Validation of a method for simultaneous isolation of Shiga toxin-producing Escherichia coli O26, O103, O111, and O145 from minced beef by an international ring-trial.

An isolation method described by Possé et al. (FEMS Microbiol Lett 2008;282:124-131) was satisfactorily validated in an international ring-trial using artificially contaminated minced beef samples. Until now, no validated method existed for the simultaneous isolation of Shiga toxin-producing Escherichia coli serogroups O26, O103, O111, and O145 in food. Twelve laboratories from five European countries participated and received 16 inoculated beef samples contaminated with cold-stressed cells of the four serogroups O26, O103, O111, and O145 in two levels (approximately 30 and 300 CFU 25 g⁻¹) in duplicate. In addition, they received four non-inoculated samples. The isolation protocol comprised a selective enrichment step, a selective isolation step on a non-O157 agar plate differentiating the serogroups by color, followed by confirmation by plating on confirmation agar media and agglutination. All laboratories were able to isolate the inoculated serogroups from the samples, both for the high and the low inoculation level. Results did not differ whether in-house-prepared or ready-to-use non-O157 agar plates were used, demonstrating that by following the instructions laboratories managed to perform the complete protocol with success.