Inactivation of the gene encoding the cationic antimicrobial peptide resistance factor MprF increases biofilm formation but reduces invasiveness of Listeria monocytogenes

Abstract

Aims: To understand the genetics involved in surface attachment and biofilm formation of Listeria monocytogenes.

Methods and results: An in vitro screen of a Himar1 transposon library of L. monocytogenes strain 15G01 identified three transposants that produced significantly different biofilm levels when compared to the wild-type strain; two mutants exhibited enhanced biofilm formation and one produced less biofilm biomass than the wild-type. The mutant 15G01 mprF::Himar1, which had a transposon insertion in the mprF gene, was selected for further analysis. The mutant produced a more densely populated biofilm on solid surfaces such as stainless steel and polystyrene, as determined using scanning electron and light microscopy. The 15G01 mprF::Himar1 mutant remained viable in biofilms, but showed an increase in sensitivity to the cationic antimicrobial gallidermin. The mutant also displayed reduced invasiveness in CaCo-2 intestinal cells, suggesting virulence properties are compromised by the inactivation of mprF.

Conclusions: Biofilm formation and gallidermin resistance of L. monocytogenes is influenced by mprF, but this trait is associated with a compromise in invasiveness.

Significance and impact of the study: The presence of pathogenic microorganisms in the food processing environment can cause a significant problem, especially when these microorganisms are established as biofilms. This study shows that the inactivation of the mprF gene results in enhanced biofilm formation and abiotic surface attachment of L. monocytogenes.

Keywords: Listeria monocytogenes; mprF; biofilm; food safety; mutagenesis library.