A paper published in Ageing describes how gut microbe transplants from longer-lived humans (with healthier gut microbiota) improve gut microbiota diversity in mice, and appear to delay ageing.
That simple faecal matter transplantation was able to achieve beneficial results in recipient mice suggests that achieving these effects in humans might not be far off. Human faecal microbiota transplants are currently performed in conditions in which pathological microbes take over the gut. Similar transfers from young donors to older individuals could restore the gut microbiota to a healthier, more diverse state, provided that harmful bacteria could be screened out.
Another less crude (and less unpleasant!) strategy would be a probiotic approach, in which desirable bacteria could be encapsulated and delivered orally.
We are close to being able to put these therapies to the test. There is solid evidence that gut health is linked to many pathologies of ageing, and so we can be hopeful that such treatments will have meaningful benefits.