Currently there is increasing interest in antibacterial effects of blended essential oils. Our study aimed to determine a suitable ratio of citronella (C), lemongrass (L), and patchouli (P) essential oils, which could be used to combat the acne causing bacteria Propionibacterium acnes and Staphylococcus epidermidis. MIC values of the oil blends were estimated by broth dilution method. Effects of blended oil against P. acnes and S. epidermidis were assessed from the fraction inhibitory concentration index (FICI). Patchouli oil exhibited the strongest activities against both microorganisms, but excessive patchouli oil in the blend was antagonistic to effects against both microorganisms. A suitable blend ratio was C:L:P (1:2:1) by weight, which showed the strongest antibacterial activities against P. acnes and S. epidermidis (MIC = 0.3125 and 0.625 mg/ml, respectively), with synergistic effects (FICI = 0.09 and 0.20, respectively). The major constituents in the oil blend were citronellal, citral, and patchouli alcohol, provided by each type of oil.
