Development of a method for the detection of verotoxin-producing Escherichia coli in food

The growing recognition of the role of non-O157 verotoxigenic Escherichia coli (VTEC) in foodborne illness underscores the importance of developing methods to detect it in the food supply. We describe here the development of a protocol for the detection, isolation, and characterization of VTEC from foods, designed for the serotype-independent enrichment, detection, and isolation of VTEC, in combination with rapid characterization of VTEC O157, O26, O103, O111, and O145. This study examined the inhibitory concentration of six antimicrobial agents used either singly or in combination for the optimal enrichment of a panel of 18 different O serogroups of VTEC in modified tryptic soy broth. Considerable variability in resistance to the different antimicrobials tested was noted among different VTEC strains. The combination enabling growth of strains of all 18 different O serogroups was vancomycin (10 μg/ml) and cefsulodin (3 μg/ml). A similar combination of antimicrobials formulated in agar plates was found beneficial in the recovery of VTEC strains from enrichment broth cultures. The efficacy of these media in the recovery of selected VTEC (O26, O103, O111, O145, and O157) from ground beef and O157 VTEC from lettuce, spinach, and apple cider was demonstrated. The selective enrichment media described herein would appear suitable for incorporation in methods for the recovery and detection of a wide range of VTEC serogroups.