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 and functions for healthy aging.
Individual gut microbiotas show distinct profiles, and this inter-individual variation is greater in older adults. Longitudinally, however, gut microbiotas of healthy adults are relatively stable even for decades. Thus, once established early in life (even within 3 years after birth), the gut microbiota seems to be rather stably maintained. Nevertheless, it is responsive to the host's dietary and health conditions, much as the host's epigenome is to various environmental cues. In fact, the gut microbiota interfaces the gut environment with the epigenome, but its communication with the host systems involves various signaling networks and their mediators. For instance, the "gut-brain axis" connects the gut microbiome with the central nervous system via neurons, hormones, or cytokines.
Despite variation between individuals, most adult age groups, from young to extremely old, seem to possess a common core function in their microbiomes that is provided by members of abundant taxa. If so, what is important in the gut microbiota for healthy aging could be a compositional change in the functional core microbiome or an enrichment of non-core functions with advancing age.
With advancing chronological age, the gut microbiota becomes more diverse. However, when biological age is considered with adjustment for chronological age, overall richness decreases, while certain bacterial taxa associated with unhealthy aging thrive. Thus, as biological age increases, the homeostatic relationship between the gut microbiota and the host deteriorates, while gut dysbiosis increases. These dysbiotic changes in the aging gut can negate the beneficial effects of the gut microbiome on the nutrient signaling pathways, and provoke proinflammatory innate immunity and other pathological conditions.