Exposure-driven adaptation of Listeria monocytogenes to disinfectants yields distinct resistance phenotypes depending on the strain

Antimicrobial resistance in Listeria monocytogenes is an increasing concern in food processing environments, especially when biocides are applied incorrectly. In this study, the susceptibility of 19 L. monocytogenes strains to three commonly used disinfectants - benzalkonium chloride (BKC), peracetic acid (PAA), and sodium hypochlorite (SHY) - was assessed by using minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) determination. Subsequently, strains were gradually exposed to increasing subinhibitory concentrations of the disinfectants to assess the impact of such exposure on their resistance profiles. All strains exhibited higher MIC and MBC values after adaptation, with increments of 0.77- to 155.93-fold, especially those from industrial environments, suggesting the selection of more tolerant phenotypes. Strain-dependent adaptation was demonstrated by principal component analysis (PCA), which revealed a clear shift in the phenotypic profiles of adapted strains, as well as increased variability. On the other hand, the susceptibility pattern of wild-type strains was maintained more homogeneous. These results confirmed that subinhibitory exposure to disinfectants induced tolerance development in L. monocytogenes, which has direct implications for hygiene protocols. Therefore, to prevent the development and persistence of cross-resistant strains in food industry, proper disinfectant control and their application are crucial.