Functions of every organ and system of the body need to be perfectly regulated for the maintenance of the milieu interior; this is performed by a wireless (endocrine) system and a wired (nervous) system in the body. Interestingly, the communication between the brain and the gut is bi-directional; though, the brain controls the gut to a large extent, the gut also influences the brain functions. In recent years, it has been realized that the gut microbiota, which is the largest organ of the human body, influences the brain functions. 1,2 However, this is not entirely a new concept. As early as pre-2nd century CE, in the Charaka Samhita, an Indian book on medicine, it was mentioned that the human being is a microcosmic replica of the universe, 3 and the 3 humors (Vayu or gas, Pitta or bile, and Kapha or mucus) are responsible for most ailments. 4 It is interesting to know how the gastrointestinal tract in general and its microbiota, in particular, is being unraveled by the scientists in recent times.Gut microbial modulation of the brain function is best demonstrated by the well-known effects of gut-microbiota directed antibiotic treatment in patients with hepatic encephalopathy. Moreover, in recent times, several animal and human experiments also demonstrated the role of gut microbiota in influencing brain function. In a study by De Palma et al, 5 germ-free mice were shown to have dysregulated hypothalamic pituitary stress response, altered brainderived neurotrophic factors (BDNF) in the hippocampus, altered neurotransmission, altered tryptophan availability and metabolism, and reduced anxiety. In another experimental study, antibiotic treatment of BALB/c mice resulted in increase in Firmicutes phylum dominated by Lactobacillus, sulfate-reducing bacterial species, and decrease in g-Proteobacteria and Bacteroidetes, which was associated with increase in hippocampal BDNF and autonomic-independent anxiolytic behavior. 6 How does gut microbiota alter brain function? In a study on wild mice separated from their mothers, which is well known to induce anxiety, it was found to be associated with altered gut-microbial colonization, long-lasting hyperactivity of the hypothalamic pituitary-axis, visceral hypersensitivity, altered cholinergic activity in the gut, and increased intestinal permeability. -522-249-4406, Fax: +91-522-2668017, E-mail: udayghoshal@gmail.com Article: Bifidobacterium longum and Lactobacillus helveticus synergistically suppress stress-related visceral hypersensitivity through hypothalamic-pituitary-adrenal axis modulation Ait-Belgnaoui A, Payard I, Rolland C, et al