Evaluation of a novel enrichment procedure for the isolation of Escherichia coli O157 from naturally contaminated raw beef, lamb and mixed meat products Original Research Article

The aim of this study was to compare enrichment at 41·5°C for 24 h in a prototype enrichment broth developed by Oxoid, UK (sTSB), with enrichment at 37°C for 6 h in buffered peptone water supplemented with vancomycin, cefixime and cefsulodin (BPW-VCC) for isolation of Escherichia coli O157 from naturally contaminated raw beef, lamb and mixed meat products. A total of 120 meat samples from previous surveillance studies, 91 of which were previously positive and 29 previously negative for E. coli O157, were used for the study. All had been stored at −70°C for between 19 and 58 months since first examination. Of the 91 previously positive samples, 67 were positive after enrichment in sTSB and 42 were positive after enrichment in BPW-VCC;E.coli O157 was not recovered from 20 previously positive samples by either method. Of the 29 previously negative samples, three were positive by enrichment in sTSB and two were positive by enrichment in BPW-VCC;E.coli O157 was not recovered from 25 previously negative samples by either method. Of the 71 isolates obtained from the meat samples, only 53 had the same toxin genotype and plasmid profile as the E.coli O157 isolated from the sample originally. On four occasions, the strain isolated originally, the strain isolated from re-culture in BPW-VCC and the strain isolated from sTSB were all different. The only feasible explanation for these differences is that two or more different strains of E.coli O157 were present in the original sample. Results of strain typing ofE.coli O157 isolated from food samples, particularly during an outbreak investigation, should therefore be interpreted with caution.