Evaluation of the USDA Protocol for Detection of Listeria monocytogenes

Estimation of microbial contamination of food from prevalence and concentration data: application to Listeria monocytogenes in fresh vegetables

A normal distribution and a mixture model of two normal distributions in a Bayesian approach using prevalence and concentration data were used to establish the distribution of contamination of the food-borne pathogenic bacteria Listeria monocytogenes in unprocessed and minimally processed fresh vegetables. A total of 165 prevalence studies, including 15 studies with concentration data, were taken from the scientific literature and from technical reports and used for statistical analysis. The predicted mean of the normal distribution of the logarithms of viable L. monocytogenes per gram of fresh vegetables was -2.63 log viable L. monocytogenes organisms/g, and its standard deviation was 1.48 log viable L. monocytogenes organisms/g. These values were determined by considering one contaminated sample in prevalence studies in which samples are in fact negative. This deliberate overestimation is necessary to complete calculations. With the mixture model, the predicted mean of the distribution of the logarithm of viable L. monocytogenes per gram of fresh vegetables was -3.38 log viable L. monocytogenes organisms/g and its standard deviation was 1.46 log viable L. monocytogenes organisms/g. The probabilities of fresh unprocessed and minimally processed vegetables being contaminated with concentrations higher than 1, 2, and 3 log viable L. monocytogenes organisms/g were 1.44, 0.63, and 0.17%, respectively. Introducing a sensitivity rate of 80 or 95% in the mixture model had a small effect on the estimation of the contamination. In contrast, introducing a low sensitivity rate (40%) resulted in marked differences, especially for high percentiles. There was a significantly lower estimation of contamination in the papers and reports of 2000 to 2005 than in those of 1988 to 1999 and a lower estimation of contamination of leafy salads than that of sprouts and other vegetables. The interest of the mixture model for the estimation of microbial contamination is discussed.

Prevalence of Listeria monocytogenes subtypes in bulk milk of the Pacific Northwest

The prevalence of Listeria monocytogenes in bulk milk from three Pacific Northwest states was assessed for 474 herds at three time points. For samples collected in November 2000 and June 2001, the L. monocytogenes prevalence levels were 4.9 and 7.0%, respectively. All isolates were subtyped by serotyping and by pulsed-field gel electrophoresis (PFGE). Forty-nine of the 55 isolates belonged to serogroup 1/2a, while 6 belonged to serogroup 4. Subtyping by PFGE revealed that isolates from 31 herds shared 10 patterns; there was a weak but significant association between PFGE subtype and geographical distance. Six herds were positive for L. monocytogenes at both time points. Of these six herds, four had indistinguishable PFGE patterns at both time points. Twenty-five of the 33 herds that were positive in June 2001 were sampled again in June 2002. L. monocytogenes was recovered from 17 of these 25 herds (68%), with the ApaI restriction enzyme digestion profiles (REDP) for 8 herds being identical to those of isolates recovered from these herds the previous year. The ApaI REDP for the bulk milk isolates were compared with those for isolates recovered from environmental and human samples that were collected by the Washington Department of Health (n = 23). Analysis of ApaI digestion profiles revealed that only two of the Washington Department of Health isolates had digestion profiles similar to those for isolates from bulk milk; however, further analysis with the use of a second enzyme (AscI) was capable of discriminating between isolates from the two sources. Thus, we found no direct REDP matches between bulk milk and clinical isolates.

Efficiency of four secondary enrichment protocols in differentiation and isolation of Listeria spp. and Listeria monocytogenes from smoked fish processing chains

Four secondary enrichment protocols (conventional methods: UVM II, Fraser 24 h and Fraser 48 h: Impedimetric method: Listeria electrical detection medium) were studied for their ability to isolate Listeria spp. and Listeria monocytogenes from fish and environmental samples collected along the processing chain of cold-smoked fish. From all methods, Listeria spp. and L. monocytogenes were respectively present in 56 and 34 of 315 samples analysed. Fraser broth incubated for 48 h gave the fewest false negative Listeria spp. results [4/56; (7.1%)], but concurrently only 15/34 (44.1%) samples were correctly identified as containing L. monocytogenes, Listeria electrical detection (LED) medium detected only 36/56 (64.3%) Listeria spp. positive samples. Despite this lower isolation rate, LED identified 20/34 (58.8%) L. monocytogenes positive samples correctly and gave fewer false positive results. The overall conclusion was that more than one isolation method is needed to accurately estimate L. monocytogenes contamination rates.