Inhibition of Listeria monocytogenes by acetate, benzoate and sorbate: Weak acid tolerance is not influenced by the glutamate decarboxylase system

Aims: Weak acids are widely used by the food industry to prevent spoilage and to inhibit the growth of pathogenic micro-organisms. In this study the inhibitory effects of three commonly used weak acids, acetic acid, benzoic acid and sorbic acid, on the growth of Listeria monocytogenes were investigated. Methods and Results: In a chemically defined medium at pH 6·4 benzoic acid had the greatest inhibitory effect (50% inhibition of growth at 4 mmol l^-1), while acetate was the least inhibitory (50% inhibition of growth at 50 mmol l^-1). Mutants lacking either sigmaB (ΔsigB) or two of the glutamate decarboxylase systems (ΔgadAB) were used to investigate the contribution these systems make to weak acid tolerance in L. monocytogenes. Conclusions: The stress-inducible sigma factor sigmaB (σ^B) was not required for protection against acetate and played only a minor role in tolerating benzoate and sorbate. The glutamate decarboxylase system, which plays an important role in tolerating inorganic acids, played no significant role in the ability of L. monocytogenes to tolerate these weak acids, and neither did the presence of glutamate in the growth medium. Significance and Impact of the Study: These results suggest that the effectiveness of weak acid preservatives in food will not be compromised by the presence of glutamate, at least under mildly acidic conditions.