title: Molecular and phenotypic characterization of Listeria monocytogenes from U.S. Department of Agriculture Food Safety and Inspection Service surveillance of ready-to-eat foods and processing facilities.
abstract: A panel of 501 Listeria monocytogenes isolates obtained from the U.S. Department of Agriculture Food Safety and Inspection Service monitoring programs for ready-to-eat (RTE) foods were subtyped by multilocus genotyping (MLGT) and by sequencing the virulence gene inlA, which codes for internalin. MLGT analyses confirmed that clonal lineages associated with previous epidemic outbreaks were rare (7.6%) contaminants of RTE meat and poultry products and their production environments. Conversely, sequence analyses revealed mutations leading to 11 different premature stop codons (PMSCs) in inlA, including three novel PMSC mutations, and revealed 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 partially explain differences in association of these subtypes with human listeriosis. Interrogation of single-nucleotide polymorphisms responsible for PMSCs in inlA improved strain resolution among isolates with the 10 most common pulsed-field gel electrophoresis (PFGE) patterns, 8 of which included isolates with a PMSC in inlA. The presence or absence of PMSCs in inlA accounted for significant differences (P < 0.05) in Caco-2 invasion efficiencies among isolates with identical PFGE patterns, and the proportion of PulseNet entries from clinical sources was significantly higher (P < 0.001) for PFGE patterns exclusively from 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.
authors.affiliation: Bacterial Foodborne Pathogens and Mycology Research Unit, U.S. Department of Agriculture, Agricultural Research Service, Peoria, Illinois 61604, USA. todd.ward@ars.usda.gov
authors.forename: Todd J
authors.initials: TJ
authors.lastname: Ward
authors.forename: Peter
authors.initials: P
authors.lastname: Evans
authors.forename: Martin
authors.initials: M
authors.lastname: Wiedmann
authors.forename: Thomas
authors.initials: T
authors.lastname: Usgaard
authors.forename: Sherry E
authors.initials: SE
authors.lastname: Roof
authors.forename: Steven G
authors.initials: SG
authors.lastname: Stroika
authors.forename: Kelley
authors.initials: K
authors.lastname: Hise
mesh_terms: D001426:Bacterial Proteins / Q000235:genetics*; D015373:Bacterial Typing Techniques; D018938:Caco-2 Cells; D018389:Codon, Nonsense; D004269:DNA, Bacterial / Q000737:chemistry / Q000235:genetics*; D016521:Electrophoresis, Gel, Pulsed-Field; D005506:Food Contamination / Q000032:analysis*; D005516:Food Microbiology; D005838:Genotype; D006801:Humans; D008089:Listeria monocytogenes / Q000145:classification* / Q000235:genetics; D008461:Meat Products / Q000382:microbiology; D009154:Mutation; D020641:Polymorphism, Single Nucleotide; D011202:Poultry Products / Q000382:microbiology; D017422:Sequence Analysis, DNA; D037521:Virulence Factors / Q000235:genetics*
chemical_list: D001426:Bacterial Proteins; D018389:Codon, Nonsense; D004269:DNA, Bacterial; D037521:Virulence Factors; C069761:internalin protein, Bacteria