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 concentrations induced cross-resistance to antibiotics in an adaptive laboratory evolution model. Methods and Results Six L. monocytogenes and two L. innocua strains underwent a 30-day sublethal BAC adaptive laboratory evolution. Susceptibility was performed against biocides and antibiotics. Variant calling was performed relative to wildtype 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.