The purpose of this study is to determine whether probiotic bacterium Bifidobacterium longum (B. longum) can improve mood and bowel symptoms in patients with Irritable bowel syndrome (IBS). The patients will be treated with probiotic or placebo for 6 weeks. Their mood, memory, general well-being and bowel symptoms will be assessed before, at the end of the treatment and 4 weeks later. Brain activation pattern, changes in gut bacteria, metabolic profile and inflammatory markers will be also measured.

Inclusion Criteria:

• Diagnosis of Irritable Bowel Syndrome based on Rome III Criteria
• Symptoms of mild to moderate anxiety and depression

Exclusion Criteria:

• Concurrent systemic disease and/or laboratory abnormalities considered by Investigators to be risky or that could interfere with data collection.
• Concurrent organic Gastrointestinal (GI) pathology other than benign polyps, diverticulosis, hemorrhoids, lipomas and melanosis coli.
• Psychiatric diagnosis other than anxiety or depression.
• Patients treated with therapeutic/standard doses of antidepressants and/or anxiolytics
• History of active cancer in the last 5 years, other than skin basal cells cancer
• Pregnant or breastfeeding women
• Treatment with antibiotics during the three months prior the study.
• Known or suspected allergies to the study products (eg maltodextrin).
• Patients who have heart pacemakers, metal implants, metal chips or clips in or around the eyeballs, artificial heart valves, metallic ear implants, bullet fragments or fixed brackets.
• High fiber diet (>35 g/day for males, > 25 g/day for females), consumption of high inulin containing foods (>5 g/day).
• Consumption of any type of yoghurts or probiotic-containing products in the 4 weeks prior to the beginning of the study.