An ELISA test and two routine culture procedures have been compared in their ability to detect Listeria spp. in food products and swabs of pig tonsils. The culture procedures used were those recommended by the Food and Drug Administration (FDA) and United States Department of Agriculture (USDA). One hundred samples of minced beef, artificially inoculated with Listeria monocytogenes, together with 149 naturally contaminated samples of minced beef, pig tonsil, pig feed and soft, whitemolded, blue-veined cheese were tested by the three methods.
The USDA procedure proved to be the most sensitive detection method when samples of artificially contaminated meat containing less than 3 colony forming units (CFU) of L. monocytogenes per gram were examined. In samples where the L. monocytogenes count was above 3 CFU per gram, the ELISA test and the USDA-detection procedure proved to be equally sensitive. When naturally contaminated samples were examined, the sensitivity of the ELISA test was 92% and the specificity 80%. The detection limit of the ELISA test for demonstration of L. monocytogenes in pure cultures was found to be approximately 106 CFU per ml. The corresponding detection limit of the culture procedure was calculated to be approximately 104 CFU per ml.