Mechanisms of the Escherichia coli and Enterococcus faecalis inactivation by ozone

The aim of this study is to investigate the inactivation mechanism of ozone on Escherichia coli and Enterococcus faecalis, which signifies a Gram-negative and a Gram-positive bacterium, respectively. For bacterial inactivation, 1 mg/L ozonated water was used with the contact times of 30 s, 1, 5, 10 and 20 min. The TTC dehydrogenase relative activity of E. faecalis was inactivated after 30 s of ozone treatment; however, 20 min were needed for almost total inactivation of E. coli according to flow cytometry analysis. 30 s of treatment resulted in the release of intracellular components (i.e. DNA and protein) for both types of cells. The rapid increase of K+ leakage after 30 s of treatment for E. faecalis and 5 min for E. coli indicated the deterioration of cell membrane integrity. Due to lipopolysaccharide content in the cell membrane, lipid peroxidation of E. coli peaked within 30 s of ozone treatment. Although the death rate was significantly higher for E. faecalis, more severe cell wall injury in E. coli was detected by TEM. These results indicate that disruption of the cell wall integrity is not the only reason for E. faecalis cell mortality; damage of intracellular components is also required.