Abstract:The antibacterial activity against Staphylococcus aureus of long-chain fatty alcohols was investigated, with a focus on normal alcohols. The antibacterial activity varied with the length of the aliphatic carbon chain and not with the water/octanol partition coefficient. 1-Nonanol, 1-decanol and 1-undecanol had bactericidal activity and membrane-damaging activity. 1-Dodecanol and 1-tridecanol had the highest antibacterial activity among the long-chain fatty alcohols tested, but had no membrane-damaging activity. Consequently, it appears that not only the antibacterial activity but also the mode of action of long-chain fatty alcohols might be determined by the length of the aliphatic carbon chain.

We have studied changes in the antibacterial activity and the mode of action of farnesol against Staphylococcus aureus when two terpene alcohols with an aliphatic carbon chain were added, individually, to a bacterial suspension that contained farnesol. Geraniol increased the growth-inhibitory activity of farnesol, but suppressed its ability to damage cell membranes, which is one of the predominant features of the growth-inhibitory activity of farnesol. Geranylgeraniol decreased the growth-inhibitory activity of farnesol and also suppressed its cell-damaging activity. It is possible that the presence of a terpene alcohol can both enhance and suppress the antibacterial activity of farnesol, and even change its mode of action. Thus, it is important to study not only the antibacterial activity of each constituent of an essential oil but also the interactions between them in efforts to characterize the antibacterial activity of the essential oil.

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To investigate the mechanism of action by which farnesol functions as an antibacterial agent and inhibits Staphylococcus aureus growth, the growth rates of S. aureus cultured in farnesol versus S. aureus cultured in farnesol and supplemented with 3-hydroxy-3-methylglutaryl (HMG)-CoA or mevalonate were compared. The investigation was designed to observe whether farnesol affected on the mevalonate pathway by using the intermediate metabolites of the pathway. The resulting growth curves demonstrated that mevalonate reduced the antibacterial activity of farnesol, but HMG-CoA did not inhibit farnesol. These results suggest that farnesol affects the mevalonate pathway. Moreover, farnesol inhibited HMG-CoA reductase activity in an in vitro enzymatic assay.

Farnesol, a sesquiterpene alcohol with an aliphatic carbon chain, inhibited the growth of Staphylococcus aureus and induced the leakage of potassium ions. We investigated the action of farnesol on the cytoplasmic membrane of S. aureus. No ion channels that would account for the loss of potassium ions were detected. Electron paramagnetic resonance measurements suggested that farnesol proceeds into the cytoplasmic membrane of S. aureus cells, where it induces the disordering and eventual disruption of the cytoplasmic membrane. This was supported by the result that the effects of farnesol decreased by the addition of carotenoid which was the stabilizing reagent for the lipid bilayer.Key words farnesol; cell membrane disordering; Staphylococcus aureus Staphylococcus aureus is an opportunistic pathogen. Novel antibiotics against this organism are needed due to the depletion of therapeutic options resulting from the development of resistance to various antibiotics and, consequently, the increasing difficulty of curing nosocomial infections. We previously investigated plant-derived essential oils and their constituents and found that some terpene alcohols with aliphatic carbon chains, such as farnesol, exhibited antibacterial activity against S. aureus.1) The cell membrane of S. aureus was damaged after only a few minutes exposure to farnesol, but the underlying mechanism of this activity was not elucidated.2,3) The aim of the present study was to determine the mechanism of farnesol-induced damage to the cell membrane of S. aureus. The findings of this study could aid in safely and effectively treating infections of this bacterium and enhance efforts to develop more effective medicines for treating S. aureus infections. MATERIALS AND METHODSGeneral Farnesol, carotenoid, 5-doxyl stearic acid (5-NS), 16-doxyl stearic acid methyl ester (16-NMS) and melittin were purchased from Sigma-Aldrich (St. Louis, MO, U.S.A.). Tetraethyl ammonium chloride and quinine hydroxide were purchased from Wako Pure Chemical Industries, Ltd. (Osaka, Japan). S. aureus FDA209P was used as the standard strain. 4)Nitro Blue Tetrazolium (NBT) Assay S. aureus cultures were incubated in brain-heart infusion (BHI) broth overnight, harvested, washed twice with phosphate-buffered saline (PBS), and resuspended in PBS. Farnesol was added to 1 mL of the bacterial suspension (ca. 10 5 colony forming units CFU mL . The cells were then incubated for 30 min at 37°C after the addition of 0.1% NBT solution. The assay was stopped by the addition of 0.1 M hydrochloric acid and the cells were pelleted by centrifugation (1500×g, 10 min, 4°C) and resuspended in dimethyl sulfoxide (600 µL) and PBS (800 µL). The absorbance of the solution at 575 nm was measured using a DU-65 spectrophotometer (Becton, Dickinson and Company, Franklin Lakes, NJ, U.S.A.)Electron Paramagnetic Resonance (EPR) Measurements EPR measurements were made using spin-labeling reagents (5-NS and 16-NMS).5) Spin-labeling reagents were dissolved individually in chloroform (0.1 mg mL −1 ), and a 25-µL ...