Small multidrug resistance protein EmrE reduces host pH and osmotic tolerance to metabolic quaternary cation osmoprotectants.

Small multidrug resistance (SMR) transporter protein EmrE in Escherichia coli are known to confer resistance to toxic antiseptics classified as quaternary cation compounds (QCC). Naturally derived QCC synthesized during metabolic activities often act as osmoprotectants, such as betaine and choline, and participate in osmotic homoestasis. The goal of this study was to determine if EmrE proteins transport biological QCC based osmoprotectants. Plasmid encoded copies of E. coli emrE and the inactive variant emrE-E14C were expressed in various E. coli strains grown in either rich or minimal media at various pH (5-9) and hypersaline (0.5- 1.0 M NaCl and KCl) conditions to identify changes in growth phenotypes induced by osmoprotectant transport. The results demonstrated that EmrE expression reduced pH tolerance of E. coli strains at or above neutral pH and when grown in hypersaline media at or above 0.75 M NaCl or KCl concentrations. Hypersaline growth conditions were used to screen QCC osmoprotectants betaine, choline, L-carnitine, L-lysine, L-proline, and L-arginine. The study indentified that betaine and choline are natural QCC substrates of EmrE.