Purpose: The purpose of this study was to evaluate a new culture media, chromID EHEC (c-EHEC), for the detection of the USDA top 7 STEC.
Methods: Isolates of each of the 7 STEC were tested using pure cultures and inoculated beef trim. For pure cultures, organisms were incubated according to MLG 5B.03. Two cultures from each serogroup were streaked on c-EHEC and MLG described media. The remaining cultures were diluted, concentrated and plated to the same media. For the matrix study, 25g of beef trim was inoculated then enriched per MLG 5B.03. The enrichments were concentrated and plated to c-EHEC media and either mRBA or RBA media per MLG methods. After incubation, suspect colonies were tested with O specific latex. Presumptive positive colonies were tested by PCR or H7 latex after incubation on blood agar.
Results: For pure cultures, one isolate taken through the IMS process and one organism streaked directly to plates did not grow on either media because of mTSB + novobiocin sensitivity. All other organisms grew well on both media. For the matrix study, neither media was able to grow all target organisms. The target organisms had less color variance on the c-EHEC media compared to the mRBA or RBA plates. When a colony from c-EHEC media was latex-positive, it was also PCR-positive more often than when colonies from RBA media were latex-positive.
Significance: The c-EHEC media worked as well as the mRBA or RBA media for the detection of the 7 STEC from pure culture and from inoculated beef trim and provides a potential alternative to augment or replace the mRBA media described in the MLG 5B.03 method.