inlAPremature Stop Codons Are Common amongListeria monocytogenesIsolates from Foods and Yield Virulence-Attenuated Strains That Confer Protection against Fully Virulent Strains

Previous studies showed that a considerable proportion ofListeria monocytogenes isolates obtained from foods carry a premature stop codon (PMSC) mutation ininlA that leads to production of a truncated and secreted InlA. To further elucidate the role these mutations play in virulence ofL. monocytogenes , we created isogenic mutants, including (i) natural isolates where aninlA PMSC was reverted to a wild-typeinlA allele (without a PMSC) and (ii) natural isolates where a PMSC mutation was introduced into a wild-typeinlA allele; isogenic mutant sets were constructed to represent two distinctinlA PMSC mutations. Phenotypical and transcriptional analysis data showed thatinlA PMSC mutations do not have a polar effect on the downstreaminlB . Isogenic and natural strains carrying aninlA PMSC showed significantly reduced invasion efficiencies in Caco-2 and HepG2 cell lines as well as reduced virulence in oral guinea pig infections. Guinea pigs were also orally infected with a natural strain carrying the most commoninlA PMSC mutation (vaccinated group), followed by challenge with a fully virulentL. monocytogenes strain 15 days postvaccination to probe potentially immunizing effects of exposure toL. monocytogenes withinlA PMSC mutations. Vaccinated guinea pigs showed reduced bacterial loads in internal organs and improved weight gain postchallenge, indicating reduced severity of infections in guinea pigs exposed to natural strains withinlA PMSC mutations. Our data support that (i)inlA PMSC mutations are causally associated with attenuated virulence in mammalian hosts and (ii) naturally occurring virulence-attenuatedL. monocytogenes strains commonly found in food confer protective immunity.