|Van Stelten, A|
Submitted to: Applied and Environmental Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/5/2010
Publication Date: 5/1/2010
Citation: Van Stelten, A., Simpson, J.M., Ward, T.J., Wiedmann, M., Nightingale, K.K. 2010. Revelation by Single-Nucleotide Polymorphism Genotyping That Mutations Leading to a Premature Stop Codon in inlA Are Common among Listeria monocytogenes Isolates from Ready-to-Eat Foods but Not Human Listeriosis Cases. Applied and Environmental Microbiology. 76(9):2783-2790. Interpretive Summary: Listeria monocytogenes is a food-borne bacterium that places a serious burden on human health and the agricultural economy. Listeria monocytogenes is responsible for over one-quarter of foodborne disease-related deaths linked to known pathogens, and L. monocytogenes contamination has been the leading cause of food recalls due to biological concerns in recent years. Significant variation exists among L. monocytogenes stains in terms of their ability to persist in different environments and to cause human illness. However, current risk assessment and dose-response models lack the information needed to account for this subtype-specific variation. This study was conducted to characterize the relative risk posed by L. monocytogenes isolates from different ready-to-eat (RTE) foods at retail. The results demonstrate that a significant proportion of L. monocytogenes isolates at retail have a reduced ability to cause human disease due to specific mutations in a key virulence gene. In addition, these results indicate that isolates with reduced virulence potential are overrepresented among isolates from deli meats, while strains associated with previous epidemic outbreaks are overrepresented among isolates from deli and seafood salads. These results will provide critical data for the development of future risk assessments and regulatory initiatives that reduce the public health and economic burdens imposed by this pathogen.
Technical Abstract: Listeria monocytogenes utilizes the virulence factor Internalin A (InlA; encoded by inlA) to cross the intestinal barrier to establish a systemic infection. At least 18 naturally occurring mutations leading to a premature stop codon (PMSC) in inlA have been identified worldwide to date and these mutations appear to be fully responsible for attenuated mammalian virulence. This study was conducted to determine the prevalence of inlA PMSC mutations among a large representative collection of >1,000 L. monocytogenes isolates from ready-to-eat (RTE) foods (n=502) and human listeriosis cases (n=507). We incorporated five novel inlA PMSC mutations discovered in isolates from France and the U.S. as additional markers in our previously described inlA single nucleotide polymorphism (SNP) genotyping assay. We employed this assay to screen for the presence/absence of 18 inlA PMSC mutations in the isolate set described above. Results showed that a significantly greater proportion (P < 0.0001) of isolates from RTE foods (45.0 %) carry a PMSC mutation in inlA as compared to human clinical isolates (5.1%). Furthermore, the proportion of L. monocytogenes with or without virulence-attenuating mutations in inlA was similar among isolates from different categories of RTE foods except for deli meats, which included a marginally higher proportion (P = 0.12) of isolates carrying a PMSC in inlA. We also investigated the distribution of epidemic clone strains, which have been linked to the majority of outbreaks worldwide and are overrepresented among sporadic listeriosis cases. We observed a significant (P < 0.05) overrepresentation of epidemic clone strains in the deli and seafood salads category and under representation of epidemic clone strains in the smoked seafood category. These results will provide critical data for revision of current, and development of future, risk assessments and regulatory initiatives regarding the presence of L. monocytogenes in RTE foods.