The detection of non-O157 E. coli in food by immunomagnetic separation

Aims: To compare immunomagnetic separation (IMS) protocols (enrichment media and temperature) for the isolation of Escherichia coli serotypes O26 and O111 from four different foods.

Methods and Results: Foods (minced beef, cheese, apple juice and pepperoni) spiked with low numbers (<100 g(-1)) of stressed nalidixic mutant E. coli serotypes O26 and O111 were enriched in media based on buffered peptone water (BPW), tryptone soya and EC broths incubated at temperatures of 37 and 42degreesC to optimize the IMS technique. BPW enrichments gave increased recoveries of both serotypes compared with tryptone soya and EC broths. Elevated temperatures of incubation at 42degreesC were superior to 37degreesC.

Conclusions: Positive detection of low numbers of stressed target pathogens in all replicate tests was only possible using BPW enrichments. The majority of tests from alternative enrichments resulted in zero or single colonies recovered post-IMS.

Significance and Impact of the Study: The optimum IMS protocol would improve isolation rates of E. coli O26 and O111 from foods and lead to increased safety for the consumer. Sub-optimal IMS protocols could lead to foods being incorrectly labelled free from these pathogens.