Purpose: To compare different culture media for enriching the “top 6” non-O157 STEC serotypes.
Methods: Two to fourteen CFU of each serotype (O26, O45, O103, O111, O121 and O145) were spiked into 375 g of beef trim, stored overnight at 4°C, and then enriched with five different media: IEH medium (A), LES medium (B), mTSB (C), mTSB+novobiocin (D), and mTSB+vancomycin (E). Sample aliquots were taken after 9, 12 and 20 h incubation at 42°C, and analyzed using an in-house, non-O157 STEC detection system that included multiplex PCR and lateral flow immunoassay (LFI).
Results: As early as after 9 h of incubation in Media A, B, C and E, most of the non-O157 serotypes were detected by multiplex PCR. All six non-O157 serotypes were detected in beef trim by using both multiplex PCR and LFI after the 12 h incubation. After enrichment in Medium D (the USDA-FSIS’ medium formulation for STEC containing novobiocin as selective inhibitor) produced positive PCR signals only after 12 and 20 h of incubation, and completely inhibited the growth of the O111 serotype. Growth of the O111 serotype in mTSB supplemented with different concentrations of novobiocin (mTSB) showed that as little as 5 ug/ml delayed the growth of the O111 serotype.
Significance: With the exception of the novobiocin-supplemented medium, the culture media used in this study effectively allowed enrichment of the non-O157 STEC strains artificially inoculated in beef trim and therefore could be used as an alternative to the most commonly used enrichment medium for the six non-O157 STEC serotypes.