Immunomagnetic separation as a sensitive method for isolatingEscherichia coliO157 from food samples

Abstract

SUMMARYMinced beef samples inoculated withEscherichia coliO157 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 againstE. coliO157 (Dynabeads anti-E. coli0157, Dynal, Oslo).E. coliO157 was recovered from initial inocula of 200 organisms/g by direct subculture and 2 organisms/g by IMS. Twelve strains ofE. coli0157 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. coliO157 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 withE. coliO157, 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 ofE. coliO157 and will be useful in epidemiological studies.