The composition of gut bacteria is thought to influence aging: there is a modest range of work on this topic in laboratory animals, but nowhere near as much as on, say, the effects of calorie restriction on aging. Here is an example of ongoing investigations:
[Scientists] have promoted health and increased lifespan in Drosophila by altering the symbiotic, or commensal, relationship between bacteria and the absorptive cells lining the intestine. The research [provides] a model for studying many of the dysfunctions that are characteristic of the aging gut and gives credence to the growing supposition that having the right balance of gut bacteria may be key to enjoying a long healthy life.
The bacterial load in fly intestines increases dramatically with age, resulting in an inflammatory condition. The imbalance is driven by chronic activation of the stress response gene FOXO (something that happens with age), which suppresses the activity of a class of molecules (PGRP-SCs, homologues of PGLYRPs in humans) that regulate the immune response to bacteria. PGRP-SC suppression deregulates signaling molecules that are important to mount an effective immune response to gut bacteria. The resulting immune imbalance allows bacterial numbers to expand, triggering an inflammatory response that includes the production of free radicals. Free radicals, in turn, cause over-proliferation of stem cells in the gut, resulting in epithelial dysplasia, a pre-cancerous state.
[Researchers] increased the expression of PGRP-SC in epithelial cells of the gut, which restored the microbial balance and limited stem cell proliferation. This enhancement of PGRP-SC function, which could be mimicked by drugs, was sufficient to increase lifespan of flies. "If we can understand how aging affects our commensal population - first in the fly and then in humans - our data suggest that we should be able to impact health span and life span quite strongly, because it is the management of the commensal population that is critical to the health of the organism."
"Quite strongly" in this context means a couple of years in humans, or a few days in flies - a very minor, modest extension of life in the grand scheme of things, in other words. This is the trouble with most of present day research in the field: it aims only to slow aging, and only to slow aging a little. Ambitions are low, and this - and much of the rest of the field - certainly isn't SENS or otherwise at all relevant to a goal of radical life extension of decades or more.