Most probable numbers of listeria species in raw meats detected by selective motility enrichment

Abstract

We evaluated the methods of direct plate count (DPC) on Modified Oxford agar and most probable number (MPN) using Fung-Yu tubes for the detection of natural contamination by Listeria spp. by examining 100 retail samples of raw meat (beef, lamb, pork, and turkey). Listeria monocytogenes was isolated from 10 samples. Other Listeria spp. (L. innocua, L. murrayi, and L. welshimeri) were isolated from 27 samples. Despite the fact that very low numbers of Listeria were found on these products, the MPN method was significantly more sensitive than the DPC method. The isolation efficiencies of the MPN and DPC methods were 31 and 17%, respectively. Quantitation of indigenous Listeria showed that many meat samples contaminated with ≤ 4 colony-forming units (CFU) per g tested negative by the DPC method but positive by the MPN method. Copyright © International Association of Milk, Food and Environmental Sanitarians.