The prevalence of verocytotoxin-producing Escherichia coli (VTEC) and E-coli O157 : H7 in beef in Sweden determined by PCR assays and an immuno-magnetic separation (IMS) method

The number of cases of enterohaemorrhagic Escherichia coli (EHEC) in Sweden has increased dramatically since 1995 but no source of the infections has been identified. The prevalence of verocytotoxin-producing E. coli (VTEC), including E, coli O157:H7 in imported and domestic raw beef in Sweden, was determined by PCR assays and immune-magnetic separation (IMS, Dynal). VTEC was detected by a multiplex PCR directed at sequences on the vt1 and vt2 ge nes, and the prevalence of E. coli O157:H7 was estimated by the IMS assay a nd a SZ-PCR which detects E. coli O157:H7 land some strains of O157:NM, O55 :H7 and O55:NM). VTEC was detected in 15.5% (57/368) and 1.1% (6/543) of th e imported and domestic beef samples, respectively, by the multiplex PCR. E . coli O157:H7 was detected in 0.3% and 2.4% of the imported beef samples b y the IMs and SZ-PCR assays, respectively. in domestic beef E. coli O157:H7 was not detected by either method, indicating that the occurrence in Swedi sh beef was less than 0.6% with a 95% certainty. The overall prevalence of VTEC was estimated to be 4.0%, by adjusting for the relative proportion of imported and domestic beef in Sweden and, similarly, the overall prevalence of E. coli O157:H7 was 0.06% (IMS) or 0.5% (SZ-PCR). We propose that a use ful strategy to analyse a large number of food samples for VTEC and E. coli O157:H7 is an initial screening of samples using the multiplex VT-PCR in c onjunction with enrichment and buoyant density centrifugation, followed by further analyses on the fraction of VT-positive samples using either the IM S method or SZ-PCR. (C) 1998 Academic Press.