Highlights Severe acute respiratory syndrome coronavirus 2 causes a respiratory infection, at times followed by severe systemic illness. Low dose (0.5–2 g/d) vitamin C may have benefits when used early in severe acute respiratory syndrome coronavirus 2 infections. Severe late forms of coronavirus disease 2019 are due to a cytokine storm involving interleukin-6 and endothelin-1. Vitamin C (especially at high doses) can reduce mediators such as interleukin-6 and endothelin-1. Thus, vitamin C may also benefit patients with severe late-stage coronavirus disease 2019.

Multi-resistant microorganisms continue to challenge medicine and fuel the search for new antimicrobials. Here we show that essential oils and their components are a promising class of antifungals that can have specific anti-Candida activity via their vapour-phase. We quantify the vapour-phase-mediated antimicrobial activity (VMAA) of 175 essential oils and 37 essential oil components, representing more than a 1,000 unique molecules, against C. albicans and C. glabrata in a novel vapour-phase-mediated susceptibility assay. Approximately half of the tested essential oils and their components show growth-inhibitory VMAA. Moreover, an average greater activity was observed against the intrinsically more resistant C. glabrata, with essential oil component citronellal having a highly significant differential VMAA. In contrast, representatives of each class of antifungals currently used in clinical practice showed no VMAA. The vapour-phase-mediated susceptibility assay presented here thus allows for the simple detection of VMAA and can advance the search for novel (applications of existing) antimicrobials. This study represents the first comprehensive characterisation of essential oils and their components as a unique class of antifungals with antimicrobial properties that differentiate them from existing antifungal classes.

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.