Transfer of cationic antibacterial agents berberine, palmatine, and benzalkonium through bimolecular planar phospholipid film and Staphylococcus aureus membrane

Some organic cations are known to be electrophoretically imported into bacterial cells and actively extruded from these cells by multidrug resistance (MDR) pumps. We have studied penetration of plant antimicrobial agents berberine and palmatine and synthetic antiseptic benzalkonium chloride through black planar phospholipid membrane (BLM) and membrane of Staphylococcus aureus cells. Gradients of these cations across BLM generated an electric potential difference. Penetrating anion tetraphenyl borate and phloretin (a plant substance decreasing membrane dipole potential) stimulated this effect. Under optimal conditions, the magnitude of the electric potential was close to theoretical, that is, 60 mV/10-fold cation gradient. Berberine accumulated in S. aureus cells as shown by, direct measurement of berberine with a berberine-sensitive electrode. The berberine accumulation was prevented by protonophore CCCP and was stimulated by mutation in the MDR pump NorA. It is concluded that the plant alkaloids and benzalkonium are penetrating cations and substrates of an MDR pump.