Multiple antibiotic resistance (mar) locus in Salmonella enterica serovar typhimurium DT104

In order to understand the role of the mar locus in Salmonella with regard to multiple antibiotic resistance, cyclohexane resistance, and outer membra ne protein F (OmpF) regulation, a marA::gfp reporter mutant was constructed in an antibiotic-sensitive salmonella enterica serovar Typhimurium DT104 b ackground. Salicylate induced marA, whereas a number of antibiotics, disinf ect ants, and various growth conditions did not. Increased antibiotic resis tance was observed upon salicylate induction, although this was shown to be by both mar-dependent and mar-independent pathways. Cyclohexane resistance , however, was induced by salicylate by a mar-dependent pathway. Complement ation studies with a plasmid that constitutively expressed marA confirmed t he involvement of map in Salmonella with low-level antibiotic resistance an d cyclohexane resistance, although the involvement of mar in down regulatio n of OmpF was unclear. However, marA overexpression did increase the expres sion of a ca. 50-kDa protein, but its identity remains to be elucidated Pas sage of the marA::gfp reporter mutant with increasing levels of tetracyclin e, a method reported to select for mar mutants in Escherichia coli, led to both multiple-antibiotic and cyclohexane resistance. Collectively, these da ta indicate that low-level antibiotic resistance, cyclohexane resistance, a nd modulation of OMPs in Salmonella, as in E. coli, can occur in both a mar -dependent and mar-independent manner.