Batch cultures of K-12 grew well in an anaerobic glucose medium at pH 5.9, but even small amounts of acetate (20 mM) inhibited growth and fermentation. O157:H7 was at least fourfold more resistant to acetate than K-12. Continuous cultures of K-12 (pH 5.9, dilution rate 0.085 h) did not wash out until the sodium acetate concentration in the input medium was 80 mM, whereas O157:H7 persisted until the sodium acetate concentration was 160 mM. K-12 cells accumulated as much as 500 mM acetate, but the intracellular acetate concentration of O157:H7 was never greater than 300 mM. Differences in acetate accumulation could be explained by intracellular pH and the transmembrane pH gradient (δpH). K-12 maintained a more or less constant δpH (intracellular pH 6.8), but O157:H7 let its δpH decrease from 0.9 to 0.2 units as sodium acetate was added to the medium. Sodium acetate increased the rate of glucose consumption, but there was little evidence to support the idea that acetate was creating a futile cycle of protons. Increases in glucose consumption rate could be explained by increases in -lactate production and decreases in ATP production. Intracellular acetate was initially lower than the amount predicted by ApH, but intracellular acetate and δpH were in equilibrium when the external acetate concentrations were high. Based on these results, the acetate tolerance of O157:H7 can be explained by fundamental differences in metabolism and intracellular pH regulation. By decreasing the intracellular pH and producing large amounts of D-lactate, O157:H7 is able to decrease δpH and prevent toxic accumulations of intracellular acetate anion.


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