ABSTRACTThe enzyme triphenylmethane reductase (TMR) reduces toxic triphenylmethane dyes into colorless, nontoxic derivatives, and TMR-producing microorganisms have been proposed as bioremediation tools. Analysis of the genome ofListeria monocytogenesH7858 (1998-1999 hot dog outbreak) revealed that the plasmid (pLM80) of this strain harboring a gene cassette (bcrABC) conferring resistance to benzalkonium chloride (BC) and other quaternary ammonium disinfectants also harbored a gene (tmr) highly homologous to TMR-encoding genes from diverse Gram-negative bacteria. The pLM80-associatedtmrwas located two genes downstream ofbcrABCas part of a putative IS1216composite transposon. To confirm the role oftmrin triphenylmethane dye detoxification, we introduced varioustmr-harboring fragments of pLM80 in a pLM80-cured derivative of strain H7550, from the same outbreak as H7858, and assessed the resistance of the constructs to the triphenylmethane dyes crystal violet (CV) and malachite green. Transcriptional and subcloning data suggest that the regulation of TMR is complex. Constructs harboring fragments spanningbcrABCandtmrwere CV resistant, and in such constructstmrtranscription was induced by sublethal levels of either BC or CV. However, constructs harboring onlytmrand its upstream intergenic region could also confer resistance to CV, albeit at lower levels. Screening a panel of BC-resistantL. monocytogenesstrains revealed that all those harboringbcrABCand adjacent pLM80 sequences, includingtmr, were resistant to CV and decolorized this dye. The findings suggest a potential role of TMR as a previously unknown adaptive attribute for environmental persistence ofL. monocytogenes.