Premature stop codons in inlA reduce Caco-2 cell invasion by Listeria monocytogenes and contribute to serotype bias in human clinical isolates.

Listeria monocytogenes is a highly lethal foodborne pathogen that exhibits strain-dependent differences in virulence. Infection begins with traversal of the intestinal barrier through a two-step process involving adhesion to intestinal epithelial cells followed by intracellular invasion. In this study, Caco-2 cells were used to investigate these early infection events in 25 Japanese L. monocytogenes isolates (9 human clinical, 14 food-derived, and 2 environmental isolates) and 1 reference clinical strain (ScottA). We examined the genetic integrity of key adhesion- and invasion-related virulence genes (lap, fbpA, inlA, and inlB) and their relationships with adhesion and invasion phenotypes. Premature stop codons (PMSCs) were identified in inlA in 11 isolates and in inlB in 15 isolates. PMSCs in inlA were absent from all clinical isolates and predominantly found in serotypes 1/2a and 1/2c, which are rarely associated with clinical listeriosis. Also, three novel inlA PMSC variants were identified. Adhesion to Caco-2 cells was relatively uniform among isolates (5.87-6.88 log10 colony-forming units [CFU]/mL), whereas invasion varied markedly (2.74-6.27 log10 CFU/mL). Isolates carrying PMSCs in inlA consistently showed reduced invasion (<4 log10 CFU/mL), whereas isolates lacking PMSCs exhibited significantly higher invasion (>5 log10 CFU/mL). Western blot analysis confirmed the absence of full-length InlA in PMSC-containing isolates. Antibody-mediated inhibition of InlA produced a dose-dependent reduction in invasion, confirming the critical role of InlA in epithelial cell invasion. These findings demonstrate that the PMSCs in inlA are a key determinant of invasion capacity and may partly explain the serotype bias observed among human clinical isolates. Our results highlight the importance of virulence factor-based risk assessment for L. monocytogenes, rather than treating all strains as equally pathogenic.