Hamadan University of Medical Sciences Avicenna Journal of Clinical Microbiology and Infection 2383-0298 6 1 2019 02 01 In Vitro Antibacterial Activity of Origanum syriacum L. Essential Oils Against Gram-Negative Bacteria 26 30 10.34172/ajcmi.2019.06 EN Ayman AL-Mariri https://orcid.org/0000-0002-9586-6573 Adnan Odeh Bassel Alobeid Hassan Boukai Journal Article 10.34172/ajcmi.2019.06 2018 10 08 2019 01 23 Background: This study aimed to determine the chemical composition of Origanum syriacum L., as well as to assess the antimicrobial activity of O. syriacum essential oil and its constituents. Methods: To this end, O. syriacum plants were collected from their native growing locations in western and coastal governorates of Syria. Next, the composition of the essential oil from every station was determined by gas chromatography (GC) and then by high-performance liquid chromatography (HPLC) to estimate the number and the quantity of all components. Preparative-HPLC was used to isolate the essential oil components. Then, the identified constituents of the essential oils were confirmed utilizing GC-mass spectrometry. Microdilution broth susceptibility assay was applied and the first well without turbidity was considered as the minimum inhibitory concentration. Results: The results showed that ß-myrcene (21.93%), carvacrol (19.20%), anisaldehyde (7.57%), thymol (7.40%), γ–terpinene (5.27%), and sabinene (4.43%) were the main components of bulk essential oils. Similarly, only minor qualitative and quantitative variation was found between locations. The antibacterial activity of bulk essential oil and its components was evaluated against gram-negative local isolates of Escherichia coli O157, Salmonella enterica, Klebsiella pneumoniae, Yersinia enterocolitica O9, Brucella melitensis, Proteus mirabilis, and Pseudomonas aeruginosa. Although the bulk essential oil inhibits all the bacteria except for E. coli O157 at the highest tested concentration (48 μL/mL), the essential oil components differ in their antibacterial activity. Conclusions: Overall, thymol and carvacrol represented the most antibacterial activity compared to the other substances.