Commercially Available Oil, Chemical Variability and Antimicrobial Activity.

L. oil is frequently recommended in the aromatherapeutic literature for treating respiratory, urinary, and skin infections; however, antimicrobial evidence is lacking. In addition, in aromatherapy, combinations of essential oils are predominantly utilized with the goal of achieving therapeutic synergy, yet no studies investigating the interaction of essential oil combinations with oil exists. This study thus aimed to address these gaps by investigating the antimicrobial activity of three oil samples, sourced from different suppliers, independently and in combination with 20 different commercial essential oils, against micro-organisms involved in respiratory, skin, and urinary tract infections associated with global resistance trends. These pathogens include several of the ESKAPE pathogens ( and spp.) The chemical profile of the oils was determined using gas chromatography coupled with mass spectrometry. The minimum inhibitory concentrations (MIC) were determined using the broth micro-dilution method. The interactive profiles for the combinations were assessed by calculating the fractional inhibitory concentration index (ΣFIC). The main compounds varied across the three samples, and included phenethyl alcohol, isopropyl myristate, 2-nonynoic acid, methyl ester, α-terpineol, α-cetone, and benzyl acetate. The oil samples displayed overall poor antimicrobial activity when tested alone; however, the antimicrobial activity of the combinations resulted in 55 synergistic interactions where the combination with resulted in the lowest MIC values as low as 0.13 mg/mL. The frequency of the synergistic interactions predominantly occurred against , , , and with noteworthy MIC values ranging from 0.25-1.00 mg/mL. This study also reports on the variability of oils sold commercially. While this warrants caution, the antimicrobial benefit in combination provides an impetus for further studies to investigate the therapeutic potential.