This study investigates the impact of postnatal microbial colonization on the hypothalamic-pituitary-adrenal (HPA) system's stress response in mice. The researchers compared the HPA responses of germfree (GF), specific pathogen free (SPF), and gnotobiotic mice. GF mice showed a significantly higher plasma ACTH and corticosterone response to restraint stress compared to SPF mice, but not to ether exposure. GF mice also exhibited reduced brain-derived neurotrophic factor (BDNF) expression levels in the cortex and hippocampus. The exaggerated HPA response in GF mice was reversed by reconstitution with Bifidobacterium infantis, while monoassociation with enteropathogenic Escherichia coli (EPEC) enhanced the stress response. Notably, the enhanced HPA response in GF mice was partly corrected by reconstitution with SPF faeces at an early developmental stage but not at a later stage. These findings suggest that commensal microbiota influence the postnatal development of the HPA stress response, indicating a bidirectional interaction between the gut and brain. The study highlights the importance of early microbial colonization in shaping the HPA system's susceptibility to inhibitory neural regulation.This study investigates the impact of postnatal microbial colonization on the hypothalamic-pituitary-adrenal (HPA) system's stress response in mice. The researchers compared the HPA responses of germfree (GF), specific pathogen free (SPF), and gnotobiotic mice. GF mice showed a significantly higher plasma ACTH and corticosterone response to restraint stress compared to SPF mice, but not to ether exposure. GF mice also exhibited reduced brain-derived neurotrophic factor (BDNF) expression levels in the cortex and hippocampus. The exaggerated HPA response in GF mice was reversed by reconstitution with Bifidobacterium infantis, while monoassociation with enteropathogenic Escherichia coli (EPEC) enhanced the stress response. Notably, the enhanced HPA response in GF mice was partly corrected by reconstitution with SPF faeces at an early developmental stage but not at a later stage. These findings suggest that commensal microbiota influence the postnatal development of the HPA stress response, indicating a bidirectional interaction between the gut and brain. The study highlights the importance of early microbial colonization in shaping the HPA system's susceptibility to inhibitory neural regulation.