DETECTION OF LISTERIA-MONOCYTOGENES IN SAMPLES CONTAINING LISTERIA-INNOCUA

A common culture procedure for the detection of Listeria monocytogenes in meats and environmental samples was evaluated in a multilaboratory study using samples inoculated with both Listeria monocytogenes and L isteria innocua. Listeria monocytogenes was recovered from 5.4% of bee f broth samples containing between 140 and 1400 L monocytogenes cells per 25-ml sample in the presence of twice as many L. innocua cells; wh ereas L. innocua was recovered from all of the samples. Listeria monoc ytogenes was isolated from 100% of the samples when L. innocua was abs ent. Similar results were obtained for swab samples containing both L. monocytogenes and L innocua. Listeria monocytogenes was recovered fro m 31% of the swabs containing 4.8 L. monocytogenes cells per swab and from 0% of the swabs containing the same number of L. monocytogenes ce lls and 4 L. innocua cells per swab. When the ratios of L. monocytogen es to L. innocua were 12:1, 120:1 and 1200:1, L. monocytogenes was iso lated from 38%, 92% and 85% of the swabs, respectively. The recovery r ates were consistent with the differences in growth rates for the two organisms in selective media. The generation times for L. monocytogene s were 74 min in primary enrichment broth and 105 min in modified seco ndary enrichment broth. Listeria innocua posted generation times of 53 and 81 min in the same two enrichment broths.