Research on the Genetic Polymorphism and Function of inlA with Premature Stop Codons in Listeria monocytogenes

Listeria monocytogenes is a Gram-positive bacterial species that causes listeriosis, a major foodborne disease worldwide. The virulence factor inlA facilitates the invasion of L. monocytogenes into intestinal epithelial cells expressing E-cadherin receptors. Naturally occurring premature stop codon (PMSC) mutations in inlA have been shown to result in the production of truncated proteins associated with attenuated virulence. Moreover, different L. monocytogenes strains contain distinct inlA variants. In this study, we first characterized inlA in 546 L. monocytogenes strains isolated from various foods in Shanghai. The results showed that 36.1% (95% Confidence Interval: 32.0~40.2%) of the food isolates harbored inlA with PMSC, which was found to be associated with clonal complex (CC) types, with the highest proportions observed in CC9 and CC121. To investigate the function of inlA, we first used the dominant CC87 isolated from patients as the test strain and constructed an inlA-deleted strain via homologous recombination. Resistance tests and virulence tests showed that while inlA did not affect the resistance of L. monocytogenes, it significantly influenced cell adhesion and invasiveness. To further explore the function of inlA, we performed virulence tests on five CC-type strains carrying inlA with PMSC and their corresponding strains with intact inlA. We found that the virulence of L. monocytogenes strains carrying inlA or inlA with PMSC was associated with their CC type. Our preliminary results showed that premature termination of inlA did not significantly affect the adhesion and invasion abilities of low-virulence CC-type L. monocytogenes strains in Caco-2 cells, but substantially promoted those of high-virulence strains such as CC8 and CC7. In summary, this study preliminarily evaluated the effects of inlA integrity and PMSC mutation variation on the virulence of L. monocytogenes, providing a foundation for further research on inlA-related pathogenic mechanisms.