Adaptation mechanisms of Listeria monocytogenes to quaternary ammonium compounds

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. Originality-Significance StatementSurvival and proliferation of Listeria monocytogenes in the food industry is an ongoing concern, and while there are various countermeasures to combat contamination of food products, the pathogen still successfully manages to withstand the harsh conditions present in food-processing facilities, resulting in reoccurring outbreaks, subsequent infection, and disease. To counteract the spread of L. monocytogenes it is crucial to understand and elucidate the underlying mechanism that permit their successful evasion. We here present various adaptation mechanisms of L. monocytogenes to withstand two important quaternary ammonium compounds.