Isolation of Listeria monocytogenes from Food Products on Four Selective Plating Media

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Abstract
The suitability of four selective media for isolation of Listeria monocytogenes strains has been evaluated. Samples of cheese, processed meat, fresh vegetables, and raw milk were inoculated with low numbers of L. monocytogenes cells. Inoculated samples were enriched at 30°C and plated on selective media for isolation. Modified McBride Agar was found to be substantially inferior in comparison with other media. Acriflavine-Ceftazidime Agar was relatively more efficient but was not satisfactory for all classes of foods and all strains of L. monocytogenes. With L. monocytogenes serotype 4b and strain 286, Lithium Chloride-Phenylethanol-Moxalactam Agar (LPM) and Oxford Agar (OA) were approximately equally effective for isolation from all classes of foods. However, LPM performed poorly with L. monocytogenes serotype 3a. OA medium was consistently superior and gave higher recovery with all strains studied. Incubation of enrichment broth culture for just one day was not sufficient and 2 d of incubation was necessary to achieve a satisfactory level of performance.