The Antimicrobial Effects of Magnolol on the Intracellular pH of Bacillus subtilis WB746 and Escherichia coli B23

Abstract

The antimicrobial activity of many cyclic hydrocarbons and phenolic compounds has been ascribed to membrane toxicity. Magnolol, extracted from Magnolia officinalis, is a member of a diverse family of plant phenols known to exhibit potent antibacterial effects on Grampositive, periodontal organisms. However, the mechanism by which this action is realized is hitherto uninvestigated. In this study, we attempted to characterize the molecular basis of the toxic attributes of magnolol by using Bacillus subtilis WB746 and Escherichia coli B23. Considering the structural features of magnolol and the biological activity of other plant phenols, an effort was made to examine the influence of the compound on membrane energetics, specifically the proton gradient. In order to elucidate this possibility, the antimicrobial efficacy of the compound was first ascertained with the minimum inhibitory concentration (MIC), the affected cells were then studied for morphological aberrations with light microscopy and, finally, the intracellular pH of each cell type was monitored for apparent changes in the ∆ pH. Accordingly, we have determined the MIC of magnolol for B. subtilis WB746 (25 µg/ml) and for E. coli B23 (50 µg/ml). Although we could not observe any significant morphological alterations to the cells at these MICs, we did witness a notable drop to the ∆ pH of B. subtilis WB746. E. coli B23, on the other hand, did not appear to experience a similar collapse in proton gradient. As our evidence indicates, it is likely that magnolol exerts its antibacterial effects on the Gram-positive organism by disruption of its energy metabolism at the membrane; however, additional means of action may be responsible, as is probably the case with E. coli B23.