Lactobacillus delbruecii treatment reverses lipopolysaccharide (LPS)‐induced depression in mice by reducing Toll‐like receptor 4 (TLR4) mRNA and protein expression in the mouse brain, thereby inhibiting TLR4 signaling, effectively reducing the inflammatory response in the brain, and exerting an antidepressant effect [133]. Here, TLR4 is linked to depressive symptom measurement.