Evaluation of in vitro antimicrobial activity of Thai basil oils and their micro‐emulsion formulas against Propionibacterium acnes

SynopsisThe aim of this study was to evaluate the efficacy of Thai basil oils and their micro‐emulsions, on in vitro activity against Propionibacterium acnes. An agar disc diffusion method was employed for screening antimicrobial activity of the essential oils of Ocimum basilicum L. (sweet basil), Ocimum sanctum L. (holy basil) and Ocimum americanum L. (hoary basil) against P. acnes. Minimum inhibitory concentration (MIC) values of the basil oils were determined using an agar dilution assay. The obtained results indicated that the MIC values of sweet basil and holy basil oils were 2.0% and 3.0% v/v, respectively, whereas hoary basil oil did not show activity against P. acnes at the highest concentration tested (5.0% v/v). Gas chromatography‐mass spectrometry analysis revealed that methyl chavicol (93.0%) was the major compound in sweet basil oil, and eugenol (41.5%), γ‐caryophyllene (23.7%) and methyl eugenol (11.8%) were major compounds in holy basil oil. Hoary basil oil contained high amounts of geraniol (32.0%) and neral (27.2%) and small amounts of methyl chavicol (0.8%). The Oil‐in‐water (o/w) micro‐emulsions of individual basil oils with concentrations corresponding to their MIC values were formulated. The stable o/w micro‐emulsion system for basil oil consisted of 55.0% v/v water phase, 10.0% v/v oil phase (2.0 or 3.0% v/v sweet basil or 3.0% v/v holy basil oil plus 7.0% v/v isopropyl myristate), 29.2% v/v polysorbate 80 and 5.8% v/v 1,2‐propylene glycol. Hydroxyethylcellulose at a concentration of 0.5% w/v was used as thickening agent. According to the disc diffusion assay, the formulations containing sweet basil oil exhibited higher activity against P. acnes than those containing holy basil oil, and the thickened formulations tended to give a lower activity against P. acnes than the non‐thickened formulations. The prepared micro‐emulsions were stable after being tested by a heat–cool cycling method for five cycles. These findings indicate the possibility to use Thai sweet and holy basil oil in suitable formulations for acne skin care.