EVALUATION OF METHODS FOR DETECTION OF LISTERIA-MONOCYTOGENES IN FOODS - NMKL COLLABORATIVE STUDY

Eleven Nordic laboratories participated in a collaborative study on qualitative methods for the detection of Listeria monocytogenes in foods. Two enrichment methods (U.S. Department of Agriculture, Food Safety and Inspection Service method for meat, and modified U.S. Food and Drug Administration/International Dairy Federation method for dairy products) and 2 agar media (lithium chloride-phenylethanol-moxalactam (LPM) and Oxford agar) were compared. The effect of treatment of the enriched culture with KOH before plating was also studied. Food samples, blue and white mold cheese and canned corned beef, were inoculated with 3 levels of L. monocytogenes. Beef was also inoculated with a microbial flora derived from raw meat. The recoveries of L. monocytogenes using the 2 enrichment methods did not differ significantly. The Oxford agar was superior to the LPM agar. KOH treatment of the enriched culture did not significantly increase the detection rate of the organism. For cheese, the detection level of the methods was below 0.2 cfu L. monocytogenes/g, and for meat, the detection level was below 5 cfu L. monocytogenes/g.