Optimization of enrichment and plating procedures for the recovery of Escherichia coli O111 and O26 from minced beef

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Abstract
Aim: Optimization of enrichment media and selective agars for the detection of Escherichia coli O26 and O111 from minced beef. Methods and Results: This study compared a number of different enrichment conditions and plating media for the recovery of E. coli O26 and E. coli O111 from minced beef. The optimum enrichment conditions for E. coli O26 was observed in beef samples enriched at 41·5°C in tryptone soya broth supplemented with cefixime (50 μg l−1), vancomycin (40 mg l−1) and potassium tellurite (2·5 mg l−1). Similar enrichment conditions were optimal for E. coli O111 with the omission of potassium tellurite. The optimum agar for recovery of E. coli O26 and giving the most effective suppression of contaminants was MacConkey agar [lactose replaced by rhamnose (20 g l−1)] and supplemented with cefixime (50 μg ml−1) and potassium tellurite (2·5 mg l−1). Optimum recovery of E. coli O111 was on chromocult agar, supplemented with cefixime (50 μg ml−1), cefsulodin (5 mg l−1) and vancomycin (8 mg l−1). Minced beef samples were inoculated with a number of strains of E. coli O26 (n = 9) and O111 (n = 8), and the developed enrichment and plating methods, used in combination with immunomagnetic separation, were shown to be an effective method for the recovery of all strains. Conclusions: Routine cultural methods for the recovery of E. coli O26 and O111 from minced beef are described. Significance and Impact of the Study: The optimized enrichment and plating procedure described for the recovery of E. coli O111 and O26 from meat can be used to extend research on these emerging pathogens in beef.