The optimization of isolation media used in immunomagnetic separation methods for the detection of Escherichia coli O157 in foods

Aims: To compare media used in immunomagnetic separation (IMS) techniques for the isolation of Escherichia coli O157 from food.

Methods and Results: Foods, both naturally contaminated and spiked, with low numbers (< 1 g(-1)) of stressed E. coli O157 were enriched in media based on buffered peptone water (BPW), tryptone soya and EC broths incubated at 30, 37, 40 and 42<degrees>C. Following immunomagnetic separation, beads were plated on a range of selective agars.

Conclusions: BPW supplemented with vancomycin (8 mg l(-1)) incubated at 42 degreesC, followed by IMS and subsequent plating of immunobeads onto cefixime tellurite sorbitol MacConkey agar plus either Rainbow(TM) or CHROMagar(TM) agars, proved optimum for the recovery of spiked, stressed E. coli O157 in minced beef, cheese, apple juice and pepperoni. The same protocol was optimum for recovery from naturally-contaminated minced beef and cheese.

Significance and Impact of the Study: The optimum protocol would increase isolation rates of E. coli O157 from foods.