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

Abstract

Cyclic hydrocarbons and phenolic compounds are known to insert into the cellular membranes of Gram-positive and Gram-negative bacteria, leading to expansion of membranes, leakage of ions and collapse of the proton motive force. Honokiol is a plant phenol extracted from Magnolia officinalis that has been used for a variety of therapeutic purposes in Chinese medicine, such as antimicrobial activity. This inhibitory action has been ascribed to membrane permeabilization based on the structure of honokiol, but the mechanism of action is yet to be determined experimentally. In this study, we sought to elucidate the molecular foundation for the observed antibacterial activity of honokiol. The minimum inhibitory concentration (MIC) was determined for the two species tested, Bacillus subtilis WB746 and Escherichia coli B23, at 25 μ g/ml and 100 μ g/ml, respectively. These MICs were used to assay the antibacterial action of honokiol, and correlate cell death to reduction in intracellular pH. Changes in intracellular pH were measured using the pH-sensitive fluorescent dye, carboxyfluorescein diacetate succinimidyl ester. Variations in intracellular pH were observed in both organisms which we theorized resulted in altered proton motive force and membrane energetics. Our findings indicated that the antimicrobial properties of honokiol may include disruption of cellular energy metabolism at the membrane.