Antibacterial activity of essential oil-bearing herbs in vapour phase against respiratory pathogens and headspace analysis optimization of Thymus vulgaris sample

While aerosolized antibiotics remain the recommended treatment for lower respiratory infections, difficulties to implement nebulization techniques have limited their widespread adoption. The volatile constituents of essential oils (EOs) appear as a promising alternative in the development of novel inhaled antibiotic therapy. In this study we first determined the in vitro antibacterial activity of five EO-bearing herbs vapour phase against respiratory pathogens including Staphylococcus aureus, Streptococcus pyogenes and Haemophilus influenzae using broth-microdilution volatilization (BMV) method. Then, with the aim of optimizing a protocol for the characterization of EO vapours, the chemical profile of the most active EO (Thymus vulgaris) was determined using two headspace sampling techniques coupled with GC/MS. While Pimpinella anisum, Mentha × piperita, Foeniculum vulgare and Eucalyptus globulus vapours didnʼt exhibit any antibacterial activity at MIC1024 µg/mL, MICs of Thymus vulgaris EO samples ranged from 512 to 1024 µg/mL. Moreover, GC/MS analysis results showed a different distribution of compounds in the headspace: thymol peak percentage area was unusually low – 5.27% (HS-SPME) and 0.60% (HS-GTS) – compared to the EOs (max. 48.65%). Multiple factors could explain these results such as parameters related to the use of a fibre coating assembly but also the experimental conditions like the matrix used. Overall, this study suggests that the procedure examined could be further exploited to better assess the benefits of EO volatile compounds for inhalation therapy against respiratory infections; however, further toxicological evaluation will be necessary to verify its potential practical use. We declare no conflict of interest.