COMPARISON OF COLD ENRICHMENT AND UNITED-STATES-DEPARTMENT-OF-AGRICULTURE METHODS FOR ISOLATING LISTERIA-MONOCYTOGENES FROM NATURALLY CONTAMINATED FOODS

We compared the cold enrichment (CE) and U.S. Department of Agriculture (USDA) methods for isolating Listeria monocytogenes by examining 402 food samples. The food samples were collected from refrigerators of listeriosis patients as part of a multistate active surveillance project to determine the role of food in sporadic listeriosis in the United States. L. monocytogenes was isolated from 51 food samples (13%). The USDA method was significantly better (P < 0.001) than the CE method. The isolation efficiencies of the USDA and CE methods were 96 and 59%, respectively. Quantitation of L. monocytogenes in the food samples revealed that many food samples containing < 0.3 CFU/g were negative as determined by the CE method but positive as determined by the USDA method.