A molecular analysis of the mechanisms of weak acid inhibition in Escherichia coli
Weak acids are commonly used in the food industry as food preservatives however a complete understanding of their mode of action is not available. This study aimed to gain an understanding of the inhibitory mechanisms of two weak acids: sodium acetate and sodium benzoate. The ability of these weak acids to perturb the internal pH of cells was determined, a phenomenon previously shown as a mode of action of weak acids. However, changes in pHi alone were not sufficient in explaining the growth inhibition observed with sodium acetate, suggesting that the weak acid anion may also contribute to growth inhibition. The effect of the accumulation of was weak acid anions was found to be a reduction of cellular amino acid pools, mainly glutamate. When cells were transferred from medium containing weak acid to acid-free medium, recovery of internal pH was shown to be dependent on glutamate synthesis. The relieving effects of methionine on weak acid inhibition were also investigated. The aim of this section of the study was to identify the mechanism by which methionine resulted in relief of weak acid inhibition. It was found that addition of weak acid resulted in a 16-fold increase of the intracellular concentrations of homocysteine. This methionine precursor has been shown to act as a competitive inhibitor of methionine for methyl-tRNA-synthetase. Addition of methionine is therefore proposed to reduce weak acid inhibition by reducing homocysteine pools by feedback inhibition and by out-competing the intermediate.