en Gastric Bypass Gastric Bypass Original Original <strong>Introduction: </strong>Bariatric surgery causes anatomical changes in the digestive system that can alter the distribution of gut microbes. We aim to evaluate the changes of <em>Firmicutes</em> after the laparoscopic one anastomosis gastric bypass/mini gastric bypass (OAGB/ MGB) surgery.<br> <strong>Methods:</strong> 50 patients with morbid obesity were operated on with OAGB/ MGB. Demographic data and <em>Firmicutes</em> counts in stool samples were obtained before, 6, and 12 months after the surgery. The logarithm of <em>Firmicutes</em> colony count based on 10 was used for analysis. Paired T-test, ANCOVA model, and Pearson correlation tests were used for statistical analysis.<br> <strong>Results:</strong> 70.6% of our patients were female. The percentage of Excess weight loss (%EWL) and excess BMI loss (%EBMIL) were 44.06%&plusmn;9.84 and 42.56%&plusmn;5.29 respectively at the 6-month follow-up and 67.55%&plusmn;5.56 and <span dir="RTL">70</span>.81%&plusmn;7.25 respectively at the 12-month follow-up. The <em>Firmicutes</em> count was dropped from 1.57 to 1.44 at 6-month (p=0.01) and 1.32 at 12-month (p=0.02) follow-up. ANCOVA model after adjustment for age, sex, preoperative BMI, and delta-BMI did not show a significant difference for either the 6 or 12 months data (p=0.74 and 0.59, respectively). Pearson correlation test did not found any relationship between <em>Firmicutes </em>count change any weight-related variables.<br> <strong>Conclusion: </strong>&nbsp;The <em>Firmicutes</em> count was significantly decreased after OAGB/MGB. However, no significant relationship was found between weight loss and <em>Firmicutes</em> count. &nbsp; Gastric Bypass, Firmicutes, Bacteroidetes, Obesity, Microbiota, Bariatric surgery 17 20 http://annbsurg.iums.ac.ir/browse.php?a_code=A-10-28-3&slc_lang=en&sid=1 shahab shahabi shshahabi@yahoo.com 10031947532846001503 10031947532846001503 Yes Minimally Invasive Surgery and Obesity Research Center, Alzahra University Hospital, Isfahan University of Medical Sciences, Isfahan, Iran mohsen mahmoudieh mahmoudieh@med.mui.ac.ir 10031947532846001504 10031947532846001504 No Minimally Invasive Surgery and Obesity Research Center, Alzahra University Hospital, Isfahan University of Medical Sciences, Isfahan, Iran behrouz keleidari keleidari@med.mui.ac.ir 10031947532846001505 10031947532846001505 No Minimally Invasive Surgery and Obesity Research Center, Alzahra University Hospital, Isfahan University of Medical Sciences, Isfahan, Iran hamid khodadadi dr.h.r.khodadadi@gmail.com 10031947532846001506 10031947532846001506 No Minimally Invasive Surgery and Obesity Research Center, Alzahra University Hospital, Isfahan University of Medical Sciences, Isfahan, Iran erfan sheikhbahaei erfan.shikhbahaei@gmail.com 10031947532846001507 10031947532846001507 No Student Research Committee, School of Medicine, Isfahan University of Medical Sciences, Isfahan, Iran reza shokrani foroushani rezashokrani128@gmail.com 10031947532846001508 10031947532846001508 No Student Research Committee, School of Medicine, Isfahan University of Medical Sciences, Isfahan, Iran