A comparison was made of four procedures to detect Listeria spp. in two food categories. The study comprised 309 assays, 71 on milk from both infected and uninfected cows, and 238 on ten types of fresh vegetables. A sample was considered positive if it could be detected by at least a single method and if isolates could be confirmed as Listeria spp. The procedures detected 98–100% of the positive milk samples. Recovery from vegetable samples ranged from 45 to 86%, probably because of low levels of Listeria spp. in the presence of mixed flora. The ELISA procedure of the Organon Teknika® corporation detected 68% of the 44 positive vegetable samples; the GENE-TRAK®DNA probe, 45%; the U.S. Food and Drug Administration (FDA) culture procedure, 75%; and the FDA probe procedure, 86%. Recovery was higher with LiCl-phenylethanol-moxalactam agar (FDA probe procedure) than with modified McBride Agar (FDA culture procedure).
1Center for Microbiological Investigations. Minneapolis. MN
2U.S. Department of Agriculture. Ames. IA.