Evaluation of enrichment and plating media for the isolation of Listeria monocytogenes from raw milk and the state of contamination of raw milk by Listeria.

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Methods of isolation of Listeria monocytogenes (L. m.) from raw milk were investigated. Three enrichment media, enrichment broth (EB) of the FDA method, L-PALCAMY (L-PAL) of Van Netten et al., and Merck's Listeria enrichment broth (LEB); enrichment by the two-stage method (2-st) of Slade et al.; and 4 plating media, modified McBride Listeria agar (MMA), LiCl phenylethanol moxalactam agar (LPM), Oxford formulation (OX), and PALCAM Listeria selective agar (PAL) were combined, and recovery tests of L. m. were performed to evaluate each combination. The best enrichment of L. m. was achieved by L-PAL; when L. m. was present in raw milk at less than 102/ml, the number of L. m. increased to 107-108/ml after 24 hours of culture. Although there was no clear difference between the 4 plating media in terms of the number of L. m. grown in the media, OX and PAL were superior to MMA and LPM in terms of the number of bacteria other than L. m. grown in the media and the ease of observation of L. m. colonies. Raw milk produced in Saitama Prefecture and brought to milk processing factories in Saitama Prefecture was positive for Listeria in 5.3%(8/150) of the raw milk samples, and L. m. was identified in 4.0%(6/150), while L. innocua was identified in 1.3%(2/150) of the samples. The serotype of the isolated L. m. was 1a for all isolates.
公益社団法人日本獣医師会の論文