Functional promiscuity of small multidrug resistance transporters from Staphylococcus aureus, Pseudomonas aeruginosa, and Francisella tularensis

Small multidrug resistance (SMR) transporters efflux toxic substrates from bacterial cells and were recently divided into two subfamilies: specific toxic metabolite transporters and promiscuous drug exporters. These drug exporters are thought to function similarly to EmrE, the model system for this subfamily of SMR transporters. Studies of EmrE homologs indicate that they are able to confer resistance to EmrE substrates in E. coli and in their native organisms. Recent work from our lab showed that functional EmrE can confer resistance or susceptibility in vivo depending on the drug substrate. Here, we test whether this functional promiscuity of EmrE extends to SMR transporters from three additional human or animal pathogens: SAsmr from Staphylococcus aureus, PAsmr from Pseudomonas aeruginosa, and FTsmr from Francisella tularensis. We find that these SMR homologs can confer either resistance or susceptibility to different toxic substrates in E. coli. This demonstrates that the ability of a single transporter to lead to opposite biological outcomes when transporting different substrates is a general property of the promiscuous multidrug transporters in the SMR family. It also suggests the potential for novel antibiotic development targeting these transporters with small molecules that trigger susceptibility. Such a strategy does not require that the target be the primary mode for antibiotic resistance because the goal is not simple inhibition of activity, but rather activation of an alternative transport function that is detrimental to bacteria.