Molecular Characterization of Virulence-Related Genes in Listeria monocytogenes Isolated from Retail Meats in Sapporo, Japan

Listeria monocytogenes is the causative agent of the globally prevalent foodborne illness listeriosis. Infection is caused by the ingestion of food contaminated with L. monocytogenes, which invades host cells via the bacterial cell surface protein internalin A (InlA). Fewer outbreaks of listeriosis have been reported in Japan than in other countries, suggesting that circulating L. monocytogenes strains in Japan have an increased prevalence of InlA mutations or mutations in other virulence factors, thereby impairing cell invasion. Herein, we investigated the molecular characteristics of inlA and other key virulence factors in L. monocytogenes isolated from retail meats sold in Japan. We isolated L. monocytogenes from retail meats in Sapporo, Japan and investigated the presence of five virulence-related genes (actA, hlyA, iap, plcA, and prfA). We also determined the sequences of the inlA gene, which encodes InlA. Ninety-three L. monocytogenes strains (31.0%) were isolated from 300 meat samples. The major serogroup of the strains was serogroup IIc (49.5%), followed by serogroup IIa (41.9%). Overall, 98.9% of the 93 strains possessed the five examined virulence-related genes. However, 51.6% of these strains exhibited premature stop codons in inlA. We showed that approximately half of the L. monocytogenes strains contaminating retail meats in Sapporo, Japan express mutated InlA, suggesting that their ability to invade host cells may be impaired. This may be one reason why fewer listeriosis outbreaks occur in Japan than in other countries, and provides new insight into geographical differences in the incidence of a common infectious disease.