Evaluation of a Selective Enrichment Most Probable Number Enumeration Method for Viable Listeria spp. in Dairy Products

Abstract

A most probable number (MPN) method for enumerating low numbers of Listeria 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 broth (1 g/10 ml). Homogenates were inoculated with L. monocytogenes Lm82, 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, and 0.001g of food sample per 10 ml. Dilution aliquots (10 ml) in triplicate or quintuplicate were incubated at 30°C for 48 h before being subcultured on Oxford agar at 35°C. Esculin-hydrolyzing colonies on Oxford agar were confirmed as the inoculum strain by their ability to grow on Trypticase soy agar containing streptomycin. Differences between inoculum and MPN values were evaluated by using tabulated 95% confidence limits. The calculated MPNs agreed with the inoculum levels in 91% (58 of 64) of noncheese dairy foods and in 49% (56 of 112) of 15 varieties of ripened cheeses. Competitive microflora affected by cheese age and the kind of milk used may account for the suboptimal performance of the MPN method with the cheeses.