Magnesium depletion by Candida albicans unleashes two unusual modes of colistin resistance in Pseudomonas aeruginosa with different fitness costs.

Increasing bacterial resistance to colistin, a vital last-resort antibiotic, is an urgent challenge. Previous studies have shown that Mg2+ depletion enables Pseudomonas aeruginosa to become resistant to colistin. Here, we show that magnesium sequestration by Candida albicans also enables P. aeruginosa to evolve a nearly hundredfold higher level of colistin resistance through genetic changes in lipid A biosynthesis-modification pathways and a putative magnesium transporter. These mutations synergize with the Mg2+-sensing PhoPQ two-component signaling system to remodel lipid A structures of the bacterial outer membrane in previously uncharacterized ways. One predominant mutational pathway involves early mutations in htrB2, a non-essential gene involved in lipid A biosynthesis, which enhances resistance but compromises outer membrane integrity, resulting in fitness costs and increased susceptibility to other antibiotics. A second pathway achieves increased colistin resistance independently of htrB2 mutations without compromising membrane integrity. In both cases, reduced colistin binding to the bacterial membrane underlies resistance. Our findings reveal that Mg2+ scarcity triggers novel evolutionary trajectories, leading to extremely high colistin resistance in P. aeruginosa.