Assay and recommendations for the detection of vapour-phase-mediated antimicrobial activities

Antimicrobial activity assays can be carried out in aqueous solutions using multi-well plates. However, some bioactive compounds are volatile and can cause effects at a distance. To detect such vapour-phase-mediated antimicrobial activity, we introduce the vapour-phase-mediated patch assay, a simple bioassay that uses standard microtitre plates. As a proof-of-principle, we use the vapour-phase-mediated patch assay to test a small but chemically diverse set of selected essential oils with known antifungal activities i.e.Origanum compactum, Artemisia dracunculus, Cinnamomum camphora ct linalool, Cinnamomum cassia and Melissa officinalis, as well as their corresponding major components carvacrol, estragole, linalool, trans-cinnamaldehyde and citral, against two pathogenic Candida species. As all but one of the tested essential oils i.e.Artemisia dracunculus and its corresponding component estragole, showed vapour-phase-mediated antimicrobial activity, we conclude that it is a rather common characteristic of essential oils and their components, that should always be taken into consideration. Additionally, we provide suggestions to prevent false positive results due to possible vapour-phase-mediated antimicrobial activities in bioactivity tests.