Fecal Microbiota Profiling in Patients with Irritable Bowel Syndrome and Small Intestinal Bacterial Overgrowth

Abstract Introduction: Small intestinal bacterial overgrowth (SIBO) is common in irritable bowel disease (IBS) and has been proposed as an etiologic factor. This study aimed to identify potential fecal microbiota profiles in patients with IBS and SIBO. Methods Patients with IBS were divided into two groups: IBS with SIBO (IBS.SIBO+) and IBS without SIBO (IBS.SIBO-). SIBO was identified using a standard glucose hydrogen breath test. A total of 52 subjects were enrolled in this study, of whom 18 were normal controls (NCs), 17 had IBS with SIBO, and 17 had IBS without SIBO. The IBS symptom severity scale, abdominal pain, and health-related quality of life (QoL) were evaluated using questionnaires. Rectal balloon distension was used to measure the visceral sensitivity. Fresh fecal samples were collected, and 16S rRNA gene sequencing analysis was performed to evaluate the structure and diversity of the microbiota among different groups. Correlation analysis was performed to assess the relationship between specific flora and abdominal pain, visceral sensitivity, and QoL. Results The richness of the intestinal microbiota in the IBS without SIBO group was significantly lower than that in the IBS with SIBO group ( P = 0.012). At the phylum level, the relative abundance of Firmicutes decreased significantly in fecal samples collected from patients with IBS and SIBO compared to those in the NC and IBS groups. No significant difference was found between the IBS with SIBO and IBS without SIBO groups, although the phylum type showed a decreasing trend in the latter. At the genus level, Fusobacterium ( P = 0.01) was significantly less abundant in the IBS with SIBO group than in the NC group; however, no significant difference was found between the IBS with SIBO and IBS without SIBO groups. The relative abundance of Firmicutes correlated negatively with the abdominal pain scale ( r = 0.436, P = 0.026) and the IBS symptom severity scale ( r = 0.494, P = 0.010), whereas a positive correlation was found between the defecation distress threshold and Firmicutes (r = 0.458, P = 0.019). Conclusions Fecal microbiota showed obvious dysbiosis in patients with IBS and SIBO compared to those without SIBO. The decreased abundance of Firmicutes may be related to IBS symptoms and abdominal pain, rather than visceral sensitivity.