Prevalence, genomic diversity, and invasion potential of Listeria monocytogenes in ready-to-eat seafood products distributed in Japan.

Listeria monocytogenes is a major foodborne pathogen capable of persisting and growing under refrigerated storage, raising food safety concerns for ready-to-eat (RTE) foods. This study investigated the occurrence, genetic and invasion characteristics of L. monocytogenes in five categories of RTE seafood products distributed in Japan: cod roe, seasoned cod roe, salmon roe, smoked salmon, and minced tuna. A total of 500 retail samples, comprising 100 samples per category, were examined in 2023. L. monocytogenes was detected most frequently in minced tuna (23%), followed by salmon roe (9%), seasoned cod roe (4%), cod roe (2%), and smoked salmon (2%). Quantification using a three-tube MPN method indicated generally low contamination levels, with most positive samples containing <10 MPN/g. However, physicochemical analyses of pH and water activity suggested that most products were capable of supporting the growth of L. monocytogenes during storage. Whole-genome sequencing of 40 isolates revealed substantial genomic diversity, classifying isolates into 14 clonal complexes and 27 cgMLST types, with no marked bias toward either BIGSdb-defined Lineage I or II. Gene content analysis identified stress survival islets, disinfectant tolerance-associated determinants, and virulence-associated loci, including LIPI-3 and LIPI-4, in specific clonal backgrounds. Epithelial invasion assays revealed wide variation in invasion phenotypes, with higher invasion among Lineage I isolates and near-zero invasion in isolates carrying premature stop codons in inlA. Overall, these findings highlight heterogeneous risk profiles among L. monocytogenes contaminating RTE seafood products in Japan.