BDNF and depressive symptom measurement: By regulating coding RNA, non-coding RNA and various signal pathways, intestinal flora can regulate not only the function of hippocampal and microglia, but also the expression level of BDNF and immune inflammatory response related to depression, which ultimately affect depression’s occurrence and development, suggesting that a potential target for treating depression could be inhibition of intestinal flora (Chen et al., 2022; Xu M. et al., 2023).