Low sensitivity of Listeria monocytogenes to quaternary ammonium compounds

Ninety-seven epidemiologically unrelated strains of Listeria monocytogenes were investigated for their sensitivities to quaternary ammonium compounds (benzalkonium chloride and cetrimide). The MICs for seven serogroup 1/2 str ains were high. Three came from the environment and four came from food; no ne were isolated from human or animal samples. All 97 strains carried the m drL gene, which encodes a multidrug efflux pump, and the orfA gene, a putat ive transcriptional repressor of mdrL. The absence of plasmids in four of t he seven resistant strains and the conservation of resistance after plasmid curing suggested that the resistance genes are not plasmid borne. Moreover , PCR amplification and Southern blot hybridization experiments failed to f ind genes phylogenetically related to the qacA and smr genes, encoding mult idrug efflux systems previously described for the genus Staphylococcus. The high association between nontypeability by phages and the loss of sensitiv ity to quaternary ammonium compounds are suggestive of an intrinsic resista nce due to modifications in the cell wall.