The antimicrobial efficacy of native Australian essential oils in liquid and vapour phase against foodborne pathogens and spoilage microorganisms

Abstract

Foodborne pathogens and spoilage microorganisms are a source of concern for public health and the food industry. Considering the problem of antimicrobial resistance in microbes, controlling these microorganisms using synthetic antimicrobials is challenging, thus, essential oils are a potential natural alternative. Previous studies have predominantly focussed on the antimicrobial activity of essential oils in the liquid phase. However, in this work we report on the in vitro antimicrobial activity of the liquid and vapour phases of essential oils from native Australian plants Tasmanian mountain pepper (Tasmannia lanceolata), lemon myrtle (Backhousia citriodora), as well as the common herb thyme (Thymus vulgaris) against 11 common food spoilage and pathogenic bacteria and fungi. Initial screening was conducted using the disc diffusion assay. The minimum inhibitory concentration (MIC) of the oils was determined using the broth microdilution assay. The vapour phase antimicrobial activity of the oils was assessed using an inverted petri plate assay and the vapour phase MIC (V-MIC) was quantified using a vapour diffusion assay. GC-MS was adopted to elucidate the compositional profile of the oil in liquid phase. Two extraction methods, namely solid-phase microextraction (HS-SPME) and gas tight syringe headspace sampling (HS) coupled with GC-MS were used for profiling the composition of the volatiles. The findings of this study confirmed the antimicrobial activity of the oils in both liquid and vapour phases, with lemon myrtle and thyme generally showing stronger antimicrobial activity than Tasmanian mountain pepper. The activity in liquid phase was generally higher than the volatiles. However, lemon myrtle and Tasmanian mountain pepper berry volatiles were more effective against Aspergillus niger and Saccharomyces cerevisiae, respectively, relative to the liquid phase highlighting their potential as vapour phase antifungals. In the liquid and gas phases, the major compounds in lemon myrtle were Z-citral (neral) and E-citral (geranial). Polygodial and thymol were the major compounds in Tasmanian mountain pepper and thyme in the liquid phase, respectively. However, the main compounds in their volatiles were p-cymene and γ-terpinene. In conclusion, our research indicates the essential oils' potent vapour phase activity, highlighting their potential application in gaseous diffusion-dependent food applications such as active antimicrobial food packaging.