Mode of action of organic acids against bacterial foodborne pathogens and investigation of improved disinfection methods

Assuring the microbiological safety of fresh produce can present a huge challenge for food producers. Organic acid washes, are commonly used as antimicrobial treatments. However, our understanding of how foodborne pathogens interact with these acids is limited. Bacteria have evolved a variety of mechanisms to promote survival under acidic conditions presented in various environments (e.g. animal stomach). These mechanisms contribute to maintaining a tolerable intracellular pH when the organism is presented with an acidic challenge. This work explores the effect of weak organic acids notably fumarate on the inhibition of amino acid decarboxylase systems which are the most potent acid resistance mechanisms. We demonstrate that sodium fumarate reduces survival in planktonic cultures and biofilms of E. coli, L. monocytogenes and Salmonella under acidic conditions. This effect stems from effects on glutamate amino acid decarboxylase system function or transcription. In E. coli fumarate inhibited the glutamate decarboxylase (GAD) activity and output resulting in lower survival. In L. monocytogenes it also inhibited GAD activity although the organism responded by gadD2 upregulation resulting in higher GABA export, suggesting that possible effects on intracellular GAD activity or other systems might be responsible. In Salmonella, fumarate inhibited the activity of the lysine decarboxylase (LDAR) system under acidic stress resulting in reduced survival. Subsequently, an examination of the possible usage of fumarate in decontamination procedures on fresh produce. Fumarate significantly improved the efficacy of a commercial acidic disinfectant, which was also significantly higher than that of chlorine. Furthermore, with the use of the phenotypic microarrays we explored the effect of significant stress genes (e.g. sigB) in carbon source utilisation and osmotic tolerance of L. monocytogenes. The work demonstrated for first time a self-preservation and nutritional competence (SPANC) balance in a Gram-positive bacterium particularly in the absence of sigB, which enhanced carbon source utilisation in expense to lower stress tolerance