Figure 1.

The interplay between the microbiome, microbial metabolites, and neuropathological processes present during neurodegenerative diseases such as AD and PD. The microbiome and microbial metabolites could regulate brain homeostasis via four different ways: 1. direct absorption of specific metabolites which have the ability to cross the BBB; 2. interaction between peripheral immune systems, which interact with brain glia cells and astrocytes; 3. influence on gut hormones and the enteroendocrine system; 4. communication via the vagus nerve. These four routes may have an influence on pathological symptoms such as increased BBB leakage and neural inflammation, enhanced beta-amyloid and alpha-synuclein formation, and finally neuron loss and motor/cognitive deficits.