BDNF and major depressive disorder: In this field, a crucial inflammatory factor, IL-1β, was observed downregulated by Lactobacillus gasseri NK109 with the simultaneous upregulated expression of brain-derived neurotrophic factor (BDNF) in the hippocampus of mice with cognitive impairment and depression induced by Escherichia coli K1, suggesting its alleviation in brain inflammation against neuropsychiatric disorders caused by bacterial infection [86].