CHROMagar™ STEC base and supplement

The isolation of non-O157 STEC from food samples has proved to be challenging. The selection of a suitable selective isolation agar remains problematic. The purpose of this study was to qualitatively and quantitatively evaluate six chromogenic agar media for the isolation of STEC: Tryptone Bile X-glucuronide agar (TBX), Rainbow® Agar O157 (RB), Rapid E. coli O157:H7 (RE), Modified MacConkey Agar (mMac), CHROMagar™ STEC (Chr ST) and chromID™ EHEC (Chr ID). During this study, 45 E. coli strains were used, including 39 STEC strains belonging to 16 different O serogroups and 6 non-STEC E. coli. All E. coli strains were able to grow on TBX and RB, whereas one STEC strain was unable to grow on Chr ID and a number of other STEC strains did not grow on mMac, CHROMagar™ STEC and Rapid E. coli O157:H7. However, only the latter three agars were selective enough to completely inhibit the growth of the non-STEC E. coli. Our conclusion was that paired use of a more selective agar such as CHROMagar™ STEC together with a less selective agar like TBX or Chr ID might be the best solution for isolating non-O157 STEC from food.

The performance and usability of CHROMagar STEC™ medium (CHROMagar Microbiology, Paris, France) for routine detection of Shiga toxin-producing Escherichia coli (STEC) strains were examined. The ability of the medium to selectively propagate STEC strains differing by their serotypes and virulence genes was studied with a collection of diarrheagenic E. coli isolates (n=365) consisting of 49 different serotypes and with non-STEC and other bacterial isolates (n=264). A total of 272 diarrheagenic E. coli (75.0%) isolates covering 24 different serotypes grew on CHROMagar™ STEC. The highest detection sensitivities were observed within the STEC serogroups O26 (90.0%), O111 (100.0%), O121 (100.0%), O145 (100.0%), and O157 (84.9%), and growth on CHROMagar™ STEC was highly associated with the presence of the tellurite resistance gene (terD). The specificity of the medium was 98.9%. In addition, CHROMagar™ STEC was used in parallel with a Shiga toxin-detecting immunoassay (Ridaquick Verotoxin/O157 Combi; R-biopharm, Darmstadt, Germany) to screen fecal specimens (n=47) collected from patients suffering from hemorrhagic diarrhea. Positive growth on CHROMagar™ STEC was confirmed by the Premier EHEC enzyme immunoassay (Meridian Bioscience, Inc., Cincinnati, OH), and discrepant results between the two screening methods were confirmed by stx gene-detecting PCR. All 16 of the 47 stool samples that showed positive growth on CHROMagar™ STEC were also positive in the confirmatory tests. CHROMagar™ STEC proved to be an interesting option for STEC screening, allowing good detection sensitivity and specificity and permitting strain isolation for further outbreak investigations when required.