The membrane potential as the driving force for the accumulation of lysine by Staphylococcus aureus

One of the most significant attributes of the chemiosmotic hypothesis [l] is that it relates directly not only to oxidative phosphorylation, but to all of the many energetic and control functions associated with cellular membranes. This feature is particularly relevant in the study of energy coupling mechanisms in bacteria with their single membranous structure. In the field of transport, one would predict that both the major components of the protonmotive force, membrane potential and pH gradient, might be capable of driving the transport of ions and nutrients across membranes against their prevailing electrochemical gradients. A flux of cations might therefore be driven by the membrane potential (inside negative), while anion transport might respond to the pH gradient (inside alkaline) through the action of a proton-symport; such a cotransport with protons would give a positively charged entity with, for example, the neutral sugars, which could then respond to both the membrane potential and the pH gradient [2,3]. We have set out to test this prediction with respect to the transport of the basic, neutral and acidic amino acids into the intracellular pool of Staphylococcus aureus. This paper deals specifically with our findings with the most experimentally accessible case, that of lysine, which, at neutral pH, exists predominantly as the positively charged species. Metabolically depleted cells, possessing relatively high proton and potassium permeabilities, maintain protons near Donnan equilibrium. In such cells there is a membrane potential which is largely an ionic diffusion potential; this may be varied by ionic manipulation, for example, by varying the extracellular potassium concentration in the presence of valinomycin. We have