Qom Univ Med Sci J 2017 December 70 Qom Univ Med Sci J 2017;11(10):70-76 The Antimicrobial Effect of Methanolic Extracts of Achillea wilhelmsii, Myrtus communis, and Allium sativum on Pseudomonas aeruginosa Mehdi Rostamirad 1 , Ali Javadi 1 , Tayebeh Akbari 2 , Azam Heidarpour 3 , Mitra Jamali 4 , Roya Mahdavi 5* , Mohammad Khodadad Motlagh 6 Original Article 1 Department of Medical Bacteriology, Tehran University of Medical Sciences, Tehran, Iran. 2 Department of Medical Microbiology, North Tehran Branch, Islamic Azad University, Tehran, Iran. 3 Department of Microbiology, Faculty of Medicine, Qom University of Medical Sciences, Qom, Iran. 4 Kerman University of Medical Sciences, Kerman, Iran. 5 Department of Medical Immunology, Faculty of Paramedical Sciences, Birjand University of Medical Sciences, Birjand, Iran. 6 Department of Microbiology, Faculty of Veterinary Medicine, Shiraz University of Medical Sciences, Shiraz, Iran. * Corresponding Author: Roya Mahdavi, Department of Medical Immunology, Faculty of Paramedical Sciences, Birjand University of Medical Sciences, Birjand, Iran. Email: royamahdavi986@gmail.com Received :8 Feb, 2017 Accepted: 6 Aug, 2017 Abstract Background and Objectives: Pseudomonas aeruginosa is a Gram- negative bacterium, which has the ability to cause opportunistic infections in patients with immunodeficiency. The conditions of the patient as well as the response of infection-producing strain to treatment, can be very effective in infection progression process, so that, infectivity process leads to bacteremia and sepsis, causing a difficult situation. Increasing resistance of this bacterium has prompted researchers to discover an effective agent to suppress the pathogenicity of this pathogen. Since ancient times, herbal extracts have been used to treat various infections. In this study, the antimicrobial effect of methanolic extracts of Achillea Wilhelmsii, Myrtus communis, and Allium sativum, was investigated on Pseudomonas aeruginosa. Methods: In this study, the effect of Achillea Wilhelmsii, Myrtus communis, and Allium sativum extracts, was investigated on 4 strains of Pseudomonas aeruginosa, and the effect of each extract, was studied using agar dilution method. Results: Among these three extracts, the Allium sativum extract showed the highest antimicrobial activity. Also, observations were indicative of difference in the susceptibility of the studied strains to different extracts, which showed different reactions to each of the extracts based on the origin and antibiotic resistance level. Conclusion: According to the results of this study, extracts are a natural and valuable sources to produce antimicrobial drugs against pseudomonas strains and other resistant pathogenic bacteria. Keywords: Pseudomonas aeruginosa; Methanolic extract; Achillea; Myrtus; Garlic.