Evaluation of a New Chromogenic Agar Medium for Detection of Shiga Toxin-Producing Escherichia coli (STEC) and Relative

This study assesses the detection performance of CHROMagar STECmedium relative to a reference cytotoxin assay and de-scribes the current relative prevalence of O157 and non-O157 Shiga toxin-producing Escherichia coli (STEC) serotypes within the province of Manitoba, Canada. Over a 10-month period, 205 nonfrozen routine stool submissions to Cadham Provincial Laboratory (CPL) were used to assess the performance of CHROMagar STEC. Of the 205 stools, 14 were identified as true posi-tives by a cytotoxin assay, with resultant CHROMagar STEC sensitivity, specificity, and positive predictive and negative predic-tive values of 85.7%, 95.8%, 60.0%, and 98.9%, respectively. Using a separate panel of 111 STEC strains, CHROMagar STEC was shown to support the growth of 96 (86.5%) isolates. To assess relative prevalence, attempts were made to isolate by any means all STEC strains identified at CPL over a 17-month period. Of 49 isolates (representing 86.0 % of all STEC infections detected), only 28.6%were O157 STEC strains. Of the 35 non-O157 STEC strains, 29 were subjected to further molecular analysis. In contrast to earlier results from our area, carriage of stx2 appears to have increased. Overall, although CHROMagar STEC is not recom-mended as a primary screen, our results indicate that it is an effective supplemental medium for the isolation of probable STEC strains. Increased isolation of these serotypes is warranted to better understand their prevalence, clinical characteristics, and epidemiology and aid in the development or enhancement of food safety control programs targeting all STEC serotypes. Shiga toxin-producing Escherichia coli (STEC) emerged inNorth America in 1983 as a ground beef-associated outbreak of serotype O157:H7. The clear link between this serotype an