The relationship between the chemical structure of a molecule and its antimicrobial activity is a promising concept in the discovery of new anti-infective agents. Due to their typical physico-chemical properties, the potential of volatile antimicrobials lies especially in the inhalation therapies for treatment of respiratory infections. Although there were some attempts to determine the antimicrobial effect of certain classes of plant-derived volatiles such as oxygenated monoterpenes [1], acyclic, bicyclic and p-menthane terpenoids [2] in liquid phase, the effect in volatile phase is still not well understood.
In this study, the growth-inhibitory effect of certain representatives of p-menthanes was tested using broth microdilution volatilisation method in liquid and vapour phase [3] against standard strains of bacterial pathogens causing respiratory infections such as Haemophilus influenzae, Staphylococcus aureus, Streptococcus pneumoniae, and Streptococcus pyogenes.
As a result, thymoquinone produced the strongest antibacterial activity against all strains tested with MICs ranging from 2 to 32 μg/mL in liquid phase and from 4 to 64 μg/mL vapour phase. Thymol and carvacrol have an antimicrobial activity varying in ranges of 64 to 256 μg/mL both in vapour and liquid phase. 1,8-cineol and p-cymene have no relevant antimicrobial activity neither in liquid, nor vapour phase. The results suggest that quinone structure contribute significantly to the antibacterial action of p-menthanes in vapour phase. In addition, presence of hydroxyl group on the p-cymene skeleton increases its antibacterial action. In comparison to previous experiments, this study was performed with broader spectrum of bacteria and the activity of tested compounds was compared with other groups of monoterpenes (e.g. pinanes). The results can lead to the development of new medicinal products being used as inhalation therapy acting directly in the respiratory tract.
