Abstract
Isoeugenol is an essential oil constituent of nutmeg, clove, and cinnamon. Despite isoeugenol's promising antimicrobial activity, no studies have yet investigated its mode of antibacterial action at the molecular level. The aim of this study is to clarify isoeugenol's antibacterial mode of action using the Gram-negative and Gram-positive model organisms Escherichia coli and Listeria innocua, respectively. We determined the antimicrobial activity of isoeugenol against the model organisms, and examined how isoeugenol affects cell morphology, cell membrane permeabilization, and how isoeugenol interacts with phospholipid membranes using vesicle and supported lipid bilayer models. Isoeugenol demonstrated a bactericidal activity against E. coli and L. innocua that did not affect cell morphology, although the cell membrane was permeabilized. We hypothesized that the cell membrane was the primary site of action, and studied this interaction in further detail using purified membrane model systems. Isoeugenol's permeabilization of calcein-encapsulated vesicles was concentration dependent, and isoeugenol's interaction with giant unilamellar vesicles indicated increased membrane fluidity and a non-disruptive permeabilization mechanism. This contradicted membrane fluidity measurements on supported lipid bilayers (SLBs), which indicated decreased membrane fluidity. However, further investigations demonstrated that the interaction between isoeugenol and bilayers was reversible, and caused membranes to display heterogeneous topography, an increased mass, and a higher degree of hydration. In conclusion, we propose that isoeugenol interacts with membranes in a reversible non-disruptive detergent-like manner, which causes membrane destabilization. Furthermore, we argue that isoeugenol increases membrane fluidity. Our work contributes to the understanding of how essential oil constituents interact with cell components.
Highlights
Essential oils from plants, herbs, or spices constitute a broad range of low molecular weight organic compounds
Antimicrobial Activity of Isoeugenol We assessed the influence of isoeugenol at different concentrations on growth and cell viability of E. coli and L. innocua cells using turbidity measurements and plate spreading, respectively
Isoeugenol at MIC or 2× MIC initially decreased cell viability, but after 16 h of treatment, E. coli cells treated at MIC regained growth, suggesting that isoeugenol exhibit a reversible inhibitory activity against E. coli cells (Figure 2A)
Summary
Essential oils from plants, herbs, or spices constitute a broad range of low molecular weight organic compounds. These constituents can be categorized into four groups based on their chemical structure: terpenes, terpenoids, phenylpropenes, and a group containing other metabolic degradation products from plants (Hyldgaard et al, 2012a). These oils and their individual constituents have received attention as a natural resource of food additives that can function as aromatic flavorings, antioxidants, and antimicrobials (Rajakumar and Rao, 1993; Brenes and Roura, 2010; Hyldgaard et al, 2012a). Isoeugenol’s antibacterial activity covers a broad range of Grampositive and Gram-negative bacteria, including Escherichia coli, Bacillus licheniformis, Micrococcus luteus, Pseudomonas aeruginosa, Salmonella type B, and Staphylococcus aureus (Zemek et al, 1979, 1987; Laekeman et al, 1990)
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