Comparative antibacterial, chemical and morphological study of essential oils of Thymbra spicata var. spicata leaves by solvent-free microwave extraction and hydro-distillation

Abstract

The essential oils of Thymbra spicata var. spicata were obtained by both hydrodistillation and solvent-free microwave extraction methods. Chemical composition and antibacterial activity of the essential oils were examined. The main constituents of both essential oils obtained by solvent-free microwave extraction and hydrodistillation from Thymbra spicata var. spicata were carvacrol (44.8-36.1%), γ-terpinene (23.8-26.1%), and p-cymene (7.3-9.6%), respectively. The essential oil obtained from solvent-free microwave extraction contained substantially higher amounts of oxygenated compounds and lower amounts of monoterpenes than hydrodistillation. The antibacterial activities of the essential oils from solvent-free microwave extraction and hydrodistillation were evaluated by the disc diffusion method against six bacterial strains. The essential oil extracted by solvent-free microwave extraction was more effective than the essential oil extracted by hydrodistillation against the tested bacteria except for E. coli ATCC 25922- and S. aureus ATCC 25923+. Especially, S. aureus ATCC 25923+ was more sensitive against the essential oils obtained from Thymbra spicata var. spicata. Scanning electron microscopy measurements show that glands (that are filled with essential oils) were subjected to more severe thermal stresses and localized high pressure by microwave heating, the pressure build up within the glands could have exceeded their capacity for expansion and break down of cell walls. Solvent-free microwave extraction resulted in more extract release more rapidly than in hydrodestillation.