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The present study describes chemical composition and antimicrobial activity of the essential oil from Origanum bilgeri, which is endemic for Turkey. The essential oil was obtained by hydrodistillation and components of the essential oil were analyzed by Gas Chromatography-Mass Spectrometry (GC-MS). The major component of O. bilgeri essential oil was carvacrol (84.8 %) and p-cymene (4.0 %), γ-terpinene (3.3 %) were the other components. Antimicrobial activity of the essential oil was investigated against standard control and resistant strains, using disc diffusion, minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) methods. O. bilgeri essential oil showed very strong antimicrobial activity against all tested pathogenic bacteria except P. aeruginosa. Especially, it is important that the essential oil was more effective than antibiotics tested against methicillin-resistant Staphylococcus aureus (MRSA), vancomycin resistant Enterococcus faecalis (VRE).

The essential oils (EOs) obtained by solvent-free microwave extraction (SFME) and hydrodistillation (HD) from endemic Origanum husnucanbaseri H. Duman, Aytac & A. Duran were investigated using the gas chromatography-mass spectrometry system. The main constituents of both oils obtained from SFME and HD from O. husnucanbaseri are borneol (15.2-12.8%), α-terpineol (12.3-10.8%) and trans-sabinene hydrate (11.8-9.92%). The EO obtained from SFME contained substantially higher amounts of oxygenated compounds and lower amounts of monoterpenes than that from HD. The antibacterial activities of the EOs from SFME and HD were evaluated by the disc diffusion method against six bacterial strains. The EO extracted by SFME was more effective than the EO extracted by HD against the tested bacteria, except for Klebsiella pneumoniae American type culture collection (ATCC) 13883. Streptococcus pyogenes ATCC 19615 and Staphylococcus aureus ATCC 25923 in particular were more sensitive against the EO extracted by SFME.