Leila Ganji - Research Center of Health Reference Laboratory, Ministry of Health and Medical Education, Tehran, Iran.
Mohammad Reza Zali - Foodborne and Waterborne Diseases Research Center, Research Institute for Gastroenterology and Liver Diseases, Shahid Beheshti University of Medical Sciences, Tehran, Iran
Masoud Alebouyeh - Pediatric Infections Research Center, Research Institute for Children‘s Health, Shahid Beheshti University of Medical Sciences, Tehran, Iran
Elahe Tajedin - Department of Food Science and Technology, National Nutrition and Food Technology Research Institute, Faculty of Nutrition Sciences and Food Technology, Shahid Beheshti University of Medical Sciences, Tehran, Iran

Background and Objectives: The pathophysiology of irritable bowel syndrome (IBS) remains unclear; however, dysbiosis may play a role in it occurrence. The aim of present study was to compare relative abundance of common members of fecal microbiota between IBS patients and healthy controls. Materials and Methods: In this study, fecal samples of 55 patients with IBS and 11 sex and age-matched control individuals were collected. The patients were classified as having IBS with constipation (IBS-C), IBS with diarrhea (IBS-D), and mixed type IBS (IBS-M) by a questionnaire following the Rome III criteria. Fecal DNA was extracted using a commercial kit, according to the manufacturer’s instructions. Relative abundance of fifteen bacterial families/genera including Methanobrevibacter, Bacteroides, Clostridium difficile, Campylobacter spp., Prevotella spp., Peptostreptococcus spp., Klebsiella pneumonia, Faecalibacterium prausnitzii, Lactobacillus spp., Firmicutes, Bifidobacter spp., Ruminococcus, Actinobacteria, Clostridia spp., and E. coli were evaluated using quantitative real-time polymerase chain reaction (qPCR). Results: Out of 55 patients, 16 had constipation (IBS-C), 12 diarrhea (IBS-D), and 27 were mixed IBS (IBS-M). A significantly lower relative abundance rate of Provetella species was observed in all IBS patients. Conversely, at the phylum level, there was a tendency for IBS patients to harbor higher abundance of Firmicutes, Clostridia spp., and Enterobacteriaceae vs healthy subjects. Conclusion: Our findings revealed that IBS patients were significantly different in the gut microbiota of the intestine, compared with healthy controls, especially as a result of a reduction in the diversity of Prevotella spp. Further studies are needed to determine role of this altered composition of microbes in the functional disorders.