No abstract

Between April 1996 and March 1997 we examined 5093 samples of raw beef and lamb products for the presence of E. coli O157. Samples were purchased from 81 small butchers' shops in south Yorkshire. In March 1997 we also examined five samples of dried mint for the presence of E. coli O157. Strains of E. coli O157 were isolated by enrichment culture in modified buffered peptone water followed by immunomagnetic separation and culture of magnetic beads onto cefixime tellurite sorbitol MacConkey agar. Strains were characterized by phage typing, toxin genotyping and plasmid analysis. Strains of E. coli O157 were isolated from 72 (1.4%) of 5093 samples; it was isolated from 36 (1.1%) of 3216 samples of beef products and from 29 (2.9%) samples of lamb products. The highest prevalence was found in lamb sausages and lamb burgers where E. coli O157 was isolated from 3 (4.1%) of 73 and 18 (3.7%) of 484 samples respectively. Strains of E. coli O157 were isolated most frequently during early summer. Strains of E. coli O157 were also isolated from 2 of 5 samples of dried mint although we did not determine how the mint had become contaminated. All isolates of E. coli O157 were Verocytotoxin-producing as determined by both Vero cell assay and DNA hybridization for the genes encoding Verocytotoxin and all were positive for the eaeA gene. A combination of phage typing, toxin genotyping and plasmid profile subdivided the 72 strains of E. coli isolated into 20 different subtypes, of which 18 were indistinguishable from strains isolated previously from cattle and sheep; of these 18 strains, 8 were indistinguishable from strains isolated from human cases of infection during the study period.

A commercial enzyme immunoassay (EIA) (E. coli O157 Visual Immunoassay; Tecra Diagnostics) performed on enrichment cultures in modified Escherichia coli broth (mECn) was compared with immunomagnetic separation (IMS) (Dynabeads anti-E. coli O157; Dynal) performed on enrichment cultures in modified buffered peptone water (BPW-VCC) for the detection of E. coli O157 in bovine fecal samples. Tests on fecal suspensions inoculated with each of 12 different strains of E. coli O157 showed that both the EIA and IMS methods were 10-to 100-fold more sensitive than direct culture or enrichment subculture methods for detection of the organism. EIA and IMS were then compared for detection of E. coli O157 in bovine rectal swabs. For confirmation of positive EIA tests, a commercial system (Immunocapture System [ICS]; Tecra Diagnostics) was compared with IMS; both were performed on mECn enrichment cultures. Of 200 rectal swabs examined, 17 gave positive results in the EIA which were confirmed by both confirmation systems, 2 gave positive results in the EIA which were confirmed by IMS but not by ICS, and 1 gave a positive result in the EIA which was confirmed by ICS but not by IMS. Of these 20, 15 were also positive by the BPW-VCC-IMS culture system; a further 3 samples were positive by this culture system but gave a negative result in the EIA. Eight samples were negative by the BPW-VCC-IMS culture system but gave a positive result in the EIA which could not be confirmed by either confirmation system. Further examination of the eight unconfirmed EIA-positive samples yielded sorbitol-fermenting E. coli O157 from three samples. Of the remaining five cultures, four were positive in an EIA for verocytotoxins (VT) and two were positive in a cell culture assay for VT 1. The remaining 170 samples were negative by both EIA and BPW-VCC-IMS. The Tecra EIA and IMS are both technically simple and sensitive methods for detecting E. coli O157 in bovine fecal samples. There was no statistically significant difference between the numbers of positives detected by the different assays (P ‫؍‬ 0.29).