Acetic Acid and Lactic Acid Inhibition of Growth ofSaccharomyces Cerevisiaeby Different Mechanisms

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Abstract
Two strains of Saccharomyces cerevisiae were grown in minimal medium with glucose as the carbon source and with added acetic acid or lactic acid. The intracellular pH (pHi) of yeast cells was not significantly affected by acetic acid up to a concentration of 0.25% w/v. The pHi was maintained at a more or less constant value through increased activity of plasma membrane H+-adenosinetriphosphatase (H+-ATPase), which pumps out protons. With lactic acid at a concentration of 0.4% w/v or higher, the pHi dropped enough to affect yeast growth. This drop in pHi was the result of reduced activity of membrane-bound H+-ATPase. Cell membrane lipids of yeast cells grown in the presence of 0.5% w/v lactic acid contained considerably reduced levels of the two unsaturated fatty acids palmitoleic and oleic. Changes in the fatty acid composition of cells grown with acetic acid were relatively small. These differences in internal pH, plasma membrane H+-ATPase activities, and membrane fatty acid composition all indicate that acetic and lactic acids inhibit the growth of S. cerevisiae by different mechanisms.