Prevalence of the multiple antibiotic resistance operon (marRAB) in the genus salmonella.

Abstract

The multiple antibiotic resistance operon (marRAB operon) is a member of the Multidrug resistance (Mdr) systems. Similar to other Mdr systems, this operon codes for resistance to structurally and functionally unrelated antibiotics. This operon has been shown to be conserved in the family Entrobacteriaceace. Within the genus Salmonella, a previous study (8) demonstrated S. typhimurium was positive for the marRAB operon while S. arizonae appeared not to contain the operon. Thus, the objective of this study was to determine the prevalence of the marRAB operon in genus Salmonella. Thirty different veterinary Salmonella isolates were examined using PCR, Southern blot, and dot blot analysis. PCR was performed on isolated chromosomal DNA from all thirty organisms using primers based on the marRAB operon of S. typhimurium. The predicted 2.2 kb band was amplified in 16 organisms, including S. arizonae. The cloned 2.2 kb PCR product of Salmonella enteritidis was subjected to DNA sequencing and shown to have approximately 99% sequence homology to the marRAB operon of S. typhimurium. Southern blot and dot blot analysis were performed on the PCR amplification products using the sequenced marRAB fragment as a probe. Southern blot data revealed that the probe hybridized with all PCR products appearing on the agarose gel. Dot blot analysis data indicated the probe hybridized with chromosomal DNA from all thirty organisms. Induction studies were performed in the presence of low concentrations of either tetracycline or chloramphenicol to determine if the isolates have a functional marRAB operon. There was an overall increase in the resistance of almost all induced organisms to tetracycline, chloramphenicol, and ampicillin. Taken together, these data demonstrate conservation of the marRAB operon in the thirty Salmonella species examined.