A comparison of immunomagnetic separation and culture, RevealTM and VIPTM for the detection of E. coli O157 in enrichment cultures of naturally-contaminated raw beef, lamb and mixed meat products

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Abstract
P.A. CHAPMAN, M. ELLIN AND R. ASHTON. 2001. Aims: The aims of this study were (i) to evaluate the specificity and sensitivity of three previously described PCR assays for the detection of E. coli O157 and, (ii) to compare PCR, culture, and two visual immunoassays (VIAs), BioSignTM and Path-StikTM, for detecting E. coli O157 after enrichment culture and immunomagnetic separation (IMS) performed on various naturally contaminated raw beef, lamb and mixed meat products. Methods and Results: Twelve sorbitol non fermenting (SNF) verocytotoxin-producing (VT+) E. coli O157, 6 SNF VT- E. coli O157, 4 sorbitol fermenting (SF) VT+ E. coli O157, 3 SF VT- E. coli O157, 23 E. coli belonging to 17 other serogroups and 12 organisms of other species were used to check the specificity of PCR reactions. Only one primer pair generated amplimers only with E. coli O157 and was used for all subsequent work. After enrichment culture and on inoculated minced beef samples, PCR was as sensitive as culture for detecting 9 of the 12 strains of E. coli O157, but up to 4 log10 more sensitive than culture for detecting three strains. Of the 120 samples of naturally contaminated meat products examined, 80 (67%) were positive by PCR, 70 (58%) were positive by BioSignTM, 69 (58%) were positive by culture and 67 (56%) were positive by Path-StikTM. Eleven samples were positive by PCR and both VIAs, but negative by culture because culture plates were heavily overgrown with SF organisms making detection of any E. coli O157 present impossible. Conclusions: PCR and both VIAs compared well with culture of beads to CT-SMAC for detecting E. coli O157 after enrichment culture and IMS. PCR appeared to be the most sensitive method, but needed specialised equipment and was also the most expensive, laborious and technically demanding technique. Although lacking the sensitivity of PCR, the VIAs were of comparable sensitivity to culture and were extremely quick and easy to perform giving a result in less than 15 minutes. Significance and Impact of the Study: Culture techniques may fail to detect E. coli O157 retrieved by IMS due to overgrowth with other organisms.

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