Sublethal exposure to benzalkonium chloride induced biocide-antibiotic cross-resistance in hypervirulent L. monocytogenes.

Aims: Listeria monocytogenes is a foodborne pathogen found ubiquitously in the food manufacturing environment (FME), where exposure to biocides at sublethal concentrations may induce adaptation and cross-resistance. This study aimed to investigate whether exposure to sublethal benzalkonium chloride (BAC) concentrations induced cross-resistance to antibiotics in an adaptive laboratory evolution (ALE) model.

Methods and results: Six L. monocytogenes and two Listeria innocua strains underwent a 30-day sublethal BAC ALE. Susceptibility was performed against biocides and antibiotics. Variant calling was performed relative to wild-type strains. Three intragenic mutations and two intergenic mutations upstream of fepR, encoding a local repressor of the fluoroquinolone efflux pump FepA are identified in BAC-adapted isolates, associated with phenotypic reduced susceptibility to BAC, fluoroquinolones, and co-trimoxazole.

Conclusions: Sublethal BAC exposure can select for fepR-associated variants linked to reduced susceptibility to both biocides and antibiotics, highlighting concerns for biocide-antibiotic cross-resistance.