Fig 1.
The chemical structures of the phenolic EO compounds.
(A) hinokitiol; (B) carvacrol; (C) thymol; (D) menthol.
Table 1.
The physical characteristics of hinokitiol, carvacrol, thymol, and menthol.
Fig 2.
The antibacterial activities of the phenolic EO compounds.
(A) MRSA treated with 500 μg phenolic EO compounds, as assessed using direct contact agar diffusion tests. (B) The phenolic EO compounds were all tested at 500 μg. The microorganisms examined were A. actinomycetemcomitans (Aa), S. mutans (Sm), MRSA, and E. coli. Dotted line, the 0.6 cm diameter of the filter disc. * P < 0.05, ** P < 0.01, *** P < 0.001 compared with A. a. in each compound group; a, b, c, and d were P < 0.05, compare with A. a., S. m., MRSA, and E. coli in the hinokitiol group, respectively; e, P < 0.01 based on a comparison of the carvacrol and thymol groups.
Table 2.
The MIC and MBC of the four phenolic EO compounds against four microorganisms (μg/mL).
Fig 3.
Phenolic EO compounds delay the microorganism growth curves in a concentration-dependent manner.
Various concentrations of the phenolic EO compounds were used to test their impact on the bacterial growth curves. The bacterial growth curves in the presence of various phenolic EO concentrations (hollow diamond, triangle, square, and circle) were compared to each control (solid circle). Broth-only treatment served as a negative control (solid square). Y axis, OD600; X axis, time (sec).
Fig 4.
The phenolic EO compounds were heat stable.
The phenolic EO compounds (500 μg) were pre-incubated at 4–100°C for 1 h before the direct contact agar diffusion test. (A) Hinokitiol; (B) carvacrol; (C) thymol. Dotted line, the 0.6 cm diameter of the filter disc.
Fig 5.
The vaporous phenolic EO compounds display antibacterial activity.
(A) The vapor phase agar diffusion experimental device. (B) MRSA treated with 500 μg phenolic EO compounds was examined by vapor phase agar diffusion tests. (C) The vapors from 500 μg phenolic EO compounds were tested by vapor phase agar diffusion. The liquid and vapor phases of (D) hinokitiol, (E) carvacrol, and (F) thymol showed different antibacterial activities after incubation at different temperatures. *P < 0.05, compared with 4°C in each curve. Dotted line, the 0.6 cm diameter of the filter disc.
Fig 6.
The antibacterial activity of vaporous hinokitiol was stable under different pH conditions.
The antibacterial activities of (A) vaporous hinokitiol, (B) vaporous carvacrol, and (C) vaporous thymol were analyzed under different pH conditions.
Fig 7.
Synergistic antibacterial effects of the phenolic EO compounds.
(A) The synergistic effects of hinokitiol (H) and zinc oxide (ZnO) against A. actinomycetemcomitans (A. a.) were tested by direct contact and vapor phase agar diffusion tests. (B) The synergism of the anti-MRSA activity of carvacrol (C) and thymol (T) was tested by direct contact agar diffusion tests. a, P < 0.01 compared with the direct contact 250H group.