Characterization of Specific Alleles in InlA and PrfA of Listeria Monocytogenes Isolated from Foods in Osaka, Japan and Their Ability to Invade Caco-2 Cells

Listeria monocytogenes expresses the surface protein internalin A (InlA), enabling the invasion of human intestinal epithelial cells to cause severe food-borne diseases. Full-length sequence analysis of inlA of 114 food isolates resulted in the detection of 29 isolates with a premature stop codon (PMSC) mutation and 6 isolates with 3-codon deletion mutations (aa 738 to 740) in inlA. The isolates with inlA PMSCs demonstrated a significantly lower level of invasion than the other food isolates in a Caco-2 cell invasion assay (P<0.01), but the isolates with the 3-codon deletion exhibited invasion comparable to the isolates with non-truncated InlA (P>0.05). According to analysis of the positive regulatory factor A (PrfA) sequences of 114 L. monocytogenes isolates, 7 isolates of serotype 1/2a from chicken samples contained a PrfA protein with a 5-nucleotide deletion from 712 to 716, including a stop codon. Although the isolates with a 5-nucleotide deletion in prfA demonstrated invasion comparable to the isolates with non-truncated InlA and PrfA after growth at 30 °C (P>0.05), they exhibited a significantly higher level of invasion than the other isolates after growth at 20 °C (P<0.01). To the authors' knowledge, this is the first report of L. monocytogenes isolates with the stop-codon deletion of PrfA.