RpoS and PmrA Induced Resistance to Polymyxin B is Independent of arnT Expression in Pseudomonas aeruginosa

Adaptive resistance of Pseudomonas aeruginosa to polymyxin B, a drug of last resort, is an emerging problem worldwide. The purpose of this study was to better understand the mechanism of transcriptional regulation of polymyxin B resistance in P. aeruginosa. One proposed regulatory pathway involved RpoS-activated PmrA which then activates ArnT, a lipid membrane modifier. We assessed the contribution of this regulatory circuit to antibiotic resistance at log phase, early stationary phase and late stationary phase by comparing plating results. Here we report that expression of rpoS and pmrA were highly correlated, supporting previous findings by other researchers that pmrA is indeed a downstream target of rpoS. Furthermore, upregulation of both rpoS and pmrA led to increased growth in the presence of sub-inhibiotry levels of polymyxin B. In contrast, arnT expression correlated poorly with expression of rpoS/pmrA suggesting the presence of other regulatory pathways. In addition, arnT expression did not lead to increased survival at the level of polymyxin B that was used. Thus, our results suggested that RpoS and PmrA induced resistance to polymyxin B occurred independently of ArnT. The direct effects of these genes on survival require further investigation.