Ability of Essential Oil Vapours to Reduce Numbers of Culturable Aerosolised Coronavirus, Bacteria and Fungi

Abstract

Transmission of pathogens present in the indoor air can occur through aerosols. This study evaluated the efficacy of an evaporated mix of essential oils to reduce the numbers of culturable aerosolized coronavirus, bacterium and fungus. The essential oil-containing gel was allowed to vaporize inside a glass chamber for 10 or 20 min. Aerosols of a surrogate of SARS-CoV-2, murine hepatitis coronavirus MHV-1, Escherichia coli or Aspergillus flavus spores were produced using a collision nebuliser and passed through the essential oil vapours, then collected on a six-stage Andersen sampler. The six-stages of the impact sampler capture aerosols in sizes ranging from 7 to 0.65 µm. The number of culturable microbes present in the aerosols collected in the different stages were enumerated and compared to the number of culturable microbes in control microbial aerosols that were not exposed to the evaporated essential oils. After 10 and 20 min evaporation, the essential oils reduced the numbers of culturable aerosolized coronavirus by 48% (log₁₀ reduction = 0.3; p = 0.002 vs. control) and 53% (log₁₀ reduction = 0.3; p = 0.001 vs. control), respectively. The essential oils vaporised for 10 min, reduced the number of viable E. coli by 51% (log₁₀ reduction = 0.3; p = 0.032 vs. control). The Aspergillus flavus spores were mostly observed in the larger aerosols (7.00 µm to 2.10 µm) and the essential oils vaporised for 10 min reduced the number of viable spores by 72% (log₁₀ reduction = 0.6; p = 0.008 vs. control). The vapours produced by a gel containing naturally occurring essential oils were able to significantly reduce the viable numbers of aerosolized coronavirus, bacteria and fungal spores. The antimicrobial gel containing the essential oils may be able to reduce aerosol transmission of microbes when used in domestic and workplace settings.

Keywords: SARS-CoV-2 surrogate; aerosol; antimicrobial; essential oils.

Figure 1

Number of murine hepatitis viral cells (MHV-1) recovered from different aerosol sizes with or without neutralisation of the vaporised essential oils for 10 min (A) or 20 min (B). The antimicrobial gel significantly (*) reduced the ability of viral aerosols to infected A9 cells in aerosols sizes of 2.10 and 1.10 µm compared to the untreated control (p = 0.011). Data points represent the mean (±95% confidence interval) of three independent experiments. BSA = bovine serum albumin.

Figure 2

Demonstration of neutralization of active ingredients (essential oils) in the gel. (A,D) in the absencde of neutralisers, (B–F) in the presence of neutralisers. TSA = tryptic soy agar; SDA = Sabouraud’s dextrose agar.

Figure 3

Numbers of E. coli K12 recovered from different aerosol sizes with or without neutralisation of the vapours of essential oils for 10 min. The gel significantly reduced the viability of bacteria in aerosols sizes 3.30, 2.10 and 1.10 µm when essential oils were non-neutralised during bacterial growth compared to the untreated control (*, p = 0.001). Data points represent the mean (±95% confidence interval) of three independent experiments. CFU = colony forming units.

Figure 4

Number of A. flavus spores recovered from different aerosol sizes with or without neutralisation of the vaporized essential oils for 10 min. The essential oils significantly reduced the viability of spores of A. flavus in aerosols sizes 3.30 and 2.10 µm in both neutralised and non-neutralised conditions compared to the untreated control (*, p < 0.05). Data points represent the mean (±95% confidence interval) of three independent experiments. CFU = colony forming units.

Figure 5

The bacterial filtration efficiency rig containing an Anderson sieve sampler. Aerosols of 3.0 ± 0.3 µm on average of viruses, bacteria or fungal spores were produced in the chamber.