Evaluation of ChromID® EHEC Agar for Detection of Seven Major Serogroups of Shiga Toxin-producing Escherichia coli from Cattle Feces

Introduction: Seven serogroups (O26, O45, O103, O111, O121, O145, and O157) of Shiga toxin-producing Escherichia coli (STEC) account for the majority of foodborne STEC infections.  Cattle are asymptomatic reservoirs for STEC.  Typically, the organisms colonize the hindgut and are shed in the feces, which serves as a source of contamination of food products.  Culture-based detection and isolation of STEC includes immunomagnetic separation followed by plating on a variety of chromogenic media, some of which are available commercially.  

Purpose: We evaluated the performance of two chromogenic media, a chromID^® (bioMeriéuex SA, Marcy-l'Etoile, France) and a MacConkey agar-based medium with sucrose, sorbose, novobiocin and tellurite (modified Possé medium; MP) for the detection of seven STEC in cattle feces. 

Methods: Fecal samples (n = 6) from beef cattle, spiked with seven serogroups, and pen-floor fecal samples (n = 144) from a feedlot were used in the evaluation.  Feces were enriched (6h at 40ºC) in Escherichia coli broth and subjected to immunomagnetic separation for each target serogroup.  Bead suspensions were spread-plated onto sorbitol MacConkey with cefixime and tellurite (CT-SMAC; for O157), MP (for six non-O157), and chromID^® (for seven STEC) media.  Chromogenic colonies from MP and chromID^® agar and sorbitol-negative colonies from CT-SMAC were tested by multiplex PCR to confirm target serogroups and major virulence genes.

Results: Based on the visual observation of the number of colonies, chromID^® appeared to have less crowded and more distinct chromogenic colonies compared to MP medium.  A higher percentage of the O45 serogroup was detected from fecal samples with chromID^® compared to MP.  Additionally, E. coli O157 was isolated from 69.4% of fecal samples (144) with chromID^® compared to 54.2% with CT-SMAC.  

Significance: Data suggested that chromID^® were more selective and better for detecting chromogenic STEC, particularly O157 and O45, from cattle feces compared to MP or CT-SMAC medium.