Phenotypic and molecular characteristics of typical and atypical Escherichia coli O157, clinical and food isolates

Enrichment, colony isolation and confirmation are three general phases of a standard diagnostic method. E. coli O 157 (the main member of EHEC group) differs metabolically from other strains of E. coli in a number of ways. Most isolates are slow- or non-fermenters of sorbitol and lack the enzyme beta-glucuronidase (GUD). But, a variety of atypical strains of E. coli O157 (sorbitol-fermenting variants, nonmotile and GUD-positive) have been reported. The discovery of these atypical pathogenic strains brings into question the validity of testing for the pathogen only by biotyping. Using classical cultivation and immunomagnetic separation, we have isolated from food a few atypical E. coli O157 (sorbitol-fermenting strains, GUD positive, nonmotile O157 strain which does not agglutinate with O157 latex and does not produce Shiga toxin). On the other hand, non-O157 VTEC (O26 serotype) producing Shiga toxin was isolated from meat. Molecular markers of E. coli O157 and virulence-associated factors of strains with aberrant biochemical properties were studied by PCR. This method helped us in the final identification of isolates. Since it was suggested that the production of verotoxins (VT) is accompanied by the production of enterohemolysin (Ehly) such correlation has also been evaluated in respect to the collection of VTEC of human, animal and food origin.