IMMUNOMAGNETIC SEPARATION AS A SENSITIVE METHOD FOR ISOLATING ESCHERICHIA-COLI O157 FROM FOOD SAMPLES

Minced beef samples inoculated with Escherichia coli O157 were culture d in buffered peptone water supplemented with vancomycin, cefsulodin a nd cefixime (BPW-VCC) and subcultured to cefixime tellurite sorbitol M acConkey (CT-SMAC) agar both directly and after immunomagnetic separat ion (IMS) of the organism with magnetic beads coated with an antibody against E. coli O157 (Dynabeads anti-E. coli O157, Dynal, Oslo). E. co li O157 was recovered from initial inocula of 200 organisms/g by direc t subculture and 2 organisms/g by IMS. Twelve strains of E. coli O157 of different combinations of phage type, H antigen and toxin genotype were all recovered from initial inocula of two organisms/g by IMS. Non specific binding of other organisms to the magnetic beads could be red uced by washing of the beads in PBS with Tween-20 0.002-0.005 % E. col i O157 was not bound by magnetic coated with an unrelated antibody. Du ring investigation of a dairy herd that was possibly linked to a small outbreak of infection with E. coli O157, the organism was isolated fr om 2 of 279 forestream milk samples from individual cattle; both isola tes were made only by the IMS technique. IMS is rapid, technically sim ple, and a specific method for isolation of E. coli O157 and will be u seful in epidemiological studies.