BDNF and depressive symptom measurement: Notably, probiotics, such as Lactobacillus paracasei, Lactobacillus acidophilus, Lactobacillus casei, Lactobacillus fermentum, Lactobacillus helveticus, Lactobacillus rhamnosus, Bifidobacterium bifidum, Bifidobacterium longum, Bifidobacterium breve and Bifidobacterium infantis, improve the central expression of BDNF, N-methyl-D-aspartatic acid (NMDA) receptor and other neuroactive peptides involved in synaptic and neural plasticity to enhance memory, cognition and behavior, and reduce microglial activation in a wide range of neurological illness including anxiety and depression [64,65,66,67].