Regulation of polymyxin resistance and adaptation to low-Mg²⁺ environments

The PmrA-PmrB two-component system of Salmonella typhimurium controls resistance to the peptide antibiotic polymyxin B and to several antimicrobial proteins from human neutrophils. Amino acid substitutions in the regulatory protein PmrA conferring resistance to polymyxin lower the overall negative charge of the lipopolysaccharide (LPS), which results in decreased bacterial binding to cationic polypeptides and increased bacterial survival within human neutrophils. We have now identified three PmrA-activated loci that are required for polymyxin resistance. These loci were previously shown to be necessarY for growth on low-Mg²⁺ solid media, indicating that LPS modifications that mediate polymyxin resistance are responsible for the adaptation to Mg²⁺-limited environments. Conditions that promote transcription of PmrA-activated genes-growth in mildly acidic pH and micromolar Mg²⁺ concentrations-increased survival in the presence of polymyxin over 16,000-fold in a wild-type organism but not in a mutant lacking pmrA. Our experiments suggest that low pH and low Mg²⁺ concentrations may induce expression of PmrA-activated genes within phagocytic cells and promote bacterial resistance to host antimicrobial proteins. We propose that the LPS is a Mg²+ reservoir and that the PmrA- controlled LPS modifications neutralize surface negative charges when Mg²⁺ is transported into the cytoplasm during growth in Mg²⁺-limited environments.