Effects of weak acids and external pH on the intracellular pH of Zygosaccharomyces bailii,and its implications in weak-acid resistance

Weak acids and hydrogen ions in different concentration combinations affect the intracellular pH value (pH_i) of Zygosaccharomyces bailii. The lowest pH_i value measured was not at the most extreme, but at intermediate conditions of inhibition. Proton and organic-acid ejection, on a cell volume basis, is greater in cells grown under inhibitory conditions and is stimulated by weak acids, whilst in cells not grown under inhibitory conditions acid efflux is lower and is depressed by weak acids; this may be important in the maintenance of tolerable pH_i values in the presence of weak acids. The concentration of benzoic acid measured internally is identical to the value expected from its pK, external pH and pH_i. Addition of fructose to starved cells causes both a decreased pH_i and a concomitant efflux of previously loaded benzoic acid, quantitatively in accord with the shift in equilibrium of the freely permeable undissociated acid. There is no evidence that weak acids are actively extruded. Protoplast volume also varies with hydrogen-ion and weak-acid concentration and this too may play a role in intracellular pH maintenace.