Background: There is increasing evidence suggesting the existence of an interaction between commensal microbiota, the gut and the brain. The aim of this study was to examine the influence of commensal microbiota on the host behaviors in a contamination‐free environment, which was verified by culture‐based methods.Methods: Open‐field and marble‐burying tests were used to analyze anxiety‐like behaviors and locomotor activity in gnotobiotic BALB/c mice with a common genetic background in a sterile isolator. The monoamine levels in several regions of the brain were measured in germfree (GF) mice and commensal fecal microbiota‐associated mice (EX‐GF).Key Results: A 24‐h exposure to the environment outside the sterile isolators rendered GF mice less anxious than those not contaminated, while there was no change in the locomotion. EX‐GF mice, the gnotobiotic mice with normal specific pathogen‐free microbiota, were less anxious and active than GF mice using open‐field and marble‐burying tests. The norepinephrine, dopamine, and serotonin turnover rates were higher in the EX‐GF mice than in the GF mice in most regions of the brain, suggesting that monoaminergic neurotransmission might increase in the EX‐GF mice comparing the GF mice. Monoassociation with Brautia coccoides reduced the anxiety level, but it did not affect the locomotor activity. In contrast, colonization with Bifidobacterium infantis decreased the locomotor activity, while having little effect on the anxiety level.Conclusions & Inferences: These results strongly support the current view that gut microorganisms modulate brain development and behavior.

1. Recently, 2 novel neuropeptides were discovered, both derived from the same precursor by proteolytic processing, which bind and activate 2 closely related orphan G protein-coupled receptors, Named orexin-A and-B (Sakurai et al., 1998). Both stimulate food intake when administered centrally to rats. 2. Our aim was to elucidate whether central injection of mammalian orexin-A or-B stimulates food intake in the chick. 3. Under conditions of free access to food, orexin-A did not alter the food intake of chicks, but cumulative food intake was significantly suppressed by orexin-B. 4. The orexin-B was then administered to chicks deprived of food for 3 h to confirm its suppressive effect. No significant effect of orexin-B on food intake was detected. 5. Central injection of orexin-B did not modify food intake when appetite was stimulated by fasting. 6. Neither of these orexins appears to stimulate feeding in chicks.