Mechanism for the antibacterial activity of a mixture of 3,4,5-trihydroxy benzoic acid and magnesium oxide

In this paper, the mechanism for the antibacterial activity of a mixture of 3,4,5-trihydroxy benzoic acid (GA) and magnesium oxide (MgO) for Escherichia coli (JCM1649, E coli) is investigated. Antibacterial activity was assayed by the liquid dilution method against E coli. E coli in the logarithmic phase and the culture medium with pepton, yeast extract, NaCl, glucose, and MgSO4 7H2O were used in all experiments. The logarithmic phase culture of E coli (1x105 CFU cm-3) was added to the culture medium with GA and/or MgO. A Clark-type oxygen electrode with 1.1 cm-3 volume cell was used for the measurements of the oxygen consumption profile of a mixture of GA and MgO. The concentration of hydrogen peroxide (H2O2) in the mixture was estimated by using porphyrinato-titanium complex. Efficient antibacterial activity was observed on a mixture of 2 mg cm-3 GA and 2 mg cm-3 MgO. The addition of 110 units cm-3 catalase, which is an enzyme of H2O2 decomposition, in a mixture of 2 mg cm-3 GA and 2 mg cm-3 MgO led to the decrease of the antibacterial activity as same as the case of 2 mg cm-3 MgO. Moreover, the production of H2O2 in the mixture was confirmed by the oxygen consumption profile under the addition of 500 units cm-3 catalase. The accumulation of 3-4 mmol dm-3 H2O2 was confirmed in a mixture of 2 mg cm-3 GA and 2 mg cm-3 MgO. The increase in the concentration of H2O2 from 3 mmol dm-3 to 4 mmol dm-3 led to a drastic increase in the antibacterial activity. Based on the results of this study, a possible mechanism for the antibacterial activity of a mixture of GA and MgO is discussed, concerning a synergy effect on the antibacterial action of MgO and that of H2O2.