Small multidrug resistance (SMR) transporters contribute to antibiotic resistance through proton-coupled efflux of toxic compounds from the bacterial cytoplasm. Previous biophysical studies of the
E. coli
SMR transporter EmrE suggested that it should also be capable of performing proton/toxin symport or uniport, leading to toxin susceptibility rather than resistance
in vivo
. Here we show EmrE does confer susceptibility to several newly characterized small-molecule substrates in
E. coli
, including harmane.
In vitro
experiments show that harmane binding to EmrE triggers uncoupled proton uniport and this protein-mediated dissipation of the transmembrane pH gradient underlies the
in vivo
phenotype. This leads to synergy with some existing antibiotics, such as kanamycin. Furthermore, this shows that it is possible to not just inhibit multidrug efflux but activate alternative transport modes that are detrimental to bacterial growth and metabolism.