A Small Multidrug Resistance Transporter in Pseudomonas aeruginosa Confers Substrate-Specific Resistance or Susceptibility

Small Multidrug Resistance (SMR) transporters are key players in bacterial defense against toxins, providing resistance to antibiotics, antiseptics, and other homeostasis-perturbing compounds in multidrug-resistant pathogens. However, recent evidence demonstrates that EmrE, an SMR from Escherichia coli and a model for understanding transport, can also induce susceptibility to some compounds by drug-gated uncontrolled proton leak. Proton Motive Force (PMF) rundown is a relatively unexplored drug target, and is orthogonal to the targets of most known antibiotics. This requires an SMR to be merely present, rather than be the primary resistance mechanism, and dissipates the energy source for many other efflux pumps. PAsmr, an EmrE homolog from P. aeruginosa, confers resistance to some EmrE substrates in cells and in purified systems. We hypothesized that PAsmr, like EmrE, could confer susceptibility to some compounds. Growth assays of E. coli expressing PAsmr displayed substrate-dependent resistance and susceptibility phenotypes, and in vitro solid-supported membrane electrophysiology experiments revealed that PAsmr performs both antiport and substrate-gated proton uniport, confirming functional homology between PAsmr and EmrE. Growth assays of P. aeruginosa strain PA14 demonstrated that PAsmr contributes resistance to some antimicrobial compounds, consistent with prior data, but confers susceptibility to at least one aminoglycoside antibiotic. Phenotypic differences between P. aeruginosa and E. coli expressing PAsmr point to differential impacts of proton leak, constituting an advance in our understanding of underlying resistance mechanisms in P. aeruginosa and prompting further investigation into the role that SMRs play in antibiotic resistance in pathogens.