BDNF and depressive symptom measurement: In a rodent model, Bifidobacterium alleviates symptom of depression and related microbiota dysbiosis, with improvement of serotonin levels and brain-derived neurotrophic factor (BDNF) concentration in brain (BDNF is essential for neuronal development and survival, synaptic plasticity, and cognitive function), and reduced serum corticosterone level and increased cecal butyrate level, which were significantly and positively correlated with depression-related indexes (Tian et al., 2019).