There is an increasing level of interest in how and why the composition of microbes in the gut changes with age, and how and why those changes affect health. It is not unreasonable to argue that these effects are in the same ballpark of significance as, say, exercise. Short-lived species, that tend to exhibit sizable effects on health and life span as a result interventions that impact aspects of aging, do appear to show a slower pace of aging as a result of engineering the gut microbiota to be more youthful in character. Gut microbes at the very least interact strongly with the immune system, but there is clearly a lot more than that going on under the hood.
The human digestive tract is inhabited by numerous microorganisms. Bacteria outnumber all other members of the gut microbial community, and the total number of bacterial species found in the gut is estimated to be about 500-1,000. The most populous bacterial phyla, constituting more than 90% of the gut microbiota are Bacteriodetes and Firmicutes. The remainder consists of many species in other phyla in lower abundance, some of which may provide important metabolites