Chromosomally mediated multidrug resistance in Salmonella typhimurium is generally attributed to the overexpression of the acrAB locus and the mutation in topoisomerase genes. The acrAB locus encode the efflux pump AcrAB,which confers multidrug tolerance,controlled by the regulon marRAB. Mutation in the gyrase and topoisomerase IV lead to fluoroquinolones resistance, mainly arise in gyrA and parC genes.