The study will characterize the biology of FMT in the context of IBS prior to wider clinical application of the method. Given the ethical concerns of unknown and long-term adverse effects of FMT therapy, the study will include patients only with moderate to severe symptoms. IBS is a heterogenous disorder and it is important to characterize the patients, and study as homogeneous patient populations as possible. Therefore the study will only include post-infectious IBS patients after the Giardia outbreak.

Primary aims:

Detailed characterisation of donor and recipient microbial community composition (by means of 16S rRNA profiling) and determination of the kinetics of changes following FMT

Secondary aims:

• Determination of interactions of importance to persisting recipient gut microbiota.
• Evaluate safety of FMT in an IBS population
• To evaluate the efficacy of FMT in relieving symptoms in selected patients with IBS in an open pilot trial in order to have the possibility to calculate number of patients needed when planning future controlled studies.

Inclusion Criteria:

• Age between 18-and 70 years
• IBS-SSS (IBS-Symptom Severity Scale) score >175 (175 - 300 represents moderate, > 300 represent severe IBS).
• All patients are required to have symptoms for at least 12 months, fulfilling the Rome III criteria with either diarrhoea-predominant irritable bowel syndrome (IBS-D) or alternating constipation and diarrhoea (IBS-A) with bloating or flatulence as predominant symptoms. Only IBS considered post-infectious after the Bergen Giardia outbreak will be included. The severity of global IBS symptoms has to be either moderate ("cannot be ignored but do not influence daily activities") or severe ("influence daily activities"). A decrease of 50 points on the IBS-SSS has been shown to correlate with improvement in clinical symptoms. All patients will undergo appropriate investigations to exclude organic disease.

Exclusion Criteria:

• History of inflammatory bowel diseases, gastrointestinal malignancy, blood in stool or antibiotic use within 1 month prior to FMT, immunocompromised patient defined as taking immuno-suppressive medications, history of opportunistic infections within 1 year prior to FMT, oral thrush, or disseminated lymphadenopathy.
• Patients who are scheduled for abdominal surgery, pregnant women or patients taking probiotics or taking antibiotics within 4 weeks prior to installation are also excluded from the protocol.