A MULTIDRUG-RESISTANCE REGULATORY CHROMOSOMAL LOCUS IS WIDESPREAD AMONG ENTERIC BACTERIA

Constitutive expression of the mar operon (marRAB) in Escherichia coli produces a multiple antibiotic resistance phenotype mediated by the e xpression of multiple genetic loci in response to regulatory proteins in the operon. A mar-specific DNA probe was used to search for the ope ron in bacterial strains representing 53 species and 27 genera. Among these, 6 other Enterobacteriaceae, Salmonella, Shigella, Klebsiella, C itrobacter, Hafnia, and Enterobacter species, contained DNA sequences that hybridized to the probe under high-stringency conditions. By use of a selection protocol developed to obtain multiple antibiotic resist ant mutants of E. coli, multiply resistant mutants that showed increas ed expression of mar-specific RNA were obtained from Enterobacter aggl omerans and Salmonella species.