- PURPOSE OF REVIEW: Interest in the microbiota-gut-brain axis is increasing apace and what was, not so long ago, a hypothetical relationship is emerging as a potentially critical factor in the regulation of intestinal and mental health. Studies are now addressing the neural circuitry and mechanisms underlying the influence of gut bacteria on the central nervous system and behavior. RECENT FINDINGS: Gut bacteria influence development of the central nervous systems (CNS) and stress responses. In adult animals, the overall composition of the microbiota or exposure to specific bacterial strains can modulate neural function, peripherally and centrally. Gut bacteria can provide protection from the central effects of infection and inflammation as well as modulate normal behavioral responses. Behavioral effects described to date are largely related to stress and anxiety and an altered hypothalamus-pituitary-adrenal axis response is a common observation in many model systems. The vagus nerve has also emerged as an important means of communicating signals from gut microbes to the CNS. SUMMARY: Studies of microbiota-gut-brain communication are providing us with a deeper understanding of the relationship between the gut bacteria and their hosts while also suggesting the potential for microbial-based therapeutic strategies that may aid in the treatment of mood disorders.