Irritable bowel syndrome (IBS) is a complex multifactorial functional disorder, whose pathophysiology is largely associated to an impairment of the intestinal microbiota composition, namely dysbiosis. Thus, the modulation of the gut microbiota has been proposed as a possible therapeutic strategy for IBS patients alongside with current available drugs. Fecal microbiota transplantation (FMT) is a promising strategy to restore intestinal eubiosis. In this randomised double-blind placebo-controlled trial patients diagnosed with IBS without constipation are assigned with 1:1 ratio to receive FMT from healthy donor or autologous FMT (placebo group) to assess the effectiveness of FMT on IBS symptoms and quality of life, to evaluate the safety of FMT among IBS patients and to estimate any change in the gut microbiota composition of IBS patients after the FMT.

Irritable bowel syndrome (IBS) is a common chronic functional disorder with a prevalence varying between 10 and 15%. IBS is characterized by recurrent chronic abdominal pain associated with altered defecation. Intestinal dysbiosis is thought to be one of the major drivers in IBS pathophysiology, since several studies have shown that there are substantial differences in microbial composition between IBS patients and healthy subjects. Moreover, the modulation of the gut microbiota in IBS patients seems a promising therapeutic approach, since a benefit on symptoms with certain antibiotics, probiotics, prebiotics, synbiotics and diet has been reported. Fecal microbiota transplantation (FMT) from healthy donors is generally considered a safe and well-tolerated procedure ant it is described to be capable of restoring eubiosis of the recipient in the long term. Recently, a certain effectiveness of FMT in ameliorating intestinal symptoms, psychological aspects and quality of life alongside with a restoration of eubiosis among IBS patients has been described. This is a prospective randomized double-blind placebo-controlled trial aimed at estimating the benefits of FMT among IBS patients without constipation. The assessment of the effectiveness on IBS symptoms and quality of life, the evaluation of the safety and the changes in the gut microbiota composition (performed with shotgun sequencing techniques) among IBS patients will be evaluated before after the FMT. 35 patients will be enrolled in 24 months. Recruited subjects will be randomized with 1:1 ratio to receive FMT from healthy donors or autologous FMT (placebo). The effectiveness of FMT in improving the IBS symptoms and IBS-related quality of life will be estimated with the IBS-SSS and IBS-QOL questionnaire, respectively, before FMT and after 30, 60, 90 and 365 days. Moreover, the frequency and form of evacuations according to the Bristol scale will be recorded at the same timepoints from diaries provided to the patients. Changes in gut microbiota composition will be estimated by comparing the composition of the faecal microbiota of the recipient before and 90 days after the FMT. The data will be evaluated using Intention-to-treat (ITT) statistical analysis and per-protocol (PP) analysis. Difference between the two groups will be calculated by two-tailed tests, Wilcoxon-Rank Sum Test for continuous variables and by the Fisher's exact probability test for categorical variables. The response rate to FMT will be evaluated by the Fisher's exact test. Adverse events will be assessed. The composition of the faecal microbiota of the recipient before and after the FMT will also be analysed and also compared with the healthy donor's one using the UniFrac method.

Inclusion Criteria:

• Ability to express and give informed consent
• Age ≥ 18 years
• Diagnosis of diarrhoea-predominant or unclassified IBS, according to the Rome IV criteria
• Have performed a colonoscopy within the last 5 years that has ruled out intestinal diseases

Exclusion Criteria:

• Taking antibiotics or probiotics within the 8 weeks before the baseline visit
• Patients with chronic inflammatory bowel disease (Crohn's disease, ulcerative colitis, indeterminate colitis or eosinophilic gastroenteritis), malignant neoplasms of the gastroenteric tract, celiac disease, diverticular disease
• Uncontrolled heart failure or severe heart disease with EF < 30%
• Severe respiratory failure
• Serious psychiatric conditions or psychological instability according to the clinician
• Contraindication to fecal microbiota transplantation (high risk of complications related to colonoscopy)
• Previous abdominal surgery on the gastroenteric tract (except cholecystectomy, appendectomy and other types of surgery not involving the digestive tract)
• Patients with cutaneous enterostomy
• Pregnancy or lactation
• Concurrent enrollment in other interventional experimental protocols
• Personality unstable or unable to adhere to protocol procedures
• Any clinical condition which, in the opinion of the investigators, may contraindicate enrollment in the study

The collected data will be analyzed in blind, as only one person not involved in the activities related to all the procedures of the study will know if faeces will be infused from a healthy donor or if an autologous FMT will be performed