Persistent contamination of food manufacturing environments by Listeria monocytogenes is an important public health risk because such contamination events defy standard sanitization protocols, for example, the application of quaternary ammonium compounds such as benzalkonium chloride (BC), providing a source for prolonged dissemination of the bacteria in food products. We performed whole-genome sequence (WGS) analyses of 1279 well-characterized L. monocytogenes isolates from a variety of foods and food manufacturing environments and identified the bcrABC gene cassette associated with BC resistance in 41.5% of isolates. Of particular interest was the finding that all but one of 177 clonal complex (CC) 321 isolates, representing one of the most commonly occurring CCs found in foods and food-production environments, harbored the intact bcrABC cassette. Thirty-nine (38.6) percent of isolates recovered from foods representing 67 different CCs, and 59.2% of strains from food-manufacturing environmental samples representing 26 different CCs, were found to harbor the intact bcrABC cassette. A representative set of 69 isolates with and without bcrABC was assayed for the ability to grow in the presence of BC, and 34 of 35 isolates harboring the bcrABC cassette were resistant to BC. Determination of bcrABC in colony isolates could be achieved using both polymerase chain reaction and whole genome sequencing techniques, providing food testing laboratories with options for the characterization of isolates. The ability to detect bcrABC provides risk managers with a valuable tool to assess the potential for persistent contamination of the food manufacturing environment, which in turn may indicate the need for more targeted surveillance to ensure the efficacy of mitigation actions.
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