Screening of benzalkonium chloride resistance in Listeria monocytogenes strains isolated during cold smoked fish production

Aims: To determine the susceptibility to disinfectants and cross-resistance to antibiotics in Listeria monocytogenes strains isolated from fish products and the fish-processing environment. Methods and Results: Minimal inhibitory concentration assessment, using the agar dilution method, showed 108 of 255 L. monocytogenes isolates with low susceptibility to benzalkonium chloride (BC), commonly used in food industries. Most of them are from raw products of farmed fish during processing, while the remaining resistant isolates were mainly from the environment and finished products irrespective of the fish species. Two BC-resistant isolates were resistant to ethidium bromide (EB). The conservation of resistance after plasmid curing suggested that the resistance genes are not plasmid associated. EB accumulation assays demonstrated that the two BCR EBR isolates used an efflux pump to expel these substrates whereas a different mechanism was probably used by the majority of the strains with BCR EBS pattern. No cross-resistance was found with antibiotics. Conclusions: This study highlights the difference in susceptibilities to BC for L. monocytogenes strains isolated from fish-processing plants and in resistance mechanisms to BC developed by these bacteria. Significance and Impact of the Study: The presence of BC resistant L. monocytogenes strains could contribute to their adaptation and so explained their survival and persistence in the fish-processing environment.