Synergistic bactericidal effect of hot water with citric acid against Escherichia coli O157:H7 biofilm formed on stainless steel.

This study investigated the antimicrobial effect of hot water with citric acid against Escherichia coli O157:H7 biofilm on stainless steel (SS). Hot water (50, 60, or 70 °C) with 2% citric acid exhibited a synergistic bactericidal effect on the pathogen biofilm. It was revealed that hot water and citric acid combination induced sub-lethally injured cells. Additionally, mechanisms of the synergistic bactericidal effects of hot water with citric acid were identified through several approaches. In terms of biofilm matrix, hot water removes exopolysaccharides, a major component of extracellular polymeric substances (EPS), thereby increasing contact between surface cells and citric acid, resulting in a synergistic bactericidal effect. In terms of the cell itself, increased permeability of citric acid through cell membranes destructed by hot water promotes the inactivation of superoxide dismutase (SOD) in E. coli O157:H7, which induce synergistic generation of reactive oxygen species (ROS) which promote inactivation of cell by activating lipid peroxidation, resulting in destruction of the cell membrane. Therefore, it is interpreted that when hot water with citric acid is applied to E. coli O157:H7 biofilm, synergy effects on the biofilm matrix and cell itself have a complex interaction with each other, thus causing a dramatic synergistic bactericidal effect.