not Inv-mediated Yop injection into B and T cells, shifting YadA-mediated Yop injection in the direction of neutrophils and other myeloid cells. Systemic Ye mouse infection experiments demonstrated that YadA is essential for Ye virulence and Yop injection into leukocytes, while Inv is dispensable for virulence and plays only a transient and minor role for Yop injection in the early phase of infection. Ye infection of mice with β1 integrin-depleted leukocytes demonstrated that β1 integrins are dispensable for YadA-mediated Yop injection into leukocytes, but contribute to Inv-mediated Yop injection. Despite reduced Yop injection into leukocytes, β1 integrin-deficient mice exhibited an increased susceptibility for Ye infection, suggesting an important role of β1 integrins in immune defense against Ye. This study demonstrates that Yop injection into leukocytes by Ye is largely mediated by YadA exploiting, as yet unknown, leukocyte receptors.

Structure, Function, and Assembly of Adhesive Organelles by Uropathogenic Bacteria

Bacteria assemble a wide range of adhesive proteins, termed adhesins, to mediate binding to receptors and colonization of surfaces. For pathogenic bacteria, adhesins are critical for early stages of infection, allowing the bacteria to initiate contact with host cells, colonize different tissues, and establish a foothold within the host. The adhesins expressed by a pathogen are also critical for bacterial-bacterial interactions and the formation of bacterial communities, including biofilms. The ability to adhere to host tissues is particularly important for bacteria that colonize sites such as the urinary tract, where the flow of urine functions to maintain sterility by washing away non-adherent pathogens. Adhesins vary from monomeric proteins that are directly anchored to the bacterial surface to polymeric, hair-like fibers that extend out from the cell surface. These latter fibers are termed pili or fimbriae, and were among the first identified virulence factors of uropathogenic Escherichia coli. Studies since then have identified a range of both pilus and non-pilus adhesins that contribute to bacterial colonization of the urinary tract, and have revealed molecular details of the structures, assembly pathways, and functions of these adhesive organelles. In this review, we describe the different types of adhesins expressed by both Gram-negative and Gram-positive uropathogens, what is known about their structures, how they are assembled on the bacterial surface, and the functions of specific adhesins in the pathogenesis of urinary tract infections.

New findings on the function and potential applications of the trimeric autotransporter adhesin

Trimeric autotransporter adhesins (TAAs) are located on the surface of many pathogenic Gram-negative bacteria. TAAs belong to the autotransporter protein family and consist of three identical monomers. These obligate homotrimeric proteins are secreted through the bacterial type Vc secretion system and share a common molecular organization that each monomer consists of a N-terminal "passenger" domain and a C-terminal translocation domain. TAAs are important virulence factors that are involved in bacterial life cycle and participate in mediating infection, invasion, dissemination and evasion of host immune responses. TAAs have also proved to be useful for many applications, such as vaccines and disease biomarkers. We here mainly focused on new findings on bio-function and application of TAAs in addition to their common structure and secretion mechanisms.

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

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

A domain dictionary of trimeric autotransporter adhesins

Trimeric autotransporter adhesins (TAAs) are modular, highly repetitive outer membrane proteins that mediate adhesion to external surfaces in many Gram-negative bacteria. In recent years, several TAAs have been investigated in considerable detail, also at the structural level. However, in their vast majority, putative TAAs in prokaryotic genomes remain poorly annotated, due to their sequence diversity and changeable domain architecture. In order to achieve an automated annotation of these proteins that is both detailed and accurate we have taken a domain dictionary approach, in which we identify recurrent domains by sequence comparisons, produce bioinformatic descriptors for each domain type, and connect these to structural information where available. We implemented this approach in a web-based platform, daTAA, in 2008 and demonstrated its applicability by reconstructing the complete fiber structure of a TAA conserved in enterobacteria. Here we review current knowledge on the domain structure of TAAs.

Ail proteins of Yersinia pestis and Y. pseudotuberculosis have different cell binding and invasion activities

The Yersinia pestis adhesin Ail mediates host cell binding and facilitates delivery of cytotoxic Yop proteins. Ail from Y. pestis and Y. pseudotuberculosis is identical except for one or two amino acids at positions 43 and 126 depending on the Y. pseudotuberculosis strain. Ail from Y. pseudotuberculosis strain YPIII has been reported to lack host cell binding ability, thus we sought to determine which amino acid difference(s) are responsible for the difference in cell adhesion. Y. pseudotuberculosis YPIII Ail expressed in Escherichia coli bound host cells, albeit at ~50% the capacity of Y. pestis Ail. Y. pestis Ail single mutants, Ail-E43D and Ail-F126V, both have decreased adhesion and invasion in E. coli when compared to wild-type Y. pestis Ail. Y. pseudotuberculosis YPIII Ail also had decreased binding to the Ail substrate fibronectin, relative to Y. pestis Ail in E. coli. When expressed in Y. pestis, there was a 30-50% decrease in adhesion and invasion depending on the substitution. Ail-mediated Yop delivery by both Y. pestis Ail and Y. pseudotuberculosis Ail were similar when expressed in Y. pestis, with only Ail-F126V giving a statistically significant reduction in Yop delivery of 25%. In contrast to results in E. coli and Y. pestis, expression of Ail in Y. pseudotuberculosis led to no measurable adhesion or invasion, suggesting the longer LPS of Y. pseudotuberculosis interferes with Ail cell-binding activity. Thus, host context affects the binding activities of Ail and both Y. pestis and Y. pseudotuberculosis Ail can mediate cell binding, cell invasion and facilitate Yop delivery.

Regulatory protein OmpR influences the serum resistance of Yersinia enterocolitica O:9 by modifying the structure of the outer membrane

The EnvZ/OmpR two-component system constitutes a regulatory pathway involved in bacterial adaptive responses to environmental cues. Our previous findings indicated that the OmpR regulator in Yersinia enterocolitica O:9 positively regulates the expression of FlhDC, the master flagellar activator, which influences adhesion/invasion properties and biofilm formation. Here we show that a strain lacking OmpR grown at 37°C exhibits extremely high resistance to the bactericidal activity of normal human serum (NHS) compared with the wild-type strain. Analysis of OMP expression in the ompR mutant revealed that OmpR reciprocally regulates Ail and OmpX, two homologous OMPs of Y. enterocolitica, without causing significant changes in the level of YadA, the major serum resistance factor. Analysis of mutants in individual genes belonging to the OmpR regulon (ail, ompX, ompC and flhDC) and strains lacking plasmid pYV, expressing YadA, demonstrated the contribution of the respective proteins to serum resistance. We show that Ail and OmpC act in an opposite way to the OmpX protein to confer serum resistance to the wild-type strain, but are not responsible for the high resistance of the ompR mutant. The serum resistance phenotype of ompR seems to be multifactorial and mainly attributable to alterations that potentiate the function of YadA. Our results indicate that a decreased level of FlhDC in the ompR mutant cells is partly responsible for the serum resistance and this effect can be suppressed by overexpression of flhDC in trans. The observation that the loss of FlhDC enhances the survival of wild-type cells in NHS supports the involvement of FlhDC regulator in this phenotype. In addition, the ompR mutant exhibited a lower level of LPS, but this was not correlated with changes in the level of FlhDC. We propose that OmpR might alter the susceptibility of Y. enterocolitica O:9 to complement-mediated killing through remodeling of the outer membrane.

Yersinia pseudotuberculosis uses Ail and YadA to circumvent neutrophils by directing Yop translocation during lung infection

A Yersinia pseudotuberculosis (Yptb) murine model of lung infection was previously developed using the serotype III IP2666NdeI strain, which robustly colonized lungs but only sporadically disseminated to the spleen and liver. We demonstrate here that a serotype Ib Yptb strain, IP32953, colonizes the lungs at higher levels and disseminates more efficiently to the spleen and liver compared with IP2666NdeI . The role of adhesins was investigated during IP32953 lung infection by constructing isogenic Δail, Δinv, ΔpsaE and ΔyadA mutants. An IP32953ΔailΔyadA mutant initially colonized but failed to persist in the lungs and disseminate to the spleen and liver. Yptb expressing these adhesins selectively bound to and targeted neutrophils for translocation of Yops. This selective targeting was critical for virulence because persistence of the ΔailΔyadA mutant was restored following intranasal infection of neutropenic mice. Furthermore, Ail and YadA prevented killing by complement-mediated mechanisms during dissemination to and/or growth in the spleen and liver, but not in the lungs. Combined, these results demonstratethat Ail and YadA are critical, redundant virulence factors during lung infection, because they thwart neutrophils by directing Yop-translocation specifically into these cells.

Adhesins and host serum factors drive Yop translocation by yersinia into professional phagocytes during animal infection

Yersinia delivers Yops into numerous types of cultured cells, but predominantly into professional phagocytes and B cells during animal infection. The basis for this cellular tropism during animal infection is not understood. This work demonstrates that efficient and specific Yop translocation into phagocytes by Yersinia pseudotuberculosis (Yptb) is a multi-factorial process requiring several adhesins and host complement. When WT Yptb or a multiple adhesin mutant strain, ΔailΔinvΔyadA, colonized tissues to comparable levels, ΔailΔinvΔyadA translocated Yops into significantly fewer cells, demonstrating that these adhesins are critical for translocation into high numbers of cells. However, phagocytes were still selectively targeted for translocation, indicating that other bacterial and/or host factors contribute to this function. Complement depletion showed that complement-restricted infection by ΔailΔinvΔyadA but not WT, indicating that adhesins disarm complement in mice either by prevention of opsonophagocytosis or by suppressing production of pro-inflammatory cytokines. Furthermore, in the absence of the three adhesins and complement, the spectrum of cells targeted for translocation was significantly altered, indicating that Yersinia adhesins and complement direct Yop translocation into neutrophils during animal infection. In summary, these findings demonstrate that in infected tissues, Yersinia uses adhesins both to disarm complement-dependent killing and to efficiently translocate Yops into phagocytes.

Many bacterial pathogens use a needle-like structure to deliver proteins into host cells to cause disease. Yersinia species use one such structure, called a type III secretion system, to deliver a set of 6–7 proteins, called Yops, into host cells. These Yops act to dismantle host defenses and establish infection. Bacterial adhesins and host factors have been suggested to promote proper delivery of Yops into specific mammalian cells. We identify three Yersinia pseudotuberculosis adhesins that significantly contribute to bacterial survival and efficient Yop delivery into host cells during animal infection. We also demonstrate that host serum factors in combination with Yersinia adhesins contribute to the number of cells that are injected with Yops and to the specific cell types targeted for injection. Our study illustrates that bacterial adhesins and host factors contribute to efficient delivery of effector proteins into targeted host cells during infection.

Autotransporter secretion: varying on a theme

Autotransporters are widely distributed among Gram-negative bacteria. They can have a large variety of functions and many of them have a role in virulence. They are synthesized as large precursors with an N-terminal signal sequence that mediates transport across the inner membrane via the Sec machinery and a translocator domain that mediates the transport of the connected passenger domain across the outer membrane to the bacterial cell surface. Like integral outer membrane proteins, the translocator domain folds in a β-barrel structure and requires the Bam machinery for its insertion into the outer membrane. After transport across the outer membrane, the passenger may stay connected via the translocator domain to the bacterial cell surface or it is proteolytically released into the extracellular milieu. Based on the size of the translocator domain and its position relative to the passenger in the precursor, autotransporters are divided into four sub-categories. We review here the current knowledge of the biogenesis, structure and function of various autotransporters.

Yersinia infection tools-characterization of structure and function of adhesins

Among the seventeen species of the Gram-negative genus Yersinia, three have been shown to be virulent and pathogenic to humans and animals-Y. enterocolitica, Y. pseudotuberculosis, and Y. pestis. In order to be so, they are armoured with various factors that help them adhere to tissues and organelles, cross the cellular barrier and escape the immune system during host invasion. The group of proteins that mediate pathogen-host interactions constitute adhesins. Invasin, Ail, YadA, YadB, YadC, Pla, and pH 6 antigen belong to the most prominent and best-known Yersinia adhesins. They act at different times and stages of infection complementing each other by their ability to bind a variety of host molecules such as collagen, fibronectin, laminin, β1 integrins, and complement regulators. All the proteins are anchored in the bacterial outer membrane (OM), often forming rod-like or fimbrial-like structures that protrude to the extracellular milieu. Structural studies have shown that the anchor region forms a β-barrel composed of 8, 10, or 12 antiparallel β-strands. Depending on the protein, the extracellular part can be composed of several domains belonging to the immunoglobulin fold superfamily, or form a coiled-coil structure with globular head domain at the end, or just constitute several loops connecting individual β-strands in the β-barrel. Those extracellular regions define the activity of each adhesin. This review focuses on the structure and function of these important molecules, and their role in pathogenesis.

Complete fiber structures of complex trimeric autotransporter adhesins conserved in enterobacteria

Trimeric autotransporter adhesins (TAAs) are modular, highly repetitive surface proteins that mediate adhesion to host cells in a broad range of Gram-negative pathogens. Although their sizes may differ by more than one order of magnitude, they all follow the same basic head-stalk-anchor architecture, where the head mediates adhesion and autoagglutination, the stalk projects the head from the bacterial surface, and the anchor provides the export function and attaches the adhesin to the bacterial outer membrane after export is complete. In complex adhesins, head and stalk domains may alternate several times before the anchor is reached. Despite extensive sequence divergence, the structures of TAA domains are highly constrained, due to the tight interleaving of their constituent polypeptide chains. We have therefore taken a "domain dictionary" approach to characterize representatives for each domain type by X-ray crystallography and use these structures to reconstruct complete TAA fibers. With SadA from Salmonella enterica, EhaG from enteropathogenic Escherichia coli (EHEC), and UpaG from uropathogenic E. coli (UPEC), we present three representative structures of a complex adhesin that occur in a conserved genomic context in Enterobacteria and is essential in the infection process of uropathogenic E. coli. Our work proves the applicability of the dictionary approach to understanding the structure of a class of proteins that are otherwise poorly tractable by high-resolution methods and provides a basis for the rapid and detailed annotation of newly identified TAAs.

Yersinia enterocolitica YadA mediates complement evasion by recruitment and inactivation of C3 products

Yersinia adhesin A (YadA) is a major virulence factor of Yersinia enterocolitica. YadA mediates host cell binding and autoaggregation and protects the pathogen from killing by the complement system. Previous studies demonstrated that YadA is the most important single factor mediating serum resistance of Y. enterocolitica, presumably by binding C4b binding protein (C4BP) and factor H, which are both complement inhibitors. Factor H acts as a cofactor for factor I-mediated cleavage of C3b into the inactive form iC3b and thus prevents formation of inflammatory effector compounds and the terminal complement complex. In this study, we challenged the current direct binding model of factor H to YadA and show that Y. enterocolitica YadA recruits C3b and iC3b directly, without the need of an active complement cascade or additional serum factors. Enhanced binding of C3b does not decrease survival of YadA-expressing Yersiniae because C3b becomes readily inactivated by factor H and factor I. Binding of factor H to YadA is greatly reduced in the absence of C3. Experiments using Yersinia lacking YadA or expressing YadA with reduced trimeric stability clearly demonstrate that both the presence and full trimeric stability of YadA are essential for complement resistance. A novel mechanism of factor H binding is presented in which YadA exploits recruitment of C3b or iC3b to attract large amounts of factor H. As a consequence, formation of the terminal complement complex is limited and bacterial survival is enhanced. These findings add a new aspect of how Y. enterocolitica effectively evades the host complement system.

Clinical isolates of Yersinia enterocolitica biotype 1A represent two phylogenetic lineages with differing pathogenicity-related properties

Background

Y. enterocolitica biotype (BT) 1A strains are often isolated from human clinical samples but their contribution to disease has remained a controversial topic. Variation and the population structure among the clinical Y. enterocolitica BT 1A isolates have been poorly characterized. We used multi-locus sequence typing (MLST), 16S rRNA gene sequencing, PCR for ystA and ystB, lipopolysaccharide analysis, phage typing, human serum complement killing assay and analysis of the symptoms of the patients to characterize 298 clinical Y. enterocolitica BT 1A isolates in order to evaluate their relatedness and pathogenic potential.

Results

A subset of 71 BT 1A strains, selected based on their varying LPS patterns, were subjected to detailed genetic analyses. The MLST on seven house-keeping genes (adk, argA, aroA, glnA, gyrB, thrA, trpE) conducted on 43 of the strains discriminated them into 39 MLST-types. By Bayesian analysis of the population structure (BAPS) the strains clustered conclusively into two distinct lineages, i.e. Genetic groups 1 and 2. The strains of Genetic group 1 were more closely related (97% similarity) to the pathogenic bio/serotype 4/O:3 strains than Genetic group 2 strains (95% similarity). Further comparison of the 16S rRNA genes of the BT 1A strains indicated that altogether 17 of the 71 strains belong to Genetic group 2. On the 16S rRNA analysis, these 17 strains were only 98% similar to the previously identified subspecies of Y. enterocolitica. The strains of Genetic group 2 were uniform in their pathogenecity-related properties: they lacked the ystB gene, belonged to the same LPS subtype or were of rough type, were all resistant to the five tested yersiniophages, were largely resistant to serum complement and did not ferment fucose. The 54 strains in Genetic group 1 showed much more variation in these properties. The most commonly detected LPS types were similar to the LPS types of reference strains with serotypes O:6,30 and O:6,31 (37%), O:7,8 (19%) and O:5 (15%).

Conclusions

The results of the present study strengthen the assertion that strains classified as Y. enterocolitica BT 1A represent more than one subspecies. Especially the BT 1A strains in our Genetic group 2 commonly showed resistance to human serum complement killing, which may indicate pathogenic potential for these strains. However, their virulence mechanisms remain unknown.

Yersinia pestis Ail: multiple roles of a single protein

Yersinia pestis is one of the most virulent bacteria identified. It is the causative agent of plague—a systemic disease that has claimed millions of human lives throughout history. Y. pestis survival in insect and mammalian host species requires fine-tuning to sense and respond to varying environmental cues. Multiple Y. pestis attributes participate in this process and contribute to its pathogenicity and highly efficient transmission between hosts. These include factors inherited from its enteric predecessors; Y. enterocolitica and Y. pseudotuberculosis, as well as phenotypes acquired or lost during Y. pestis speciation. Representatives of a large Enterobacteriaceae Ail/OmpX/PagC/Lom family of outer membrane proteins (OMPs) are found in the genomes of all pathogenic Yersiniae. This review describes the current knowledge regarding the role of Ail in Y. pestis pathogenesis and virulence. The pronounced role of Ail in the following areas are discussed (1) inhibition of the bactericidal properties of complement, (2) attachment and Yersinia outer proteins (Yop) delivery to host tissue, (3) prevention of PMNL recruitment to the lymph nodes, and (4) inhibition of the inflammatory response. Finally, Ail homologs in Y. enterocolitica and Y. pseudotuberculosis are compared to illustrate differences that may have contributed to the drastic bacterial lifestyle change that shifted Y. pestis from an enteric to a vector-born systemic pathogen.

The rickettsial OmpB β-peptide of Rickettsia conorii is sufficient to facilitate factor H-mediated serum resistance

Pathogenic species of the spotted fever group Rickettsia are subjected to repeated exposures to the host complement system through cyclic infections of mammalian and tick hosts. The serum complement machinery is a formidable obstacle for bacteria to overcome if they endeavor to endure this endozoonotic cycle. We have previously demonstrated that that the etiologic agent of Mediterranean spotted fever, Rickettsia conorii, is susceptible to complement-mediated killing only in the presence of specific monoclonal antibodies. We have also shown that in the absence of particular neutralizing antibody, R. conorii is resistant to the effects of serum complement. We therefore hypothesized that the interactions between fluid-phase complement regulators and conserved rickettsial outer membrane-associated proteins are critical to mediate serum resistance. We demonstrate here that R. conorii specifically interacts with the soluble host complement inhibitor, factor H. Depletion of factor H from normal human serum renders R. conorii more susceptible to C3 and membrane attack complex deposition and to complement-mediated killing. We identified the autotransporter protein rickettsial OmpB (rOmpB) as a factor H ligand and further demonstrate that the rOmpB β-peptide is sufficient to mediate resistance to the bactericidal properties of human serum. Taken together, these data reveal an additional function for the highly conserved rickettsial surface cell antigen, rOmpB, and suggest that the ability to evade complement-mediated clearance from the hematogenous circulation is a novel virulence attribute for this class of pathogens.

Strategies of exploitation of mammalian reservoirs by Bartonella species

Numerous mammal species, including domestic and wild animals such as ruminants, dogs, cats and rodents, as well as humans, serve as reservoir hosts for various Bartonella species. Some of those species that exploit non-human mammals as reservoir hosts have zoonotic potential. Our understanding of interactions between bartonellae and reservoir hosts has been greatly improved by the development of animal models for infection and the use of molecular tools allowing large scale mutagenesis of Bartonella species. By reviewing and combining the results of these and other approaches we can obtain a comprehensive insight into the molecular interactions that underlie the exploitation of reservoir hosts by Bartonella species, particularly the well-studied interactions with vascular endothelial cells and erythrocytes.

Analysis of the BadA stalk from Bartonella henselae reveals domain-specific and domain-overlapping functions in the host cell infection process

Human pathogenic Bartonella henselae cause cat scratch disease and vasculoproliferative disorders. An important pathogenicity factor of B. henselae is the trimeric autotransporter adhesin Bartonella adhesin A (BadA) which is modularly constructed and consists of a head, a long and repetitive neck-stalk module with 22 repetitive neck/stalk repeats and a membrane anchor. The BadA head is crucial for bacterial adherence to host cells, binding to several extracellular matrix proteins and for the induction of vascular endothelial growth factor (VEGF) secretion. Here, we analysed the biological role of the BadA stalk in the infection process in greater detail. For this purpose, BadA head-bearing and headless deletion mutants with different lengths (containing one or four neck/stalk repeats in the neck-stalk module) were produced and functionally analysed for their ability to bind to fibronectin, collagen and endothelial cells and to induce VEGF secretion. Whereas a head-bearing short version (one neck/stalk element) of BadA lacks exclusively fibronectin binding, a substantially truncated headless BadA mutant was deficient for all of these biological functions. The expression of a longer headless BadA mutant (four neck/stalk repeats) restored fibronectin and collagen binding, adherence to host cells and the induction of VEGF secretion. Our data suggest that (i) the stalk of BadA is exclusively responsible for fibronectin binding and that (ii) both the head and stalk of BadA mediate adherence to collagen and host cells and the induction of VEGF secretion. This indicates overlapping functions of the BadA head and stalk.

The structure of E. coli IgG-binding protein D suggests a general model for bending and binding in trimeric autotransporter adhesins

The Escherichia coli Ig-binding (Eib) proteins are trimeric autotransporter adhesins (TAAs) and receptors for IgG Fc. We present the structure of a large fragment of the passenger domain of EibD, the first TAA structure to have both a YadA-like head domain and the entire coiled-coil stalk. The stalk begins as a right-handed superhelix, but switches handedness halfway down. An unexpected β-minidomain joins the two and inserts a ∼120° rotation such that there is no net twist between the beginning and end of the stalk. This may be important in folding and autotransport. The surprisingly large cavities we found in EibD and other TAAs may explain how TAAs bend to bind their ligands. We identified how IgA and IgG bind and modeled the EibD-IgG Fc complex. We further show that EibD promotes autoagglutination and biofilm formation and forms a fibrillar layer covering the cell surface making zipper-like contacts between cells.

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.

Pathogenesis of Y. enterocolitica and Y. pseudotuberculosis in Human Yersiniosis

Yersiniosis is a food-borne illness that has become more prevalent in recent years due to human transmission via the fecal-oral route and prevalence in farm animals. Yersiniosis is primarily caused by Yersinia enterocolitica and less frequently by Yersinia pseudotuberculosis. Infection is usually characterized by a self-limiting acute infection beginning in the intestine and spreading to the mesenteric lymph nodes. However, more serious infections and chronic conditions can also occur, particularly in immunocompromised individuals. Y. enterocolitica and Y. pseudotuberculosis are both heterogeneous organisms that vary considerably in their degrees of pathogenicity, although some generalizations can be ascribed to pathogenic variants. Adhesion molecules and a type III secretion system are critical for the establishment and progression of infection. Additionally, host innate and adaptive immune responses are both required for yersiniae clearance. Despite the ubiquity of enteric Yersinia species and their association as important causes of food poisoning world-wide, few national enteric pathogen surveillance programs include the yersiniae as notifiable pathogens. Moreover, no standard exists whereby identification and reporting systems can be effectively compared and global trends developed. This review discusses yersinial virulence factors, mechanisms of infection, and host responses in addition to the current state of surveillance, detection, and prevention of yersiniosis.

Apolipoprotein A-I exerts bactericidal activity against Yersinia enterocolitica serotype O:3

Apolipoprotein A-I (apoA-I), the main protein component of high density lipoprotein (HDL), is well recognized for its antiatherogenic, antioxidant, and antiinflammatory properties. Here, we report a novel role for apoA-I as a host defense molecule that contributes to the complement-mediated killing of an important gastrointestinal pathogen, Gram-negative bacterium Yersinia enterocolitica. We specifically show that the C-terminal domain of apoA-I is the effector site providing the bactericidal activity. Although the presence of the lipopolysaccharide O-antigen on the bacterial surface is absolutely required for apoA-I to kill the bacteria, apoA-I does not interact with the bacteria directly. To the contrary, exposure of the bacteria by serum proteins triggers apoA-I deposition on the bacterial surface. As our data show that both purified lipid-free and HDL-associated apoA-I displays anti-bacterial potential, apoA-I mimetic peptides may be a promising therapeutic agent for the treatment of certain Gram-negative infections.

Unique cell adhesion and invasion properties of Yersinia enterocolitica O:3, the most frequent cause of human Yersiniosis

Many enteric pathogens are equipped with multiple cell adhesion factors which are important for host tissue colonization and virulence. Y. enterocolitica, a common food-borne pathogen with invasive properties, uses the surface proteins invasin and YadA for host cell binding and entry. In this study, we demonstrate unique cell adhesion and invasion properties of Y. enterocolitica serotype O:3 strains, the most frequent cause of human yersiniosis, and show that these differences are mainly attributable to variations affecting the function and expression of invasin in response to temperature. In contrast to other enteric Yersinia strains, invasin production in O:3 strains is constitutive and largely enhanced compared to other Y. enterocolitica serotypes, in which invA expression is temperature-regulated and significantly reduced at 37°C. Increase of invasin levels is caused by (i) an IS1667 insertion into the invA promoter region, which includes an additional promoter and RovA and H-NS binding sites, and (ii) a P98S substitution in the invA activator protein RovA rendering the regulator less susceptible to proteolysis. Both variations were shown to influence bacterial colonization in a murine infection model. Furthermore, we found that co-expression of YadA and down-regulation of the O-antigen at 37°C is required to allow efficient internalization by the InvA protein. We conclude that even small variations in the expression of virulence factors can provoke a major difference in the virulence properties of closely related pathogens which may confer better survival or a higher pathogenic potential in a certain host or host environment.

Adhesins of human pathogens from the genus Yersinia

Bacteria of the Gram-negative genus Yersinia are environmentally ubiquitous. Three species are of medical importance: the intestinal pathogens Y. enterocolitica and Y. pseudotuberculosis, and the plague bacillus Y. pestis. The two former species, spread by contaminated food or water, cause a range of gastrointestinal symptoms and, rarely, sepsis. On occasion, the primary infection is followed by autoimmune sequelae such as reactive arthritis. Plague is a systemic disease with high mortality. It is a zoonosis spread by fleas, or more rarely by droplets from individuals suffering from pneumonic plague. Y. pestis is one of the most virulent of bacteria, and recent findings of antibiotic-resistant strains together with its potential use as a bioweapon have increased interest in the species. In addition to being significant pathogens in their own right, the yersiniae have been used as model systems for a number of aspects of pathogenicity. This chapter reviews the molecular mechanisms of adhesion in yersiniae. The enteropathogenic species share three adhesins: invasin, YadA and Ail. Invasin is the first adhesin required for enteric infection; it binds to β(1) integrins on microfold cells in the distal ileum, leading to the ingestion of the bacteria and allows them to cross the intestinal epithelium. YadA is the major adhesin in host tissues. It is a multifunctional protein, conferring adherence to cells and extracellular matrix components, serum and phagocytosis resistance, and the ability to autoagglutinate. Ail has a minor role in adhesion and serum resistance. Y. pestis lacks both invasin and YadA, but expresses several other adhesins. These include the pH 6 antigen and autotransporter adhesins. Also the plasminogen activator of Y. pestis can mediate adherence to host cells. Although the adhesins of the pathogenic yersiniae have been studied extensively, their exact roles in the biology of infection remain elusive.

Complement control protein factor H: the good, the bad, and the inadequate

The complement system is an essential component of the innate immune system that participates in elimination of pathogens and altered host cells and comprises an essential link between the innate and adaptive immune system. Soluble and membrane-bound complement regulators protect cells and tissues from unintended complement-mediated injury. Complement factor H is a soluble complement regulator essential for controlling the alternative pathway in blood and on cell surfaces. Normal recognition of self-cell markers (i.e. polyanions) and C3b/C3d fragments is necessary for factor H function. Inadequate recognition of host cell surfaces by factor H due to mutations and polymorphisms have been associated with complement-mediated tissue damage and disease. On the other hand, unwanted recognition of pathogens and altered self-cells (i.e. cancer) by factor H is used as an immune evasion strategy. This review will focus on the current knowledge related to these versatile recognition properties of factor H.

First analysis of a bacterial collagen-binding protein with collagen Toolkits: promiscuous binding of YadA to collagens may explain how YadA interferes with host processes

The Yersinia adhesin YadA mediates the adhesion of the human enteropathogen Yersinia enterocolitica to collagens and other components of the extracellular matrix. Though YadA has been proposed to bind to a specific site in collagens, the exact binding determinants for YadA in native collagen have not previously been elucidated. We investigated the binding of YadA to collagen Toolkits, which are libraries of triple-helical peptides spanning the sequences of type II and III human collagens. YadA bound to many of them, in particular to peptides rich in hydroxyproline but with few charged residues. We were able to block the binding of YadA to collagen type IV with the triple-helical peptide (Pro-Hyp-Gly)(10), suggesting that the same site in YadA binds to triple-helical regions in network-forming collagens as well. We showed that a single Gly-Pro-Hyp triplet in a triple-helical peptide was sufficient to support YadA binding, but more than six triplets were required to form a tight YadA binding site. This is significantly longer than the case for eukaryotic collagen-binding proteins. YadA-expressing bacteria bound promiscuously to Toolkit peptides. Promiscuous binding could be advantageous for pathogenicity in Y. enterocolitica and, indeed, for other pathogenic bacteria. Many of the tightly binding peptides are also targets for eukaryotic collagen-binding proteins, and YadA was able to inhibit the interaction between selected Toolkit peptides and platelets. This leads to the intriguing possibility that YadA may interfere in vivo with host processes mediated by endogenous collagen-binding proteins.

Trimer stability of YadA is critical for virulence of Yersinia enterocolitica

Yersinia adhesin A (YadA) is a trimeric autotransporter adhesin with multiple functions in host-pathogen interactions. The aim of this study was to dissect the virulence functions promoted by YadA in vitro and in vivo. To accomplish this, we generated Yersinia enterocolitica O:8 mutants expressing point mutations in YadA G389, a highly conserved residue in the membrane anchor of YadA, and analyzed their impact on YadA expression and virulence functions. We found that point mutations of YadA G389 led to impaired transport, stability, and surface display of YadA. YadA G389A and G389S mutants showed comparable YadA surface expression, autoagglutination, and adhesion to those of wild-type YadA but displayed reduced trimer stability and complement resistance in vitro and were 10- to 1,000-fold attenuated in experimental Y. enterocolitica infection in mice. The G389T, G389N, and G389H mutants lost trimer stability, exhibited strongly reduced surface display, autoagglutination, adhesion properties, and complement resistance, and were avirulent (>10,000-fold attenuation) in mice. Our data demonstrate that G389 is a critical residue of YadA, required for optimal trimer stability, transport, surface display, and serum resistance. We also show that stable trimeric YadA protein is essential for virulence of Y. enterocolitica.

Interaction of Yersinia with the gut: mechanisms of pathogenesis and immune evasion

Yersinia entercolitica and Yersinia pseudotuberculosis are human foodborne pathogens that interact extensively with tissues of the gut and the host's immune system to cause disease. As part of their pathogenic strategies, the Yersinia have evolved numerous ways to invade host tissues, gain essential nutrients, and evade host immunity. Technological advances over the last 10 years have revolutionized our understanding of host-pathogen interactions. The application of these new technologies has also shown that even well-understood pathogens such as the Yersinia have many surprises waiting to be revealed. The complex interaction with the host has made Yersinia a paradigm for understanding bacterial pathogenesis and the host response to invasive bacterial infections. This review examines the mechanisms of immune evasion employed by the Yersinia and highlights recent advances in understanding the host-pathogen interaction.

Localization of the domains of the Haemophilus ducreyi trimeric autotransporter DsrA involved in serum resistance and binding to the extracellular matrix proteins fibronectin and vitronectin

Resisting the bactericidal activity of naturally occurring antibodies and complement of normal human serum is an important element in the evasion of innate immunity by bacteria. In the gram-negative mucosal pathogen Haemophilus ducreyi, serum resistance is mediated primarily by the trimeric autotransporter DsrA. DsrA also functions as an adhesin for the extracellular matrix proteins fibronectin and vitronectin and mediates attachment of H. ducreyi to keratinocytes. We sought to determine the domain(s) of the 236-residue DsrA protein required for serum resistance and extracellular matrix protein binding. A 140-amino-acid truncated protein containing only the C-terminal portion of the passenger domain and the entire translocator domain of DsrA exhibited binding to fibronectin and vitronectin and conferred serum resistance to an H. ducreyi serum-sensitive strain. A shorter DsrA construct consisting of only 128 amino acids was unable to bind to extracellular matrix proteins but was serum resistant. We concluded that neither fibronectin binding nor vitronectin binding is required for high-level serum resistance in H. ducreyi.