Listeria monocytogenes: Low Levels Equal Low Risk

Abstract

Because of the public health significance of L. monocytogenes, U.S. regulatory agencies established a policy whereby ready-to-eat foods contaminated with the organism at a detectable level are deemed adulterated. This "zero tolerance" policy, however, makes no distinction between foods contaminated at high and low levels. We have reported elsewhere that a survey of over 31,000 ready-to-eat retail food samples, representing eight product categories, showed an overall prevalence rate of 1.82% for these foods. In this study, we used the food survey data in combination with concurrent data regarding illness in the population consuming the foods, together with other variable factors, to derive a dose-response model. The confidence interval for prevalence was 1.68 to 1.97%. L. monocytogenes levels, which ranged from -2 to 6 log CFU/g, were adequately described by the distribution beta (0.29, 2.68, -1.69, 6.1). An exponential dose-response model was obtained, with an R value (essentially the probability of a single cell causing illness) of 1.76 x 10(-10) for the population at the highest risk. A microbial risk assessment based on the model shows that an alternative to the zero tolerance strategy has a greater risk reduction potential and suggests that a management strategy focusing on the concentration of L. monocytogenes rather than its presence alone may have a greater impact on the improvement of public health by facilitating the development of control measures to limit the maximum levels of L. monocytogenes in foods.

Listeria monocytogenesIsolates from Foods and Humans Form Distinct but Overlapping Populations

“…These findings support the existence of virulence and transmissibility differences among L. monocytogenes subtypes. An understanding of the variability of strains encountered in humans, foods, animals, and the environment and the range of virulence characteristics expressed by different strains will contribute to improved assessment of the public health risk posed by L. monocytogenes (6,9,33).…”

“…L. monocytogenes food isolates were obtained through a previously described large-scale food survey (9,13). As part of this survey, more than 30,000 samples representing seven selected RTE food categories (luncheon meats, deli salads, fresh soft "Hispanic-style" cheeses, bagged salads, blue-veined and mold-ripened cheeses, smoked seafood, and seafood salads) were collected in Maryland and Northern California during 2000 and 2001.…”

Factors moderating Listeria monocytogenes growth in raw milk and in soft cheese made from raw milk

“…The Lag phase observed with a high initial concentration of L. monocytogenes (10 5 cfu·mL -1 ) was shorter than those observed with low initial concentrations [26]. Some studies report the effect of inoculum concentration on the growth of L. monocytogenes [3,12,17]. Our results differed from one other study [4], where the initial concentration was 3 × 10 2 cfu·mL -1 , lag phase was between 10 and 15 d and population increased 10 3 -fold over 40 d. The difference between these and our results may be related to the strains studied, all isolated from raw milk.…”

Relatedness of Listeria monocytogenes Isolates Recovered from Selected Ready-To-Eat Foods and Listeriosis Patients in the United States

“…Of the 577 food samples that tested positive for L. monocytogenes using DNA-based screening methods, 505 yielded a viable isolate of L. monocytogenes, and a single isolate was retained from each sample (13). An additional 42 human clinical isolates from the same geographic regions and time periods were obtained for assessing the risk of L. monocytogenes from RTE foods in the United States and for a comparative analysis with the 505 food isolates (8,13). The goal of the present study was to identify the "types" of the isolates mentioned above and to determine if certain subtypes were more often associated with certain foods and/or more often involved in cases of listeriosis.…”