Inhibition of Staphylococcus aureus on a laboratory medium and black peppercorns by individual and combinations of essential oil vapors

The objectives of this study were to screen essential oil (EO) vapors for inhibitory activities against Staphylococcus aureus on a laboratory medium and to validate their antibacterial effects on dried black peppercorns (Piper nigrum). Among 84 commercially available EOs, using a vapor disc assay, vapors from caraway seed, carrot seed, cinnamon bark, citronella, kanuka, lemongrass, may chang, oregano, and thyme thymol EOs were found to be more inhibitory to S. aureus. The minimal inhibitory concentration (MIC) and minimal lethal concentration (MLC) of nine EO vapors from these plant materials were measured on a laboratory medium. Citronella, lemongrass, and oregano EO vapors had the lowest MIC and MLC values, at 0.0781 and 0.1563 μL/mL, respectively. Using a checkerboard assay, partial synergism in inhibitory activity of a combination of citronella and lemongrass EO vapors was observed (fractional inhibitory concentration index = 0.6250). When tested against S. aureus on black peppercorns, citronella, lemongrass, and oregano EO vapors, both individually and in combination, showed a significant antibacterial effect (P ≤ 0.05). Notably, a combination of citronella and lemongrass EO vapors (1:4 vol ratio) showed significant synergistic activity, reducing the S. aureus population by ca. ≥ 1.3–1.8 log CFU/g more than treatment with single EOs (P ≤ 0.05). The results of this study provide important basic information for developing technologies to control S. aureus on the surfaces of dried foods using EO vapors.