2.1. Anti-bacterial activity
We first analyzed the anti-bacterial activities of essential oils towards
P. acnes. The data obtained from the disc diffusion method indicated that thyme essential oil exhibited the strongest inhibitory activities. The inhibition zone diameter measured was 40.0 ± 1.2 mm. Cinnamon essential oil also possessed considerable antibacterial activity, and the inhibition diameter determined was 33.5 ± 1.5 mm. Jasmine essential oil exhibited the lowest inhibitory activity (
Figure 1). The results of minimum inhibitory concentrations (MICs) revealed that the thyme, cinnamon and rose essential oils exhibited the best anti-bacterial activities towards
P. acnes. The MIC values were 0.016% (v/v), 0.016% (v/v) and 0.031% (v/v), respectively (
Table 1). The anti-bacterial activity of jasmine essential oil was lower than that of other essential oils, and its minimum bactericidal concentration (MBC) was 0.5% (v/v). The MBC values of all 10 essential oils were comparable to their corresponding MIC values (
Table 1).
Figure 1.
Inhibition diameters of 10 essential oils towards P. acnes.
Figure 1.
Inhibition diameters of 10 essential oils towards P. acnes.
Table 1.
Minimal inhibitory concentrations (MICs, %v/v) and minimal bactericidal concentrations (MBCs, %v/v) of 10 essential oils towards P. acnes.
Table 1.
Minimal inhibitory concentrations (MICs, %v/v) and minimal bactericidal concentrations (MBCs, %v/v) of 10 essential oils towards P. acnes.
| cinnamon | rose | thyme | chamomile | lavender | jasmine | grapefruit | lemon | ginger | mint |
MIC | 0.016 | 0.031 | 0.016 | 0.125 | 0.125 | 0.500 | 0.250 | 0.250 | 0.250 | 0.250 |
MBC | 0.016 | 0.031 | 0.016 | 0.125 | 0.125 | 0.500 | 0.250 | 0.250 | 0.250 | 0.250 |
2.2. Time-kill curves
The time-kill curves of the 10 essential oils are shown in
Figure 2. Rose, cinnamon, thyme, and lavender essential oils exhibited the strongest bactericidal activities at a concentration of 0.25% (v/v); the bacteria were completely killed within 5 min. The bactericidal activities of the other essential oils decreased in the order: chamomile > grapefruit = lemon > ginger > mint > jasmine. The bacteria were completely killed after 20 min by chamomile essential oil, 30 min by grapefruit and lemon essential oils, and 45 min by ginger essential oil. However, mint essential oil did not kill bacteria even after 120 min. The jasmine essential oil exhibited the lowest bactericidal activity, and the number of
P.acnes still retained 10
5 CFU/mL after 120 min.
Figure 2.
Time-kill curves of 10 essential oils (0.25% v/v) towards P.acnes in 2h. (A): essential oils of mint, ginger, rose, cinnamon, and grapefruit; (B): essential oils of jasmine, lavender, chamomile, thyme, and lemon.
Figure 2.
Time-kill curves of 10 essential oils (0.25% v/v) towards P.acnes in 2h. (A): essential oils of mint, ginger, rose, cinnamon, and grapefruit; (B): essential oils of jasmine, lavender, chamomile, thyme, and lemon.
2.3. Cytotoxic activity towards cancer cells
To investigate the cytotoxic activities, three human tumor cell lines, A-549, PC- 3 and MCF-7, were exposed to increasing concentrations of essential oils. Cell viability was determined by the MTT assay. As shown in
Figure 3 and
Table 2 the essential oils revealed different cytotoxic activities towards the three human cancer cell lines investigated. In general, a dose-dependent decrease in the survival of the three tumor cell lines was observed. However, mint essential oil exhibited no effect on A549 cell over a concentration range of 0.002% to 0.2% (v/v).
At a concentration of 0.002% (v/v), the essential oils did not considerably affect the viability of the three human tumor cell lines compared with untreated control cells. The cell survival after treatment with essential oils was more than 80%.
At a concentration of 0.200% (v/v), however, all essential oils exhibited strong cytotoxicities towards PC-3 cells. Cell viability was lower than 4%. Most essential oils exhibited strong cytotoxicities towards A549 cells. However, cells treated with mint essential oil still grew well, and the number of survival cells was comparable to that of untreated control cells. For MCF-7 cell, the cytotoxictiies of cinnamon, thyme, chamomile, and jasmine essential oils was significantly stronger than that of the other six essential oils. The fractions of viable cells were reduced to 5.31%, 3.47%, 6.93% and 4.34%, respectively. Essential oils from grapefruit and ginger exhibited the lowest cytotoxicities towards MCF-7 cells. The percentages of cells viability were 75.03% and 81.85%, respectively.
Of all essential oils investigated, thyme essential oil exhibited the strongest cytotoxicities towards cancer cells. The IC50 values for thyme essential oil against PC-3, A549 and MCF-7 cells were 0.010%, 0.011% and 0.030% (v/v), respectively. Moreover, cinnamon and jasmine essential oils possessed stronger cytotoxic activities towards PC-3 and A549 cell lines. IC50 values for cinnamon essential oil against PC-3 and A549 cells were 0.012% (v/v) and 0.017% (v/v). The IC50 values for jasmine essential oil against these two cell lines were 0.022% (v/v) and 0.012% (v/v) respectively. However, the IC50 values for cinnamon and jasmine essential oils against MCF-7 cells were 0.076% (v/v) and 0.077% (v/v). MCF-7 cell was less sensitive than the other cell lines towards the essential oils.
Figure 3.
Dose-dependent cytotoxicity of 10 essential oils (72 h exposure) towards PC-3 (A), A549 (B) and MCF-7 (C) cell lines as determined by the MTT assay. Values are expressed as means ± SD of three independent experiments. Standard deviations were less than 15%.
Figure 3.
Dose-dependent cytotoxicity of 10 essential oils (72 h exposure) towards PC-3 (A), A549 (B) and MCF-7 (C) cell lines as determined by the MTT assay. Values are expressed as means ± SD of three independent experiments. Standard deviations were less than 15%.
Table 2.
Inhibition concentrations 50% (IC50, %v/v) values for 10 essential oils of PC-3, A549 and MCF-7 cancer cell lines as determined by the MTT assay.
Table 2.
Inhibition concentrations 50% (IC50, %v/v) values for 10 essential oils of PC-3, A549 and MCF-7 cancer cell lines as determined by the MTT assay.
| cinnamon | rose | thyme | chamomile | lavender | jasmine | grapefruit | lemon | ginger | mint |
PC-3 | 0.012 | 0.040 | 0.010 | 0.071 | 0.050 | 0.022 | 0.094 | 0.083 | 0.077 | 0.088 |
A549 | 0.017 | 0.055 | 0.011 | 0.067 | 0.133 | 0.012 | 0.100 | 0.061 | 0.107 | ------- |
MCF-7 | 0.076 | 0.074 | 0.030 | 0.072 | 0.142 | 0.077 | ------- | 0.143 | ------- | ------- |
Anti-bacterial and cytotoxic activities of essential oils can be attributed to their different constituents. Essential oils comprise complex mixtures, including monoterpenes and sesquiterpenes, such as limonene, menthol, α-pinene, 3-carene, and α-farnesol, etc. Some compositions have been reported for their anti-bacterial activities towards bacteria and fungi [
21,
22]. The anti-cancer activities of some monoterpenes and sesquiterpenes was also reported in the literature [
23,
24].
Until now, various authors have reported antitumor activities of essential oils as well as their components. For instance, the lavender essential oil was found to be active against COL-2 [
25], the aldehyde compounds of
Citrus paradisi essential oil induced apoptosis strongly in HL-60 cells [
26], and thyme essential oil, which contains carvacrol, as the major component has an important
in vitro cytotoxic activity against tumor cells [
27]. In our results, ten essential oils (except for grapefruit, ginger and mint) also showed excellent antitumor activities against PC-3, A549, MCF-7. However, further studies are urgently needed for screening for the mechanism of the antitumor activity.