Epidemiology and Infection

Research Article

Immunomagnetic separation as a sensitive method for isolating Escherichia coli O157 from food samples

D. J. Wrighta1, P. A. Chapmana1 c1 and C. A. Siddonsa1

a1 Public Health Laboratory, Herries Road, Sheffield S5 7BQ


Minced beef samples inoculated with Escherichia coli O157 were cultured in buffered peptone water supplemented with vancomycin, cefsulodin and cefixime (BPW-VCC) and subcultured to cefixime tellurite sorbitol MacConkey (CT-SMAC) agar both directly and after immunomagnetic separation (IMS) of the organism with magnetic beads coated with an antibody against E. coli O157 (Dynabeads anti-E. coli 0157, Dynal, Oslo). E. coli O157 was recovered from initial inocula of 200 organisms/g by direct subculture and 2 organisms/g by IMS. Twelve strains of E. coli 0157 of different combinations of phage type, H antigen and toxin genotype were all recovered from initial inocula of two organisms/g by IMS. Nonspecific binding of other organisms to the magnetic beads could be reduced by washing of the beads in PBS with Tween-20 0·002–0.005% E. coli O157 was not bound by magnetic coated with an unrelated antibody.

During investigation of a dairy herd that was possibly linked to a small outbreak of infection with E. coli O157, the organism was isolated from 2 of 279 forestream milk samples from individual cattle; both isolates were made only by the IMS technique. IMS is rapid, technically simple, and a specific method for isolation of E. coli O157 and will be useful in epidemiological studies.

(Accepted March 21 1994)


c1 Author for correspondence.