Purpose: The purpose of this study was to compare among three commercial agar media, chromID® EHEC agar, EZ-CHROM STEC agar and Rainbow® agar, in their ability to effectively differentiate among colonies of these seven STEC strains.
Methods: Pure overnight culture of each STEC serotype was streaked onto chromID® EHEC agar, EZ-CHROM STEC agar and Rainbow® agar. Combinations of each strain, up to a multiple of six strains, were also streaked and colony differentiation was noted on each agar medium. Final confirmation of each well isolated colony of each serotype was performed using serotype-specific primers and real-time PCR.
Results: EZ-CHROM STEC generated similarly colored colonies for most serotypes which were hard to differentiate and, hence, was discontinued from the study. chromID® EHEC agar was able to clearly differentiate a cocktail of up to five serotypes, while the color differentiation obtained for the five serotype mixture was not distinguishable on Rainbow® agar. PCR results for the colonies picked from mixed culture chromID® EHEC agar plates were in accordance with the morphological characteristics of each serotype, except for E. coli O145 and E. coli O45.
Significance: Out of the three chromogenic media tested, chromID® EHEC agar produced the most distinctive and easily distinguishable colored STEC colonies and was the most effective for the screening and identification of the STEC serotypes in this study.