Gut microbiota produce different metabolites within the human body, which include neurotransmitters. Animal studies have demonstrated a critical role for the gut microbiota in various aspects of brain and behavioural function, and a smaller number of studies in humans have shown differences of gut microbiota composition in psychiatric conditions. However, almost nothing is known about the impact of neurotransmitters produced by the gut microbiota on human brain and behaviour. The way in which differences in brain, behaviour and personality traits are associated with the gut microbiota, and how they are influenced by a probiotic will be explored, with a special focus on GABA (Gamma Amino Butyric Acid). Abnormalities of microbiota composition have been identified in metabolic disorders, such as inflammatory bowel disease and obesity, and psychiatric conditions, such as depression and anxiety. The aim of this intervention trial will be to answer the following fundamental questions: 1. Does the population of gut bacteria capable of producing GABA modulate brain-based measures of GABA? 2. Does the population of gut bacteria capable of producing GABA influence performance in behavioural tasks known to depend on GABA-ergic function? The impact of a GABA producing probiotic on the measures of GABA in the brain and serum, relevant metabolites in blood, faecal and urine samples and performance in GABA dependent behavioural tasks will be investigated in this trial.