20 JOURNAL OF COSMETIC SCIENCE (IZD: 16 mm), cinnamon (IZD: 36.75 mm), and tea tree (IZD: 14.67 mm) EOs exhibited antimicrobial activity against C acnes (43). Esmael et al. (2020) reported that IZDs of tea tree oil were 21.02 mm and 20.85 mm, and IZDs of rosemary oil were 15.18 mm and 14.77 mm against S epidermidis and C acnes, respectively. Cinnamon, lavender, and lemon EOs were found to be resistant to both strains (44). In this study, both bacteria were found extremely sensitive (IZDs 20 mm) to oregano, cinnamon, lavender, and tea tree. These results were higher than previous reports. The other EOs (eucalyptus, laurel, sandalwood, and lemon) presented moderate activity similar to previous articles. MINIMUM INHIBITORY CONCENTRATION AND MINIMUM BACTERICIDAL CONCENTRATION VALUES OF ESSENTIAL OILS EOs having strong antibacterial potency (IZD ≥ 12) were selected to evaluate their bacteriostatic and bactericidal features using MIC/MBC assays, and results are shown in Table III. Oregano was the most active agent with the MIC/MBC values of 0.03125% (v/v) toward S epidermidis and 0.0625% (v/v) toward C acnes. The MIC/MBC values of tested EOs were in the range of 0.03125–64% (v/v). Taleb et al. (2018) reported oregano MIC/MBC values of 0.34/0.672 and 0.67/1.34 mg/ml tea tree 1.28/2.55 and 1.27/5.10 mg/ml and lavender 2.52 and 2.52 mg/ml against P acnes and S epidermidis, respectively (40). Previous studies demonstrated that the MIC of cinnamon was 0.13 and 0.25 mg/ml (45) sandalwood 0.50 and 0.25 mg/ml (46) and tea tree 0.31–0.63 and 0.63–1.25 mg/ml (47) against P acnes Table I Principal Phytochemical Components of the Organic EOs From Turkey EO (Plant species) Percentage of principal components (%)Lemon (C lemon) limonene (33.34%), p-cymene (18.45%), β-pinene (10.56%), α-pinene (3.70%), myrcene (2.45%), and limonene oxide (2.40%) Tea tree (M alternifolia) p-cymene (19.30%), α-pinene (7.25%), β-pinene (1.68%), limonene (5.90%), γ-terpinene (6.30%), α-terpinene (2.53%), terpinene-4-ol (10.37%), α-terpineol (9.96%), carvone (3.30%), isoborneol (2.79%), and caryophyllene oxide (4.37%) Lavender (L officinalis) linalol (20.67%), isoborneol (10.21%), 1.8-cineole (9.01%), delta-3-carene (6.11%), terpinene-4-ol (6.10%), β-caryophyllene (5.45%), α-pinene (3.55%), myrcene (1.27%), and α-bisabolol (1.87%) Laurel (L nobilis) 1.8-cineole (eucalyptol) (30.61%), sabinene (9.62%), p-cymene (7.90%), α-pinene (6.27%), β-pinene (4.87%), terpinolene (9.57%), terpinene-4-ol (5.49%), and linalol (1.68%) Eucalyptus (E globulus) eucalyptol (55.56%), p-cymene (25.96%), α-pinene (8.58%), limonene (1.36%), and α-terpineol (1.46%) Sandalwood (S album) β-santalol (28.90%), α-santalol (9.33%), α-bergamotol (11.65%), vanillin (1.37%), and isobornyl acetate (1.27%) Cinnamon (C verum) cinnamaldehyde (66.15%), cinnamil acetate (5.93%), α-pinene (5.66%), naphthalene (5.08%), delta-3-carene (3.62%), β-caryophyllene (2.64%), p-cymene (1.71%), copaene (1.04%), eugenol (1.32%), and carvone (2.06%) Oregano (O vulgare) carvacrol (42.7%), thymol (1.99%), p-cymene (16.40%), α-pinene (1.58%), myrcene (2.60%), α-terpinene (2.53%), terpinene-4-ol (2.23%), β-caryophyllene (3.02%), isoborneol (2.73%), linalol (4.50%), and naphthalene (1.25%)

21 CHARACTERIZATION AND ACTIVITY OF ESSENTIAL OILS and S epidermidis, respectively. In another study, the MIC and MBC values of lavender were 0.125, cinnamon 0.016, and lemon 0.250% (v/v) against P acnes (42). FRACTIONAL INHIBITORY CONCENTRATION INDEX OF THE BINARY COMBINATION OF ESSENTIAL OILS Possible synergistic effects of eight antibacterial effective EOs in dual combinations were investigated using the checkerboard method. FICI values were calculated and shown in Tables IV and V. Cinnamon/lavender and cinnamon/sandalwood EOs demonstrated the best activity with a synergistic effect (FICI ≤ 0.5) against both strains. In addition, cinnamon EO combined with lemon (FICI: 0.25) and eucalyptus (FICI: 0.27) eucalyptus EO combined with lavender (FICI: 0.50) and tea tree (FICI: 0.38) EOs displayed a synergistic action oregano/sandalwood (FICI: 1.00), cinnamon/tea tree (FICI: 0.63), and sandalwood/ lemon (FICI: 0.75) exhibited additive effect against S epidermidis. For C acnes, a synergistic inhibition effect and an additive effect were observed in the laurel/lavender (FICI: 0.047) and cinnamon/laurel (FICI: 0.563) EO mixtures. According to previous studies, tea tree in dual combination with lemon (FICI: 1.00), lavender (FICI: 1.00), eucalyptus (FICI: 0.75), and sandalwood (FICI: 0.98) and lemon with lavender (FICI: 1.00) and eucalyptus (FICI: 0.83) showed an additive effect against C acnes. On the other hand, tea tree in combination with lemon (FICI: 0.66), lavender (FICI: 0.56), and eucalyptus (FICI: 0.19) showed a synergistic Table II Inhibition Zone Diameters (mm) of EOs EOs S epidermidis ATCC 12228 p value C acnes ATCC 6919 p value Or 74.00 ± 5.888 p 0.0.1 37.50 ± 0.577 p 0.0.1 Lau 11.75 ± 0.957 p: 1.00 20.75 ± 0.957 p 0.0.1 Gi – – – – Ci 53.75 ± 1.708 p 0.0.1 40.00 ± 1.633 p 0.0.1 O – – – – La 28.50 ± 2.646 p 0.0.1 28.25 ± 1.258 p 0.0.1 Ch – – 8.75 ± 0.50 p: 0.478 (Ery) p: 0.540 (Clin) Sa 13.00 ± 0.817 p: 0.996 (Ery) p: 0.974 (Clin) 15.50 ± 0.577 p 0.0.1 Te 23.50 ± 3.109 p 0.0.1 22.00 ± 0.817 p 0.0.1 Le 19.75 ± 0.957 p 0.0.5 – – Eu 18.00 ± 1.414 p: 0.22 (Ery) p: 0.11 (Clin) 20.00 ± 0.817 p 0.0.1 Ro – – 8.75 ± 0.957 p: 0.629 (Ery) p: 0.677 (Clin) Ga – – 8.25 ± 0.50 p: 0.478 (Ery) p: 0.540 (Clin) Co – – – Cl – – 8.75 ± 0.50 p: 0.478 (Ery) p: 0.540 (Clin) Sag – – 9.00 ± 0.817 p: 0.577 (Ery) p: 0.629 (Clin) Ery 11.50 ± 0.577 11.00 ± 0.817 11.00 ± 0.817 11.25 ± 0.957 Clin Tween 80 – – Or: oregano Lau: laurel Gi: ginger Ci: cinnamon O: orange La: lavender Ch: chamomile Sa: sandalwood Te: tea tree Le: lemon Eu: eucalyptus Ro: rosemary Ga: garlic Co: coconut Cl: clove Sag: sage Ery: erythromycin and Clin: clindamycin as positive controls negative control: Tween 80 -:no inhibition results are mean—± standard deviation (SD).