Performance Testing of Chromogenic Agar Media for Improved Detection in Milk and Other Food Samples.

In this study, the performance of four alternative selective chromogenic agar was compared to the reference mannitol-yolk polymyxin (MYP) agar (ISO 7932) using inclusion and exclusion test strains ( = 110) and by analyzing naturally contaminated milk and other food samples ( = 64). Subsequently, the group affiliation and toxin gene profile of () isolates were determined. Our results corroborate that the overall best performing media CHROMagar™ (93.6% inclusivity; 82.7% exclusivity) and BACARA (98.2% inclusivity, 62.7% exclusivity) are more sensitive and specific compared to Brilliance™ , MYP and ChromoSelect Agar. Both media allow unequivocal detection of with low risks of misidentification. Media containing ß-D-glucosidase for the detection of presumptive may form atypical colony morphologies resulting in a false negative evaluation of the sample. Naturally contaminated samples presented high numbers of background flora, while numbers of presumptive were below the detection limit (<10 CFU g or mL). Recovery after freezing resulted in the highest detection of . on BACARA (57.8%), CHROMagar™ (56.3%) and MYP agar (54.7%). The /toxin profile combination IV/A was the most abundant (33.0%), followed by III/F (21.7%) and VI/C (10.4%). More and toxin combinations were present in 15.6% of samples when reanalyzed after freezing. In order to improve detection and confirmation of in food samples, we recommend the parallel use of two complementary selective media followed by molecular characterization (e.g., typing combined with toxin gene profiling). When determining psychrotolerant or thermophilic members of the group, the selective agar media should additionally be incubated at appropriate temperatures (5 °C, ≥45 °C). If high-risk toxin genes (e.g., or ) are detected, the strain-specific ability to produce toxin should be examined to decisively assess risk.