Detection of Shiga toxin-producing Escherichia coli (STEC) in ground beef and bean sprouts: Evaluation of culture enrichment conditions

The main purpose of this work was to evaluate culture enrichment conditions, with particular regard to those reported in ISO/TS 13136:2012, for STEC detection in food. The culture media evaluated included mTSB with novobiocin 0-16 mg/l (mTSB + N0-16) or acriflavin 12 mg/l (mTSB + A(12)); BPW; mBPWp with acriflavin 10 mg/l, cefsulodin 10 mg/l, vancomycin 8 mg/l (mBPWp + ACV); and mBPWp with cefsulodin 10 mg/l, vancomycin 8 mg/l (mBPWp + CV). They were used for the growth of STEC 0157, 026, 0103, 0111, 0145 and 0104 in pure cultures or in artificially contaminated food matrices (ground beef, mung bean sprouts). STEC detection was accomplished using commercially available multiplex real-time PCR assays targeting stxl-stx2 and eae, and serogroup-associated genes. More rapid multiplication of STEC in pure cultures occurred in mBPWp + CV, while an inhibitory effect of novobiocin and acriflavin was observed for some STEC serogroups in media with these selective agents. mBPWp + CV allowed the detection of all serogroups in bean sprouts when inoculated at levels as low as 1 CFU/25 g. A reduced novobiocin concentration of 2 mg/l in mTSB was required for STEC detection in ground beef samples. A temperature of 42 degrees C for the entire duration of the enrichment or 44 degrees C after an initial phase of 6 h at 37 degrees C was important to limit the multiplication of non-target bacteria. Results of this study suggest that media and protocols should be adapted to the food being analyzed, since protocols provided in official reference methods may produce insufficient sensitivity.