Tehran University of Medical Sciences Journal of Medical Bacteriology 2251-8649 4 1-2 2015 03 02 21 26 EN Hassan Hashemi Health Faculty, Shiraz University of Medical Sciences, Shiraz, Iran. Mahdi Safari Kurdistan Environmental Health Research Center, Kurdistan University of Medical Sciences, Sanandaj, Iran. Abbas Khodabakhshi Health Faculty, Shahrekord University of Medical Sciences, Shahr-e-kord, Iran. 2016 08 07 2016 08 07 Introduction: Antibiotics have the potential to adversely affect the microbial community. For anaerobic digestion, a sufficient methanogenic population needs to be preserved in the system. The main aim of this study was determination of inhibitory concentration of oxytetracycline on methanogenic bacteria. Methods: A 120 mL jacketed bioreactor with a 90 mL working volume was inoculated granular sludge from an anaerobic digester, substrate and different concentration of oxytetracycline with 10 days cycles and intermittent mixing. The reactor was operated at 35 ± 2 ° C. The inhibitory effect of antibiotic was evaluated by monitoring biogas production. Results: Based on the findings from each batch, complete inhibitory concentration of oxy tetracycline was in concentration of 800 mg L-1. Significant relation was seen between inoculated antibiotic concentrations and methane production (r=-0.86). Conclusion: The addition of antibiotics to the biomass affected the utilization of fatty acids, resulting in unfavorable effects on methanogenesis. Thus, overusing of antibiotics can adverse effects of intestinal flora. https://jmb.tums.ac.ir/index.php/jmb/article/view/171 https://jmb.tums.ac.ir/index.php/jmb/article/download/171/117