Staphylococcus epidermidis, a common bacterium on the human epithelia, is considered an opportunistic pathogen. To explore potential treatments of S. epidermidis infection, essential oils, which display antibacterial properties, were tested. The uncertainty of active ingredient concentrations and differences in extraction protocols in essential oils from bottle to bottle have made it difficult to accurately determine the amount of essential oil needed to inhibit bacterial growth and the active ingredients involved in growth inhibition. This study aims to quantitate and identify these essential oil active ingredients involved in S. epidermidis antibacterial activity using GCMS (Gas Chromatography Mass Spectrometry) and determine their MIC (minimum inhibitory concentration). To this end, Lemongrass, Clove Bud, and Cinnamon Leaf essential oils and their separate active ingredients, were tested for antibacterial activity by performing two different Kirby-Bauer Disk Diffusion methods. First, a wet method was used in which disks were soaked in dilute essential oil suspensions with ultra-pure water as the diluent and placed wet onto inoculated tryptic soy agar plates. While Lemongrass displayed S. epidermidis growth inhibition, Clove Bud did not show inhibition with the wet disk method, most likely due to solubility limitations. Therefore, a second method was employed where essential oils and active ingredients were dissolved in methanol, pipetted onto the disks, and placed on inoculated tryptic soy agar plates once the methanol had evaporated. The dry method showed lower MICs likely due to evaporation of active ingredient; however, unlike the wet method, inhibition of growth was seen by the eugenol in the Clove Bud essential oil. It was determined that Clove, Lemongrass, and Cinnamon leaf oils all inhibited bacterial growth, with eugenol, citral, and cinnamaldehyde being the main antibacterial ingredients. Kirby-Bauer test results, MICs, as well as GCMS-determined active ingredient concentrations in essential oils, will be presented.

Determination of minimum inhibitory concentrations for active ingredients responsible for the antibacterial behavior of essential oils

ACS Spring 2022

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