Adherence characteristics of attaching and effacing strains of Escherichia coli from rabbits

Enterobacteria and host resistance to infection

Enterobacteriaceae are a large family of Gram-negative, non-spore-forming bacteria. Although many species exist as part of the natural flora of animals including humans, some members are associated with both intestinal and extraintestinal diseases. In this review, we focus on members of this family that have important roles in human disease: Salmonella, Escherichia, Shigella, and Yersinia, providing a brief overview of the disease caused by these bacteria, highlighting the contribution of animal models to our understanding of their pathogenesis and of host genetic determinants involved in susceptibility or resistance to infection.

Establishment and characterization of a skin epidermal cell line from mud loach, Misgurnus anguillicaudatus, (MASE) and its interaction with three bacterial pathogens

A continuous skin epidermal cell line from mud Loach (Misgurnus anguillicaudatus) (MASE cell line) was established with its application in bacteria infection demonstrated in this study. Primary MASE cell culture was initiated at 26 °C in Dulbecco's modified Eagle medium/F12 medium (1:1; pH7.2) supplemented with 20% fetal bovine serum (FBS). The primary MASE cells in spindle morphology proliferated into a confluent monolayer within 2 weeks, and were continuously subcultured even in 10% FBS- DMEM/F12 after 10 passages. Impacts of medium and temperature on the growth of the cells were examined. The optimum growth was found in DMEM/F12 with 20% FBS and at 26 °C. The MASE cells have been subcultured steadily over Passage 90 with a population doubling time of 53.3 h at Passage 60. Chromosome analysis revealed that 60.5% of MASE cells at Passage 60 maintained the normal diploid chromosome number (50) with a normal karyotype of 10m+4sm + 36t. Bacteria from the three species (Aeromonas veronii, Vibrio parahaemolyticus and Escherichia coli) were used to investigate the interactions between bacteria and cellular hosts. The three strains could be attached to the MASE cells and replicate at different levels. A. veronii could induce apoptosis in the MASE cells, with highest adherence rate among the three strains, whereas V. parahaemolyticus could cause highest cell death rate through a non-apoptotic cell death pathway, with high level of replication. The results revealed that different bacteria could interact with the MASE cells in different manners, and divergent pathways might lie in mediating cell death when cellular hosts confronted with pathogen infection. Therefore, the MASE cell line may serve as a useful tool for studying the interaction between skin bacteria and fish cells.

Pathogenesis of human enterovirulent bacteria: lessons from cultured, fully differentiated human colon cancer cell lines

Hosts are protected from attack by potentially harmful enteric microorganisms, viruses, and parasites by the polarized fully differentiated epithelial cells that make up the epithelium, providing a physical and functional barrier. Enterovirulent bacteria interact with the epithelial polarized cells lining the intestinal barrier, and some invade the cells. A better understanding of the cross talk between enterovirulent bacteria and the polarized intestinal cells has resulted in the identification of essential enterovirulent bacterial structures and virulence gene products playing pivotal roles in pathogenesis. Cultured animal cell lines and cultured human nonintestinal, undifferentiated epithelial cells have been extensively used for understanding the mechanisms by which some human enterovirulent bacteria induce intestinal disorders. Human colon carcinoma cell lines which are able to express in culture the functional and structural characteristics of mature enterocytes and goblet cells have been established, mimicking structurally and functionally an intestinal epithelial barrier. Moreover, Caco-2-derived M-like cells have been established, mimicking the bacterial capture property of M cells of Peyer's patches. This review intends to analyze the cellular and molecular mechanisms of pathogenesis of human enterovirulent bacteria observed in infected cultured human colon carcinoma enterocyte-like HT-29 subpopulations, enterocyte-like Caco-2 and clone cells, the colonic T84 cell line, HT-29 mucus-secreting cell subpopulations, and Caco-2-derived M-like cells, including cell association, cell entry, intracellular lifestyle, structural lesions at the brush border, functional lesions in enterocytes and goblet cells, functional and structural lesions at the junctional domain, and host cellular defense responses.

In vitro and in vivo model systems for studying enteropathogenic Escherichia coli infections

Enteropathogenic Escherichia coli (EPEC) and enterohemorrhagic E. coli (EHEC) belong to a group of bacteria known as attaching and effacing (A/E) pathogens that cause disease by adhering to the lumenal surfaces of their host's intestinal epithelium. EPEC and EHEC are major causes of infectious diarrhea that result in significant childhood morbidity and mortality worldwide. Recent advances in in vitro and in vivo modeling of these pathogens have contributed to our knowledge of how EPEC and EHEC attach to host cells and subvert host-cell signaling pathways to promote infection and cause disease. A more detailed understanding of how these pathogenic microbes infect their hosts and how the host responds to infection could ultimately lead to new therapeutic strategies to help control these significant enteric pathogens.

Actin cytoskeleton manipulation by effector proteins secreted by diarrheagenic Escherichia coli pathotypes

The actin cytoskeleton is a dynamic structure necessary for cell and tissue organization, including the maintenance of epithelial barriers. Disruption of the epithelial barrier coincides with alterations of the actin cytoskeleton in several disease states. These disruptions primarily affect the paracellular space, which is normally regulated by tight junctions. Thereby, the actin cytoskeleton is a common and recurring target of bacterial virulence factors. In order to manipulate the actin cytoskeleton, bacteria secrete and inject toxins and effectors to hijack the host cell machinery, which interferes with host-cell pathways and with a number of actin binding proteins. An interesting model to study actin manipulation by bacterial effectors is Escherichia coli since due to its genome plasticity it has acquired diverse genetic mobile elements, which allow having different E. coli varieties in one bacterial species. These E. coli pathotypes, including intracellular and extracellular bacteria, interact with epithelial cells, and their interactions depend on a specific combination of virulence factors. In this paper we focus on E. coli effectors that mimic host cell proteins to manipulate the actin cytoskeleton. The study of bacterial effector-cytoskeleton interaction will contribute not only to the comprehension of the molecular causes of infectious diseases but also to increase our knowledge of cell biology.

Mutations that impact the enteropathogenic Escherichia coli Cpx envelope stress response attenuate virulence in Galleria mellonella

In this paper, we show that the larvae of the greater wax moth, Galleria mellonella, can be used as a model to study enteropathogenic Escherichia coli (EPEC) virulence. G. mellonella larvae are killed after infection with EPEC type strain E2348/69 but not by an attenuated derivative that expresses diminished levels of the major virulence determinants or by a mutant specifically defective in type III secretion (T3S). Infecting EPEC inhabit the larval hemocoel only briefly and then become localized to melanized capsules, where they remain extracellular. Previously, it was shown that mutations affecting the Cpx envelope stress response lead to diminished expression of the bundle-forming pilus (BFP) and the type III secretion system (T3SS). We demonstrate that mutations that activate the Cpx pathway have a dramatic effect on the ability of the bacterium to establish a lethal infection, and this is correlated with an inability to grow in vivo. Infection with all E. coli strains led to increased expression of the antimicrobial peptides (AMPs) gloverin and cecropin, although strain- and AMP-specific differences were observed, suggesting that the G. mellonella host perceives attenuated strains and Cpx mutants in unique manners. Overall, this study shows that G. mellonella is an economical, alternative infection model for the preliminary study of EPEC host-pathogen interactions, and that induction of the Cpx envelope stress response leads to defects in virulence.

Simultaneous inactivation of espB and tir abrogates the strong, but non-protective, inflammatory response induced by EPEC

Enteropathogenic Escherichia coli (EPEC) belong to the attaching and effacing (A/E) family of bacterial pathogens that represent a worldwide health concern. These non-invasive bacteria attach to intestinal enterocytes through a type III secretion system (T3SS), leading to intestinal inflammation and severe diarrhea. To dissect the signals leading to the induction of the inflammatory response and to understand its role in the pathogenesis of infection, we used the rabbit model, which represents a close model of human infections. Rabbits were orally inoculated with either the wild type O103:K-:H2 E22 EPEC strain or with the E22Δtir/espB strain, which bears mutations in two genes involved in the injectisome structure and function. To monitor the development of the inflammatory response, we developed a quantitative real-time RT-PCR (qPCR) assay specific for a panel of rabbit genes. Using combined immunohistochemistry and qPCR, we show here that the inflammatory response triggered by wild type EPEC occurs very early, preceding the bacterial colonization of the epithelium. However, this early response is unable to prevent bacterial attachment on enterocytes. Moreover, our results show that expression of a complete bacterial injectisome is required for the development of inflammation. Finally, infection by the virulent strain, but not by the doubly mutated strain, rapidly induces the development of a specific immune response in the mesenteric lymph nodes, which is not associated with protection. Our findings suggest that the induction of a strong inflammatory response by T3SS dependent components represents a selective advantage for T3SS+ bacteria, thereby facilitating their colonization.

Characterisation of atypical enteropathogenic E. coli strains of clinical origin

BACKGROUND

Enteropathogenic E. coli (EPEC) is a prominent cause of diarrhoea, and is characterised in part by its carriage of a pathogenicity island: the locus for enterocyte effacement (LEE). EPEC is divided into two subtypes according to the presence of bundle-forming pili (BFP), a fimbrial adhesin that is a virulence determinant of typical EPEC (tEPEC), but is absent from atypical EPEC (aEPEC). Because aEPEC lack BFP, their virulence has been questioned, as they may represent LEE-positive Shiga toxin-producing E. coli (STEC) that have lost the toxin-encoding prophage, or tEPEC that have lost the genes for BFP. To determine if aEPEC isolated from humans in Australia or New Zealand fall into either of these categories, we undertook phylogenetic analysis of 75 aEPEC strains, and compared them with reference strains of EPEC and STEC. We also used PCR and DNA hybridisation to determine if aEPEC carry virulence determinants that could compensate for their lack of BFP.

RESULTS

The results showed that aEPEC are highly heterogeneous. Multilocus sequence typing revealed that 61 of 75 aEPEC strains did not belong to known tEPEC or STEC clades, and of those that did, none expressed an O:H serotype that is frequent in tEPEC or STEC strains associated with disease. PCR for each of 18 known virulence-associated determinants of E. coli was positive in less than 15% of strains, apart from NleB which was detected in 30%. Type I fimbriae were expressed by all aEPEC strains, and 12 strains hybridised with DNA probes prepared from either bfpA or bfpB despite being negative in the PCR for bfpA.

CONCLUSION

Our findings indicate that clinical isolates of aEPEC obtained from patients in Australia or New Zealand are not derived from tEPEC or STEC, and suggest that functional equivalents of BFP and possibly type I fimbriae may contribute to the virulence of some aEPEC strains.

Rabbit-specific fimbriae, Ral, alter the patterns of in vitro adherence and intestinal colonisation of rabbits by human-specific enteropathogenic E. coli

Enteropathogenic Escherichia coli (EPEC) poses a significant threat to human health, causing diarrhoea in children worldwide, and is a leading cause of infant mortality in developing countries. The pathogenic effects of EPEC and other attaching-effacing (A/E) bacteria result from adhesion to the intestinal mucosa by a variety of mechanisms, including fimbrial adhesins, which are believed to contribute to the host and tissue specificity of EPEC by their interaction with specific receptors on cell surfaces. In this study we investigated the contribution of a fimbrial adhesin, Ral, of rabbit-specific EPEC (REPEC) to host specificity by introducing Ral into derivatives of human-specific EPEC (hEPEC) strain, E2348/69, in which expression of the fimbrial adhesin, Bfp, had been interrupted. Although unable to cause diarrhoeal disease in rabbits, Ral-bearing hEPEC strains colonised rabbit intestine more efficiently and showed altered intestinal localisation when compared to an isogenic Ral-negative strain. These findings suggest that Ral enhances the initial interaction between a DeltabfpA mutant of hEPEC and rabbit intestine and may influence tissue specificity, but is not sufficient on its own to transform hEPEC into a rabbit pathogen. This study affords new insights into the complex mechanisms which determine the host range of bacterial pathogens.

Infection of rabbit kidney cells (RK13) by enteropathogenicEscherichia colias a model to study the dynamics of actin cytoskeleton

Enteropathogenic Escherichia coli (EPEC) colonizes the intestinal mucosa and causes a cell lesion known as attachment and effacement (A/E) lesion. The molecular mechanisms for A/E lesions include injection of Tir, which is a receptor for an adhesin named intimin. The Tir–intimin interaction causes rearrangement of the cytoskeleton forming actin-rich structures called pedestals. Unfortunately, the formation of the A/E lesions and the dynamics of the actin cytoskeleton during this rearrangement induced by EPEC cannot be studied in the natural host. However, there are EPEC strains that infect rabbit (REPEC) that are genetically and pathologically similar to EPEC. Here, we used REPEC for the infection of rabbit kidney epithelial cells, line RK13, as a model to understand the actin cytoskeleton dynamics during pedestal formation. Actin-rich pedestal formation during the infection of RK13 cells by REPEC was analyzed by electron and confocal microscopy. The kinetics of infection along with the use of antibiotics for eliminating the bacteria, as well as reinfection, evidenced the plasticity of the actin cytoskeleton during pedestal formation. Thus, this model is a helpful tool for studying the dynamics of actin cytoskeleton and for correlating the data with those observed in in vivo models in rabbits experimentally infected with REPEC.

Contribution of a novel gene, rpeA, encoding a putative autotransporter adhesin to intestinal colonization by rabbit-specific enteropathogenic Escherichia coli

Rabbit-specific enteropathogenic Escherichia coli (REPEC) is an attaching and effacing pathogen of young rabbits. Using signature-tagged mutagenesis, we identified several known colonization factors of REPEC as well as a gene predicted to encode a novel autotransporter protein. This novel gene was termed rpeA for REPEC plasmid-encoded autotransporter.

Maternal milk contains antimicrobial factors that protect young rabbits from enteropathogenic Escherichia coli infection

Enteropathogenic Escherichia coli (EPEC) colibacillosis represents a major cause of lethal diarrhea in young children in developing countries. EPEC strains also infect numerous mammal species and represent a major economical problem in rabbit industry. Protection against this pathogen is a challenging goal both in humans and in other mammal species. Despite a good knowledge of the pathogenicity mechanisms of EPEC, the intrinsic and environmental factors that control the expression of EPEC virulence in mammals remain unknown. For instance, the exacerbated sensitivity of young mammals to EPEC infection is still unexplained. Our goal was to investigate if age or other factors, like milk consumption, could be determinants that trigger the disease. We used rabbits as an animal model to study the role of milk in the sensitivity to an EPEC infection. Weaned and suckling rabbits were orally inoculated with EPEC strain E22 (O103:H2:K-) at 28 days of age, and the evolution of the disease was investigated in the two groups. In addition, in order to better characterize the interactions between milk and EPEC, we determined in vitro bacterial growth and the abilities of EPEC cells to adhere to epithelial cells in the presence of milk. Our results demonstrate a protective role of milk in vivo in association with in vitro antibacterial activity. These effects are independent of the presence of specific anti-EPEC antibodies.

Attaching and effacing Escherichia coli isolated from dogs in Brazil: characteristics and serotypic relationship to human enteropathogenic E. coli (EPEC)

Escherichia coli isolates recovered from 182 fecal specimens from dogs up to five months old from the cities of São Paulo and Campinas, SP, Brazil, were examined by polymerase chain reaction (PCR) for several virulence factors and properties. The eae gene was found in 23 isolates of E. coli from 22 dogs, 19 of 146 (13%) from dogs with diarrhea and 3 of 36 (8.3%) from dogs with no diarrhea. Two different eae+ isolates were recovered from one dog with diarrhea. Isolates from two dogs with diarrhea harbored the bfpA gene, and none of the isolates possessed genes for enterotoxins, the EAF plasmid or Shiga toxins. PCR showed that, among the 23 isolates, eight were positive for beta intimin, six for gamma, two for, one for alpha, one for kappa, and five showed no amplification with any of the nine pairs of specific intimin primers used. PCR also showed that the LEE (locus of enterocyte effacement) was inserted in selC in four isolates, likely in pheU in seven isolates, and in undetermined sites in twelve isolates. Fifteen isolates adhered to HEp-2 cells and were fluorescence actin staining (FAS) positive. The predominant adherence pattern was the localized adherence-like (LAL) pattern. The eae-positive isolates belonged to a wide diversity of serotypes, including O111:H25, O119:H2 and O142:H6, which are serotypes that are common among human EPEC. These results confirmed the presence of EPEC in dogs (DEPEC) with and without diarrhea. The virulence factors found in these strains were similar to those in human EPEC, leading to the possibility that EPEC may move back and forth among human and canine populations.

Contribution of long polar fimbriae to the virulence of rabbit-specific enteropathogenic Escherichia coli

Enteropathogenic Escherichia coli (EPEC) is a major of cause of diarrhea among children in developing countries. Although EPEC is a human specific pathogen, some related strains are natural pathogens of animals, including laboratory-bred rabbits. We have identified two chromosomal loci in rabbit-specific EPEC (REPEC) O15:H- strain 83/39, which are predicted to encode long polar fimbriae (LPF). lpf(R154) was identical to a fimbrial gene cluster, lpf(O113), identified previously in enterohemorrhagic E. coli (EHEC) O113:H21. The second locus, lpf(R141), comprised a novel sequence with five predicted open reading frames, lpfA to lpfE, that encoded long fine fimbriae in nonfimbriated E. coli ORN103. The predicted products of lpf(R141) shared identity with components of the lpfABCC'DE gene cluster from EHEC O157:H7, and the fimbriae were similar in morphology and length to LPF from EHEC O157:H7. Interruption of lpf(R141) resulted in significant attenuation of REPEC 83/39 for rabbits with respect to the early stages of colonization and severity of diarrhea. However, there was no significant difference in the number of bacteria shed at later time points or in overall body weight and mortality rate of rabbits infected with lpf(R141) mutant strains or wild-type REPEC 83/39. Although rabbits infected with the lpf(R141) mutants did not develop severe diarrhea, there was evidence of attaching and effacing histopathology, which was indistinguishable in morphology, location, and extent compared to rabbits infected with wild-type REPEC 83/39. The results suggested that lpf(R141) contributes to the early stages of REPEC-mediated disease and that this is important for the development of severe diarrhea in susceptible animals.

Genetically engineered enteropathogenic Escherichia coli strain elicits a specific immune response and protects against a virulent challenge

Enteropathogenic Escherichia coli (EPEC), a major cause of severe disease with diarrhea in infants, is also involved in weaned rabbit colibacillosis. EPEC O103 is frequent in rabbit-fattening units of Western Europe. It causes high mortality and growth retardation, leading to substantial economic losses. We report here the construction by allelic exchange of an EPEC O103 strain mutated in espB and tir, two essential virulence genes. Upon live oral administration to weaned rabbits, the E22DeltaTir/EspB mutant strain efficiently colonized the intestinal tract without any adverse consequences. The rabbits were challenged with the highly pathogenic parental strain E22. The mutant provided complete protection to rabbits and total resistance to intestinal colonization by E22. In addition, E22DeltaTir/EspB strain induced a specific humoral response against the bacterial adhesin AF/R2. These Abs prevent bacterial attachment to epithelial cells in vitro. These results open the way for the development of an efficient vaccine strategy against rabbit EPEC infections.

New flow cytometric method to quantify the inhibition of enteropathogenic Escherichia coli adhesion by anti-adhesin antibodies

BACKGROUND

Pathogenesis of enteropathogenic Escherichia coli (EPEC) infections can be divided in three stages. The first one is the intestinal colonization mediated by bacterial adhesins. The second and third stages are characterized by an intimate attachment of bacteria to the enterocytes. Little information is available on the specific immune response against EPEC. Here, we describe and validate a new approach to quantify the function of anti-EPEC adhesin antibodies (Abs).

METHODS

We developed a new method to quantify the function of anti-adhesin Abs by flow cytometry. We used pEGFP-E22 (a rabbit EPEC E22 strain expressing the GFP protein) and HeLa cells. The adhesion of E22 bacteria to HeLa cells is mediated by AF/R2, the specific E22 adhesin. We performed short-time interaction (30 min) between pEGFP-E22 and HeLa cells. After extensive washes, 10,000 HeLa cells were acquired by flow cytometry and bacterial adhesion was quantified. Different sera were used to inhibit bacterial adhesion and recombinant MPB-Afr2G (Afr2G is the main AF/R2 subunit) was also tested in this system.

RESULTS

We first verified that GFP expression by E22 did not modify bacterial adhesion. We then showed that this flow cytometry approach allowed easy quantification of bacterial adhesion and inhibition mediated by a specific anti-AF/R2 serum. Moreover, recombinant AF/R2 protein reversed the effect of the anti-AF/R2 serum. Finally, we validated our method using sera from E22 orally infected rabbits. We detected and quantified with this method functional specific anti-AF/R2 Abs in their sera. In addition, we correlated our results with an anti-AF/R2 enzyme-linked immunosorbent assay.

CONCLUSIONS

We have developed a new method to detect and quantify specific anti-EPEC adhesin Abs by flow cytometry. This method is easy to use and highly reproducible. Its development could be extended to the search of specific anti-adhesin Abs in human EPEC infections.

Contribution of Efa1/LifA to the adherence of enteropathogenic Escherichia coli to epithelial cells

Enteropathogenic E. coli(EPEC) is an important diarrhoeal pathogen that induces characteristic lesions on the host intestine termed attaching and effacing (A/E) lesions. In this study we have examined the contribution of a large gene, efa1, which is present in all A/E pathogens, to the adherence phenotype of EPEC. An efa- derivative of EPEC JPN15 was constructed and this mutant was significantly less adherent to epithelial cells than the parent strain. The JPN15 efa- derivative was FAS-positive, produced EspA filaments and showed comparable levels of EspA secretion to JPN15. In addition, polyclonal antibodies raised to Efa1 partially inhibited the adherence of JPN15 to cultured epithelial cells. In further work, we showed that human and rabbit hosts infected with an A/E pathogen produced antibodies to Efa1 and we observed that the truncated form of efa1 present in EHEC O157:H7 was specific to that serotype. Generally efa1 was present in its entirety in the genomes of other A/E pathogens. Overall our data suggest that Efa1 has host cell binding activity, at least in tissue culture, and that it is produced during infection. These findings suggest that Efa1 may play a direct role in the pathogenesis of infections caused by A/E pathogens.

CesD2 of enteropathogenic Escherichia coli is a second chaperone for the type III secretion translocator protein EspD

Enteropathogenic Escherichia coli (EPEC) and enterohemorrhagic E. coli are extracellular pathogens that employ a type III secretion system to export translocator and effector proteins, proteins which facilitates colonization of the mucosal surface of the intestine via formation of attaching and effacing (A/E) lesions. The genes encoding the proteins for A/E lesion formation are located on a pathogenicity island, termed the locus of enterocyte effacement (LEE), which contains eae encoding intimin as well as the type III secretion system and effector genes. Many type III secreted proteins are stabilized and maintained in a secretion-competent conformation in the bacterial cytosol by specific chaperone proteins. Three type III chaperones have been described thus far within the EPEC LEE region: CesD, for the translocator proteins EspB and EspD; CesT, for the effector proteins Tir and Map; and CesF, for EspF. In this study we report the characterization of CesD2 (previously Orf27), a second LEE-encoded chaperone for EspD. We show specific CesD2-EspD protein interaction which appears to be necessary for proper EspD secretion in vitro and pathogenesis in vivo as demonstrated in the A/E-lesion-forming mouse pathogen Citrobacter rodentium.

Interaction of Ler at the LEE5 (tir) operon of enteropathogenic Escherichia coli

The genome of enteropathogenic Escherichia coli (EPEC) encodes a global regulator, Ler (locus of enterocyte effacement [LEE]-encoded regulator), which activates expression of several polycistronic operons within the 35.6-kb LEE pathogenicity island, including the LEE2-LEE3 divergent operon pair containing overlapping -10 regions and the LEE5 (tir) operon. Ler is a predicted 15-kDa protein that exhibits amino acid similarity with the nucleoid protein H-NS. In order to study Ler-mediated activation of virulence operons in EPEC, we used a molecular approach to characterize the interactions of purified Ler protein with the upstream regulatory sequences of the LEE5 operon. We determined the cis-acting DNA sequences necessary for Ler binding at LEE5 by mobility shift and DNase I protection assays, demonstrating that Ler acts directly at LEE5 by binding sequences between positions -190 and -73 in relation to the transcriptional start site. Based on the molecular weight of Ler, the similarity to H-NS, and the extended region of protection observed in a DNase I footprint at LEE5, we hypothesized that multiple Ler proteins bind upstream of the LEE5 promoter to increase transcriptional activity from a distance. Using an hns deletion strain, we demonstrated that like the LEE2-LEE3 operon pair, H-NS represses LEE5 transcription. We describe a model in which Ler activates transcription at both divergent overlapping paired and single promoters by displacing H-NS, which results in the disruption of a repressing nucleoprotein complex.

Modulation of inducible nitric oxide synthase expression by the attaching and effacing bacterial pathogen citrobacter rodentium in infected mice

Citrobacter rodentium belongs to the attaching and effacing family of enteric bacterial pathogens that includes both enteropathogenic and enterohemorrhagic Escherichia coli. These bacteria infect their hosts by colonizing the intestinal mucosal surface and intimately attaching to underlying epithelial cells. The abilities of these pathogens to exploit the cytoskeleton and signaling pathways of host cells are well documented, but their interactions with the host's antimicrobial defenses, such as inducible nitric oxide synthase (iNOS), are poorly understood. To address this issue, we infected mice with C. rodentium and found that iNOS mRNA expression in the colon significantly increased during infection. Immunostaining identified epithelial cells as the major source for immunoreactive iNOS. Finding that nitric oxide (NO) donors were bacteriostatic for C. rodentium in vitro, we examined whether iNOS expression contributed to host defense by infecting iNOS-deficient mice. Loss of iNOS expression caused a small but significant delay in bacterial clearance without affecting tissue pathology. Finally, immunofluorescence staining was used to determine if iNOS expression was localized to infected cells by staining for the C. rodentium virulence factor, translocated intimin receptor (Tir), as well as iNOS. Interestingly, while more than 85% of uninfected epithelial cells expressed iNOS, fewer than 15% of infected (Tir-positive) cells expressed detectable iNOS. These results demonstrate that both iNOS and intestinal epithelial cells play an active role in host defense during C. rodentium infection. However, the selective expression of iNOS by uninfected but not infected cells suggests that this pathogen has developed mechanisms to locally limit its exposure to host-derived NO.

Characterization and evidence of mobilization of the LEE pathogenicity island of rabbit-specific strains of enteropathogenic Escherichia coli

We have characterized the LEE pathogenicity islands (PAIs) of two rabbit-specific strains of enteropathogenic E. coli (REPEC), 83/39 (serotype O15:H-) and 84/110-1 (O103:H2), and have compared them to homologous loci from the human enteropathogenic and enterohaemorrhagic E. coli strains, E2348/69 and EDL933, and another REPEC strain, RDEC-1. All five PAIs contain a 34 kb core region that is highly conserved in gene order and nucleotide sequence. However, the LEE of 83/39 is significantly larger (59 540 basepairs) than those of the human strains, which are less than 44 kb, and has inserted into pheU tRNA. The regions flanking the 34 kb core of 83/39 contain homologues of two putative virulence determinants, efa1/lifA and senA. The LEE of 84/110-1 is approximately 85 kb and is located at pheV tRNA. Its core is almost identical to those of 83/39 and RDEC-1, apart from a larger espF gene, but its flanking regions contain trcA, a putative virulence determinant of EPEC. All three REPEC LEE PAIs contain a gene for an integrase, Int-phe. The LEE PAI of 84/110-1 is also flanked by short direct repeats (representing the 3'-end of pheV tRNA), suggesting that it may be unstable. To investigate this possibility, we constructed a LEE::sacB derivative of 84/110-1 and showed that the PAI was capable of spontaneous deletion. We also showed that Int-phe can mediate site-specific integration of foreign DNA at the pheU tRNA locus of E. coli DH1. Together these results indicate possible mechanisms of mobilization and integration of the LEE PAI.

Site-directed mutagenesis of intimin alpha modulates intimin-mediated tissue tropism and host specificity

The hallmark of enteropathogenic (EPEC) and enterohaemorrhagic (EHEC) Escherchia coli adhesion to host cells is intimate attachment leading to the formation of distinctive 'attaching and effacing' lesions. This event is mediated, in part, by binding of the bacterial adhesion molecule intimin to a second bacterial protein, Tir, delivered by a type III secretion system into the host cell plasma membrane. The receptor-binding activity of intimin is localized to the C-terminal 280 amino acids (Int280) and at least five distinct intimin types (alpha, beta, gamma, delta and epsilon) have been identified thus far. In addition to binding to Tir, intimin can also bind to a component encoded by the host. The consequence of latter intimin-binding activity may determine tissue tropism and host specificity. In this study we selected three amino acids in intimin, which are implicated in Tir binding, for site-directed mutagenesis. We used the yeast two-hybrid system and gel overlays to study intimin-Tir protein interaction. In addition, the biological consequences of the mutagenesis was tested using a number of infection models (cultured epithelial cells, human intestinal explants and a mouse model). We report that while an I237/897A substitution (positions numbered according to Int280alpha/whole intimin alpha) in intimin alpha did not have any affect on its biological activity, a T255/914A substitution attenuated intimin activity in vivo. In contrast, the mutation V252/911A affected tissue targeting in the human intestinal explant model and attenuated the biological activity of intimin in the mouse model. This study provides the first clues of the molecular basis of how intimin mediates tissue tropism and host specificity.

Enteropathogenic Escherichia coli strain RDEC-1 produces a novel electrogenic factor active on rabbit ileum in vitro

BACKGROUND

Attaching and effacing Escherichia coli demonstrate marked species specificity in inducing diarrhea, although its mechanism remains largely unclear. The purpose of this study was to investigate the existence of a soluble, species-specific factor that induces diarrhea in an in vitro model.

METHODS

Stripped rabbit ileum was mounted in Ussing chambers, and changes in potential difference and short-circuit current were monitored after the addition of bacterial culture supernatant.

RESULTS

The culture supernatant from rabbit-specific strain RDEC-1, but not from human-specific enteropathogenic Escherichia coli strain E2348/69, induced an increase in potential difference and short-circuit current in rabbit ileum mounted in Ussing chambers. This electrical signal was related to chloride ion secretion, was absent in colonic tissue, and was retained in the 30 to 100-KDa fraction of the supernatant. Preliminary experiments failed to show an involvement of calcium or cyclic nucleotides as intracellular messengers. RDEC-1 cured of a 42-MDa plasmid lost the enterotoxicity whereas conjugation of the plasmid into the negative E. coli recipient HB101 resulted in the expression of toxicity.

CONCLUSIONS

The authors describe a novel, species-specific factor that helps to explain RDEC-1 diarrhea, which may be relevant to the pathogenesis of enteropathogenic Escherichia coli infection.

eae-negative attaching and effacing Escherichia coli from piglets with diarrhea

One hundred and ninety strains of Escherichia coli that were isolated from pigs with diarrhea in the state of São Paulo, Brazil, and that were negative for enterotoxins and cytotoxins were investigated. Strains which adhered to HeLa cells were examined for fluorescence actin staining (FAS), the ability to induce attaching and effacing (A/E) lesions on HEp-2 cells detectable by transmission electron microscopy and the presence of eae gene sequences detected by PCR. Intimin production was detected by western blot and serogrouping was performed. Forty-seven isolates adhered to HeLa cells in several patterns, but none adhered in a localized adherence pattern. However, seven of the 47 adherent strains were positive for the FAS reaction, although the reactions were usually weak or atypical. One FAS-negative and three FAS-positive strains, which were examined for their ability to induce A/E lesions, were all positive. Subsequently, testing of these strains for the eae gene showed that they all lacked this gene. These findings, along with earlier reports of eae-negative A/E E. coli, suggest that higher quantities of E. coli in this category might be detected if more reliance were placed on phenotypic tests rather than on gene detection tests alone.

Contribution of plasmid-encoded fimbriae and intimin to capacity of rabbit-specific enteropathogenic Escherichia coli to attach to and colonize rabbit intestine

Attachment to the intestinal mucosa is an essential step in the pathogenesis of diarrhea caused by enteropathogenic Escherichia coli (EPEC). Fimbriae and intimin, the outer membrane protein product of the chromosomal eae gene, contribute to this process, but their relative roles and the nature of their interaction are not known. The aim of this study was to determine the relative contribution of plasmid-encoded fimbriae, termed Ral, and intimin to the capacity of rabbit-specific EPEC (REPEC) to attach to the intestinal mucosa of rabbits. To achieve this, we constructed a series of mutants in REPEC strain 83/39 (O15:H-), in which the ralE and eae genes were insertionally inactivated. These strains were then inoculated into ligated loops of rabbit ileum, which were resected 18 h later and examined by light and electron microscopy. The results showed that intimin, but not Ral, is essential for the elicitation of attaching-effacing lesions by REPEC. Nevertheless, a delta eae Ral-bearing mutant adhered to the intestinal epithelium to the same extent as its eae-positive parent and far more extensively than an eae(+) delta ral strain. To examine the contribution of Ral and intimin to colonization of rabbit intestine, we fed these strains to weanling rabbits, which were killed 4 days later, so that the number of bacteria in various regions of the intestine could be determined. The results indicated that strain 83/39 requires both Ral and intimin to colonize the intestine successfully and that a delta eae delta ralE double mutant was incapable of colonizing the intestine. Taken together, these findings indicate that Ral and intimin act independently as adhesion factors of REPEC strain 83/39 and that this strain carries no other significant colonization factor. When both Ral and intimin are present, they appear to act cooperatively, with Ral-mediated adhesion preceding that mediated by intimin.

Role of tir and intimin in the virulence of rabbit enteropathogenic Escherichia coli serotype O103:H2

Attaching and effacing (A/E) rabbit enteropathogenic Escherichia coli (REPEC) strains belonging to serogroup O103 are an important cause of diarrhea in weaned rabbits. Like human EPEC strains, they possess the locus of enterocyte effacement clustering the genes involved in the formation of the A/E lesions. In addition, pathogenic REPEC O103 strains produce an Esp-dependent but Eae (intimin)-independent alteration of the host cell cytoskeleton characterized by the formation of focal adhesion complexes and the reorganization of the actin cytoskeleton into bundles of stress fibers. To investigate the role of intimin and its translocated coreceptor (Tir) in the pathogenicity of REPEC, we have used a newly constructed isogenic tir null mutant together with a previously described eae null mutant. When human HeLa epithelial cells were infected, the tir mutant was still able to induce the formation of stress fibers as previously reported for the eae null mutant. When the rabbit epithelial cell line RK13 was used, REPEC O103 produced a classical fluorescent actin staining (FAS) effect, whereas both the eae and tir mutants were FAS negative. In a rabbit ligated ileal loop model, neither mutant was able to induce A/E lesions. In contrast to the parental strain, which intimately adhered to the enterocytes and destroyed the brush border microvilli, bacteria of both mutants were clustered in the mucus without reaching and damaging the microvilli. The role of intimin and Tir was then analyzed in vivo by oral inoculation of weaned rabbits. Although both mutants were still present in the intestinal flora of the rabbits 3 weeks after oral inoculation, neither mutant strain induced any clinical signs or significant weight loss in the inoculated rabbits whereas the parental strain caused the death of 90% of the inoculated rabbits. Nevertheless, an inflammatory infiltrate was present in the lamina propria of the rabbits infected with both mutants, with an inflammatory response greater for the eae null mutant. In conclusion, we have confirmed the role of intimin in virulence, and we have shown, for the first time, that Tir is also a key factor in vivo for pathogenicity.

The Per regulon of enteropathogenic Escherichia coli : identification of a regulatory cascade and a novel transcriptional activator, the locus of enterocyte effacement (LEE)-encoded regulator (Ler)

Enteropathogenic Escherichia coli (EPEC) is the prototype organism of a group of pathogenic Gram-negative bacteria that cause attaching and effacing (AE) intestinal lesions. All EPEC genes necessary for the AE phenotype are encoded within a 35.6 kb pathogenicity island termed the locus of enterocyte effacement (LEE). The LEE encodes 41 predicted open reading frames (ORFs), including components of a type III secretion apparatus and secreted molecules involved in the disruption of the host cell cytoskeleton. To initiate our studies on regulation of genes within the LEE, we determined the genetic organization of the LEE, defining transcriptional units and mapping transcriptional start points. We found that components of the type III secretion system are transcribed from three polycistronic operons designated LEE1, LEE2 and LEE3. The secreted Esp molecules are part of a fourth polycistronic operon designated LEE4. Using reporter gene fusion assays, we found that the previously described plasmid-encoded regulator (Per) activated operons LEE1, LEE2 and LEE3, and modestly increased the expression of LEE4 in EPEC. Using single-copy lacZ fusions in K-12-derived strains, we determined that Per only directly activated the LEE1:lacZ fusion, and did not directly activate the other operons. Orf1 of the LEE1 operon activated the expression of single-copy LEE2:lacZ and LEE3:lacZ fusions in trans and modestly increased the expression of LEE4:lacZ in K-12 strains. Orf1 was therefore designated Ler, for LEE-encoded regulator. Thus, the four polycistronic operons of the LEE that encode type III secretion components and secreted molecules are now included in the Per regulon, where Ler participates in this novel regulatory cascade in EPEC.

Characterization of nonenterotoxigenic Escherichia coli strains producing F17 fimbriae isolated from diarrheic lambs and goat kids

Forty-five ovine and caprine nonenterotoxigenic Escherichia coli strains producing F17-related fimbriae were characterized with respect to the fimbrial structural subunit and adhesin subtypes produced. In addition, several characteristics related to the virulence of strains producing F17 fimbriae were studied. Most of the strains (73%) possessed the f17cA structural subunit gene, whereas the f17aA and f17dA genes were detected only on three (6%) and two (4%) strains, respectively. The f17bA gene was not detected. All but one of these strains possessed the f17G genes of the adhesin subfamily II. The only strain having the f17G gene of subfamily I possessed the structural subunit gene f17dA. Sequencing of the f17A and f17G genes of four selected strains confirmed the association of f17cA and f17dA structural subunit genes with the f17G genes of the adhesin subfamily II. These results indicated that adhesins of the subfamily II are prominent among ovine and caprine isolates and that they are indistinctly associated with the F17 structural subunit subtypes on these field strains. CS31A- and CNF2-related genes were not detected. Most of the strains adhered in vitro to ovine intestinal brush borders (36 of 45) and agglutinated the erythrocytes of different species in the presence of D-mannose (39 of 45). F17-positive strains produced colicin V (57%) and were resistant to the bactericidal effect of serum (91%) in significantly higher percentages than F17-negative strains (34% produced colicin V, and 66% were serum resistant). Thus, most of the studied ovine and caprine strains showed phenotypic characteristics of septicemic strains.

Fimbrial expression in enteric bacteria: a critical step in intestinal pathogenesis

The ability of species of enteric bacteria to recognize and colonize unique niches along the intestine is mainly based on receptor distribution and interpretation of a combination of environmental signals leading to the expression of specific adherence factors. Such elaborate orchestration of events is critical during the initial steps of pathogenesis.

Identification and characterization of a K88- and CS31A-like operon of a rabbit enteropathogenic Escherichia coli strain which encodes fimbriae involved in the colonization of rabbit intestine

Initiation of attaching-effacing lesions, which characterize infections with rabbit enteropathogenic Escherichia coli (REPEC), requires bacteria to adhere to the intestinal epithelium. This adherence is reflected in vitro by the affinity of these E. coli strains for various types of eukaryotic cells. TnphoA mutants of REPEC 83/39 (O15:H-) which had lost the ability to adhere to HEp-2 epithelial cells, guinea pig ileal brush borders, and mouse erythrocytes were generated. DNA sequencing of the region surrounding the inactivating transposon insertions within a 95-kb plasmid, designated pRAP for REPEC adherence plasmid, revealed extensive homology between that region and the structural genes of enterotoxigenic E. coli operons encoding the K88 and CS31A fimbrial adhesins and the genes for the afr2 adhesin from REPEC B10 (O103:H2). Seven genes of the ral operon (for REPEC adherence locus), including three putative minor fimbrial subunit genes (ralC, ralF, and ralH), a major fimbrial subunit gene (ralG), a gene of unknown function (ralI), and genes for two fimbrial subunit chaperones (ralD and ralE), were sequenced. When inoculated perorally into weanling rabbits, a mutant with a TnphoA insertion in the ralE gene showed a 10-fold reduction in colonizing ability, with only 1 of 10 rabbits excreting bacteria compared to all 5 of those infected with the wild-type parent strain (P = 0.002). The severity of the diarrheal illness caused by the mutant strain was also reduced. Western blotting of surface protein extracts of strain 83/39 with hyperimmune anti-83/39 antiserum, adsorbed with the ralE mutant, revealed a 32-kDa protein which was absent from protein extracts of two nonadherent mutants. The adsorbed antiserum also bound to the surface of strain 83/39 but not to nonadherent mutants, as detected by immunogold labeling. These results indicate that the ral operon of REPEC 83/39 contains genes necessary for the biosynthesis of fine fimbriae which are responsible for in vitro adherence of the bacteria and play a role in their colonization of, and hence virulence for, rabbits. The putative major fimbrial subunit is a protein with an observed molecular size of approximately 32 kDa which, when assembled, appears to form a capsule of fimbriae surrounding the bacterium similar to that described for CS31A.

Peyer's patch adherence of enteropathogenic Escherichia coli strains in rabbits

RDEC-1 (serotype O15) is an attaching and effacing strain of rabbit enteropathogenic Escherichia coli (REPEC) that causes diarrhea in postweanling rabbits. It expresses AF/R1 pili that mediate Peyer's patch M-cell adherence. We investigated Peyer's patch adherence, the presence of virulence genes, ileal brush border aggregation, and pilus expression in 9 strains representing several serotypes of REPEC as well as in two commensal strains. Postweanling rabbits were inoculated with 10(6) organisms and sacrificed at 24 h, and tissues were prepared for examination by light microscopy. Strains B10 and RDEC-1 were also studied at 12 and 72 h postinoculation. All REPEC strains were eaeA positive, expressed pili, and adhered to ileal brush borders. Both commensal strains expressed pili, and one strain adhered to brush borders. All REPEC strains demonstrated some degree of Peyer's patch lymphoid follicle adherence, ranging from diffuse coverage to small patches covering two to three dome epithelial cells. Strains C102 and C110 had genes homologous with the structural subunit gene of the AF/R1 pilus (afrA) of RDEC-1, which correlated with greater degrees of lymphoid follicle adherence and lesser degrees of ileal villus adherence. The observation that all REPEC strains adhere to Peyer's patch epithelium suggests the possibility that human strains of enteropathogenic E. coli (EPEC) might do likewise. EPEC strains might thus serve as mucosal vaccine vectors in humans. Better understanding of the molecular mechanism of REPEC adherence should provide a model for the targeting of the Peyer's patch in humans.

Enteropathogenic Escherichia coli O103 from rabbit elicits actin stress fibers and focal adhesions in HeLa epithelial cells, cytopathic effects that are linked to an analog of the locus of enterocyte effacement

Escherichia coli O103, a major agent of weaned-rabbit diarrhea in Western Europe, was previously shown to produce diarrhea and attaching-and-effacing intestinal lesions in experimentally infected rabbits and to possess a homolog of the eaeA gene of enteropathogenic E. coli (EPEC). In the present study, we have shown that although negative in the fluorescent-actin staining test on HeLa cells, prototype rabbit E. coli O103 strain B10 was able to induce an original cytopathic effect (CPE) in the same interaction model. This CPE was characterized by a generalized reorganization of the actin cytoskeleton and the formation of focal adhesions on the entire surface of the target cells. These effects amplified with time, leading to cell death about 5 days after the interaction. They were produced by all rabbit E. coli O103 strains tested, by rabbit-infecting E. coli RDEC-1, and also by two human EPEC isolates. We localized genes associated with CPE by using TnphoA insertion mutagenesis in strain B10. In all five independent CPE-negative mutants that we were able to generate, the insertion was located in a region of the genome homologous to the 35-kb locus of enterocyte effacement (LEE locus) of EPEC E2348/69. The mutants concurrently lost the ability to secrete four major supernatant proteins of 25, 37, 39, and 40 kDa, which were shown by immunoprecipitation to share antigenic determinants with secreted proteins of human EPEC E2348/69. The virulence of one of these mutants (strain B10/CA1) was compared with that of the parental strain in the weaned-rabbit diarrhea model. The mutant was totally deprived of virulence, although it colonized the intestine as efficiently as the parental strain did. This study points to a new pathogenic trait of EPEC strains, which is associated with the LEE locus and, possibly, with in vivo virulence.

Virulence properties of Escherichia coli strains belonging to enteropathogenic (EPEC) serogroups isolated from calves with diarrhea

Nineteen Escherichia coli strains belonging to enteropathogenic (EPEC) serogroups were isolated from calves with diarrhea in Paraná State. Brazil, and studied for virulence markers associated with EPEC or enterohemorrhagic E. coli (EHEC). The 19 isolates belonged to 12 serotypes with isolates of O26:H11, O119:H25 and O114:H- being the most prevalent Localized adherence (LA) was demonstrated for 37% of the isolates, consisting of all four O26:H11, both O114:H- and one O114:H40 isolates. All the LA strains were positive in the fluorescent-actin staining (FAS) test and possessed attaching-effacing E. coli (eae) sequences, but only O114 strains hybridized with the EPEC adherence factor (EAF) probe. None of the strains produced Shiga-like toxins (Verotoxin). Only the O26:H11 strains hybridized with the EHEC plasmid specific (CVD419) probe and were enterohemolytic, properties associated with EHEC strains. This investigation demonstrates that among the bovine strains isolated only those of serogroup O114 behaved as typical EPEC.

Adhesive factor/rabbit 2, a new fimbrial adhesin and a virulence factor from Escherichia coli O103, a serogroup enteropathogenic for rabbits

Enteropathogenic Escherichia coli-like E. coli strains belonging to serovar O103:K-:H2 and rhamnose-negative biotypes are highly pathogenic diarrhea-inducing strains for weaned European rabbits. We describe here the cloning and sequencing of the major subunit gene of a new fimbrial adhesin, adhesive factor/rabbit 2 (AF/R2), which confers on these strains the ability to attach to rabbit enterocytes and to HeLa cells in a diffuse manner and which is associated with in vivo virulence. The chromosomal operon that encodes functional AF/R2 has been cloned from strain B10. The major subunit gene afr2G, as well as an adjacent open reading frame, afr2H, has been sequenced. The Afr2G protein shows homologies with FaeG and ClpG, which are the respective major subunits of fimbrial adhesin K88 (F4) and afimbrial adhesin CS31A. Plasmid carrying the operon transcomplements an AF/R2-negative TnphoA mutant for its ability to express AF/R2. As a whole, AF/R2 is a new member of the E. coli K88 adhesin family which is associated with virulence and which may serve in the design of vaccines.

Transfer of attaching and effacing from a strain of enteropathogenic Escherichia coli to E. coli K-12

The ability to cause attaching and effacing (AE) lesions in intestinal epithelial cells is an essential virulence trait of enteropathogenic E. coli (EPEC) that requires several chromosomal genes acting in concert with one another. In this study, we show that the ability to cause AE lesions can be transferred by conjugal mating from a high frequency recombinant (Hfr) derivative of a rabbit EPEC strain, E. coli RDEC-1, to a strain of E. coli K-12. Although the recipient acquired a considerable amount of donor DNA during the transfer process, it expressed the AE phenotype phenotype only weakly. The findings suggest the AE is a multigene phenomenon, the genes for which may not reside on a single region of the bacterial chromosome.

O serogroups, biotypes, and eae genes in Escherichia coli strains isolated from diarrheic and healthy rabbits

A total of 305 Escherichia coli strains isolated from diarrheic and healthy rabbits in 10 industrial fattening farms from different areas of Spain were serotyped, biotyped, and tested for the presence of the eae gene and toxin production. The characteristics found in strains isolated from healthy rabbits were generally different from those observed in E. coli strains associated with disease. Thus, strains with the eae gene (74% versus 22%); strains belonging to serogroups O26, O49, O92, O103, and O128 (64% versus 12%); rhamnose-negative strains (51% versus 5%); and rhamnose-negative O103 strains with eae genes present (41% versus 1%) were significantly (P < 0.001 in all cases) more frequently detected in isolates from diarrheic animals than in those from healthy rabbits. Whereas a total of 35 serogroups and 17 biotypes were distinguished, the majority of the strains obtained from diarrheic rabbits belonged to only four serobiotypes, which in order of frequency were O103:B14 (72 strains), O103:B6 (16 strains), O26:B13 (12 strains), and O128:B30 (12 strains). These four serobiotypes accounted for 48% (112 of 231) and 5% (4 of 74) of the E. coli strains isolated from diarrheic and healthy rabbits, respectively. Only six strains were toxigenic (three CNF1+, two CNF2+, and one VT1+). We conclude that enteropathogenic E. coli strains that possess the eae gene are a common cause of diarrhea in Spanish rabbit farms and that the rhamnose-negative highly pathogenic strains of serotype O103:K-:H2 and biotype B14 are especially predominant. Detection of the eae gene is a useful method for the identification of enteropathogenic E. coli strains from rabbits. However, a combination of serogrouping and biotyping may be sufficient to accurately identify the highly pathogenic strains for rabbits.

Adherence and invasive capacities of the fish pathogen Pasteurella piscicida

Pasteurella piscicida strains were weakly or moderately adherent to cell lines, the levels of attachment being variable depending on the cells employed. All the isolates exhibited the highest binding capacity to CHSE-214 cells. Adhesive capacities were affected by heat and sugars but not by proteinase K or by treatment with antisera raised against the lipopolysaccharides of P. piscicida, implicating components of glycoprotein(s) as ligands in the adhesion process. The isolates showed a great binding capacity to intestines from the marine fish hosts gilthead sea bream, sea bass and turbot, with values ranging from 10(4) to 10(5) bacteria/g. Although the P. piscicida strains showed a weak invasiveness in the poikilothermic cell lines employed as in vitro model, the bacteria remained viable inside the infected cells at least for 2 days. The invasion process was inhibited by cytochalasin D indicating the active participation of the host cytoskeleton in the internalization of P. piscicida.

Genotypic and phenotypic characterization of Escherichia coli isolates from dogs manifesting attaching and effacing lesions

Thirteen Escherichia coli isolates from dogs manifesting attaching and effacing lesions were characterized genetically with respect to the presence of the following virulence determinants associated with human enteropathogenic E. coli (EPEC): eaeA, encoding the outer membrane protein intimin; eaeB, which is necessary for inducing signal transduction; bfpA, encoding the bundle-forming pilus; and the EAF (stands for EPEC adherence factor) plasmid. These isolates were also analyzed phenotypically with respect to adherence to mammalian cells in vivo and in vitro. Nine of these 13 isolates were found to be eaeA positive by PCR: four of these nine were eaeB positive. The 5' end, but not the 3' end, of the eaeA gene was amplified by PCR when primers derived from the eaeA gene of EPEC were used. Six and eight of these 13 isolates were found to be bfpA positive and EAF positive, respectively. The bfpA gene and EAF locus were found on high-molecular-weight plasmids, whereas the eaeA and eaeB genes were chromosomally located when present. Only one canine E. coli isolate, 4221, which was positive for eaeA, eaeB, bfpA, and EAF, adhered to HEp-2 cells in a localized manner and was positive in the fluorescence actin staining test. The nine eaeA-positive isolates adhered to the mucosal surface of piglet ileal explants and induced some microvillus effacement. However, when tested in experimentally inoculated gnotobiotic piglets, isolate 4221 did not induce attaching and effacing lesions at any level of the intestinal tract. Our results indicate that canine E. coli isolates associated with attaching and effacing lesions share some properties with human EPEC but form a heterogeneous group.

Interaction with pig ileal explants of Escherichia coli O45 isolates from swine with postweaning diarrhea

We have previously observed that Escherichia coli O45 isolates from swine postweaning diarrhea (PWD) induced attaching-effacing (A/E) lesions in experimentally inoculated gnotobiotic piglets. In the present work, ileal explant culture has been used as an in vitro model for the study of the development of A/E lesions due to these isolates. The characteristic intimate bacterial attachment and microvilli effacement with cupping and pedestal formation, identical to that observed in gnotobiotic piglets, was demonstrated in pig ileal explants inoculated with O45 E. coli isolates. The initial attachment of bacteria to the enterocytes was observed from 2 to 4 h postinoculation (PI) and full development of A/E lesions was observed within 8 h PI. In this model, we observed that 22 of 25 eaeA-positive O45 isolates induced A/E lesions. However, A/E lesions were not observed for any of 7 eaeA-negative O45 isolates. Thus, we describe a useful in vitro model for the study of A/E capacity of porcine E. coli. Use of this model has enabled us to demonstrate the relatedness of the eaeA gene to A/E capability among porcine O45 E. coli from PWD.

Host specificity of enteropathogenic Escherichia coli from rabbits: lack of correlation between adherence in vitro and pathogenicity for laboratory animals

The pathogenicity of four attaching and effacing strains of enteropathogenic Escherichia coli originally isolated from diarrheic rabbits was investigated by inoculating them perorally into rabbits, guinea pigs, and mice. The ability of the four strains to adhere to cultured epithelial cells, erythrocytes, and intestinal brush borders from various animal species, including rabbits, guinea pigs, and mice, varied considerably. Only one strain carried AF/R1 fimbriae, which are believed to determine the host specificity of these bacteria. Despite these differences, the pattern of behavior of the four strains in experimentally infected animals was similar. Each strain caused fatal diarrhea in rabbits (although the virulence of individual strains for rabbits differed significantly), and none was virulent for guinea pigs or mice. None of the strains colonized the intestinal tract of guinea pigs, but all were able to cause attaching-effacing lesions in ligated loops of guinea pig small intestine. By contrast, all four strains colonized mice, in particular the distal intestine, but none induced attaching-effacing lesions in mouse intestinal loops. These findings suggest that there may be previously unrecognized host-restricted adhesins in enteropathogenic E. coli and indicate that adherence to erythrocytes or intestinal brush borders in vitro does not necessarily reflect colonizing ability or pathogenicity in vivo.