<i>Limosilactobacillus fermentum</i> 2L Ameliorates Chronic Stress-Induced Neuroinflammation through Gut-Brain Axis Modulation in Mice

Chronic stress contributes to neuroinflammation and psychiatric disorders through gut-brain axis dysregulation. This study investigated the therapeutic potential of <i>Limosilactobacillus fermentum</i> 2L in ameliorating stress-induced neuroinflammation using an unpredictable chronic mild stress (UCMS) model. Five <i>L. fermentum</i> strains were screened for probiotic properties, with strain 2L selected based on superior acid and bile tolerance, cellular adhesion, and antioxidant activity. C57BL/6J mice underwent 7-week UCMS with concurrent 10-weeks of 2L treatment (10⁹ CFU/day). Behavioral assessments, histological analysis, qRT-PCR, Western blotting, and gut microbiome analysis were performed. Strain 2L demonstrated superior gastrointestinal survival and anti-inflammatory properties in LPS-challenged HT-29 cells. In UCMS mice, 2L treatment significantly ameliorated anxiety- and depression-like behaviors, preserved hippocampal neuronal morphology, and normalized hypothalamic-pituitary-adrenal (HPA) axis dysfunction by reducing elevated corticosterone levels (155.9 ± 17.2 to 121.3 ± 3.1 ng/ml, <i>p</i> < 0.001). Molecular analysis revealed restored hippocampal BDNF expression, normalized serotonin receptors (HTR1A, 5HT7R), and attenuated stress-activated MAPK pathways (ERK1/2, JNK1/2). Treatment restored intestinal barrier integrity through tight junction protein upregulation and reduced pro-inflammatory cytokine expression. Microbiome analysis showed successful <i>Limosilactobacillus</i> colonization with restoration of beneficial bacteria (<i>Faecalibaculum</i>, <i>Akkermansia</i>) and normalization of stress-elevated <i>Prevotella</i>. <i>L. fermentum</i> 2L provides multifaceted neuroprotection through gut-brain axis modulation, involving microbiota restoration, intestinal barrier strengthening, HPA axis normalization, and enhanced neuroplasticity. These findings support the therapeutic potential of targeted probiotic interventions for stress-related neuropsychiatric disorders.