DEVELOPMENT AND USE OF PHYLOGENETIC SYSTEMS TO ENHANCE FOOD SAFETY AND FOOD SECURITY
Location: Bacterial Foodborne Pathogens & Mycology Research Unit
Title: Molecular and Phenotypic Characterization of Listeria monocytogenes from U.S. Department of Agriculture Food Safety Inspection Service Surveillance of Ready-to-Eat Foods and Processing Facilities
Submitted to: Journal of Food Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 21, 2010
Publication Date: May 1, 2010
Citation: Ward, T.J., Evans, P., Wiedmann, M., Usgaard, T.R., Roof, S.E., Stroika, S.G., Hise, K. 2010. Molecular and Phenotypic Characterization of Listeria monocytogenes from U.S. Department of Agriculture Food Safety Inspection Service Surveillance of Ready-to-Eat Foods and Processing Facilities. Journal of Food Protection. 73(5):861-869.
Interpretive Summary: Listeria monocytogenes is a food-borne bacterium responsible for serious invasive illness in humans and other animals. The vast majority of human listeriosis cases result from consumption of contaminated foods, and although listeriosis is relatively rare, L. monocytogenes is responsible for over one-quarter of foodborne disease-related deaths linked to known pathogens. In addition, L. monocytogenes contamination has been a leading cause of food recalls in recent years. Regulatory agencies and food processors lack the information needed to account for this subtype-specific variation in their inspection and sanitation programs. The objective of this study was to develop an enhanced framework for prediction of relative-risk associated with L. monocytogenes strains collected by the Food Safety and Inspection Service from ready-to-eat (RTE) foods and food processing facilities. The results demonstrated that strains associated with previous epidemic outbreaks were rare contaminants of RTE foods, while isolates with a reduced ability to cause human disease were substantially more common than previously appreciated. In addition, these results indicated that integration of DNA sequence-based subtyping along with current subtyping methods provides an improved framework for prediction of relative-risk associated with L. monocytogenes strains from RTE foods. These results provide information and molecular tools critical to allocating inspection and sanitation resources, identifying sources of contamination, and developing science-based intervention strategies and regulations that reduce the public health and economic burdens imposed by this pathogen.
A panel of 501 Listeria monocytogenes obtained from Food Safety and Inspection Service monitoring of ready-to-eat (RTE) foods were subtyped by multilocus genotyping (MLGT) and by sequencing the virulence gene inlA. MLGT analyses confirmed that clonal lineages associated with previous epidemic outbreaks were rare (7.6%) contaminants of RTE foods. Conversely, sequence analyses revealed 11 different premature stop codon (PMSC) mutations in inlA, including three novel PMSC mutations, and demonstrated that the frequency of these virulence-attenuating mutations among RTE isolates (48.5%) was substantially higher than previously appreciated. Significant differences (P < 0.001) in the frequency of inlA PMSCs were observed between lineages and between major serogroups, which could explain differences in association of these subtypes with human listeriosis. As PFGE is used by PulseNet to subtype L. monocytogenes, we also investigated the utility of combining MLGT with PFGE data. Interrogation of SNPs responsible for PMSCs in inlA improved strain resolution among isolates with the 10 most common PFGE patterns, eight of which included isolates with a PMSC in inlA. In addition, the presence or absence of PMSCs in inlA accounted for significant differences (P < 0.05) in Caco-2 invasion efficiencies among isolates with the same PFGE pattern, and the proportion of PulseNet entries from clinical sources was significantly higher (P < 0.001) for PFGE patterns comprised exclusively of isolates with full-length inlA. These results indicated that integration of PFGE and DNA sequence-based subtyping provides an improved framework for prediction of relative-risk associated with L. monocytogenes strains from RTE foods.