Identification of plasmid-encoded mannose-resistant hemagglutinin and HEp-2 and HeLa cell adherence factors of two diarrheagenic Escherichia coli strains belonging to an enteropathogenic serogroup

Factors influencing bacterial attachment to the ruminal epithelium in vitro

Some factors influencing bacterial attachment to the rumen epithelium were studied in vitro using mixed rumen bacteria (upper- or lower-layer bacteria formed at bacterial sediment by centrifugation), isolated from steers fed a roughage diet, and rumen epithelial cells collected from beef cattle given low-concentrate (50%; LC) and high-concentrate (90%; HC) diets. Optimal incubation conditions for bacterial attachment to rumen epithelial cells were 39 degrees C for 30 min. The bacteria isolated from the upper layer had a higher attaching activity to the LC epithelial cells than those of the lower layer. A higher degree of bacterial attachment was observed using the rumen epithelium from steers fed the LC diet rather than the HC diet (p<0.01). Ethylenediamine dihydroiodide (EDDI) added at 10 through 40 &mgr;g/ml increased bacterial attachment to the HC epithelial cells. Ammonia at 50 through 100 &mgr;g/ml positively affected bacterial attachment to both LC and HC epithelial cells. Bacterial attachment to the HC epithelial cells was enhanced (p<0.01) by the addition of a reducing agent (L-cysteine.HCl) but no increase was noted with LC cells. L- or D-lactate, volatile and unsaturated fatty acids markedly decreased bacterial attachment to rumen epithelial cells.

Expression of antigenically distinct fimbriae with hemagglutination and HeLa cell adherence properties by an enteroaggregative Escherichia coli strain belonging to the enteropathogenic serogroup

An enteroaggregative Escherichia coli (EAggEC) strain (DS92), isolated from a case of infantile diarrhea, was shown to express mannose-resistant hemagglutination and HeLa cell adhering properties when grown at 37 degrees C but not at 28 degrees C. Cellular adherence properties of DS92, which belonged to enteropathogenic serogroup 0125, were shown to correlate well with the expression of fimbriae that were encoded by a 112 kb plasmid. The fimbriae of the EAggEC strain DS92 were composed of 20 kDa subunit proteins and were serologically distinct from fimbrial or non-fimbrial cell surface antigen(s) of other diarrheagenic E. coli strains including the reference EAggEC strain 17-2. Interestingly, the 20-kDa fimbrial protein was found to be antigenically related to 18- and 14.5-kDa cell surface proteins of two other locally isolated EAggEC strains belonging to the enteropathogenic serogroup 086.

Adhesion and its role in the virulence of enteropathogenic Escherichia coli

Enteropathogenic Escherichia coli (EPEC) organisms are an important cause of diarrheal disease in young children. The virulence of EPEC is a multifactorial process and involves a number of distinct stages. Initial adherence to intestinal mucosa is mediated by fimbriae which bring about a distinct form of adhesion, localized adhesion. Intimate adhesion of the bacterium to the eukaryotic membrane occurs, resulting in the activation of signal transduction pathways. Microvilli are disrupted and effaced from the apical membrane which then cups around the organism to form pedestal structures, the attaching and effacing lesion. Diarrhea may be produced by alteration of the permeability of the apical membrane and also through a malabsorption mechanism. The pathways involved in the production of the attaching and effacing lesion are described. EPEC organisms were originally thought to belong to a number of distinct serogroups; it is now apparent that many isolates belonging to these serogroups are not pathogenic or belong to other pathogenic groups of E. coli. In addition, isolates falling outside of these serogroups are considered to be true EPEC. The definition of EPEC based on serotyping is inaccurate and should be replaced by methods that specifically detect the virulence properties of EPEC.

Novel invasion determinant of enteropathogenic Escherichia coli plasmid pLV501 encodes the ability to invade intestinal epithelial cells and HEp-2 cells

An Escherichia coli K-12 transformant carrying 96.5-kb plasmid pLV501 from enteropathogenic E. coli (EPEC) strain K798 is able to produce the same characteristic attaching-effacing lesions in a rabbit ileal biopsy explant model as its parent strain. Cloned EcoRI-SalI DNA restriction fragments from this plasmid failed to reproduce the attaching-effacing lesions, but one recombinant plasmid, pLV527, containing 4.5 kb of pLV501 DNA, conferred on E. coli DH1 transformants the ability to invade enterocytes in the rabbit explant model. DH1(pLV527) was also able to adhere to and invade HEp-2 cells. The relative invasive ability of DH1(pLV527) was quantified by recovery of internalized bacteria following gentamicin treatment of infected HEp-2 monolayers. DH1(pLV527) was 1,000-fold more invasive than DH1 carrying pBR322 or a recombinant plasmid which had no physiological effect on ileal biopsy explants but was less invasive than an enteroinvasive E. coli strain or a transformant carrying the cloned invasion genes of Shigella flexneri. Invasion by DH1(pLV501) could also be detected but occurred at a level 30 times lower than that by DH1(pLV527). Colony-hybridization of the pLV527 insert against a panel of 49 EPEC and related strains revealed that only 11 contained pLV527-hybridizing sequences; thus, the invasion determinant is not an essential component of the attachment-effacement pathogenic mechanism. One pLV527-hybridizing strain displayed both attachment-effacement and invasiveness in the rabbit ileal biopsy explant model. No significant hybridization was observed to non-EPEC invasive pathogenic enteric bacteria, indicating that the invasion determinant encoded on pLV527 is distinct from those used by these organisms.

Ability of enteroaggregative Escherichia coli strains to adhere in vitro to human intestinal mucosa

A collection of 44 enteroaggregative Escherichia coli (EAggEC) strains isolated from infants with diarrhea in India and the United Kingdom were examined for their ability to adhere in vitro to human intestinal mucosa and by electron microscopy for production of putative adherence factors. None of the strains adhered to human duodenal mucosa, and six strains tested did not adhere to ileal mucosa; all 44 strains, however, adhered to human colonic mucosa in localized aggregates. Electron microscopy of infected colonic mucosa indicated fimbrially mediated adhesion of the EAggEC strains. Four morphologically distinct kinds of fimbriae, including a new morphological type of E. coli fimbriae consisting of bundles of fine filaments, were identified among the EAggEC strains; this new type of fimbria was observed in 43 of the 44 EAggEC strains. Forty-three of the 44 EAggEC strains were positive with a DNA probe developed to identify EAggEC, and most of the strains belonged to serotypes unrelated to the other major classes of diarrheic E. coli. These results suggest that EAggEC may be a large-bowel pathogen and colonize the colon by a fimbrially mediated adhesion mechanism.