P1-28 Comparison of Different Enrichment Media for Non-O157 Shiga Toxin-producing Escherichia coli Strains in Beef Trim

Monday, July 23, 2012
Exhibit Hall (Rhode Island Convention Center)
Jason Cantera, IEH Laboratories and Consulting Group, Lake Forest Park, WA
Ruth Cantera, IEH Laboratories and Consulting Group, Lake Forest Park, WA
Cesar Nadala, IEH Laboratories and Consulting Group, Lake Forest Park, WA
Mansour Samadpour, LifeForce Foods, Lake Forest Park, WA
Introduction:  Shiga toxin-producing Escherichia coli (STEC) strains other than serotype O157 represent some of the most important causes of foodborne infections worldwide. Detection methods for STEC in foods include culture enrichments followed by molecular, immunological and cultural characterization. 

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.