Abstract
While the inhalation of Thymus vulgaris L. essential oil (EO) is commonly approved for the treatment of mild respiratory infections, there is still a lack of data regarding the antimicrobial activity and chemical composition of its vapours. The antibacterial activity of the three T. vulgaris EOs against respiratory pathogens, including Haemophilus influenzae, Staphylococcus aureus, and Streptococcus pyogenes, was assessed in both liquid and vapour phases using the broth microdilution volatilisation (BMV) method. With the aim of optimising a protocol for the characterisation of EO vapours, their chemical profiles were determined using two headspace sampling techniques coupled with GC/MS: solid-phase microextraction (HS-SPME) and syringe headspace sampling technique (HS-GTS). All EO sample vapours exhibited antibacterial activity with minimum inhibitory concentrations (MIC) ranging from 512 to 1024 μg/mL. According to the sampling technique used, results showed a different distribution of volatile compounds. Notably, thymol was found in lower amounts in the headspace—peak percentage areas below 5.27% (HS-SPME) and 0.60% (HS-GTS)—than in EOs (max. 48.65%), suggesting that its antimicrobial effect is higher in vapour. Furthermore, both headspace sampling techniques were proved to be complementary for the analysis of EO vapours, whereas HS-SPME yielded more accurate qualitative results and HS-GTS proved a better technique for quantitative analysis.
Highlights
Pneumonia is an acute lower respiratory infection (ALRI) mainly caused by bacterial pathogens, including Gram-positive bacteria such as Staphylococcus aureus, Streptococcus pneumoniae, and Streptococcus pyogenes, or Gram-negative strains such as Haemophilus influenzae [1,2,3]
Essential oil samples from dry plant material of T. vulgaris from three different suppliers were tested against three standard bacterial strains associated with respiratory infections using the broth microdilution volatilisation (BMV) method (Table 1)
The least effective essential oil (EO) source was from supplier A: results showed mild efficacy against H. influenzae (512 μg/mL), whereas a weaker inhibitory effect of 1024 μg/mL was observed against both S. aureus and S. pyogenes
Summary
Pneumonia is an acute lower respiratory infection (ALRI) mainly caused by bacterial pathogens, including Gram-positive bacteria such as Staphylococcus aureus, Streptococcus pneumoniae, and Streptococcus pyogenes, or Gram-negative strains such as Haemophilus influenzae [1,2,3] They have been amongst the major causes of morbidity and mortality worldwide; this is significant in low-income countries where vulnerable populations such as children under five years old, the elderly, and the immunocompromised population are at higher risk [4]. Nebulised antibiotics offer significant advantages over intravenous and oral therapies; amongst others, inhalation therapy allows to deliver high drug concentrations and to have more efficient and faster action while reducing the risk of side effects compared to systemic administrations [8] Today, such treatments are already included in care protocols, especially for cystic fibrosis. Drug delivery is affected by the quality of inhalations which hinge on the patient’s age (especially children and elderly), the severity of the respiratory disease, or even the physical capability to perform a breathing manoeuvre [15]
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