Gut bacteria play a significant role in stimulating the growth of the immune system and regulating its activity. One way they do it is by maintaining the integrity of the cells lining the gut; they prevent escape into the bloodstream of any microbes or toxic substances that might otherwise leak out and cause chronic systemic inflammation, a contributor to disorders ranging from inflammatory bowel disease to depression. Dysfunction of the gut barrier, a condition known as “leaky gut,” is linked to autism in ways still not understood.
The composition of the microbiome also directly influences immunity. Animal studies show, for example, that the presence of specific beneficial bacteria in the microbiome (Akkermansia is one type) can tamp down the activity of immune cells responsible for inflammation in the brain. Having a vibrant population of Akkermansia in the gut may help people resist frailty in aging. Other beneficial bacteria have been shown to lower levels of a type of inflammatory cells known as cytokines.