A Small Multidrug Resistance Transporter inPseudomonas aeruginosaConfers Substrate-Specific Resistance or Susceptibility

Abstract

ABSTRACTSmall 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 fromEscherichia coliand 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 fromP. 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 ofE. coliexpressing PAsmr displayed substrate-dependent resistance and susceptibility phenotypes, andin vitrosolid-supported membrane electrophysiology experiments revealed that PAsmr performs both antiport and substrate-gated proton uniport, confirming functional homology between PAsmr and EmrE. Growth assays ofP. aeruginosastrain PA14 demonstrated that PAsmr contributes resistance to some antimicrobial compounds, consistent with prior data, but conferssusceptibilityto at least one aminoglycoside antibiotic. Phenotypic differences betweenP. aeruginosaandE. coliexpressing PAsmr point to differential impacts of proton leak, constituting an advance in our understanding of underlying resistance mechanisms inP. aeruginosaand prompting further investigation into the role that SMRs play in antibiotic resistance in pathogens.IMPORTANCESmall multidrug resistance transporters are a class of efflux pumps found in many pathogens, but whose contributions to antibiotic resistance are poorly understood. We hypothesize that in addition to conferring resistance to toxic compounds, these transporters may also confer susceptibility by dissipating the proton-motive force. This means that an SMR merely needs to be present in the pathogen in order to be targeted (as opposed to being the primary resistance mechanism). Here, we test this hypothesis with an SMR found in the pathogenPseudomonas aeruginosaand find that it can confer both resistance and susceptibility depending upon the compounds it interacts with.