Evaluation of the USDA Protocol for Detection of Listeria monocytogenes

This study compared the recovery of Listeria monocytogenes from Fraser broth incubated 26 h versus 48 h. L. monocytogenes was isolated from 60 of 1,088 samples analyzed. Samples included meat products and environmental swabs from meat and dairy plants. Recovery of L. monocytogenes occurred in two samples after 48 h incubation but not at 26 h, resulting in 3.3% false negatives at 26 h. L. monocytogenes was detected in two samples at 26 h incubation in which the Fraser broth did not blacken. A 6.7% incidence of false negatives resulted if one considers only blackened Fraser broth. Additionally, 659 of the primary enrichments were streaked directly onto selective media. This procedure recovered L. monocytogenes in 76.7% of the total number of samples positive; failure to recover L. monocytogenes occurred in eight Fraser broth enrichments from positive primary enrichments.