Tehran University of Medical Sciences Iranian Journal of Microbiology 2008-3289 11 2 2019 05 30 120 128 EN Nazanin Zargar Iranian Center for Endodontic Research, Research Institute of Dental Sciences, School of Dentistry, Shahid Beheshti University of Medical Sciences, Tehran, Iran Mahmoud Amin Marashi Medical Microbiology Research Center, Qazvin University of Medical Sciences, Qazvin, Iran Hengameh Ashraf Department of Endodontics, School of Dentistry, Shahid Beheshti University of Medical Sciences, Tehran, Iran Rene Hakopian Iranian Center for Endodontic Research, Research Institute of Dental Sciences, School of Dentistry, Shahid Beheshti University of Medical Sciences, Tehran, Iran Peyman Beigi Department of Hematology, Faculty of Medicine, Tarbiat Modares University, Tehran, Iran 2017 08 02 2019 05 24 Background and Objectives: Bacterial agents are commonly accepted as the main etiology of endodontic infections. A significant proportion of oral bacteria cannot be cultured using existing methods. Since diversity and abundance of bacterial species are different in different populations, the present study was aimed to identify effective microorganisms in persistent endodontic infections in Iranian patients based on culture and molecular biology methods using sequence analysis of 16S rDNA gene. Materials and Methods: Thirty patients with previous failure of endodontic treatment were enrolled in the study. After isolation and disinfection of the tooth surrounding area with 3% sodium hypochlorite and 30% hydrogen peroxide, sampling from the root canals was carried out using two sterile Hedstrom files and two sterile paper points, and then the specimens were transferred to the microbiology laboratory in thioglycolate transport medium so that they undergo aerobic-anaerobic culture, PCR, and 16S rDNA gene sequencing. Results: Of 30 patients (15 women and 15 men), 15 patients had radiographic lesions smaller than 5 mm and other 15 patients had radiographic lesions larger than 5 mm. The mean age of patients was 40.20 ± 13.76 years. A total of 26 patients were asymptomatic. Only four patients had clinical signs such as pain and percussion sensitivity and Tannerella forsythia was the most common bacterium found in this group of patients. 13 bacterial species were found in 11 different genus, one virus strain and one fungus strain. From 30 studied specimens, Enterococcus faecalis was the most common microorganism with prevalence rate of 63.63%. Conclusion: This study showed the type and prevalence of effective bacteria in secondary/persistent endodontic infections in Iranian patients. E. faecalis is the most commonly found microorganism in Iranian patients. https://ijm.tums.ac.ir/index.php/ijm/article/view/1450 https://ijm.tums.ac.ir/index.php/ijm/article/download/1450/1139