Molecular evaluation of colistin-resistant gene expression changes in Acinetobacter baumannii with real-time polymerase chain reaction Shahriar Sepahvand,1 Mohammad Ali Davarpanah,2 Amir Roudgari,3 Abbas Bahador,4 Vajihe Karbasizade,5 Zahra Kargar Jahromi6 1Department of Microbiology, Falavarjan Branch, Islamic Azad University, Isfahan, Iran; 2Shiraz HIV/AIDS Research Center, Institute of Health, Shiraz University of Medical Sciences, Shiraz, Iran; 3Shiraz Trauma Center, Shiraz University of Medical Sciences, Shiraz, Iran; 4Department of Microbiology, Faculty of Medicine, Tehran University of Medical Sciences, Tehran, Iran; 5Department of Microbiology, School of Medicine, Isfahan University of Medical Sciences, Isfahan, Iran; 6Zoonoses Research Center, Jahrom University of Medical Sciences, Jahrom, Iran Background: Acinetobacter baumannii is an important human pathogen which has recently gained increased attention due to the occurrence of drug-resistant nosocomial infections in patients suffering from immune system disorders, and those in hospital intensive care units. The aim of this research was to identify and isolate A. baumannii strains resistant to colistin, determine antibiotic resistance pattern of this bacteria, investigate the presence of colistin-resistant genes, and finally assess the effect of expression changes in pmrA and pmrB genes resistant to A. baumannii against colistin via real-time polymerase chain reaction.Methods: The samples were initially purified and isolated using biochemical tests and Microgen kit. Later, the resistance pattern evaluation of validated samples to different antibiotics and colistin was carried out using two methods viz., disc diffusion and E-test. This was followed by the assessment of genes resistant to colistin via polymerase chain reaction besides gene expression changes via real-time polymerase chain reaction. Results: The results of this study indicated that eleven strains of A. baumannii isolated from Shahid Rajaee Trauma Hospital were resistant to colistin. However, in the resistance pattern evaluation of A. baumannii isolated from Ali Asghar Hospital, all the strains were sensitive to colistin. In the evaluation of genes resistant to pmrA and pmrB, most of the strains resistant to colistin were carriers of these genes. Besides, in the expression assessment of these genes, it was demonstrated that expression of pmrA in the strains resistant to colistin significantly increased in relation to sensitive strains, but the expression of pmrB increased at a lower rate in the strains resistant to colistin as compared to the sensitive strains. Conclusion: Thus, it can be safely mentioned that increased expression of pmrA was due to the resistance of A. baumannii to colistin. Keywords: Acinetobacter baumannii, colistin, real-time PCR, gene expression, PCR