The multiple antibiotic resistance locus in salmonella from animals.
In order to understand the role of the Multiple Antibiotic Resistance
(mar) locus in the Biology of Salmonella, a Typhimurium DT104
marA:: g fp:: cam reporter mutant was constructed. Expression of mar in
Salmonella, as observed for E. coli, did induce low level multiple antibiotic
resistance and cyclohexane resistance. However, mar expression in
Salmonella did not appear to down regulate OmpF as has been observed for
E. coli. Both in-vitro and in-vivo tests suggested that mar contributed to the
virulence of Salmonella. The exact mechanism whereby mar contributes to
virulence is uncertain. It could be that insertional inactivation of mar limits
the ability of Salmonella to up regulate a ToIC type porin, which in
Enteritidis has been linked to virulence.
Wild type Salmonella were screened for cyclohexane resistance as a
possible way of detecting naturally occurring mar mutants. Cyclohexane
resistance occurred in about one to five percent of wild type Salmonella
tested. These strains had a typical mar phenotype as regards low level
multiple antibiotic resistance, cyclohexane resistance and down regulated
OmpF, but none of the strains tested had mutations in marOR.
Passage of wild type Salmonella strains on media with tetracycline and
chloramphenicol gave rise to multiple antibiotic resistant and cyclohexane
resistant mutants. The mutants with higher level antibiotic resistance
generally lacked OmpF although these mutants lacked mutations in marOR
The high degree of specificity required to induce mar and the low level of
antibiotic resistance conferred by its induction suggest that mar in itself is
unlikely to confer clinical antibiotic resistance in Salmonella. However,
higher levels of resistance were seen in the mar like mutants generated and
for strains grown with salicylate. Coupled with the involvement of mar in
virulence, it is possible that mar and related regulatory systems such as
acrAB, rob and sox could have important roles in antibiotic resistance and
virulence of Salmonella.