Virulence Attenuation and Relative Fitness of Listeria monocytogenes from the Processing Plant Environment

Listeria monocytogenes is a gram positive opportunistic food-borne pathogen that can cause morbidity and mortality in humans and animals. L. monocytogenes internalin A (InlA), a surface protein, has been associated with the invasion of human intestinal epithelial cells. Truncated forms of internalin A are soluble, and bacteria harboring premature stop codons (PMSC) resulting in truncations are markedly impaired in invasiveness. Increasing evidence suggests that inlA PMSC mutations are frequent in strains from foods, but rare among clinical isolates. However, limited information is available on the prevalence of such mutations among isolates from the processing plant environment. In this study, we sequenced the entire inlA of forty-nine genetically distinct strains from the environment of turkey processing plants in order to assess the prevalence of PMSC mutations. EGD-e (1/2a, 3a), FSL R2-503 (1/2b, 3b) and F2365 (4b) were chosen as reference strains for each serogroup in order to retrieve an accurate account of the mutations from one serotype to the next. PMSCs were localized at positions 189 and 700 among the serotype 1/2a strains, and at position 606 among those of serotype 1/2b or 3b. Among serotype 1/2a isolates, clonal groups consisting exclusively of isolates with a specific PMSC in inlA were identified, whereas serotype 1/2b isolates with or without a PMSC appeared to be members of the same clonal group. These data suggest that prevalence, location, and clonal associations of PMSCs in inlA correlate strongly with the serotype of environmental isolates. In the course of identifying these mutations we examined the survival and possible growth of sixteen L. monocytogenes isolates in raw and pasteurized milk over a nine week period at 4°C. Four isolates from serogroup 1/2 contained PMSCs in InlA. The data were statistically analyzed to determine the impact of serotype, time (9 weeks) and presence of PMSCs. The presence or absence of a PMSC appeared to have no visible effect on the growth of the isolates. Raw milk had a significant impact on the isolates ability to grow, while pasteurized milk appeared to be more of an ideal medium. Significant differences could be seen amongst serogroups and time; however, the presence of PMSCs didn't appear to be a major factor. These findings were of interest because several of the strains were isolated from raw or pasteurized milk. These isolates were predicted to have optimal growth in this medium and in turn were unable to develop. The poor performance in raw milk is presumed to be associated with the competition of other bacteria present.