Irritable Bowel Syndrome (IBS), characterized by the Rome IV criteria, is a functional bowel disorder combining abdominal pain with changes in bowel habits and/or stool consistency. This condition is common, affecting 5% to 10% of the population in developed countries. The etiology of IBS is multifactorial, involving intestinal motility disorders, visceral hypersensitivity, micro-inflammation of the intestinal mucosa, and dysbiosis. It has been demonstrated that the sub-category of IBS patients with constipation predominantly have increased amounts of Methanobrevibacter smithii, the most common methanogenic archaea found in the intestinal lumen, compared to other IBS patients. Breath tests can evaluate methane production by the intestinal microbiota, indirectly assessing the presence and quantity of methanogenic archaea. The acronym IMO (intestinal methanogen overgrowth) defines the association of digestive symptoms (notably bloating and constipation) with a high concentration of methane in exhaled gases (≥ 10 ppm). The links between constipation, methane production, and fecal microbiota are uncertain, necessitating further studies that could lead to precise diagnostic and treatment recommendations. Main objective: Describe the initial composition of the fecal microbiota of constipated methano-producing IBS patients. Secondary objectives: 2\. Describe the initial composition of blood metabolites linked to the microbiota of constipated methano-producing IBS patients 3. Describe the evolution of the fecal microbiota and blood metabolites linked to the fecal microbiota during conventional therapeutic management (before and after) of constipated methano-producing IBS patients. 4\. Compare the composition of the fecal microbiota before and after conventional therapeutic management of responding IBS-C patients (-30% on the IBS-SSS symptom severity score) compared to non-responding patients. 5\. Evaluate the impact of Methanobrevicter smithii in the response to symptomatic treatment. Exploratory observational single-center study (cohort follow-up) descriptive in patients with irritable bowel syndrome with constipation (IBS-C, IBS-m or IBS-U) with excessive methane production detected on a glucose breath test. Patients will be invited to participate once the results of the breath test are known. Please note: In the context of this study, only two microbiota samples and 4 additional tubes and one tube of blood will be added to the usual practice. All treatments will be prescribed as part of the care and are not conditioned by the research protocol. The primary endpoint is the analysis of the initial composition (16S rRNA gene sequencing) of the fecal microbiota of constipated methano-producing IBS patients. The study population consists of constipated IBS patients with excessive methane production seen in the digestive functional exploration department for a breath test prescribed as part of an external procedure or a day hospital session. A total of 40 patients will be included over 18 months, with a participation duration of 2 months +/- 2months per subject. Patients with IBS constitute a heterogeneous population for whom only symptomatic treatment is currently offered with variable and unpredictable efficacy. Through this work, we seek to find new therapeutic axes to relieve or even treat their symptoms.