Evaluation of BBL CHROMagar Listeria agar for the isolation and identification of Listeria monocytogenes from food and environmental samples

The performance of BBL CHROMagar Listeria chromogenic agar for the detection of Listeria monocytogenes was evaluated for its ability to isolate and identify L. monocytogenes from food and environmental samples. The medium was compared to non-chromogenic selective agars commonly used for Listeria isolation: Oxford, Modified Oxford, and PALCAM. BBL CHROMagar Listeria had a sensitivity of 99% and 100% for the detection of L. monocytogenes from 200 natural and artificially inoculated food samples, respectively, with a colony confirmation rate of 100%. The sensitivity of non-chromogenic selective media for the detection of L. monocytogenes from these same samples was 97-99% with colony confirmation rates of 65-67.5%. From 93 environmental samples, BBL CHROMagar Listeria agar results correlated 100% with a Listeria spp. visual immunoassay (TECRA) performed on these same samples and the USDA-FSIS standard culture method for the isolation of L. monocytogenes. From environmental samples, the L. monocytogenes confirmation rate was 100% for BBL CHROMagar Listeria as compared to 50% for conventional agars tested. On BBL CHROMagar Listeria, L. monocytogenes forms a translucent white precipitation zone (halo) surrounding blue-pigmented colonies of 2-3 mm in diameter, with an entire border. BBL CHROMagar Listeria offers a high degree of specificity for the confirmation of suspect L. monocytogenes colonies, whereas non-chromogenic selective agars evaluated were not differential for L. monocytogenes from other Listeria species.