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Applied and Environmental Microbiology, March 2001, p. 1190-1197, Vol. 67, No. 3
0099-2240/01/$04.00+0   DOI: 10.1128/AEM.67.3.1190-1197.2001

Multiple Antibiotic Resistance (mar) Locus in Salmonella enterica Serovar Typhimurium DT104

Luke P. Randall and Martin J. Woodward*

Department of Bacterial Disease, Veterinary Laboratories Agency (Weybridge), New Haw, Addlestone, Surrey KT15 3NB, United Kingdom

Received 16 August 2000/Accepted 2 January 2001

In order to understand the role of the mar locus in Salmonella with regard to multiple antibiotic resistance, cyclohexane resistance, and outer membrane protein F (OmpF) regulation, a marA::gfp reporter mutant was constructed in an antibiotic-sensitive Salmonella enterica serovar Typhimurium DT104 background. Salicylate induced marA, whereas a number of antibiotics, disinfectants, and various growth conditions did not. Increased antibiotic resistance 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. Complementation studies with a plasmid that constitutively expressed marA confirmed the involvement of mar in Salmonella with low-level antibiotic resistance and cyclohexane resistance, although the involvement of mar in down regulation of OmpF was unclear. However, marA overexpression did increase the expression of a ca. 50-kDa protein, but its identity remains to be elucidated. Passage of the marA::gfp reporter mutant with increasing levels of tetracycline, a method reported to select for mar mutants in Escherichia coli, led to both multiple-antibiotic and cyclohexane resistance. Collectively, these data indicate that low-level antibiotic resistance, cyclohexane resistance, and modulation of OMPs in Salmonella, as in E. coli, can occur in both a mar-dependent and mar-independent manner.

* Corresponding author. Mailing address: Department of Bacterial Disease, Veterinary Laboratories Agency (Weybridge), New Haw, Addlestone, Surrey KT15 3NB, United Kingdom. Phone: 44 (0) 1932 357582. Fax: 44 (0) 1932 347046. E-mail: M.J.Wooward{at}vla.maff.gsi.gov.uk.

Applied and Environmental Microbiology, March 2001, p. 1190-1197, Vol. 67, No. 3
0099-2240/01/$04.00+0   DOI: 10.1128/AEM.67.3.1190-1197.2001


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