The performances of three chromogenic agars were evaluated for the recovery of Escherichia coli O157:H7 from spiked de-chlorinated tap, ground and surface water, and treated drinking water samples. The chromogenic agars: ChromAgar O157 (CHROM), Rainbow Agar O157 (RB) and HiCrome EC O157 (HC) were compared to cefixime-tellurite Sorbitol MacConkey (CT-SMAC), commonly used for the isolation of E. coli O157:H7. Confirmation of suspect E. coli O157:H7 colonies were performed by colony real-time PCR (C-RTi-PCR) based on the presence of Shiga-toxin genes (stx1 and stx2). Recovery of inoculated E. coli O157:H7 from de-chlorinated tap water indicated that RB and CHROM agars demonstrated improved recovery when compared to HC or CT-SMAC. There was a significant drop in recovery on all agars tested after 120 hours (day 5). Twenty de-chlorinated tap and/or treated drinking water samples were inoculated with a pure culture of E. coli O157:H7 (ATCC 43894), and a mixed culture of E. coli O157:H7 (ATCC 43894), E. coli strain K-12, and Enterococcus faecalis (ATCC 063589). After a 48 hour holding time, the recovery using CHROM (99 %) and HC (12 %) from samples contaminated with the pure culture were found to be significantly different (p < 0.05). Recovery results using CHROM (39 %) and CT-SMAC (32 %) from samples contaminated with the mixed culture after 48 hour holding time were not significantly different (p > 0.05). Analysis by C-RTi-PCR of forty five environmental water samples (surface, sewage, and final effluents) which were negative for E. coli O157:H7 showed an incidence of false suspect positive colonies of 38 % (CHROM), 53 % (RB), 58 % (HC), and 91 % (CT-SMAC). Further analysis of eight of the environmental samples inoculated with E. coli (ATCC 43894) showed 100% recovery when utilizing CHROM, 50% when using RB and 40 % when using HC. In addition, the C-RTi-PCR positive confirmation rate was 100% for CHROM and HC and 65% for RB. CHROM demonstrated improved recovery of E. coli O157:H7 over RB, HC, and CT-SMAC in terms of sensitivity and specificity.
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