Evaluation of adaptive response in E. coli O157:H7 to UV light and gallic acid based antimicrobial treatments

Ability of bacteria to develop cross-protection to various stresses is well-known. However, exploration into cross-protection between conventional physiological stresses and emerging antimicrobial treatments is limited. Previously, we developed two new antimicrobial treatments based on a combined UV-A light and gallic acid (UVA + GA) treatment and enhanced antimicrobial action of gallic acid by UV-C exposure (UVC-GA) against Escherichia coli O157:H7. In this study, we investigated (a) whether prior exposure to sub-lethal stresses (heat, acid, osmotic and oxidation) can increase the resistance of E. coli O157:H7 towards these newly developed treatments, and (b) whether repeated exposure to these two treatments selects for E. coli O157:H7 sub-population resistant towards heat, acid, and oxidative challenge. Prior exposure to heat or acid stress increased (p < 0.05) the resistance of E. coli O157:H7 towards UVA + GA treatment, while osmotic stress increased (p < 0.05) their sensitivity to that treatment. Interestingly, only heat stress showed protective (p < 0.05) effect to subsequent UVC-GA treatment, while acid or osmotic stress increased (p < 0.05) the sensitivity. Prior exposure to oxidative stress had no effect (p > 0.05) on the sensitivity of the bacteria to either of the treatments. Repeated exposure to UVA + GA or UVC-GA treatment selected for a sub-population that demonstrated higher (p < 0.05) resistance towards these two treatments as well as heat or acid challenge. Further experiments showed that increased synthesis of enzymes such as superoxide dismutase and up-regulation of general stress response regulator were likely associated with the development of cross-protection. Thus, sub-lethal conditions encountered during food processing should be considered in the development and optimization of emerging non-thermal technologies.