Genetic Characterization of Plasmid-Associated Benzalkonium Chloride Resistance Determinants in aListeria monocytogenesStrain from the 1998-1999 Outbreak

Abstract

ABSTRACTQuaternary ammonium compounds such as benzalkonium chloride (BC) are widely used as disinfectants in both food processing and medical environments. BC-resistant strains ofListeria monocytogeneshave been implicated in multistate outbreaks of listeriosis and have been frequently isolated from food processing plants. However, the genetic basis for BC resistance inL. monocytogenesremains poorly understood. In this study, we have characterized a plasmid (pLM80)-associated BC resistance cassette inL. monocytogenesH7550, a strain implicated in the 1998-1999 multistate outbreak involving contaminated hot dogs. The BC resistance cassette (bcrABC) restored resistance to BC (MIC, 40 μg/ml) in a plasmid-cured derivative of H7550. All three genes of the cassette were essential for imparting BC resistance. The transcription of H7550 BC resistance genes was increased under sublethal (10 μg/ml) BC exposure and was higher at reduced temperatures (4, 8, or 25°C) than at 37°C. The level of transcription was higher at 10 μg/ml than at 20 or 40 μg/ml.In silicoanalysis suggested that the BC resistance cassette was harbored by an IS1216composite transposon along with other genes whose functions are yet to be determined. The findings from this study will further our understanding of the adaptations of this organism to disinfectants such as BC and may contribute to the elucidation of possible BC resistance dissemination inL. monocytogenes.