Determinants for thermoinducible cell binding and plasmid-encoded cellular penetration detected in the absence of the Yersinia pseudotuberculosis invasin protein

Yersinia deploys type III-secreted effectors to evade caspase-4 inflammasome activation in human cells

IMPORTANCE

Yersinia are responsible for significant disease burden in humans, ranging from recurrent disease outbreaks (yersiniosis) to pandemics (Yersinia pestis plague). Together with rising antibiotic resistance rates, there is a critical need to better understand Yersinia pathogenesis and host immune mechanisms, as this information will aid in developing improved immunomodulatory therapeutics. Inflammasome responses in human cells are less studied relative to murine models of infection, though recent studies have uncovered key differences in inflammasome responses between mice and humans. Here, we dissect human intestinal epithelial cell and macrophage inflammasome responses to Yersinia pseudotuberculosis. Our findings provide insight into species- and cell type-specific differences in inflammasome responses to Yersinia.

Yersinia Type III-Secreted Effectors Evade the Caspase-4 Inflammasome in Human Cells

Yersinia are gram-negative zoonotic bacteria that use a type III secretion system (T3SS) to inject Yersinia outer proteins (Yops) into the host cytosol to subvert essential components of innate immune signaling. However, Yersinia virulence activities can elicit activation of inflammasomes, which lead to inflammatory cell death and cytokine release to contain infection. Yersinia activation and evasion of inflammasomes have been characterized in murine macrophages but remain poorly defined in human cells, particularly intestinal epithelial cells (IECs), a primary site of intestinal Yersinia infection. In contrast to murine macrophages, we find that in both human IECs and macrophages, Yersinia pseudotuberculosis T3SS effectors enable evasion of the caspase-4 inflammasome, which senses cytosolic lipopolysaccharide (LPS). The antiphagocytic YopE and YopH, as well as the translocation regulator YopK, were collectively responsible for evading inflammasome activation, in part by inhibiting Yersinia internalization mediated by YadA and β1-integrin signaling. These data provide insight into the mechanisms of Yersinia-mediated inflammasome activation and evasion in human cells, and reveal species-specific differences underlying regulation of inflammasome responses to Yersinia .

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.

Transcriptomic profiling of Yersinia pseudotuberculosis reveals reprogramming of the Crp regulon by temperature and uncovers Crp as a master regulator of small RNAs

One hallmark of pathogenic yersiniae is their ability to rapidly adjust their life-style and pathogenesis upon host entry. In order to capture the range, magnitude and complexity of the underlying gene control mechanisms we used comparative RNA-seq-based transcriptomic profiling of the enteric pathogen Y. pseudotuberculosis under environmental and infection-relevant conditions. We identified 1151 individual transcription start sites, multiple riboswitch-like RNA elements, and a global set of antisense RNAs and previously unrecognized trans-acting RNAs. Taking advantage of these data, we revealed a temperature-induced and growth phase-dependent reprogramming of a large set of catabolic/energy production genes and uncovered the existence of a thermo-regulated ‘acetate switch’, which appear to prime the bacteria for growth in the digestive tract. To elucidate the regulatory architecture linking nutritional status to virulence we also refined the CRP regulon. We identified a massive remodelling of the CRP-controlled network in response to temperature and discovered CRP as a transcriptional master regulator of numerous conserved and newly identified non-coding RNAs which participate in this process. This finding highlights a novel level of complexity of the regulatory network in which the concerted action of transcriptional regulators and multiple non-coding RNAs under control of CRP adjusts the control of Yersinia fitness and virulence to the requirements of their environmental and virulent life-styles.

Many bacterial pathogens cycle between environmental sources and mammalian hosts. Adaptation to the different natural habitats and host niches is achieved through complex regulatory networks which adjust synthesis of the large repertoire of crucial virulence factors and fitness determinants. To uncover underlying control circuits, we determined the first in-depth single-nucleotide resolution transcriptome of Yersinia. This revealed important novel genetic information, such as global locations of transcriptional start sites, non-coding RNAs, potential riboswitches and provided a set of virulence-relevant expression profiles, which constitute a valuable tool for the research community. The analysis further uncovered a temperature-induced global reprogramming of central metabolic functions, likely to support intestinal colonization of the pathogen. This is accompanied by a major reorganization of the CRP regulon, which involves a multitude of regulatory RNAs. The primary consequence is a fine-tuned, coordinated control of metabolism and virulence through a plethora of environmentally controlled regulatory RNAs allowing rapid adaptation and high flexibility during life-style changes.

In vivo-induced InvA-like autotransporters Ifp and InvC of Yersinia pseudotuberculosis promote interactions with intestinal epithelial cells and contribute to virulence

The Yersinia pseudotuberculosis Ifp and InvC molecules are putative autotransporter proteins with a high homology to the invasin (InvA) protein. To characterize the function of these surface proteins, we expressed both factors in Escherichia coli K-12 and demonstrated the attachment of Ifp- and InvC-expressing bacteria to human-, mouse-, and pig-derived intestinal epithelial cells. Ifp also was found to mediate microcolony formation and internalization into polarized human enterocytes. The ifp and invC genes were not expressed under in vitro conditions but were found to be induced in the Peyer's patches of the mouse intestinal tract. In a murine coinfection model, the colonization of the Peyer's patches and the mesenteric lymph nodes of mice by the ifp-deficient strain was significantly reduced, and considerably fewer bacteria reached liver and spleen. The absence of InvC did not have a severe influence on bacterial colonization in the murine infection model, and it resulted in only a slightly reduced number of invC mutants in the Peyer's patches. The analysis of the host immune response demonstrated that the presence of Ifp and InvC reduced the recruitment of professional phagocytes, especially neutrophils, in the Peyer's patches. These findings support a role for the adhesins in modulating host-pathogen interactions that are important for immune defense.

The subcutaneous inoculation of pH 6 antigen mutants of Yersinia pestis does not affect virulence and immune response in mice

Two isogenic sets of Yersinia pestis strains were generated, composed of wild-type strains 231 and I-1996, their non-polar pH 6(-) mutants with deletions in the psaA gene that codes for its structural subunit or the whole operon, as well as strains with restored ability for temperature- and pH-dependent synthesis of adhesion pili or constitutive production of pH 6 antigen. The mutants were generated by site-directed mutagenesis of the psa operon and subsequent complementation in trans. It was shown that the loss of synthesis or constitutive production of pH 6 antigen did not influence Y. pestis virulence or the average survival time of subcutaneously inoculated BALB/c naïve mice or animals immunized with this antigen.

The pH 6 antigen is an antiphagocytic factor produced by Yersinia pestis independent of Yersinia outer proteins and capsule antigen

The pH 6 antigen (pH 6 Ag; PsaA) of Yersinia pestis has been shown to be a virulence factor. In this study, we set out to investigate the possible function of Y. pestis PsaA in a host cell line, RAW264.7 mouse macrophages, in order to better understand the role it might play in virulence. Y. pestis KIM5 derivatives with and without the pCD1 plasmid and their psaA isogenic counterparts and Escherichia coli HB101 and DEta5alpha carrying a psaA clone or a vector control were used for macrophage infections. Macrophage-related bacteria and gentamicin-resistant intracellular bacteria generated from plate counting and direct microscopic examinations were used to evaluate these RAW264.7 macrophage infections. Y. pestis psaA isogenic strains did not show any significant difference in their abilities to associate with or bind to mouse macrophage cells. However, expression of psaA appeared to significantly reduce phagocytosis of both Y. pestis and E. coli by mouse macrophages (P < 0.05). Furthermore, we found that complementation of psaA mutant Y. pestis strains could completely restore the ability of the bacteria to resist phagocytosis. Fluorescence microscopy following differential labeling of intracellular and extracellular Y. pestis revealed that significantly lower numbers of psaA-expressing bacteria were located inside the macrophages. Enhanced phagocytosis resistance was specific for bacteria expressing psaA and did not influence the ability of the macrophages to engulf other bacteria. Our data demonstrate that Y. pestis pH 6 Ag does not enhance adhesion to mouse macrophages but rather promotes resistance to phagocytosis.

The YadA protein of Yersinia pseudotuberculosis mediates high-efficiency uptake into human cells under environmental conditions in which invasin is repressed

The YadA protein is a major adhesin of Yersinia pseudotuberculosis that promotes tight adhesion to mammalian cells by binding to extracellular matrix proteins. In this study, we first addressed the possibility of competitive interference of YadA and the major invasive factor invasin and found that expression of YadA in the presence of invasin affected neither the export nor the function of invasin in the outer membrane. Furthermore, expression of YadA promoted both bacterial adhesion and high-efficiency invasion entirely independently of invasin. Antibodies against fibronectin and beta(1) integrins blocked invasion, indicating that invasion occurs via extracellular-matrix-dependent bridging between YadA and the host cell beta(1) integrin receptors. Inhibitor studies also demonstrated that tyrosine and Ser/Thr kinases, as well as phosphatidylinositol 3-kinase, are involved in the uptake process. Further expression studies revealed that yadA is regulated in response to several environmental parameters, including temperature, ion and nutrient concentrations, and the bacterial growth phase. In complex medium, YadA production was generally repressed but could be induced by addition of Mg(2+). Maximal expression of yadA was obtained in exponential-phase cells grown in minimal medium at 37 degrees C, conditions under which the invasin gene is repressed. These results suggest that YadA of Y. pseudotuberculosis constitutes another independent high-level uptake pathway that might complement other cell entry mechanisms (e.g., invasin) at certain sites or stages during the infection process.

Cas, Fak and Pyk2 function in diverse signaling cascades to promote Yersinia uptake

The interplay between pathogen-encoded virulence factors and host cell signaling networks is critical for both the establishment and clearance of microbial infections. Yersinia uptake into host cells serves as an in vitro model for exploring how host cells respond to Yersinia adherence. In this study, we provide insight into the molecular nature and regulation of signaling networks that contribute to the uptake process. Using a reconstitution approach in Fak-/- fibroblasts, we have been able to specifically address the interplay between Fak, Cas and Pyk2 in this process. We show that both Fak and Cas play roles in the Yersinia uptake process and that Cas can function in a novel pathway that is independent of Fak. Fak-dependent Yersinia uptake does not appear to involve Cas-Crk signaling. By contrast, Cas-mediated uptake in the absence of Fak requires Crk as well as the protein tyrosine kinases Pyk2 and Src. In spite of these differences, the requirement for Rac1 activity is a common feature of both pathways. Furthermore, blocking the function of either Fak or Cas induces similar morphological defects in Yersinia internalization, which are manifested by incomplete membrane protrusive activity that is consistent with an inhibition of Rac1 activity. Pyk2 also functions in Yersinia uptake by macrophages, which are physiologically important for clearing Yersinia infections. Taken together, these data provide new insight into the host cellular signaling networks that are initiated upon infection with Y. pseudotuberculosis. Importantly, these findings also contribute to a better understanding of other cellular processes that involve actin remodeling, including the host response to other microbial pathogens, cell adhesion and migration.

The RhoGAP activity of the Yersinia pseudotuberculosis cytotoxin YopE is required for antiphagocytic function and virulence

A variety of pathogenic bacteria use type III secretion pathways to translocate virulence proteins into host eukaryotic cells. YopE is an important virulence factor that is translocated into mammalian cells via a plasmid-encoded type III system in Yersinia spp. YopE action in mammalian cells promotes the disruption of actin filaments, cell rounding and blockage of phagocytosis. It was reported recently that two proteins with sequence similarity to YopE, SptP of Salmonella typhimurium and ExoS of Pseudomonas aeruginosa, function as GTPase-activating proteins (GAPs) for Rho GTPases. YopE contains an 'arginine finger' motif that is present in SptP, ExoS and other Rho GAPs and is essential for catalysis by this class of proteins. We show here that a GST-YopE fusion protein stimulated in vitro GTP hydrolysis by the Rho family members Cdc42, RhoA and Rac1, but not by Ras. Conversion of the essential arginine in the arginine finger motif to alanine (R144A) eliminated the in vitro GAP activity of GST-YopE. Infection assays carried out with a Yersinia pseudotuberculosis strain producing YopER144A demonstrated that GAP function was essential for the disruption of actin filaments, cell rounding and inhibition of phagocytosis by YopE in HeLa cells. Furthermore, the GAP function of YopE was important for Y. pseudotuberculosis pathogenesis in a mouse infection assay. Transfection of HeLa cells with a vector that produces a constitutively active form of RhoA (RhoA-V14) prevented the disruption of actin filaments and cell rounding by YopE. Production of an activated form of Rac1 (Rac1-V12), but not RhoA-V14, in HeLa cells interfered with YopE antiphagocytic activity. These results demonstrate that YopE functions as a RhoGAP to downregulate multiple Rho GTPases, leading to the disruption of actin filaments and inhibition of bacterial uptake into host cells.

Role of complement receptors in uptake of Mycobacterium avium by macrophages in vivo: evidence from studies using CD18-deficient mice

Mycobacterium avium is an intracellular pathogen that has been shown to invade macrophages by using complement receptors in vitro, but mycobacteria released from one cell can enter a second macrophage by using receptors different from complement receptors. Infection of CD18 (beta(2) integrin) knockout mice and the C57 BL/6 control mice led to comparable levels of tissue infection at 1 day, 2 days, 1 week, and 3 weeks following administration of bacteria. A histopathological study revealed similar granulomatous lesions in the two mouse strains, with comparable numbers of organisms. In addition, transmission electron microscopy of spleen tissues from both strains of mice showed bacteria inside macrophages. Our in vivo findings support the hypothesis that M. avium in the host is likely to use receptors other than CR3 and CR4 receptors to enter macrophages with increased efficiency.

Coordinate Involvement of Invasin and Yop Proteins in a Yersinia pseudotuberculosis-Specific Class I-Restricted Cytotoxic T Cell-Mediated Response

Yersinia pseudotuberculosis is a pathogenic enteric bacteria that evades host cellular immune response and resides extracellularly in vivo. Nevertheless, an important contribution of T cells to defense against Yersinia has been previously established. In this study we demonstrate that Lewis rats infected with virulent strains of Y. pseudotuberculosis, mount a Yersinia-specific, RT1-A-restricted, CD8+ T cell-mediated, cytotoxic response. Sensitization of lymphoblast target cells for cytolysis by Yersinia-specific CTLs required their incubation with live Yersinia and was independent of endocytosis. Although fully virulent Yersinia did not invade those cells, they attached to their surface. In contrast, invasin-deficient strain failed to bind to blast targets or to sensitize them for cytolysis. Furthermore, an intact virulence plasmid was an absolute requirement for Yersinia to sensitize blast targets for cytolysis. Using a series of Y. pseudotuberculosis mutants selectively deficient in virulence plasmid-encoded proteins, we found no evidence for a specific role played by YadA, YopH, YpkA, or YopJ in the sensitization process of blast targets. In contrast, mutations suppressing YopB, YopD, or YopE expression abolished the capacity of Yersinia to sensitize blast targets. These results are consistent with a model in which extracellular Yersinia bound to lymphoblast targets via invasin translocate inside eukaryotic cytosol YopE, which is presented in a class I-restricted fashion to CD8+ cytotoxic T cells. This system could represent a more general mechanism by which bacteria harboring a host cell contact-dependent or type III secretion apparatus trigger a class I-restricted CD8+ T cell response.

Models of invasion of enteric and periodontal pathogens into epithelial cells: a comparative analysis

Bacterial invasion of epithelial cells is associated with the initiation of infection by many bacteria. To carry out this action, bacteria have developed remarkable processes and mechanisms that co-opt host cell function and stimulate their own uptake and adaptation to the environment of the host cell. Two general types of invasion processes have been observed. In one type, the pathogens (e.g., Salmonella and Yersinia spp.) remain in the vacuole in which they are internalized and replicate within the vacuole. In the other type, the organism (e.g., Actinobacillus actinomycetemcomitans, Shigella flexneri, and Listeria monocytogenes) is able to escape from the vacuole, replicate in the host cell cytoplasm, and spread to adjacent host cells. The much-studied enteropathogenic bacteria usurp primarily host cell microfilaments for entry. Those organisms which can escape from the vacuole do so by means of hemolytic factors and C type phospholipases. The cell-to-cell spread of these organisms is mediated by microfilaments. The investigation of invasion by periodontopathogens is in its infancy in comparison with that of the enteric pathogens. However, studies to date on two invasive periodontopathogens. A actinomycetemcomitans and Porphyromonas (Bacteroides) gingivalis, reveal that these bacteria have developed invasion strategies and mechanisms similar to those of the enteropathogens. Entry of A. actinomycetemcomitans is mediated by microfilaments, whereas entry of P. gingivalis is mediated by both microfilaments and microtubules. A. actinomycetemcomitans, like Shigella and Listeria, can escape from the vacuole and spread to adjacent cells. However, the spread of A. actinomycetemcomitans is linked to host cell microtubules, not microfilaments. The paradigms presented establish that bacteria which cause chronic infections, such as periodontitis, and bacteria which cause acute diseases, such as dysentery, have developed similar invasion strategies.

Invasin-dependent and invasin-independent pathways for translocation of Yersinia pseudotuberculosis across the Peyer's patch intestinal epithelium

Yersinia pseudotuberculosis initiates systemic disease after translocation across the intestinal epithelium. Three Y. pseudotuberculosis factors, previously identified by their ability to promote association with cultured cells, were evaluated for their relative roles in translocation. To this end, mutants defective for invasin, YadA, or pH 6 antigen were tested for movement from the intestinal lumen into the subepithelium. Within 45 min after introduction of bacteria into the lumen, wild-type bacteria were found in the Peyer's patch. Mutants expressing defective invasin derivatives were unable to promote efficient translocation into the Peyer's patch and instead colonized on the luminal surface of the intestinal epithelium. In particular, a translocation defect was observed in a Y. pseudotuberculosis strain that expressed an uptake-defective invasin protein retaining considerable receptor binding activity. To attempt to reduce binding to luminal mucus, Y. pseudotuberculosis yadA and inv yadA strains were analyzed. Both strains had reduced mucus binding, with the inv yadA mutant revealing an alternate uptake pathway that was invasin independent. A mutant defective in the production of the pH 6 antigen adhesin also showed reduced binding to luminal mucus, with specific localization of bacteria in M cells. These results indicate that Y. pseudotuberculosis adhesive factors control the site of bacterial interaction within the intestinal environment and that loss of one factor causes drastic changes in the preferred site of localization of the bacterium in this locale.

Growth within macrophages increases the efficiency of Mycobacterium avium in invading other macrophages by a complement receptor-independent pathway

Infections caused by organisms of the Mycobacterium avium complex occur in approximately 50 to 60% of patients with AIDS. M. avium is an intracellular pathogen that survives and multiplies within mononuclear phagocytes. In this study, we investigated the uptake of M. avium grown within macrophages (intracellular growth M. avium [IG]) by a second macrophage compared with M. avium cultured in broth (extracellular growth M. avium [EG]). The results showed that IG was six- to eightfold more efficient than EG in entering macrophages. In addition, while an anti-CR3 antibody was able to inhibit approximately 60% of EG uptake by macrophages, it failed to inhibit the entry of IG. In contrast to EG, IG uptake into macrophages was significantly inhibited in the presence of anti-beta1-integrin and anti-transferrin receptor antibodies. Entry into macrophages by alternate receptors was associated with resistance to tumor necrosis factor alpha (TNF-alpha) stimulation. While stimulation with TNF-alpha resulted in inhibition of the growth of EG, it was not associated with inhibition of intracellular growth of IG. Investigation of the reason why M. avium is able to sense the changes in the intracellular environment triggering a change to the invasive phenotype suggests a direct relationship with macrophage apoptosis. These results suggest that intracellular growth is associated with novel mechanisms of M. avium uptake of macrophages and that those mechanisms appear to offer advantages to the bacteria in escaping the host defense.

Unmasking of intestinal epithelial lateral membrane beta1 integrin consequent to transepithelial neutrophil migration in vitro facilitates inv-mediated invasion by Yersinia pseudotuberculosis

Idiopathic intestinal disease states characterized by active inflammation associated with transepithelial migration of neutrophils may, paradoxically, be associated with an increased risk of infection by enteric pathogens. Although the specific ligands with which various intestinal pathogens associate remain largely unknown, it is thought that many reside on the basolateral membrane. For example, beta1 integrin, a basolateral membrane protein, mediates the specific interaction between epithelial cells and the inv gene product (invasin) on the surface of Yersinia pseudotuberculosis. Our observations indicate that neutrophil migration across model T84 cell intestinal epithelia produced transient separation of epithelial cells at sites of neutrophil migration, resulting in microdiscontinuities that remained unsealed for several hours. We hypothesized that such sites of microdiscontinuities would yield a potential route for luminal pathogens to gain access to basolateral ligands and, thus, provide a window of risk for enteric infection. The surface biotinylation and fluorescence localization studies reported here revealed that, as in natural intestinal epithelia, beta1 integrin was strictly polarized to the basolateral membrane in confluent T84 monolayers. However, the transient microdiscontinuities resulting from neutrophil migration permitted access to beta1 integrin from the apical reservoir. Coincident with such basolateral exposure of beta1 integrin, monolayers became susceptible to invasion by Y. pseudotuberculosis. Fluorescence localization indicated that Y. pseudotuberculosis selectively associated with monolayers at sites where small discontinuities resulting from neutrophil transmigration were found. An increased risk for Y. pseudotuberculosis infection was specifically related to exposure of beta1 integrin (normally concealed by tight junctions) to the apical compartment, as Y. pseudotuberculosis cells lacking the inv gene were unable to invade following neutrophil transepithelial migration. Following closure of the microdiscontinuities associated with neutrophil migration, a small pool of beta1 integrin remained apically localized, presumably due to incomplete repolarization. However, this small apical pool of beta1 integrin was insufficient to support a detectable increased risk of Yersinia infection. Together, these observations indicate that by transiently perturbing monolayer continuity, neutrophil transepithelial migration is associated with a window of risk in which luminal pathogens can access basolateral ligands such as beta1 integrin.

The psa locus is responsible for thermoinducible binding of Yersinia pseudotuberculosis to cultured cells

Yersinia pseudotuberculosis inv mutant strains cured of the virulence plasmid exhibit thermoinducible adhesion to cultured mammalian cells. To identify the genes responsible for this phenotype, Y. pseudotuberculosis homologs of the Y. enterocolitica ail and the Y. pestis psa loci were identified. Mutations in the Y. pseudotuberculosis ail and psa loci were constructed and tested for thermoinducible binding. Results of cellular binding assays indicated that only mutations in psa, not in ail, resulted in defects for thermoinducible binding, with inv yadA psa strains showing no detectable cell adhesion. In addition, an inv psa strain was defective for hemagglutination of sheep erythrocytes, in contrast to an inv psa+ strain which was fully competent for hemagglutination. The introduction of a plasmid containing a 6.7-kb KpnI-ClaI fragment of Y. pseudotuberculosis encompassing the psa locus was sufficient to complement both the cell adhesion and hemagglutination defects of the psa mutant. Results from subcloning and transposon mutagenesis indicated that the complete 6.7-kb region was required for thermoinducible binding and hemagglutination.

Pathogenesis of defined invasion mutants of Yersinia enterocolitica in a BALB/c mouse model of infection

It has been hypothesized for many years that the ability of Yersinia spp. to invade tissue culture cells is reflective of their ability to penetrate the intestinal epithelium and that this capacity is an important aspect of the disease process. Three different genes from Yersinia spp. that are involved in the tissue culture invasion phenotype have been identified: inv, ail, and yadA. It was previously shown that inv is necessary for efficient penetration of the intestinal epithelium by Yersinia enterocolitica. The present study was initiated to determine whether other known Yersinia invasion factors could promote uptake of the bacteria by mice in the absence of invasion. In addition, the roles of these three invasion factors in the survival of the bacteria, lethality for mice, and development of pathology were compared. We found that YadA is necessary for persistence of Y. enterocolitica in Peyer's patches, and consistent with this observation, the yadA mutant was avirulent for mice infected either orally or intraperitoneally. In addition, the inv yadA double mutant was avirulent. Histological and immunohistological examination of the Peyer's patches of infected mice indicated that despite the presence of large numbers of CFU at 24 h the yadA and ail yadA mutants cause only minimal pathology and recruitment of macrophages. At 42 h postinfection, Peyer's patches from mice infected with the inv mutant showed no pathology, despite the prediction that some of the mice by this time would be colonized. However, at 72 h, inflammation and necrosis were evident in some Peyer's patches. Together, these observations suggest that for visible pathology to develop, a threshold number of bacteria (> 10(5)) is needed and the bacteria need to persist for more than 24 h. Lastly, YadA but not Ail may play a role in the less efficient, delayed invasion of the intestinal epithelium observed for the inv mutant.

Role of RpoS in survival of Yersinia enterocolitica to a variety of environmental stresses

rpoS, a gene that encodes an alternative sigma factor (also known as katF), is critical for the ability of Yersinia enterocolitica grown at 37 degrees C, but not at 26 degrees C, to survive diverse environmental insults such as high temperature, hydrogen peroxide, osmolarity, and low pH. However, a Y. enterocolitica rpoS mutant was not affected in expression of inv or ail, invasion of tissue culture cells, or virulence in mice.

In vitro and in vivo characterization of an ail mutant of Yersinia enterocolitica

Ail is a 17-kDa protein of Yersinia enterocolitica previously identified on the basis of its ability to confer upon Escherichia coli the phenotype of attachment and invasion of cultured epithelial cells. Here we report an examination of the contribution of ail to the pathogenicity of Y. enterocolitica. A low-copy-number ail plasmid that promoted serum resistance in E. coli HB101 was constructed. The serum resistance phenotype conferred by ail to E. coli was affected by the growth phase of the culture as well as by the gene copy number. In contrast, the copy number of ail (and the relative quantity of Ail) was found to have little effect on the amount of Ail-promoted invasion of cultured epithelial cells. An ail mutant of Y. enterocolitica was constructed and characterized in vitro. This mutant produced no detectable Ail and had a reduced ability to invade CHO cells. Serum resistance of Y. enterocolitica was Ail dependent and was affected by growth phase and ail copy number. The phenotype of the ail mutant was examined in vivo by using a murine model for infection. The ail mutant phenotype was identical to that of the wild-type strain in oral 50% lethal dose studies and early colonization of Peyer's patches as well as in kinetic studies. Western blot (immunoblot) analysis of Ail produced by bacteria growing in vivo at 48 h postinfection indicated that ail was expressed at this time point. Thus, our findings confirm that Ail contributes to the serum resistance and invasion phenotypes of Y. enterocolitica in vitro and indicate that Ail is not required to establish an infection or to cause systemic infection of BALB/c or DBA/2 mice.

Comparison of the ability of Mycobacterium avium, M. smegmatis and M. tuberculosis to invade and replicate within HEp-2 epithelial cells

OBJECTIVE

Previous studies have demonstrated that mycobacteria can interact with epithelial cells, a property which can be important for establishing infection. In this study we investigated comparatively the ability of Mycobacterium avium, M. tuberculosis and M. smegmatis to invade and multiply within HEp-2 epithelial cells.

DESIGN

The ability to invade and to multiply intracellularly in HEp-2 cells was examined using a virulent strain of M. avium, a virulent (H37Rv) and an attenuated (H37Ra) strain of M. tuberculosis and a strain of M. smegmatis. The locus responsible for M. avium invasion was also cloned in Escherichia coli and M. smegmatis.

RESULTS

It was observed that M. avium invaded HEp-2 cells with greater efficiency than M. tuberculosis and M. smegmatis, while the H37Rv strain of M. tuberculosis was more efficient in invading HEp-2 than H37Ra and M. smegmatis. Both M. avium and M. tuberculosis were capable of multiplying within HEp-2 cells, while M. smegmatis was not. E. coli K12 and M. smegmatis were transformed with M. avium DNA. The invasive locus of M. avium provided E. coli K12 and M. smegmatis strains S5M101-1 and S5M101-2 with the ability to invade HEp-2 epithelial cells. Transformed M. smegmatis strains were able to grow intracellularly.

CONCLUSION

'Virulent' strains of M. avium and M. tuberculosis were shown to invade and to multiply within HEp-2 epithelial cells. This property was transferred to E. coli K12 and M. smegmatis by transformation with the invasive locus of M. avium. The ability of certain strains of mycobacteria to invade epithelial cells (bronchial, alveolar, intestinal) may represent an important phenotypic characteristic and could be directly related to pathogenicity.

Inhibition of the Fc receptor-mediated oxidative burst in macrophages by the Yersinia pseudotuberculosis tyrosine phosphatase

Suppression of host-cell-mediated immunity is a hallmark feature of Yersinia pseudotuberculosis infection. To better understand this process, the interaction of Y. pseudotuberculosis with macrophages and the effect of the virulence plasmid-encoded Yersinia tyrosine phosphatase (YopH) on the oxidative burst was analyzed in a chemiluminescence assay. An oxidative burst was generated upon infection of macrophages with a plasmid-cured strain of Y. pseudotuberculosis opsonized with immunoglobulin G antibody. Infection with plasmid-containing Y. pseudotuberculosis inhibited the oxidative burst triggered by secondary infection with opsonized bacteria. The tyrosine phosphatase activity of YopH was necessary for this inhibition. These results indicate that YopH inhibits Fc receptor-mediated signal transduction in macrophages in a global fashion. In addition, bacterial protein synthesis was not required for macrophage inhibition, suggesting that YopH export and translocation are controlled at the posttranslational level.

How intestinal epithelial cell differentiation inhibits the cell-entry of Yersinia pseudotuberculosis in colon carcinoma Caco-2 cell line in culture

In the human intestine, target cells of enteropathogens differentiate during cell migration along the crypt-villus axis. We have recently provided evidence that intestinal cell differentiation up-regulates intestinal cell infection by the noninvasive enterotoxigenic Escherichia coli [5, 23]. Several enterovirulent bacteria can penetrate intestinal epithelial cells, which are normally nonphagocytic. To document the role of intestinal epithelial cell differentiation in the pathogenesis of enteroinvasive bacteria, we examined here the intestinal cell-association and cell-entry of Yersinia pseudotuberculosis as a function of cell differentiation. For this purpose we used the colon carcinoma Caco-2 cell line in culture, which provides the most useful tool for the study of intestinal epithelial cell differentiation, because of its unique ability to spontaneously differentiate upon reaching confluence in normal culture condition. We report here that the thermoregulated inv and ail loci of Y. pseudotuberculosis have distinct roles in infection of Caco-2 cells. The ail locus initiates the cell-association and the inv locus initiates both the cell-association and the cell-entry processes. Moreover, we observed that: (i) both the bacterial cell-association (ail) and the bacterial cell-invasion (inv) occur at subconfluence when the Caco-2 cells are undifferentiated, and (ii) these processes are arrested when the differentiation commences. Since the integrin-beta 1 heterodimers are involved in cell-entry of Y. pseudotuberculosis in several mammalian cells, we further examined which beta 1 integrin promotes bacterial cell-entry in Caco-2 cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Growth of Legionella pneumophila in Acanthamoeba castellanii enhances invasion

Legionella pneumophila is considered to be a facultative intracellular parasite. Therefore, the ability of these bacteria to enter, i.e., invade, eukaryotic cells is expected to be a key pathogenic determinant. We compared the invasive ability of bacteria grown under standard laboratory conditions with that of bacteria grown in Acanthamoeba castellanii, one of the protozoan species that serves as a natural host for L. pneumophila in the environment. Amoeba-grown L. pneumophila cells were found to be at least 100-fold more invasive for epithelial cells and 10-fold more invasive for macrophages and A. castellanii than were L. pneumophila cells grown on agar. Comparison of agar- and amoeba-grown L. pneumophila cells by light and electron microscopy demonstrated dramatic differences in the morphology and structure of the bacteria. Analyses of protein expression in the two strains of bacteria suggest that these phenotypic differences may be due to the expression of new proteins in amoeba-grown L. pneumophila cells. In addition, the amoeba-grown bacteria were found to enter macrophages via coiling phagocytosis at a higher frequency than agar-grown bacteria did. Replication of L. pneumophila in protozoans present in domestic water supplies may be necessary to produce bacteria that are competent to enter mammalian cells and produce human disease.

Entrance and survival of Salmonella typhimurium and Yersinia enterocolitica within human B- and T-cell lines

Lymphocytes, located within the Peyer's patches, might be involved in the dissemination of enteropathogenic Salmonella typhimurium and Yersinia enterocolitica bacteria. To test this hypothesis, we have investigated the susceptibility of human B- and T-cell lines to bacterial adhesion and invasion. The two S. typhimurium strains analyzed were highly invasive, while the two Y. enterocolitica (O:8) strains adhered to the B- and T-cell lines but did not enter the cell lines in significant amounts. We hypothesize that the incapability of the Y. enterocolitica (O:8) strains to enter the human B- and T-cell lines is most probably due to the bacterial inability to induce the internalization process upon adhesion to both cell lines. Although immortalized B- and T-cell lines were used in this study, the results presented suggest the possibility that both cell types could play a role in the dissemination of intracellularly residing S. typhimurium in vivo.

Growth phase and low pH affect the thermal regulation of the Yersinia enterocolitica inv gene

The inv gene encodes the protein invasin, which is the primary invasion factor for Yersinia enterocolitica in vitro and in vivo. Previous studies of Yersinia species have shown that inv expression and entry into mammalian cells are temperature regulated. Invasin production is reduced at the host temperature of 37 degrees C as compared to production at ambient temperature; consequently, this study was initiated to determine whether other host environmental signals might induce inv expression at 37 degrees C. An inv::phoA translational fusion was recombined on to the Y. enterocolitica chromosome by allelic exchange to monitor inv expression. Molecular characterization of expression of the wild-type inv gene and the inv::phoA fusion showed that invasin is not produced until early stationary phase in bacteria grown at 23 degrees C. Y. enterocolitica grown at 37 degrees C and pH 5.5 showed levels of inv expression comparable to those observed in bacteria grown at 23 degrees C. An increase in Na+ ions caused a slight increase in expression at 37 degrees C. However, expression at 37 degrees C was unaffected by anaerobiosis, growth medium, calcium levels, or iron levels. Additionally, Y. enterocolitica expressed invasin in Peyer's patches two days after being introduced intragastrically into BALB/c mice. These results suggest that invasin expression in Y. enterocolitica may remain elevated early during interaction with the intestinal epithelium, a site at which invasin was shown to be necessary.

Cellular internalization in the absence of invasin expression is promoted by the Yersinia pseudotuberculosis yadA product

The Yersinia pseudotuberculosis invasin protein is able to promote bacterial penetration into mammalian cells. Insertion mutations that eliminate production of this protein show residual internalization that is dependent on the presence of the Yersinia virulence plasmid. An enrichment procedure was used to isolate molecular clones containing regions of the virulence plasmid that confer this low-level uptake on Y. pseudotuberculosis inv mutants. All of the Y. pseudotuberculosis strains isolated from this procedure harbored plasmids containing a region encompassing the yadA gene, which encodes a previously identified adhesin associated with attachment to extracellular matrix proteins. All of the mutations isolated that affected internalization of one of the strains that survived the enrichment disrupted the yadA open reading frame. Furthermore, a strain that contained yadA sequences and no other region of the virulence plasmid was able to promote internalization of a Y. pseudotuberculosis inv mutant. Consistent with these results, an intact virulence plasmid containing an insertion mutation in yadA was as defective as a plasmid-cured strain at promoting uptake of Y. pseudotuberculosis inv mutants. These results indicate that the product of the yadA gene is responsible for the plasmid-dependent entry observed in Y. pseudotuberculosis inv mutants.

The Yersinia pseudotuberculosis adhesin YadA mediates intimate bacterial attachment to and entry into HEp-2 cells

We characterized a bacterium-host cell interaction that is mediated by the Yersinia adhesin YadA. Derivatives of the virulence plasmid pIB1 harboring mutations in yadA, yopE, or yopH or in a low-calcium-response regulatory locus were introduced into a Yersinia pseudotuberculosis YPIII strain defective for Inv. The mutant strains were tested for the capacity to attach to and enter HEp-2 cells and express the cytotoxic activities of YopE and YopH. As previously shown, expression of YadA was necessary for bacterial attachment and Yop activity in the absence of Inv (R. Rosqvist, A. Forsberg, M. Rimpilainen, T. Bergman, and H. Wolf-Watz, Mol. Microbiol. 4:657-667, 1990). In addition, bacterial entry into HEp-2 cells occurred efficiently when YadA was expressed in the absence of YopE and YopH. These results demonstrated that YadA mediates intimate attachment of Y. pseudotuberculosis to HEp-2 cells and that phagocytic uptake of bacteria by this pathway is inhibited by the synergistic activities of YopH and YopE. A role for beta 1 integrins as host cell receptors for this bacterial attachment and entry mechanism was supported by HEp-2 cell adhesion and monoclonal antibody neutralization studies.

Yersinia enterocolitica invasin: a primary role in the initiation of infection

The ability to invade the intestinal epithelium of mammals is an essential virulence determinant of Yersinia enterocolitica. The chromosomally encoded Y. enterocolitica 8081v invasion gene, inv, was disrupted to assess its role in pathogenesis. The inv mutant (JP273v) was approximately 80-fold less invasive than wild type for cultured epithelial cells. When mice were infected intragastrically, up to 10(7) fewer JP273v were recovered from Peyer's patches early (6-18 hr) after infection compared with wild type. Analysis of the course of infection revealed that the inv mutant had distinct differences relative to wild type in the distribution of visible infectious foci and in tissue colonization; however, the mutant and wild-type strains had similar LD50 values for both orally and intraperitoneally infected mice. The invasion defect of the inv mutant was fully complemented in vitro and in vivo by introduction of the wild-type inv gene in trans. The inv gene product, invasin, appears to play a vital role in promoting entry during the initial stage of infection. During the subsequent establishment of a systemic infection, invasin may be of secondary importance, since the Y. enterocolitica inv mutant was as proficient as wild type at causing a fatal infection in mice. Based on these data, we discuss the role of invasin in a naturally occurring Y. enterocolitica infection.

Invasin expression in Yersinia pseudotuberculosis

A 3.2-kb region on the chromosome of Yersinia pseudotuberculosis, called inv, encodes invasin, a 103-kDa protein of the bacterial outer membrane. Invasin mediates bacterial entry into cultured animal cells. Six Y. pseudotuberculosis strains isolated from animal or human infections were analyzed for the presence of inv-related sequences with a radiolabeled inv clone, pRI203. We found that inv-specific sequences were present in all strains studied. Strains cured of virulence plasmid pYV were studied by Western immunoblot analysis with a monoclonal antibody directed against invasin. All but one strain produced invasin, but some strains produced more invasin than others. A strong correlation was found between the level of invasin production by these strains and their ability to enter into HEp-2 or CHO cells. The virulence of these strains was assessed in a murine model by measuring the number of bacteria in the spleen after intravenous challenge or in the mesenteric lymph nodes after intragastric challenge. The capacities of strains to invade cultured mammalian cells and to colonize the spleen were strongly correlative. In contrast, the ability of strains to translocate from the intestinal lumen to the mesenteric lymph nodes after intragastric inoculation did not correlate with their in vitro invasiveness.

Molecular and functional characterization of the Salmonella invasion gene invA: homology of InvA to members of a new protein family

One of the earliest steps in the pathogenic cycle of the facultative intracellular pathogen Salmonella spp. is the invasion of the cells of the intestinal epithelium. We have previously identified a genetic locus, inv, that allows Salmonella spp. to enter cultured epithelial cells. invA is a member of this locus, and it is the first gene of an operon consisting of at least two additional invasion genes. We have constructed strains carrying nonpolar mutations in invA and examined the individual contribution of this gene to the invasion phenotype of Salmonella typhimurium. Nonpolar S. typhimurium invA mutants were deficient in invasion of cultured epithelial cells although they were fully capable of attaching to the same cells. In addition, unlike wild-type S. typhimurium, invA mutants did not alter the normal architecture of the microvilli of polarized epithelial cells nor did they cause any alterations in the distribution of actin microfilaments of infected cells. The invasion phenotype of invA mutants was readily rescued by wild-type S. typhimurium when cultured epithelial cells were simultaneously infected with both strains. On the contrary, in a similar experiment, the adherent Escherichia coli strain RDEC-1 was not internalized into cultured cells when coinfected with wild-type S. typhimurium. The invA locus was found to be located at about 59 min on the Salmonella chromosome, 7% linked to mutS. The nucleotide sequence of invA showed an open reading frame capable of encoding a polypeptide of 686 amino acids with eight possible membrane-spanning regions and a predicted molecular weight of 75,974. A protein of this size was visualized when invA was expressed in a bacteriophage T7 RNA polymerase-based expression system. The predicted sequence of InvA was found to be homologous to Caulobacter crescentus FlbF, Yersinia LcrD, Shigella flexneri VirH, and E. coli FlhA proteins. These proteins may form part of a family of proteins with a common function, quite possibly the translocation of specific proteins across the bacterial cell membrane.

Adherence and internalization of Helicobacter pylori by HEp-2 cells

Helicobacter pylori colonizes the mucous layer of the stomach and the surface of gastric mucous cells. Although H. pylori is not generally thought of as invasive, it has been observed in the lamina propria and within vacuoles in the cytoplasm of epithelial cells. The authors report that isolates of H. pylori can enter into the cytoplasm of tissue culture epithelial cell lines such as HEp-2 cells. Intracellular uptake of H. pylori by HEp-2 cells is rapid and appears to require both the N-acetylneuraminyllactose-binding adhesin and another factor present only in living bacteria. Uptake of H. pylori was inhibited by ammonium chloride and chloroquine at concentrations that did not effect either adherence or bacterial viability. Dansylcadaverine, an inhibitor of receptor clustering and internalization, also inhibited uptake but not adherence of H. pylori. Uptake was completely inhibited when H. pylori and HEp-2 cells were incubated at 4 degrees C under conditions that did not effect bacterial adherence. Cytochalasin B, an inhibitor of phagocytosis, did not inhibit uptake. It is concluded that H. pylori is internalized either by receptor-mediated endocytosis or by a closely related pathway.

Bacterial resistance to complement killing mediated by the Ail protein of Yersinia enterocolitica

Ail is a 17-kDa outer membrane Yersinia protein that mediates bacterial attachment to, and invasion of, cultured epithelial cells. We report here an alternative role for Ail in the pathogenesis of Yersinia infection. We found that Escherichia coli HB101 harboring the 4-kilobase recombinant ail clone pVM102 were highly resistant to killing in up to 50% normal human serum. A 674-base-pair fragment of DNA from pVM102, which encodes the ail gene, was inserted into pUC18 and shown to promote full resistance to complement killing in E. coli HB101. Cellular attachment and resistance to complement killing in a plasmid-cured inv- strain of Yersinia enterocolitica (0:8) was correlated with the thermoinduced expression of Ail at 37 degrees C. Insertional inactivation of ail in Y. enterocolitica resulted in loss of both thermoinduced bacterial properties. Cellular attachment and serum resistance were restored by complementation of the defect by plasmid-encoded ail. Complementation of cell attachment activity required bacterial growth at 37 degrees C, indicating that an additional thermoinduced factor is required for this Ail function. In addition, these studies reveal that functional homology exists between Ail and the structurally related protein Rck, which promotes resistance to complement killing in Salmonella typhimurium.

Binding of cultured mammalian cells to immobilized bacteria

The invasin protein of Yersinia pseudotuberculosis binds to integrin receptors on mammalian cells and promotes cellular penetration. We demonstrate here that the cell attachment activity of invasin can be detected in bacterial colonies that have been immobilized on filter membranes. Invasin expressed in either Escherichia coli K-12 or Y. pseudotuberculosis mediated binding to membranes, and as few as 10(5) Y. pseudotuberculosis resulted in detectable attachment of cultured epithelial cells. A similar binding activity was detected in clinical isolates of the related pathogen Y. enterocolitica but not in environmental isolates. Although there exist multiple mechanisms for the binding of wild-type organisms to host cells, efficient mammalian cell binding to immobilized Y. pseudotuberculosis required expression of a functional invasin protein. Several pathogens that are known to bind or penetrate mammalian cells were also tested, and only one of these bound cultured mammalian cells efficiently.

Distribution of the invA, -B, -C, and -D genes of Salmonella typhimurium among other Salmonella serovars: invA mutants of Salmonella typhi are deficient for entry into mammalian cells

Invasion of intestinal epithelial cells is an essential virulence factor of salmonellae. A group of genes, invABC and invD, that allow Salmonella typhimurium to penetrate cultured epithelial cells have previously been characterized (J. E. Galán and R. Curtiss III, Proc. Natl. Acad. Sci. USA 86:6383-6387, 1989). The distribution of these genes among Salmonella isolates belonging to 37 different species or serovars was investigated by Southern and colony blot hybridization analyses. Regions of high sequence similarity to the invABC genes were present in all Salonella isolates examined, while regions of sequence similarity to the invD gene were present in all but one (S. arizonae) of the isolates tested, with little restriction fragment length polymorphism. Sequences similar to these genes were not detected in strains of Escherichia coli, Yersinia spp., or Shigella spp. invA mutants (unable to express the invABC genes) of several Salmonella species or serovars, including S. typhi, were constructed and examined for their ability to penetrate Henle-407 cells. All mutants were deficient for entry into cultured epithelial cells, indicating that the invABC genes were not only present in these strains but also functional.

Enhancement of the invasive ability of Neisseria gonorrhoeae by contact with HecIB, an adenocarcinoma endometrial cell line

Since Neisseria gonorrhoeae is an obligate pathogen, there is no animal model for identification of virulence factors for this bacterium. An alternative model for assessment of gonococcal virulence is invasion of the adenocarcinoma endometrial cell line, HecIB. Preincubation of gonococci with glutaraldehyde-fixed HecIB cells eliminated the six- to eight-hour lag in entry of bacteria into a fresh HeIIB monolayer seen with unpreincubated gonococci or gonococci preincubated in tissue-culture medium alone. Gonococci tightly bound to fixed HecIB cells were more invasive than cells free in the tissue-culture medium, suggesting that actual contact with HecIB cells was required for the enhancement of invasive ability. Chloramphenicol addition during the preincubation prevented the enhanced invasion. Preincubated gonococci were not more adherent to HecIB cells, suggesting that a stage in invasion after binding of gonococci to HecIB cells was enhanced. The enhanced invasion occurred only when gonococci were preincubated with HecIB cells and not with HEp-2, HeLa, Chang or CHO cells. This eukaryotic cell specificity for induction of enhanced invasion may indicate a role for invasion in gonococcal infection of the endometrium.

Discrimination between intracellular uptake and surface adhesion of bacterial pathogens

Most bacterial pathogens initiate infectious diseases by adhering to host cells. Bacterial adherence to nonphagocytic cells usually leads to extracellular colonization; however, many invasive microorganisms enter host cells after binding to the host cell surface. It is unclear why bacterial adherence can result in these two different fates for the microorganism. Analyses of model systems, such as the uptake of enteropathogenic Yersinia into cultured cells, indicate that the particular mammalian cell receptors bound and the nature of the binding event dictate whether the bacterium remains extracellular or enters host cells.

Outer membranes are competitive inhibitors of Escherichia coli O157:H7 adherence to epithelial cells

Escherichia coli of serotype O157:H7 are Vero cytotoxin-producing enteric pathogens that have been associated recently with sporadic cases and outbreaks of hemorrhagic colitis and with the hemolytic-uremic syndrome. Adherence of many enteropathogenic bacteria to mucosal surfaces is a critical step in the pathogenesis of diarrheal disease. We showed previously that adherence of E. coli O157:H7 strain CL-56 to epithelial cells in vitro is inhibited by outer membranes. In this study we examined whether outer membranes from a series of E. coli O157:H7 strains mediated competitive inhibition of bacterial binding to epithelial cells grown in tissue culture. We also determined which constituents of the outer membrane mediated inhibition of CL-56 adherence. Binding of six O157:H7 strains to HEp-2 cells was determined by quantitating the number of adherent bacteria in the presence and absence of outer membranes which were extracted from each strain with N-lauroyl sarcosinate (1.7%, wt/vol). After separation of outer membranes by gel electrophoresis, four bands (94, 40, 36, and 30 kDa) were collected by electroelution. Immune sera were raised in rabbits to each of the four eluted bands. Outer membrane extracts from each of the six O157:H7 strains inhibited binding of homologous organisms to the HEp-2 cells. At dilutions which did not cause bacterial agglutination, antiserum raised against the 94-kDa outer membrane protein showed maximal inhibition of bacterial adherence (17.0 +/- 7.3% adherence of control levels). Growth of bacteria in iron-depleted broth did not affect their binding to HEp-2 cells, suggesting that iron-regulated outer membranes were not involved. Fluid accumulation in ileal ligated loops of rabbits in response to E. coli O157:H7 challenge was diminished following both parenteral immunization with outer membranes extracted from the homologous strain and coincubation of organisms with immune serum which contained antibodies to outer membrane extracts. These data indicate that outer membranes are competitive inhibitors of E. coli O157:H7 adherence. Specific constituents of the outer membrane may function as bacterial attachment factors (i.e., adhesins) for E. coli O157:H7 adherence to epithelial cell surfaces.

Role of the Yersinia outer membrane protein YadA in adhesion to rabbit intestinal tissue and rabbit intestinal brush border membrane vesicles

The Yersinia virulence plasmid confers on strains of Yersinia pseudotuberculosis and Y. enterocolitica an adhesive potential superior to the one encoded by the chromosome alone. We have evaluated the role of the plasmid-encoded outer membrane protein YadA (formerly called Yopl) in adhesion. Insertional inactivation of the yadA gene (formerly called yopA), which encodes YadA, led to a reduction in the capacity of plasmid-carrying strains of Y. pseudotuberculosis 0:III and Y. enterocolitica 0:9 to adhere to intestinal tissue, brush border membranes and polystyrene surfaces. The adhesive characteristics of the mutants were comparable to those of their plasmid-cured counterparts. When the yadA gene from Y. pseudotuberculosis serotype 0:III or Y. enterocolitica serotype 0:3 or 0:8 was cloned into an Escherichia coli strain, increased ability to adhere to intestinal tissue, brush border membrane vesicles and polystyrene was transferred concomitantly. The introduction of the yadA gene from Y. pestis, which is unable to express YadA due to a one base pair deletion, did not change the adhesive characteristics of E. coli. Expression of YadA in the outer membrane may, therefore, make an important contribution to intestinal adherence of the two enteropathogenic members of the Yersinia species, Y. pseudotuberculosis and Y. enterocolitica.

Tyrosine phosphate hydrolysis of host proteins by an essential Yersinia virulence determinant

The plasmid-encoded YopH protein is a protein-tyrosine phosphatase (PTPase; EC 3.1.3.48) that is essential for Yersinia virulence. We have investigated the molecular basis for the role of PTPase activity in Yersinia pathogenesis. Allelic recombination was employed to introduce a defined mutation into the yopH plasmid gene. Conversion of the essential Cys-403 to Ala in the catalytic domain of the protein abolished YopH PTPase activity and significantly reduced the virulence of Yersinia pseudotuberculosis in a murine infection model. 32P-labeled phosphotyrosine-containing proteins were immunoprecipitated from extracts of Y. pseudotuberculosis-infected cell monolayers and analyzed by SDS/PAGE to assess the impact of YopH on host protein phosphorylation. Major proteins of 200, 120, and 60 kDa were dephosphorylated in macrophages associated with wild-type Y. pseudotuberculosis. Selective removal of phosphate from the 120- and 60-kDa proteins was shown to be specific to the YopH PTPase activity. Phagocytosis of the bacteria was not required for this dephosphorylation activity, suggesting that YopH is functionally expressed by extracellular bacteria. These observations indicate that the essential function of YopH in Yersinia pathogenesis is host-protein dephosphorylation.

An O antigen can interfere with the function of the Yersinia pseudotuberculosis invasin protein

Escherichia coli strains harbouring the Yersinia pseudotuberculosis inv gene are able to enter cultured mammalial cells. We show here that this property is not shared by all enteric bacteria, since Shigella flexneri 2a cured of its virulence-associated plasmid and harbouring the inv gene is unable to enter mammalian cells efficiently. Mapping studies showed that the region of the chromosome responsible for this phenotype includes rfaB, a locus involved in the production of O antigen. S. flexneri 2a strains that express O antigen were unable to enter mammalian cells, even though invasin was efficiently expressed and localized, showing that this structure interferes with invasin activity. The O antigen either masks invasin or sterically hinders the ability of the mammalian cell receptor to bind this protein.

Interactions between Yersinia enterocolitica and rabbit ileal mucus: growth, adhesion, penetration, and subsequent changes in surface hydrophobicity and ability to adhere to ileal brush border membrane vesicles

Interactions between Yersinia enterocolitica and rabbit ileal mucus were examined. Strains carrying the Yersinia virulence plasmid, pYV, adhered to crude mucus but not to intestinal luminal contents that had been immobilized on polystyrene. Using an Y. enterocolitica O:9 mutant in which the yadA gene (formerly called yopA), encoding the high-molecular-weight outer membrane protein YadA (formerly called protein P1 or Yop1), had been inactivated and an Escherichia coli strain carrying the cloned yadA gene, we demonstrated that the ability to adhere to mucus correlated closely to expression of YadA. Thereafter, we evaluated possible consequences of binding between pYV-carrying Y. enterocolitica O:3 strains and constituents in the mucus layer. pYV-carrying strains were able to multiply at a high rate in mucus but not in luminal contents, and the ability to adhere to mucus could therefore facilitate bacterial colonization of the mucosa. However, we also showed in vitro that mucus acted as a barrier for a mucus-adherent, pYV-carrying Y. enterocolitica strain. Furthermore, penetration through, or preincubation with, mucus reduced subsequent adhesion of the pYV-carrying strain to brush border membrane vesicles without simultaneously causing bacterial aggregation. Preincubation with mucus also changed the bacterial surface of the same strain from hydrophobic to hydrophilic. Immunoglobulins present in mucus did not seem to be of importance for our observations. Interaction of Y. enterocolitica with intestinal mucus may thus reflect a host defense mechanism that reduces the pYV-mediated adhesion to the epithelial cell membrane, possibly by rendering the bacteria less hydrophobic.

A new assay for invasion of HeLa 229 cells by Bordetella pertussis: effects of inhibitors, phenotypic modulation, and genetic alterations

Invasion and intracellular survival of Bordetella pertussis in HeLa 229 cells was studied by a new assay that utilizes polymyxin B instead of gentamicin to rapidly kill extracellular organisms. Invasion measured by this assay was time and temperature dependent and was inhibited by the microfilament drug cytochalasin D. The invasion process was also dependent on a functional vir locus (also known as bvg), the positive regulator of virulence gene expression in B. pertussis. Four spontaneous Vir- phase variants of B. pertussis and a mutant with a transposon insertion mutation in the vir locus did not invade. Cells that were environmentally modulated and thus did not express virulence determinants also did not invade. Two Vir- mutants, a vir-directed plasmid insertion mutant and a UV-light-induced mutant, were capable of invasion, although they did not produce other known virulence factors such as pertussis toxin and hemolysin but did produce small amounts of filamentous hemagglutinin (FHA) and the 69-kilodalton outer membrane protein. None of 70 Tn5 IS50L::phoA (TnphoA) insertion mutants of strain Bp18323 (including three mutants defective in FHA) tested showed any reproducible defect in invasion. A mutant carrying a site-directed deletion mutation in FHA was also capable of invasion in our assay. These data suggest that there is redundancy in the invasion functions of B. pertussis and that one or more of these are coordinately regulated with FHA and the 69-kilodalton outer membrane protein more tightly than with other vir-activated gene products.

Genetic evidence that the yopA gene-encoded Yersinia outer membrane protein Yop1 mediates inhibition of the anti-invasive effect of interferon

HEp-2 cell monolayers were challenged with genetic variants of Yersinia pseudotuberculosis YPIII(pIB1) and Yersinia enterocolitica W22708(pYL4). Both strains were represented by (i) variants harboring the 70-kilobase virulence plasmid, (ii) their isogenic plasmid-cured derivatives, and (iii) two transposon mutants constructed by insertional inactivation of the plasmid genes encoding outer membrane protein Yop1 and Ca2+ dependency in strains YPIII(pIB1) and W22708(pYL4). When the HEp-2 cells were pretreated with recombinant alpha interferon subtype A, all invasive variants of Y. enterocolitica and Y. pseudotuberculosis, except those variants which expressed Yop1, showed a significantly reduced ability to localize intracellularly. The anti-invasive effect of interferon was abolished when the gene was expressed or when a sterile filtered sonic extract of a Yop1-producing strain was added to the cell cultures. To obtain further evidence of a potential role of Yop1, a DNA fragment encoding Yop1 cloned into the vector pBR322 was used. After introduction of the resultant recombinant plasmid pYMS2 into the plasmid-cured variant YPIII and the Yop1-negative transposon mutant YPIII(pIB102) of Y. pseudotuberculosis, both transformants regained the ability to produce Yop1 and showed complete inhibition of the interferon effect. Moreover, the sterile sonic extract of an Escherichia coli strain, which carried pYMS2, neutralized the anti-invasive effect of interferon. The results provide direct genetic evidence that Yop1 mediates inhibition of the anti-invasive effect of interferon in HEp-2 cell cultures. The results also demonstrated that Yop1 itself reduces the ability of Yersinia spp. to localize intracellularly in HEp-2 cells.