Purpose: The objective of this study was to compare the sensitivity, specificity, and accuracy of CHROMagar™ STEC (CS) and R&F®Non-O157 STEC chromogenic plating medium (RF) for detection of STEC using pure cultures.
Methods: A total of 79 STEC (divided among O26, O45, O103, O104, O111, O121, O145, and O157) and 38 non-STEC strains were included in the study. Strains were grown in trypticase soy broth, then streaked onto agar media using a semi-quantitative loop technique. After 22h incubation, plates were evaluated for growth (scored from 0-4), and colony phenotype and compared to the expected results as provided by the manufacturer. Growth on these media was also compared to that on trypticase soy agar (TSA).
Results: The sensitivity of RF (95.0%; 95% confidence interval [CI], 89.9 to 98.0%) was significantly greater than that of CS (82.4%; CI, 75.9 to 77.7%; P < 0.05), whereas the specificities of CS (80.7%; CI, 70.9 to 88.8%) and RF (76.8%; CI, 66.2 to 85.4%) were not significantly different. The accuracy of RF (88.2%; CI, 83.2 to 92.2%) was marginally higher than that of CS (81.8%; CI, 76.6 to 86.3%; P = 0.06). Growth of STEC on RF (3.26) was significantly greater (P < 0.05) than that on CS (2.60), as was growth of non-STEC controls (2.05 versus 0.81). Growth of non-STEC controls (3.49) on TSA was significantly higher than on the other media.
Significance: R&F® Non-O157 STEC chromogenic plating medium yielded the best combination of sensitivity, specificity and accuracy for detection of non-O157 STEC. However, further studies are needed to assess performance in different food and environmental matrices, as these could significantly influence results.