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Antimicrobial Agents and Chemotherapy, June 2005, p. 2474-2478, Vol. 49, No. 6
0066-4804/05/$08.00+0     doi:10.1128/AAC.49.6.2474-2478.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.

Mechanisms of Antibacterial Action of Three Monoterpenes

Department Farmaco-Biologico, University of Messina, Messina,¹ Department of Chemical Sciences, University of Catania, Catania,² Department of Pharmacology of Natural Substances and General Physiology, University of Rome "La Sapienza," Rome, Italy³

Received 11 October 2004/ Returned for modification 23 October 2004/ Accepted 6 February 2005

In the present paper, we report the antimicrobial efficacy of three monoterpenes [linalyl acetate, (+)menthol, and thymol] against the gram-positive bacterium Staphylococcus aureus and the gram-negative bacterium Escherichia coli. For a better understanding of their mechanisms of action, the capability of these three monoterpenes to damage biomembranes was evaluated by monitoring the release, following exposure to the compounds under study, of the water-soluble fluorescent marker carboxyfluorescein from unilamellar vesicles with different lipidic compositions (phosphatidylcholine, phosphatidylcholine/phosphatidylserine [9:1], phosphatidylcholine/stearylamine [9:1], and phosphatidylglycerol/cardiolipin [9:1]). Furthermore, the interaction of the terpenes tested with dimyristoylphosphatidylcholine multilamellar vesicles as model membranes was monitored by means of differential scanning calorimetry. Finally, the results were related to the relative lipophilicity and water solubility of the compounds examined. Taken together, our findings lead us to speculate that the antimicrobial effect of (+)menthol, thymol, and linalyl acetate may result, at least partially, from a perturbation of the lipid fraction of microorganism plasma membrane, resulting in alterations of membrane permeability and in leakage of intracellular materials. Besides being related to physicochemical characteristics of the drugs (such as lipophilicity and water solubility), this effect seems to be dependent on lipid composition and net surface charge of microbial membranes. Furthermore, the drugs might cross the cell membranes, penetrating into the interior of the cell and interacting with intracellular sites critical for antibacterial activity.

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