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246 PROCEEDINGS OF INTERNATIONAL SCIENTIFIC CONFERENCE ON APPLIED BIOTECHNOLOGYProbiotic Bacillus strains influence mental health through the gut-brain axis. Imbalances in the gut microbiota are linked to depression, causing brain inflammation and neurotransmitter imbalances. Bacillus exerts anti-inflammatory effects, restoring balance by regulating pro-inflammatory cytokines and promoting anti-inflammatory molecules, thereby reducing systemic inflammation. These probiotics interact with immune cells in the gut, modulating inflammation and supporting brain health [5, 6].Another important mechanism of Bacillus probiotics is the production of short-chain fatty acids (SCFAs) such as acetate, propionate, and butyrate during fiber fermentation. SCFAs maintain gut health, provide energy to gut epithelial cells, strengthen the gut barrier, and regulate immune function. They also influence the brain via the vagus nerve, a major pathway of the gut-brain axis. Studies show that SCFAs, especially butyrate, can improve mood and cognition, and reduce brain inflammation, helping to alleviate depression symptoms [4].Bacillus probiotics, such as B. licheniformis and B. subtilis, participate in the synthesis and regulation of important neurotransmitters like serotonin, dopamine, and GABA, which are essential for mood regulation and emotional health. Bacillus can produce L-tryptophan, a precursor of serotonin, a neurotransmitter crucial in mood regulation, anxiety, and depression. The production of serotonin in the gut and its effect on the brain through the gut-brain axis is a key mechanism for reducing depression symptoms. Bacillus also influences the synthesis of dopamine and norepinephrine, which are involved in stress response and mood regulation [7, 8].Bacillus probiotics have demonstrated the ability to regulate the hypothalamic-pituitary-adrenal (HPA) axis, an important system in the stress response. The HPA axis regulates the release