Adaptation mechanisms ofListeria monocytogenesto quaternary ammonium compounds

Abstract

Listeria monocytogenes is ubiquitously found in nature and can easily enter food-processing facilities due to contaminations of raw materials. Several countermeasures are used to combat contamination of food products, for instance the use of disinfectants that contain quaternary ammonium compounds, such as benzalkonium chloride (BAC) and cetyltrimethylammonium bromide (CTAB). In this study, we assessed the potential of the commonly used wildtype strain EGD-e to adapt to BAC and CTAB under laboratory growth conditions. All BAC-tolerant suppressors exclusively carried mutations in fepR or its promoter region likely resulting in the overproduction of the efflux pump FepA. In contrast, CTAB-tolerance was associated with mutations in sugR, which regulates the expression of the efflux pumps SugE1 and SugE2. L. monocytogenes strains lacking either FepA or SugE1/2 could still acquire tolerance towards BAC and CTAB. Genomic analysis revealed that the overproduction of the remaining efflux system could compensate for the deleted one. Even in the absence of both efflux systems, tolerant strains could be isolated, which all carried mutations in the diacylglycerol kinase encoding gene lmo1753 (dgkB). DgkB converts diacylglycerol to phosphatidic acid, which is subsequently re-used for the synthesis of phospholipids suggesting that alterations in membrane composition could be the third adaptation mechanism.