Bacterial resistance to disinfectants containing quaternary ammonium compounds

Quaternary ammonium compounds (QAC) are widely used as disinfectants in both medical and food environments. Microbial contaminants are, therefore, regularly exposed to their action and the isolation from clinical and food sources of resistant bacteria continues to be reported in many countries. Resistance to QAC in clinical strains of staphylococci is encoded by one of at least three resistance genes, designated qacA, qacB and qacC. Using hybridisation analysis, we have shown that these QAC resistance genes are also distributed among staphylococcal strains in the food industry. In addition, we have discovered two new resistance determinants in these food isolates, which are now being characterised and sequenced. Although the general level of resistance of pure cultures is low, the resistant strains have originally been isolated after exposure to the recommended user concentration of a commercial brand of QAC. We have also studied resistance to QAC in pseudomonads isolated from the food industry. Their level of resistance is much higher than that found in staphylococci. About 30% of the collected strains were able to grow in 200 μg•ml−1 benzalkonium chloride, the lowest recommended use concentration for this commonly used type of QAC.