Effect of Shiga toxin and Shiga-like toxins on eukaryotic cells

Shigella dysenteriae and Shiga-toxin-producing Escherichia coli (STEC) elaborate the AB holotoxins, Shiga or Shiga-like toxins (Stx). Stx play a major role in the pathogenesis of haemorrhagic colitis and haemolytic uremic syndrome. This review provides an overview of the mechanisms of action of Stx and a model of the pathogenesis of Stx-induced disease.

EspG, a novel type III system-secreted protein from enteropathogenic Escherichia coli with similarities to VirA of Shigella flexneri

The function of the rorf2 gene located on the locus of enterocyte effacement (LEE) pathogenicity island of enteropathogenic Escherichia coli (EPEC) has not been described. We report that rorf2 encodes a novel protein, named EspG, which is secreted by the type III secretory system and which is translocated into host epithelial cells. EspG is homologous with Shigella flexneri protein VirA, and the cloned espG (rorf2) gene can rescue invasion in a Shigella virA mutant, indicating that these proteins are functionally equivalent in Shigella. An EPEC espG mutant had no apparent defects in in vitro assays of virulence phenotypes, but a rabbit diarrheagenic E. coli strain carrying a mutant espG showed diminished intestinal colonization and yet diarrheal attack rates similar to those of the wild type. A second EspG homolog, Orf3, is encoded on the EspC pathogenicity islet. The cloned orf3 gene could also rescue invasion in a Shigella virA mutant, but an EPEC espG orf3 double mutant was not diminished in any tested in vitro assays for EPEC virulence factors. Our results indicate that EspG plays an accessory but as yet undefined role in EPEC virulence that may involve intestinal colonization.

Immune response to infection with Mycobacterium ulcerans

Mycobacterium ulcerans is a slow-growing, acid-fast bacillus that causes chronic necrotizing skin ulcers known as Buruli ulcers. Previously reported information on immunity to this mycobacterium is limited. We examined immune responses to M. ulcerans and M. bovis BCG in patients with M. ulcerans disease and in 20 healthy control subjects (10 tuberculin test positive and 10 tuberculin test negative). Cell-mediated immunity was assessed by stimulating peripheral blood mononuclear cells (PBMC) with whole mycobacteria and then measuring PBMC proliferation and the production of gamma interferon (IFN-gamma). Humoral immunity was assessed by immunoblotting. PBMC from all subjects showed significantly greater proliferation and IFN-gamma production in response to stimulation with living mycobacteria compared with killed cells. However, PBMC from subjects with past or current M. ulcerans disease showed significantly reduced proliferation and production of IFN-gamma in response to stimulation with live M. ulcerans or M. bovis than PBMC from healthy, tuberculin test-positive subjects (P < 0.001) and showed results in these assays comparable to those of tuberculin test-negative subjects (P > 0.2). Serum from 9 of 11 patients with M. ulcerans disease, but no control subject, contained antibodies to M. ulcerans. The results indicate that patients with M. ulcerans infection mount an immune response to M. ulcerans as evidenced by antibody production, but they demonstrate profound systemic T-cell anergy to mycobacterial antigens. These findings may explain some of the distinct clinical and pathological features of M. ulcerans-induced disease.

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.

Identification of a novel genetic locus that is required for in vitro adhesion of a clinical isolate of enterohaemorrhagic Escherichia coli to epithelial cells

Enterohaemorrhagic Escherichia coli (EHEC) are food-borne intestinal pathogens with a low infectious dose. Adhesion of some EHEC strains to epithelial cells is attributed, in part, to intimin, but other factors may be required for the intestinal colonizing ability of these bacteria. In order to identify additional adherence factors of EHEC, we generated transposon mutants of a clinical EHEC isolate of serotype O111:H-, which displayed high levels of adherence to cultured Chinese hamster ovary (CHO) cells. One mutant was markedly deficient in CHO cell adherence, human red blood cell agglutination and autoaggregation. Sequence analysis of the gene disrupted in this mutant revealed a 9669 bp novel chromosomal open reading frame (ORF), which was designated efa1, for EHEC factor for adherence. efa1 displayed 28% amino acid identity with the predicted product of a recently described ORF from the haemolysin-encoding plasmid of EHEC O157:H7. The amino termini of the putative products of these two genes exhibit up to 38% amino acid similarity to Clostridium difficile toxins A and B. efa1 occurred within a novel genetic locus, at least 15 kb in length, which featured a low G+C content, several insertion sequence homologues and a homologue of the Shigella flexneri enterotoxin ShET2. DNA probes prepared from different regions of efa1 hybridized with all of 116 strains of attaching-effacing E. coli (AEEC) of a variety of serotypes, including enteropathogenic E. coli (EPEC) and EHEC, but with none of 91 non-AEEC strains. Nevertheless, efa1 was not required for the attachment-effacement phenotype, and the efa1 locus was not physically linked to the locus for enterocyte effacement (LEE) pathogenicity island, which is responsible for this phenotype in EPEC. These findings suggest that efa1 encodes a novel virulence-associated determinant of AEEC, which contributes to the adhesive capacity of these bacteria.

Campylobacter upsaliensis gastroenteritis in childhood

BACKGROUND

Campylobacter upsaliensis can cause gastroenteritis and bacteremia. Data on its epidemiology and role in pediatric gastroenteritis are limited.

OBJECTIVE

To describe the incidence and clinical features of enteric C. upsaliensis infection in children and to compare these with similar data for Campylobacter jejuni.

DESIGN AND METHODS

Medical records of all patients with enteric C. upsaliensis infection between 1992 and 1999 at the Royal Children's Hospital, Melbourne, were reviewed. A case-control study (age-matched 1:2) was performed to compare the severity of clinical disease and associated risk factors for infection with C. upsaliensis and C. jejuni.

RESULTS

Of 18,516 specimens 666 (3.6%) were positive for C. jejuni and 19 (0.1%) were positive for C. upsaliensis. Records were available for 18 patients with C. upsaliensis gastroenteritis (mean age, 1.6 years; median age, 1.3 years; range, 3 months to 7 years; 14 male). Eleven patients (61%) presented with acute and 7 (39%) with chronic or intermittent diarrhea. The case-control study showed that fever (P = 0.03), acute diarrhea (P = 0.05) and rectal bleeding (P = 0.0006) were significantly less common in C. upsaliensis than in C. jejuni infection.

CONCLUSION

C. upsaliensis is a rare cause of gastroenteritis in young children and, compared with C. jejuni infection, is associated with significantly lower rates of fever, acute diarrhea and rectal bleeding.

Characterization of the interaction between Yersinia enterocolitica biotype 1A and phagocytes and epithelial cells in vitro

Yersinia enterocolitica strains of biotype 1A are increasingly being recognized as etiological agents of gastroenteritis. However, the mechanisms by which these bacteria cause disease differ from those of highly invasive, virulence plasmid-bearing Y. enterocolitica strains and are poorly understood. We have investigated several biotype 1A strains of diverse origin for their ability to resist killing by professional phagocytes. All strains were rapidly killed by polymorphonuclear leukocytes but persisted within macrophages (activated with gamma interferon) to a significantly greater extent (survival = 40.5% +/- 17.4%) than did Escherichia coli HB101 (9.3% +/- 0.7%; P = 0.0001). Strains isolated from symptomatic patients were significantly more resistant to killing by macrophages (survival = 48.9% +/- 19.5%) than were strains obtained from food or the environment (survival = 32.1% +/- 10.3%; P = 0.04). Some strains which had been ingested by macrophages or HEp-2 epithelial cells showed a tendency to reemerge into the tissue culture medium over a period lasting several hours. This phenomenon, which we termed "escape," was observed in 14 of 15 strains of clinical origin but in only 3 of 12 nonclinical isolates (P = 0.001). The capacity of bacteria to escape from cells was not directly related to their invasive ability. To determine if escape was due to host cell lysis, we used a variety of techniques, including lactate dehydrogenase release, trypan blue exclusion, and examination of infected cells by light and electron microscopy, to measure cell viability and lysis. These studies established that biotype 1A Y. enterocolitica strains were able to escape from macrophages or epithelial cells without causing detectable cytolysis, suggesting that escape was achieved by a process resembling exocytosis. The observations that biotype 1A Y. enterocolitica strains of clinical origin are significantly more resistant to killing by macrophages and significantly more likely to escape from host cells than are strains of nonclinical origin suggest that these properties may account for the virulence of these bacteria.

Identification and characterization of IS2404 and IS2606: two distinct repeated sequences for detection of Mycobacterium ulcerans by PCR

Molecular analysis of Mycobacterium ulcerans has revealed two new insertion sequences (ISs), IS2404 and IS2606. IS2404 was identified by complete sequencing of a previously described repetitive DNA segment from M. ulcerans. This element is 1,274 bp long, contains 12-bp inverted repeats and a single open reading frame (ORF) potentially encoding a protein of 327 amino acids (aa), and apparently generates 7-bp direct repeats upon transposition. Amino acid similarity was found between the putative transposase and those encoded by ISs in other bacterial sequences from Aeromonas salmonicida (AsIs1), Escherichia coli (H repeat element), Vibrio cholerae (VcIS1), and Porphyromonas gingivalis (PGIS2). The second IS, IS2606, was discovered by sequence analysis of a HaeIII fragment of M. ulcerans genomic DNA containing a repetitive sequence. This element is 1,404 bp long, with 12-bp inverted repeats and a single ORF potentially encoding a protein of 445 aa. Database searches revealed a high degree of amino acid identity (70%) with the putative transposase of IS1554 from M. tuberculosis. Significant amino acid identity (40%) was also observed with transposases from several other microorganisms, including Rhizobium meliloti (ISRm3), Burkholderia cepacia (IS1356), Corynebacterium diphtheriae, and Yersinia pestis. PCR screening of DNA from 45 other species of mycobacteria with primers for IS2404 confirm that this element is found only in M. ulcerans. However, by PCR, IS2606 was also found in Mycobacterium lentiflavum, another slow-growing member of the genus Mycobacterium that is apparently genetically distinct from M. ulcerans. Testing the sensitivity of PCR based on IS2404 and IS2606 primers demonstrated the ability to detect 0.1 and 1 M. ulcerans genome equivalents, respectively. The ability to detect small numbers of cells by using two gene targets will be particularly useful for analyzing environmental samples, where there may be low concentrations of M. ulcerans among large numbers of other environmental mycobacteria.

Production of enterotoxin by Yersinia bercovieri, a recently identified Yersinia enterocolitica-like species

Yersinia bercovieri, a recently identified Y. enterocolitica-like species, produces a heat-stable enterotoxin (designated YbST) which has biologic activity in infant mice and increases short circuit current in Ussing chambers. Although YbST has some properties in common with the heat-stable enterotoxins of Y. enterocolitica (YST I and YST II), it appears to be a novel toxin because (i) it was not neutralized by anti-YST I antiserum, (ii) YbST-neutralizing antiserum did not neutralize YST I, and (iii) Y. bercovieri strains did not hybridize with genetic probes for yst I, yst II, and other known enterotoxins.

Ability of clinical isolates of group A streptococci to adhere to and invade HEp-2 epithelial cells

Individual strains of group A streptococci (GAS) differ in virulence, but the reasons for these differences are incompletely understood. To determine if the ability of GAS to cause invasive disease corresponded with their capacity to adhere to or invade epithelial cells, 63 clinical isolates of GAS (40 from patients with systemic infection and 23 from superficial disease) were examined in quantitative assays of bacterial adhesion to and invasion of HEp-2 cells, a continuous line of human pharyngeal epithelial cells. The results showed that individual isolates of GAS varied considerably in their ability to adhere to and penetrate HEp-2 cells. However, on the whole, strains from patients with invasive disease adhered to cells in numbers c.1.5 greater than those from superficial infection. Paradoxically, strains from patients with invasive disease invaded HEp-2 cells to a significantly lesser extent than those from superficial sites, with a two-fold difference in invasion index (defined as the percentage of cell-associated bacteria located intracellularly). To determine if these differences were caused by differences in the production of hyaluronic acid capsule or M protein by the two groups of bacteria, the adherence and invasive capacities of bacteria carrying defined mutations in the genes for these factors were examined. Although M6-protein-deficient [corrected] bacteria were less adherent to HEp-2 cells than the wild-type, neither the hyaluronic acid capsule nor the M protein had a significant influence on the ability of GAS to adhere to or invade HEp-2 cells. The results of this study demonstrate that there are biological differences between GAS isolates associated with invasive and superficial diseases and that these differences can be demonstrated by an assay of bacterial adherence to and invasion of HEp-2 epithelial cells.

Examination of Serpulina pilosicoli for attachment and invasion determinants of Enterobacteria

The spirochaete, Serpulina pilosicoli, is the agent of intestinal spirochaetosis, a diarrhoeal disease of humans and other species. By mechanisms as yet unknown, large numbers of these spirochaetes intimately attach to the colonic mucosa by one cell end. In some infected individuals, the spirochaetes may invade the lamina propria and adjacent tissues, and they may cause spirochaetaemia. To examine S. pilosicoli for pathogenic determinants homologous with Enterobacteria, DNA was extracted from six strains of S. pilosicoli and hybridised at low stringency with DNA probes derived from the inv, ail and yadA genes of Yersinia enterocolitica, the eae gene from enteropathogenic Escherichia coli and a probe derived from the virulence plasmid of Shigella flexneri. No hybridisation of the enterobacterial probes to S. pilosicoli DNA was detected, indicating that these gene sequences, which are known to be involved in the attachment and invasion processes of the other intestinal pathogens, were not present in the spirochaetes.

Hemolytic-uremic syndrome following urinary tract infection with enterohemorrhagic Escherichia coli: case report and review

A 6-week-old child with acute urinary tract infection caused by Shiga toxin-producing Escherichia coli (STEC) O5:H-developed hemolytic-uremic syndrome (HUS). Molecular and phenotypic analysis of the urinary isolate indicated that it lacked uropathic properties and that it was probably of intestinal origin. Nevertheless, the patient did not experience a diarrheal prodrome, nor was STEC or Shiga toxin detected in his feces at any time. Examination of the patient's serum pointed to recent infection with E. coli O5, with no evidence of exposure to E. coli O157, O111, or O26. A review of 13 previously reported cases of HUS associated with acute urinary tract infection indicated that this was the first case of nondiarrheal HUS in which infection with the most common STEC serogroups was specifically excluded. This case illustrates the need to investigate patients with nondiarrheal HUS for infection with STEC.

Effect of bacterial invasion of macrophages on the outcome of assays to assess bacterium-macrophage interactions

In vitro assays to quantify killing of bacteria by macrophages provide useful insights into host-pathogen relations. In the present study, we used strains of Yersinia enterocolitica and Escherichia coli which varied in their ability to invade mammalian cells to evaluate these assays. The results showed that 30 min and 24 h after incubation with murine bone marrow-derived macrophages, strains of Y. enterocolitica and E. coli which expressed invasin (an outer membrane protein which allows bacteria to penetrate mammalian cells) achieved significantly greater numbers in macrophages than otherwise isogenic bacteria which lacked this protein (P < 0.01). When the 24-h data were corrected for the number of bacteria ingested by macrophages initially, the differences between invasin-positive and -negative bacteria were no longer evident (P> 0.2). This study has shown (1) that invasin-mediated penetration of macrophages by bacteria is not associated with enhanced intracellular survival, and (2) that invasion of macrophages by bacteria may influence the interpretation of assays for bactericidal capacity unless allowance is made for the number of bacteria ingested during the early phase of the assay.

In vitro association between the virulence proteins, YopD and YopE, of Yersinia enterocolitica

The virulence plasmid (pYV) of Y. enterocolitica encodes a set of anti-host proteins, known as Yops, which contribute to survival of the bacteria in the host. Several Yops directly influence the interaction of pathogenic bacteria with host cells which in the case of at least four Yops, YopE, YopH, YopM and YpkA (YopO), involves translocation of the Yop across the eukaryotic cell membrane into the cytoplasm of target cells. Translocation requires the presence of two other pYV-encoded proteins, YopB and YopD, but it is not clear how these proteins mediate translocation of the effector Yops. We have used an affinity blot technique (overlay) to examine potential binding between YopD and other Yops. The results indicated that under the in vitro conditions used in this study, YopD bound preferentially to YopE and YopB. To investigate the interaction of YopD and YopE in more detail we produced different regions of the YopD protein in Escherichia coli by creating fusions of YopD with glutathione S-transferase. These studies showed that YopE bound the central hydrophobic region of YopD. This is the first demonstration of protein interactions between an effector Yop and a putative Yop translocator.

Characterization of the roles of hemolysin and other toxins in enteropathy caused by alpha-hemolytic Escherichia coli linked to human diarrhea

Escherichia coli strains producing alpha-hemolysin have been associated with diarrhea in several studies, but it has not been clearly demonstrated that these strains are enteropathogens or that alpha-hemolysin is an enteric virulence factor. Such strains are generally regarded as avirulent commensals. We examined a collection of diarrhea-associated hemolytic E. coli (DHEC) strains for virulence factors. No strain produced classic enterotoxins, but they all produced an alpha-hemolysin that was indistinguishable from that of uropathogenic E. coli strains. DHEC strains also produced other toxins including cytotoxic necrotizing factor 1 (CNF1) and novel toxins, including a cell-detaching cytotoxin and a toxin that causes HeLa cell elongation. DHEC strains were enteropathogenic in the RITARD (reversible intestinal tie adult rabbit diarrhea) model of diarrhea, causing characteristic enteropathies, including inflammation, necrosis, and colonic cell hyperplasia in both small and large intestines. Alpha-hemolysin appeared to be a major virulence factor in this model since it conferred virulence to nonpathogenic E. coli strains. Other virulence factors also appear to be contributing to virulence. These findings support the epidemiologic link to diarrhea and suggest that further research into the role of DHEC and alpha-hemolysin in enteric disease is warranted.

Identification of virulence-associated characteristics in clinical isolates of Yersinia enterocolitica lacking classical virulence markers

Yersinia enterocolitica is an important enteric pathogen which has well-defined virulence determinants that allow the bacteria to become established in their hosts and overcome host defenses. A number of strains obtained from patients with diarrhea, however, lack these genes. Accordingly, the mechanisms by which they cause disease are uncertain. Most of these isolates belong to biotype 1A. Strains of this biotype are also frequently isolated from a variety of nonclinical sources, such as food, soil, water, and healthy animals, and there is evidence that some of these strains are avirulent. In this study we investigated 111 strains of Y. enterocolitica biotype 1A, 79 from symptomatic humans and 32 from nonclinical sources, for virulence-associated characteristics. DNA hybridization studies showed that none of the strains carried sequences homologous with pYV, the approximately 70-kb Yersinia virulence plasmid. Some strains hybridized with DNA probes for one of the following chromosomal virulence-associated genes: ail (7.2%), myfA (11.7%), ystA (0.9%), and ystB (85%). In addition, 33 strains (29.7%) produced an enterotoxin that was reactive in infant mice. However, the frequencies of these virulence-associated properties in clinical and nonclinical isolates were similar. Clinical isolates invaded HEp-2 cells and Chinese hamster ovary cells to a significantly greater extent than nonclinical strains (P < or = 0.002). In addition, clinical strains colonized the intestinal tracts of perorally inoculated mice for significantly longer periods than nonclinical isolates (P < or = 0.01). Light and electron microscopic examination of tissue culture cells incubated with invasive yersiniae revealed that the bacteria invaded selected cells in large numbers but spared others, suggesting that biotype-1A strains of Y. enterocolitica may invade cells by a novel mechanism. These results indicate that some clinical isolates of Y. enterocolitica which lack classical virulence markers may be able to cause disease via virulence mechanisms which differ from those previously characterized in enteropathogenic Yersinia species.

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.

Colonic structural and ion transport abnormalities in suckling rabbits infected with Escherichia coli K12

BACKGROUND

Escherichia coli K12 is a laboratory strain considered nonpathogenic. The purpose of this study was to examine the effect of E. coli K12 infection on colonic structure and function.

METHODS

Suckling rabbits were infected at 10 days of age with 6 x 10(9) CFU E. coli by intragastric inoculation and were examined 4 to 5 days later. Segments of ileum and proximal and distal colon were removed for light and electron microscopy, and NaCl transport was examined in vitro under short-circuited conditions in Ussing chambers.

RESULTS

Infection did not cause weight loss or diarrhea. Colonic mucosa was inflamed with infiltration by polymorphonuclear neutrophils mainly in the lamina propria. The proximal and distal colon exhibited reduced Na+ absorption. The proximal colon also showed increased Cl- secretion; the ileum was unaffected.

CONCLUSIONS

Infection with E. coli K12 disrupts the epithelium and alters ion transport in the colon, probably as a result of mucosal inflammation. The changes indicate that nonpathogenic E. coli have the potential to cause intestinal disease.

Detection of Mycobacterium ulcerans in environmental samples during an outbreak of ulcerative disease

Mycobacterium ulcerans is an environmental bacterium which causes chronic skin ulcers. Despite significant epidemiological evidence to suggest that water is the source of infection, the organism has never been identified in the environment. Environmental water samples were collected from a small town in which an outbreak of 29 cases had occurred in a 3-year period. These were examined by mycobacterial culture and PCR amplification. Similar to previous studies, M. ulcerans was not cultured from the water samples. However, five samples were positive for M. ulcerans by PCR. These samples were collected from a swamp and a golf course irrigation system within the outbreak area. This is the first time that M. ulcerans has been demonstrated to be present in the environment and supports the postulated epidemiology of disease due to this organism.

Development of a PCR assay for rapid diagnosis of Mycobacterium ulcerans infection

The diagnosis of Mycobacterium ulcerans infection is hampered by the slow growth of the bacterium in culture, resulting in a delay of several months before a specific diagnosis can be obtained. In addition, M. ulcerans cannot be isolated from water even when there is convincing epidemiological evidence implicating this as the source of infection. The aim of the present study was to develop a PCR assay to circumvent the problems of delayed diagnosis and insensitivity of standard bacterial culture for M. ulcerans. For the PCR, we isolated an M. ulcerans-specific DNA fragment, 1,109 bp long, which is repeated at least 50 times throughout the genome. Use of this sequence as a target for PCR allowed us to detect as few as 2 molecules of genomic DNA in vitro. The PCR was used to detect M. ulcerans DNA in fresh tissue and paraffin-embedded sections from all seven patients with culture-confirmed cases of infection.

Development of a highly specific assay for rapid identification of pathogenic strains of Yersinia enterocolitica based on PCR amplification of the Yersinia heat-stable enterotoxin gene (yst)

The chromosomal gene yst, which encodes a heat-stable enterotoxin of Yersinia enterocolitica, is a useful diagnostic marker because it occurs only in invasive strains of this species. A homologous gene also occurs in some strains of Yersinia kristensenii. Sequence analysis of the yst genes from two different strains of Y. enterocolitica and from Y. kristensenii revealed a substantial number of mismatches at the 3' ends of the yst genes of the so-called American and European biotypes of Y. enterocolitica. Moreover, several mismatches and a deletion of 5 codons were found in the yst of Y. kristensenii. These findings were used to develop a PCR-based assay for yst of Y. enterocolitica which yielded a detectable product in as little as 50 min. The assay was 100% specific in terms of its ability to identify potentially pathogenic strains of Y. enterocolitica regardless of biotype or serotype. The PCR yielded an amplicon that was visible on agarose gel electrophoresis from as few as 100 CFU, or 10 CFU when the PCR was combined with dot blot hybridization with a digoxigenin-labeled oligonucleotide probe that corresponded to an internal sequence of yst. These results establish the value of the yst gene as a target for the identification of pathogenic bioserotypes of Y. enterocolitica and the usefulness of PCR for this purpose.

A novel mechanism of urease regulation in Yersinia enterocolitica

Yersinia enterocolitica produces the enzyme urease which hydrolyses urea, resulting in the production of carbonic acid and ammonia and a net increase in pH. In the presence of urea, urease enhances survival of Y. enterocolitica in the stomach and presumably in other acidic environments the bacteria encounter during the course of infection. In this study we show that Y. enterocolitica urease is a cytosolic enzyme which has a low Km value (0.15 +/- 0.01 mM urea), suggesting that it functions at close to maximum velocity even at the low concentrations of urea available to Y. enterocolitica in gastric fluid and other tissues. Y. enterocolitica urease was active over a wide pH range, but unlike most other bacterial ureases, displayed an optimal activity at pH 3.5-4.5, suggesting a physiological role in protecting the bacteria from acid. Higher levels of urease activity were attained at 28 degrees C than at 37 degrees C, and investigation of the regulation of urease production revealed that the enzyme was not induced by urea, or by nitrogen limitation. Instead maximal activity was attained during the stationary phase of growth which coincides with the period of maximum acid tolerance of the bacteria. This type of regulation has not been described for any other ureolytic bacteria and seems to be unique to Y. enterocolitica.

A cluster of atypical Yersinia strains with a distinctive 16S rRNA signature

Thirty-eight bacterial isolates from raw milk samples in Queensland, Australia were identified as members of the genus Yersinia on the basis of biochemical profile, ability to hybridize with a genus-specific DNA probe, comparative 16S rDNA sequence analysis, and the presence of characteristic 16S rDNA signature nucleotides which occur in all Yersinia spp. Twenty-five of these isolates reacted with typing sera (O:22 or O:58) of Y. enterocolitica; the remainder were non-typable. None of the isolates displayed any of the phenotypic or genetic virulence-associated characteristics of Y. enterocolitica. Comparative 16S rDNA sequence analysis revealed that members of this group appear to represent a new sub-line within the genus Yersinia, most closely related to Y. frederiksenii hybridization group 2 (unnamed genomospecies 2). This findings was confirmed by DNA hybridization studies which indicated that the strains belonged to the unnamed genomospecies, Yersinia frederiksenii genomospecies 2, which is biochemically indistinguishable from Y. frederiksenii (Y. frederiksenii genomospecies 1). A 23-nucleotide 16S rDNA signature stretch which characterised these strains was identified.

Analysis of the urease gene complex of members of the genus Yersinia

The urease gene complex of Yersinia enterocolitica is relatively conserved within the species, although this conservation may not extend to other members of the genus. Spontaneous urease-negative isolates of Y. enterocolitica appear to have arisen as a result of large deletions within this complex, while Y. pestis shows no significant deletions within the complex, despite being urease negative.

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.

Contribution of YopB to virulence of Yersinia enterocolitica

The 70-kb virulence plasmid, pYV, of Yersinia enterocolitica encodes a number of secreted proteins (Yops) which are essential for virulence. YopD, the 33-kDa product of the lcrGVHyopBD operon, appears to be involved in delivering YopE and YopH (the Yersinia protein tyrosine phosphatase) into target cells. These proteins then act in concert to cause cytotoxicity in host cells. Previously, we reported that bacteria carrying transposon insertions in yopD are not cytotoxic for macrophages, show impaired tyrosine phosphatase activity in host cells, and are avirulent for mice (E. L. Hartland, S. P. Green, W. A. Phillips, and R. M. Robins-Browne, Infect. Immun. 62:4445-4453, 1994). trans complementation of yopD mutants of Y. enterocolitica with the yopD gene restores all these properties. In this study, we show that polar mutations in proximal genes of the lcrGVHyopBD operon also abrogated bacterial virulence and the capacity to induce cytotoxicity in mouse bone marrow-derived macrophages and HEp-2 epithelial cells. Moreover, trans complementation of a yopBD mutant with the yopD gene alone was not sufficient to restore the ability of the bacteria to cause cytotoxicity. Further work showed that YopB was required for cytotoxicity, dephosphorylation of host proteins, and virulence for mice. These findings indicate that YopB and YopD may serve a related function in Y. enterocolitica and that they may act together to deliver intracellularly acting Yops to their respective targets in host cells.

Differential secretion of interleukin-8 by human epithelial cell lines upon entry of virulent or nonvirulent Yersinia enterocolitica

Epithelial cells of the intestinal mucosa are among the first cells encountered by invasive pathogens. Bacterial invasion of the mucosa gives rise to an inflammatory response, characterized by the influx of polymorphonuclear leukocytes. The chemotactic stimulus responsible for this accumulation is unknown, but several in vitro studies have demonstrated that epithelial cells secrete the chemokine interleukin-8 (IL-8), a potent chemoattractant of polymorphonuclear leukocytes, upon bacterial entry. In this study we analyzed the secretion of IL-8 by human intestinal (T84) and cervical (HeLa) epithelial cell lines in response to infection with the enteric pathogen Yersinia enterocolitica. IL-8 was secreted by T84 and HeLa cells in response to invasion by Y. enterocolitica. Virulent Y. enterocolitica induced a significantly lower level of IL-8 secretion than nonvirulent Y. enterocolitica. Subsequent analysis employing a mutant defective in Yop secretion and various yop mutants showed that the reduced secretion of IL-8 is due to the presence of Yop proteins. Our data suggest that YopB and YopD are required for the suppressive effect.

Sporadic invasion of cultured epithelial cells by Haemophilus influenzae type b

While working with an in vitro invasion assay, we observed that Haemophilus influenzae type b occasionally exhibits highly invasive behavior. The phenomenon is not inhibited by colchicine or cytochalasin but is dependent on the presence of supplemental CO2. We propose that sporadic invasiveness may correlate with the unknown events that precede Haemophilus influenzae type b bacteremia.

Contribution of urease to acid tolerance in Yersinia enterocolitica

The stomach serves as a barrier to enteric infection because of the antibacterial effect of the hydrochloric acid in gastric juice. In this study, we tested the ability of the enteric pathogen Yersinia enterocolitica to tolerate a pH range of 2.0 to 6.0 and found that under the conditions of a normal human fasting stomach (pH < 3 and a gastric emptying time of 2 h), Y. enterocolitica is highly acid resistant, showing approximately 85% survival. The resistance of Y. enterocolitica to acid in vitro depended on the bacterial growth phase and the concentration of urea in the medium, being maximal during stationary phase in the presence of at least 0.3 mM urea. Urease-negative mutants of Y. enterocolitica were constructed by disrupting the urease gene complex of a virulent strain of serogroup O9. Compared with the wild type, these mutants showed an approximately 1,000-fold decrease in the ability to tolerate acid in vitro (< 0.08% survival) and a 10-fold reduction in viability after passage through the stomachs of mice. Complementation of the disrupted urease genes in trans restored the ability of urease-negative mutants to tolerate low pH in vitro and gastric acidity to approximately wild-type levels. These findings indicate that urease is responsible for acid resistance in Y. enterocolitica and suggest that urease contributes to the virulence of Y. enterocolitica by enhancing the likelihood of bacterial survival during passage through the stomach.

Role of YopH in the suppression of tyrosine phosphorylation and respiratory burst activity in murine macrophages infected with Yersinia enterocolitica

Tyrosine phosphorylation is an important component of the signaling pathways responsible for the activation of the macrophage respiratory burst. Because the virulence plasmid of Yersinia enterocolitica encodes a phosphotyrosine phosphatase, YopH, it is possible that the pathogenic strategy of Y. enterocolitica involves the disruption of tyrosine phosphorylation in the macrophage leading to inhibition of respiratory burst activity. We have investigated the effects of Yersinia infection on tyrosine phosphorylation and respiratory burst activity in murine bone marrow-derived macrophages. Infection of macrophages with virulent [Ye(pYV+)] but not avirulent [Ye(pYV-)] strains of Y. enterocolitica was found to suppress both tyrosine phosphorylation and respiratory burst activity in response to zymosan. Mutational inactivation of YopH reversed the suppressive effect of Ye(pYV+) on zymosan-induced tyrosine phosphorylation, indicating that YopH is responsible for the dephosphorylation of macrophage phosphotyrosine-containing proteins observed in macrophages infected with the virulent strain of Y. enterocolitica. In contrast, mutational loss of YopH failed to reverse the inhibitory effect of Ye(pYV+) on the zymosan-triggered respiratory burst. We conclude that the inhibition of the macrophage respiratory burst by Y. enterocolitica involves a plasmid-encoded virulence protein(s) other than, or in addition to, YopH.

Essential role of YopD in inhibition of the respiratory burst of macrophages by Yersinia enterocolitica

The respiratory burst is a key element of the bactericidal armamentarium of phagocytes. In this study we have shown that a virulent strain of Yersinia enterocolitica serogroup O:9 completely inhibited the ability of murine bone marrow-derived macrophages to mount a respiratory burst in response to stimulation by zymosan. This property of the bacterium was abrogated by curing the strain of its 71.5-kb virulence plasmid and by transposon mutagenesis of the plasmid-borne yopD gene. Derivatives of the bacterium which were unable to inhibit the respiratory burst were also less able to disrupt cytoskeletal actin and to resist phagocytosis. yopD mutants also showed an impaired ability to dephosphorylate phosphotyrosine residues in macrophage proteins and were completely avirulent for mice. All of these defects were fully or partly restored by trans-complementation of a yopD mutant with a cloned yopD gene. The results of this study and those of previous work with YopD (R. Rosqvist, A. Forsberg, and H. Wolf-Watz, Infect. Immun. 59:4562-4569, 1991) suggest that YopD functions chiefly by facilitating the transport of virulence plasmid-encoded proteins, such as YopE, a cytotoxin, and YopH, a protein tyrosine phosphatase, across the cytoplasmic membrane to their targets within host cells. The combined action of these Yops on cytoplasmic proteins, especially actin, could explain the effects of virulent Y. enterocolitica on macrophage morphology, phagocytic capacity, and respiratory burst activity, all of which rely on cytoskeletal integrity to function normally.

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.

Characterisation of the urease-encoding gene complex of Yersinia enterocolitica

A cosmid gene library of chromosomal DNA from Yersinia enterocolitica A2635 (serogroup O:8) was constructed in Escherichia coli. Subcloning of a urease-positive (Ure+) clone revealed a region of 6.6 kb that was sufficient for expression of Ure activity in E. coli. Sequencing of this fragment disclosed seven ORFs transcribed in the same direction. On the basis of homology to known Ure, these were designated ureA, ureB, ureC, ureE, ureF, ureG and ureD, which are predicted to encode polypeptides of 11.1, 17.9, 61.0, 29.5, 25.0, 24.1 and 36.4 kDa, respectively. The polypeptides encoded by the ure gene complex of Y. enterocolitica are significantly divergent from those encoded by the ure operons of other Enterobacteriaceae, which appear to be closely related to each other. This suggests that the ure genes were acquired by Y. enterocolitica from an unrelated organism or alternatively, that they diverged from those of other Enterobacteriaceae some considerable time ago.

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

Twelve strains of Escherichia coli previously reported to cause diarrhea in rabbits were examined for properties associated with virulence. Ten strains met the criteria for classification as enteropathogenic E. coli in that they were diarrheagenic strains that evoked attaching-effacing lesions in the small intestine and did not produce detectable enterotoxins or cytotoxins. These bacteria exhibited a variety of patterns when investigated for adherence to HEp-2 epithelial cells. Although several strains displayed localized and/or diffuse adherence to epithelial cells, they did not hybridize with DNA probes that recognize the genes responsible for these phenotypes in diarrheagenic E. coli from humans. The bacteria also varied in their ability to bind to erythrocytes and intestinal brush borders from various animal species. Six strains adhered to rabbit brush borders; two of these also adhered to brush borders from other animals. Two strains that did not adhere to rabbit brush borders adhered to those from guinea pigs or sheep. Only one of the strains investigated carried AF/R1 fimbriae, which are believed to govern the host specificity of this category of diarrheagenic E. coli. This strain was E. coli RDEC-1, which remains the only E. coli strain to date that is known to carry fimbriae of this type. The results indicate that although diarrheagenic E. coli strains from rabbits may have common properties associated with the ability to produce attaching-effacing lesions, they differ from each other and from enteropathogenic E. coli of humans in terms of some of the adhesins that mediate binding to eukaryotic cells.

Genetic relationships and virulence factors among classical enteropathogenic Escherichia coli serogroup O126 strains

Thirty-nine Escherichia coli strains of the enteropathogenic (EPEC) serogroup O126 isolated from sporadic and outbreak cases of infantile diarrhoea between 1982 and 1988 were studied. These strains consisted of four serotypes showing close genetic relationships between their virulence markers, outer-membrane protein and lipopolysaccharide profiles, and electrophoretic types by multilocus enzyme electrophoresis. None of these strains exhibited localised adherence to HEp-2 cells or the attaching-effacing properties of classical type I EPEC. Of the 39 strains, 31 were of serotype O126:H12 and enterotoxigenic; one strain was serotype O126:H10 and enteroaggregative. The remaining six strains of serotype O126:H21 and one strain of serotype O126:H8 harboured no known virulence factors for diarrhoeagenic E. coli.

Serological response of sheep to plasmid-encoded proteins of Yersinia species following natural infection with Y. enterocolitica and Y. pseudotuberculosis

A prospective study of the serological response to natural infection with Yersinia enterocolitica and Y. pseudotuberculosis was performed in an experimental flock of sheep. A preliminary investigation with immunoblotting techniques showed that lambs infected with virulent Yersinia spp. produced antibodies to several yersinia outer-membrane proteins (yops) encoded by a virulence plasmid (pYV) of Y. enterocolitica or Y. pseudotuberculosis. Thereafter, an enzyme immunoassay (EIA) was developed to measure antibodies to yops. Criteria for interpreting the EIA were established by examining sera from a negative control population of lambs which had not been infected with Yersinia spp. since birth. Test samples comprised 25 pairs of pre- and post-infection sera from animals with bacteriologically proven infections with Yersinia spp. The results showed that infection of lambs with pYV-bearing strains of Y. enterocolitica or Y. pseudotuberculosis invariably evoked a significant antibody response to yops, even though all the infections were subclinical. No animal infected with so-called "environmental", pYV-negative Yersinia spp. seroconverted to yops. EIA with yops as antigen provided a sensitive and specific means to diagnose subclinical infection of lambs with virulent Yersinia spp.

Examination of archetypal strains of enteropathogenic Escherichia coli for properties associated with bacterial virulence

Nine strains of Escherichia coli isolated from infants with diarrhoea between 1947 and 1960 and designated "enteropathogenic" were examined for phenotypic and genetic characters associated with virulence. Each strain belonged to a different serotype. All the isolates were historically significant in that they were amongst the first strains of E. coli reported to be causally associated with infantile diarrhoea. Five strains possessed the virulence properties of class I enteropathogenic E. coli (EPEC). All these strains were isolated originally from symptomatic children during outbreaks of diarrhoea. Two isolates from sporadic cases of diarrhoea fulfilled the criteria for classification as class II EPEC. One strain was identified as enteroaggregative E. coli and the other carried no known virulence-associated properties. These findings indicate that most early isolates of E. coli which were designated "enteropathogenic" were indeed EPEC, as currently defined.

Elimination of extracellular bacteria by antibiotics in quantitative assays of bacterial ingestion and killing by phagocytes

In vitro assays to quantify the bactericidal capacity of phagocytes require the removal of extracellular bacteria from the reaction mixture before intracellular bacteria are released from the phagocytic cells and counted. This may be achieved by using an antibiotic, such as gentamicin, which has a rapid bactericidal action coupled with limited ability to cross cytoplasmic membranes. In this study, we investigated the susceptibility of Escherichia coli and Yersinia enterocolitica to killing by mouse peritoneal macrophages. Extracellular bacteria in the reaction mixture were eliminated with gentamicin, whereafter the antibiotic was enzymatically inactivated using gentamicin acetyl transferase (GAT). The method has few washing stages, thus reducing the likelihood of inadvertent removal of macrophages or bacteria from the reaction mixture. The assay disclosed clear differences in the ability of E. coli and Y. enterocolitica to survive phagocytosis by macrophages.

Assessment of enterotoxin production by Yersinia enterocolitica and identification of a novel heat-stable enterotoxin produced by a noninvasive Y. enterocolitica strain isolated from clinical material

Twenty-eight clinical isolates of Yersinia enterocolitica were investigated for their abilities to produce heat-stable enterotoxin (YST). All 21 invasive strains (serogroup O3 biotype 4) carried the previously described gene for YST (yst), with toxin detectable in culture supernatants from 20 strains. One of seven noninvasive, biotype 1A strains also had enterotoxic activity, despite failure to hybridize with a probe for yst. The toxin produced by this noninvasive (serogroup O6) strain resembled YST in terms of molecular size, heat stability, and solubility in methanol. It differed from YST, however, with respect to regulation of its production by temperature and its mechanism of action, which did not appear to involve cyclic GMP.

Studies with enteroaggregative Escherichia coli in the gnotobiotic piglet gastroenteritis model

Two strains of enteroaggregative Escherichia coli of human origin fed to gnotobiotic piglets caused diarrhea or death in the majority of them. Histological examination revealed moderate hyperemia of the distal small intestine and cecum, swelling of small intestinal villi, and layers of aggregated bacteria stacked together in a mucus gel-like matrix overlying intact epithelium. These findings confirm that enteroaggregative E. coli strains produce distinctive intestinal lesions different from those caused by other major categories of diarrheagenic E. coli.

Influence of a 70 kilobase virulence plasmid on the ability of Yersinia enterocolitica to survive phagocytosis in vitro

During the course of infection, Yersinia enterocolitica invades tissues where macrophages and polymorphonuclear leucocytes (PMNs) constitute the first line of defence. As expression of virulence in Y. enterocolitica is governed in part by a c. 70 kilobase virulence plasmid (pYV), we investigated the influence of this plasmid on the interaction between Y. enterocolitica and phagocytes in vitro. The results showed that, irrespective of plasmid-carriage, yersiniae survived phagocytosis by macrophages and PMNs. Plasmidless Y. enterocolitica that had grown intracellularly in macrophages, however, were susceptible to killing by PMNs, whereas plasmid-bearing bacteria were resistant. In vitro cultivation of Y. enterocolitica in a Ca(2+)-deficient medium resembling that found within macrophages, did not influence the susceptibility of plasmid-bearing and plasmidless strains to killing by PMNs. These results indicate that passage through macrophages renders plasmidless strains of Y. enterocolitica susceptible to killing by PMNs. This finding may explain some of the differences in the behaviour of plasmid-bearing and plasmidless strains of Yersinia species in vivo.

Quantitative assessment of the ability of Escherichia coli to invade cultured animal cells

Assays to quantify bacterial invasion of epithelial cells generally fail to take account of the ability of the bacteria to adhere to the cells prior to invasion. We have developed a modified invasion assay to allow for this factor. We then used the assay to investigate diarrhoeagenic strains of Escherichia coli with differing ability to adhere to and invade HEp-2 epithelial cells. The results showed that enteroinvasive strains of E. coli were the most invasive variety, followed in order by enteropathogenic E. coli and enterotoxigenic E. coli. These findings correspond to what is known of the ability of the bacteria to invade the intestinal tract in vivo. The results also indicated that adhesins of diarrhoeagenic E. coli play no direct role in invasion, although they may facilitate invasion indirectly by promoting initial contact between bacteria and animal cells.

Yersinia enterocolitica isolated from two cohorts of young children in Santiago, Chile: incidence of and lack of correlation between illness and proposed virulence factors

Yersinia enterocolitica was isolated from children in two cohorts in Santiago, Chile. In a cohort containing a cross section of children aged 0 to 4 years, Y. enterocolitica was isolated from stool samples of 1.1% of children with diarrhea and 0.2% of age-matched control children. In a subgroup of this cohort from which weekly stool samples were obtained from all children irrespective of clinical status, 6% of children had asymptomatic Yersinia infections. In a birth cohort (with a greater representation of children less than 1 year of age and a significantly higher rate of diarrhea), Y. enterocolitica was isolated from 1.9% of children with diarrhea and 0.6% of controls (P = 0.05). Biogroup 1A strains (which lacked traditional phenotypic and molecular markers for pathogenicity) were isolated from seven children with diarrhea but from no control children in the birth cohort (P = 0.02). All other isolates, including all isolates from asymptomatic children, were "pathogenic" strains in biogroup 4, serogroup O3; no association between these isolates and occurrence of disease was found. Y. enterocolitica is found among young children in Santiago, with asymptomatic infections not uncommon occurrences. However, questions about the association between previously described virulence factors and diarrheal illness remain.

Identification of a conjunctivitis-associated gene locus from the virulence plasmid of Yersinia enterocolitica

The virulence plasmid (pYV) of Yersinia enterocolitica is necessary for production of conjunctivitis in guinea pigs and for mouse lethality. To identify the genes responsible for production of conjunctivitis in guinea pigs, we subcloned the BamHI and SalI restriction fragments of the virulence plasmid of Y. enterocolitica A2635 (serotype O:8) into derivatives of the broad-host-range plasmid pRK290 and introduced the constructions into plasmid-negative Y. enterocolitica strains. A mild, transient conjunctivitis was evident 24 h after inoculation with strains containing a 2.8-kilobase (kb) BamHI fragment of pYV. These strains were cytotoxic to HEp-2 cells but did not cause death in iron-loaded adult mice. When the 2.8- and adjacent 0.5-kb BamHI fragments were deleted from the virulence plasmid of a fully virulent Y. enterocolitica isolate, the resultant strain did not cause conjunctivitis in guinea pigs and was not cytotoxic to HEp-2 cells. However, the strain with the deletion appeared to be more virulent for mice, with more rapid dissemination after orogastric inoculation, compared with that of the parent strain. When the deletion was complemented by introduction of a plasmid containing the 2.8-kb BamHI fragment, the strain again caused conjunctivitis but had decreased virulence for mice.

Evaluation of DNA colony hybridization and other techniques for detection of virulence in Yersinia species

The virulence of yersiniae varies according to (i) species and biotype and (ii) possession of a 67- to 72-kilobase virulence plasmid. Y. pestis, Y. pseudotuberculosis, and biotypes 1B, 2, 3, 4, and 5 of Y. enterocolitica are inherently virulent but express full virulence only when in possession of a virulence plasmid. Other Yersinia species and biotypes 1A and 3B of Y. enterocolitica are seldom implicated in disease. In this study, we prepared DNA probes from eight nonoverlapping regions of the virulence plasmid of a strain of Y. enterocolitica and from the inv and ail chromosomal loci responsible for the invasive capacity of Y. enterocolitica and Y. pseudotuberculosis. The probes were used in colony hybridization experiments to investigate 156 yersiniae of various species and biotypes and of differing virulence. Probes prepared from the inv gene of Y. pseudotuberculosis hybridized with Y. pseudotuberculosis and Y. pestis only, whereas an analogous probe prepared from Y. enterocolitica hybridized with all species and biotypes of yersiniae (but not with other bacteria) regardless of virulence or potential virulence. Probes prepared from the ail region of Y. enterocolitica reacted almost exclusively with Y. enterocolitica strains of pathogenic biotypes. Probes prepared from the virulence plasmid of a serogroup O:8, biotype 1B isolate of Y. enterocolitica identified virulent yersiniae in all species with a high degree of sensitivity and specificity. These probes did not react with yersiniae of avirulent biotypes or species. Of the other assays of virulence evaluated (calcium dependence, binding of crystal violet, and pyrazinamidase activity), binding of crystal violet provided a simple means for identifying plasmid-bearing strains.

Role of a 60-megadalton plasmid and Shiga-like toxins in the pathogenesis of infection caused by enterohemorrhagic Escherichia coli O157:H7 in gnotobiotic piglets

Enterohemorrhagic Escherichia coli (EHEC) of serotype O157:H7 has two putative virulence factors: (i) a fimbrial adhesin, specified by a 60-megadalton (MDa) plasmid, and (ii) bacteriophage-specified cytotoxin(s), known as Shiga-like toxin (SLT) or verotoxin. The contribution of these factors to the pathogenesis of EHEC-induced disease in gnotobiotic piglets was examined. The bacterial strains included the following: two EHEC strains and their corresponding plasmid-cured derivatives; another EHEC isolate and its derivative which had spontaneously lost the ability to produce SLT; one E. coli K-12 transconjugatant containing a 60-MDa plasmid from an EHEC strain; two K-12 strains into which an SLT-producing phage had been transduced (one of these strains also carried a 60-MDa EHEC-derived plasmid); and the parent K-12 strain. Each strain was fed to four piglets, which were observed for diarrhea and examined for development of characteristic mucosal lesions 3 or 5 days after inoculation. All 24 piglets inoculated with the three EHEC strains and their respective derivatives (two plasmid cured and one SLT negative) showed the typical mucosal lesions of bacterial attachment: effacement of microvillous border and cell membrane dissolution culminating in destruction of surface and glandular epithelium in the cecum and colon. No such lesions were observed in 12 piglets inoculated with three strains of E. coli K-12, including the strain which carried both the 60-MDa plasmid and a phage which specified production of SLT. Moderate to severe diarrhea was observed in 16 piglets inoculated with two EHEC strains and their derivatives (one plasmid cured and one SLT negative). The third EHEC strain and its plasmid-cured derivative produced fewer typical mucosal lesions and no diarrhea. The reason for the reduced virulence of this strain was not clear. These results demonstrate that neither the 60-MDa plasmid nor the capacity to produce SLT is essential for expression of virulence by E. coli O157:H7 in gnotobiotic piglets.

In vitro and in vivo pathogenicity of Plesiomonas shigelloides

Epidemiologic evidence suggests that Plesiomonas shigelloides is an enteric pathogen. We conducted in vitro, animal, and volunteer studies on P. shigelloides isolates from patients with diarrhea. Five strains gave a negative keratoconjunctivitis reaction in guinea pigs and did not invade HeLa cells. Genetic probes for heat-stable enterotoxins related to those of enterotoxigenic Escherichia coli and for gene sequences common to the invasiveness plasmids of Shigella spp. and enteroinvasive E. coli were negative. Heat-labile enterotoxins were not found when a modified GM1-enzyme-linked immunosorbent assay was used. Rabbits did not develop diarrhea but were transiently colonized when inoculated with up to 10(11) P. shigelloides CFU using the reversible intestinal tie adult rabbit diarrhea model. A very large plasmid (between 118 and 312 megadaltons) was found in all isolates. Strain P012 was cured of its plasmid by novobiocin. This strain, but not its cured derivative, invaded the mucosa of the distal ileum of gnotobiotic piglets given 10(10) CFU. At a lower inoculum (10(9) CFU), strain P012 induced inflammation of the colonic mucosa and diarrhea at day 6. The same isolate was fed to 33 healthy volunteers in doses of 1 X 10(3) to 4 X 10(9) CFU. Thirty-six percent of the volunteers shed the organism, but none became ill. These data are only weakly supportive of a role for P. shigelloides in diarrheal illness and suggest the need for more studies with other strains to better understand its pathogenicity.