Abstract
The aim of the present study was to determine the antibacterial activities of the phenolic essential oil (EO) compounds hinokitiol, carvacrol, thymol, and menthol against oral pathogens. Aggregatibacter actinomycetemcomitans, Streptococcus mutans, Methicillin-resistant Staphylococcus aureus (MRSA), and Escherichia. coli were used in this study. The minimum inhibitory concentrations (MICs), minimum bactericidal concentrations (MBCs), bacterial growth curves, temperature and pH stabilities, and synergistic effects of the liquid and vapor EO compounds were tested. The MIC/MBC of the EO compounds, ranging from the strongest to weakest, were hinokitiol (40–60 μg/mL/40-100 μg/mL), thymol (100–200 μg/mL/200-400 μg/mL), carvacrol (200–400 μg/mL/200-600 μg/mL), and menthol (500-more than 2500 μg/mL/1000-more than 2500 μg/mL). The antibacterial activities of the four EO phenolic compound based on the agar diffusion test and bacterial growth curves showed that the four EO phenolic compounds were stable under different temperatures for 24 h, but the thymol activity decreased when the temperature was higher than 80°C. The combination of liquid carvacrol with thymol did not show any synergistic effects. The activities of the vaporous carvacrol and thymol were inhibited by the presence of water. Continual violent shaking during culture enhanced the activity of menthol. Both liquid and vaporous hinokitiol were stable at different temperatures and pH conditions. The combination of vaporous hinokitiol with zinc oxide did not show synergistic effects. These results showed that the liquid and vapor phases of hinokitiol have strong anti-oral bacteria abilities. Hinokitiol has the potential to be applied in oral health care products, dental materials, and infection controls to exert antimicrobial activity.
Highlights
Essential oils (EOs) are volatile oily liquids obtained from different parts of plants
A. actinomycetemcomitans was more sensitive to the phenolic EO compounds than the other bacteria
The phenomenon was not observed in the CHX group. These results suggested that the EO phenolic compounds might evaporate to interfere with bacterial growth, and the molecular diffusion could be excluded as a factor affecting the findings
Summary
Essential oils (EOs) are volatile oily liquids obtained from different parts of plants. EOs are widely used in food preservation and health care products because of their potent antibacterial activity [1,2,3], reduction of oxidative stress [4], and anti-inflammatory activities [5]. Many EOs are generally recognized as safe by the Food and Drug Administration (FDA) of the United States and have been used as artificial flavorings and preservatives. EOs are often diluted in solvents for sprays and rinses or are heated to volatilize them to prohibit bacterial growth and eliminate unpleasant odors. Many EOs contain terpenoids, which include phenols, aldehydes, ketones, alcohols, ethers, and hydrocarbons. Phenolic EOs have stronger antibacterial activity than other constituents. The antibacterial activities of the terpenoids are affected by their functional groups, hydrophobicity, and environmental conditions
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