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 Listeria innocua . Listeria monocytogenes was recovered from 5.4% of beef broth samples containing between 140 and 1400 L. monocytogenes cells per 25-ml sample in the presence of twice as many L. innocua cells; whereas L. innocua was recovered from all of the samples. Listeria monocytogenes was isolated from 100% of the samples when L. innocua was absent. Similar results were obtained for swab samples containing both L. monocytogenes and L. innocua . Listeria monocytogenes was recovered from 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 cells and 4 L. innocua cells per swab. When the ratios of L. monocytogenes to L. innocua were 12:1, 120:1 and 1200:1, L. monocytogenes was isolated from 38%, 92% and 85% of the swabs, respectively. The recovery rates were consistent with the differences in growth rates for the two organisms in selective media. The generation times for L. monocytogenes were 74 min in primary enrichment broth and 105 min in modified secondary enrichment broth. Listeria innocua posted generation times of 53 and 81 min in the same two enrichment broths.