A comparison of immunomagnetic separation and direct culture for the isolation of verocytotoxin-producing Escherichia coli O157 from cases of bloody diarrhoea, non-bloody diarrhoea and asymptomatic contacts.

Enrichment culture in modified buffered peptone water followed by immunomagnetic separation (IMS) with magnetic beads coated with an antibody against Escherichia coli O157 was compared with direct culture on cefixime rhamnose sorbitol MacConkey agar (CR-SMAC) and cefixime tellurite sorbitol MacConkey agar (CT-SMAC) for the isolation of E. coli O157 from human faeces. In total, 690 samples were examined; E. coli O157 was isolated from 25 samples by IMS but from only 15 and 12 by direct culture on CT-SMAC and CR-SMAC, respectively. The difference in sensitivity of detection was at its most marked on screening repeat faecal samples from known cases and samples from asymptomatic contacts, when of 12 strains of E. coli O157 isolated by IMS, only five were isolated by direct culture. IMS is a sensitive and simple technique for the isolation of E. coli O157 from human faecal samples and should prove useful in elucidating further the epidemiology of this micro-organism.