Irritable bowel syndrome (IBS) is a prevalent gastrointestinal disorder affecting 15-20% of people in industrialized countries, often presenting as abdominal discomfort, pain, and altered bowel habits, significantly impacting quality of life. Among IBS subtypes, IBS-D (diarrhea-predominant) is marked by frequent, loose bowel movements, identified using the Bristol Stool Form Scale (BSFS) and classified according to Rome IV criteria. Emerging research indicates that alterations in gut microbiota, particularly a decline in butyrate-producing bacteria like *Clostridium butyricum*, play a critical role in IBS-D. Butyrate is an essential short-chain fatty acid that provides energy to colonocytes and supports intestinal health, but its deficiency may contribute to intestinal inflammation, impaired sodium and water absorption, and diarrhea. *Clostridium butyricum CBM588®* is a unique butyrate-producing bacterium that withstands oxygen, making it viable for therapeutic use. Originating from Japan, CBM588® has shown promise in enhancing gut microbiota balance, improving symptoms in IBS-D, and supporting intestinal integrity. Although primarily studied in Asian populations, additional research in Caucasian patients is warranted to validate its broader applicability and potential benefits in IBS-D symptom management.

The aim of this prospective, open-label, interventional study, based on clinical data collected from real-world clinical settings, is to assess the safety and efficacy of Clostridium butyricum CBM588® in patients with IBS-D. The study consists of two cohort groups:

A cohort, where patients will receive probiotic Butirrisan® (Clostridium butyricum CBM588®) as three tablets daily, taken in the morning either before or after breakfast, for a continuous period of eight weeks.

A control cohort, consisting of IBS-D patients who received the standard of care treatment (Trimebutin maleate followed by Lactose-free, no-slag diet).

The study evaluates changes in IBS symptom severity, fecal consistency, daily frequency of evacuation and diarrhea episodes, and quality of life at the end of the treatment. The primary outcome measures include changes in the Irritable Bowel Syndrome Symptom Severity Scale (IBS-SSS), fecal consistency assessed through the Bristol Stool Form Scale (BSFS), and the frequency of evacuation and diarrhea episodes. Additionally, safety will be evaluated by assessing the number of patients reporting adverse effects. The secondary outcome measure is an evaluation of quality of life, assessed using a qualitative questionnaire. Comparative analyses will be performed between the prospective Butirrisan®-treated group and the control group (Trimebutin maleate + Lactose-free, no-slag diet) to determine the relative effectiveness of probiotic therapy versus standard care.

Inclusion Criteria:

• male and female, aged ≥18 years
• diagnosed with IBS-D according to the Rome IV criteria

Exclusion Criteria:

• the use of antibiotic therapies
• use of laxative therapies
• the presence of organic gastrointestinal diseases
• resection of the digestive system
• the state of pregnancy or breastfeeding
• history of drug use and alcohol abuse
• the presence of an ascertained food allergy
• the presence of lactose intolerance