Gut microbiota (GM) plays a significant role in the pathophysiology of neuropsychiatric diseases like depression. A complex two-way system between gut and brain termed as gut-brain axis (GBA) dependent on intestinal GM and central nervous system (CNS). Alterations in the GM can affect behavior and brain neurochemistry including tryptophan metabolism through immune mediated pathways. Researchers focused to understand immune challenges involving kynurenine action via indoleamine-2,3-dioxygenase (IDO) induces depressive like "sickness behavior". This study examined the role of Lactobacillus fermentum (LF) and 1-methyl-D-tryptophan (1-MT) on IDO regulation, proinflammatory cytokine responses, and gut microbial diversity in chronic unpredictable mild stress (CUMS) depression model. LF (oral administration, 1 × 10CFU/day/mouse for 28 days) and 1-MT (intraperitoneal, 15 mg/KgBW/day for 21 days) supplementation decreases the expression of cytokines and IDO in cortex, hippocampus, and medulla. Likewise, the mRNA and protein level of cytokines and IDO were modulated after LF and 1-MT administration. Additionally, 16 S rRNA gene sequencing showed that Lactobacillus fermentum restored gut microbial β-diversity and increased overall community richness, indicating a shift toward a balanced microbiome. These results suggest that LF alleviates stress-induced neuroinflammatory and immune changes by modulating IDO activity in the tryptophan pathway. The findings highlight the therapeutic potential of LF as a microbiota-based intervention for regulating neuroinflammation and mood disorders such as depression. 8
