The ugtL gene mediates polymyxin B resistance independently of the PmrA-PmrB system phoP Salmonella are > 20 times more polymyxin sensitive than a pmrA mutant when bacteria are grown in low ygjU 3394555 3392617 rfaB rfaI 3914096 3915562 slsA STM3760 cigR 3959077 3960805 rhaT rhaR 4260472 4262386 STM0725 STM0726 STM0724 792487 790384 Mg2 + ( Wosten et al. , 2000 ) , indicating that a PmrA-inde-pendent PhoP-regulated gene ( s ) is necessary for poly-myxin resistance .