Background: One of the most important causes of death due to tuberculosis is drug resistant Mycobacterium tuberculosis. Rifampin is one of the most important drugs that are commonly used for treatment of tuberculosis. Mutations in rpoB gene of Mycobacterium tuberculosis cause rifampin drug resistant. The aim of this study was to investigate the type and frequencies of the mutation in this region in Mycobacterium tuberculosis isolated from patients in Tehran.

 Materials and methods: In this experimental study, 50 patients who did not respond to 6- month chemotherapy with anti- tuberculosis drugs were selected and tested for resistance to this drug. Patients’ sputum were obtained and cultured on Lowenstein-Jensen solid medium. After susceptibility testing, DNA of resistant strains was extracted and rpoB gene was amplified with PCR. Results were analyzed by computer using DNAMAN program.

 Results: 14 cases had mutations in this region of Mycobacterium tubrculosis. However, 6 isolates (30%) had no mutation in this part of the genome. The most frequent mutations occurred in codons 531(40%) and 515 (20%). We report mutation of codon 515 for the first time. Mutations in codons 526 (10%) and 510 (15%) were found.

Conclusion: Mutation in codon 515 probably shows the different pattern of mutation in Tehran. Our data indicate 6 isolates without mutation in the 81bp region. According to this study, resistance to rifampin may have non genetic and genetic causes on genes outside of 81bp region.