Rifaximin has the clearest guideline-supported role for IBS with diarrhea among microbiota-directed interventionsRifaximin and Soluble Fiber Show Support for IBS Symptoms

Irritable bowel syndrome (IBS) is a heterogeneous disorder of gut–brain interaction in which bowel-habit subtype, dominant symptoms, post-infectious onset, bile-acid handling, visceral sensitivity, and diet–microbiota interactions may influence the treatment response. This narrative mini-review asks whether available clinical evidence is sufficient to select or sequence microbiota-directed and microbiota-modulating interventions according to Rome bowel-habit subtype and clinically observable dominant symptoms. Literature was identified through PubMed/MEDLINE and major guidelines, systematic reviews, meta-analyses, and pivotal trials published through June 2026. Evidence was interpreted at the level of the exact intervention or product, studied population, target outcome, consistency, independent replication, safety, and guideline position. Mechanistic data support plausible links through fermentation, epithelial barrier function, mucosal immunity, bile-acid metabolism, visceral sensitivity, and gut–brain signaling, but mechanistic or microbiome changes do not establish the clinical efficacy. Rifaximin has the clearest replicated and guideline-supported role in IBS with diarrhea, while soluble fiber has guideline support for global or constipation-associated symptoms. Evidence for individual probiotics, postbiotics, partially hydrolyzed guar gum, galactooligosaccharides, and fermented matrices remains product-specific or emerging and is rarely independently replicated. Symptom improvement with a diet low in fermentable oligosaccharides, disaccharides, monosaccharides, and polyols (low-FODMAP diet) does not establish microbiota mediation. Fecal microbiota transplantation (FMT) and microbiome-guided prescribing remain investigational. Current evidence, therefore, supports only limited phenotype-informed matching and does not establish a validated treatment sequence or microbiome-defined therapeutic phenotype.