EVALUATION OF A SELECTIVE ENRICHMENT MOST PROBABLE NUMBER ENUMERATIONMETHOD FOR VIABLE LISTERIA SPP IN DAIRY-PRODUCTS

A most probable number (MPN) method for enumerating low numbers of Lis teria spp. in dairy foods was developed by adapting the U.S. Food and Drug Administration (FDA) Listeria isolation methodology. Milk, cheese , and other milk products were diluted and homogenized in enrichment b roth (1 g/10 mi). Homogenates were inoculated with L. monocytogenes Lm 82, a streptomycin-resistant variant of strain Scott A, at < 1 to 320 CFU/g and further diluted in FDA enrichment broth to give 0.1, 0.01, a nd 0.001 g of food sample per 10 mi. Dilution aliquots (10 mi) in trip licate or quintuplicate were incubated at 30 degrees C for 48 h before being subcultured on Oxford agar at 35 degrees C. Esculin-hydrolyzing colonies on Oxford agar were confirmed as the inoculum strain by thei r ability to grow on Trypticase soy agar containing streptomycin. Diff erences between inoculum and MPN values were evaluated by using tabula ted 95% confidence limits. The calculated MPNs agreed with the inoculu m levels in 91% (58 of 64) of noncheese dairy foods and in 49% (56 of 112) of 15 varieties of ripened cheeses. Competitive microflora affect ed by cheese age and the kind of milk used may account for the subopti mal performance of the MPN method with the cheeses.