fig2

Figure 2. Model of the gut-immune-brain axis. Microbes can directly shape the brain microenvironment through their metabolites or indirectly through modulation of the host immune system. The absence of microflora in germ-free (GF) or antibiotics-treated (ABX) mice have been demonstrated to induce blood-brain barrier (BBB) leakage^[85,86]. Segmented filamentous bacteria (SFB) can stimulate the production of IL17a by Th17 cells, which drives neuroinflammation^{[105,108-110]}. On the other hand, beneficial probiotics such as L. reuteri ATCC PTA 6475 or B. longum subsp. infantis DSM 24737 can protect from neurological disorders through metabolic pathways and the induction of immune homeostasis. Brain-to-gut signals through the vagus nerve control colonic regulator T cell (Treg) populations to regulate gastrointestinal homeostasis.