Detrimental changes in the gut microbiome might prove to be one of the easier issues to fix in the aging human body. When we talk about an aged gut microbiome, we mean that the balance of microbial populations has shifted. There are more harmful microbes that either produce damaging metabolites or otherwise engage with tissue and the immune system to provoke chronic inflammation. At the same time there are fewer beneficial microbes working to produce metabolites that are necessary for tissue function, such as the butyrate that promotes neurogenesis. Rejuvenation in the context of the gut microbiome means only rebalancing these populations, nothing more complicated than that is needed.
How to go about achieving the goal of putting a youthful microbiome into an old body? The approach with the most robust evidence in animal studies is fecal microbiota transplantation, which is to say literally taking the microbiome from a young individual and placing it into an old individual. This restores a youthful microbiome for a lasting period of time, reduces inflammation, improves other measures of health, and in short-lived species acts to extend life span. Fecal microbiota transplantation is already practiced in human medicine, but only as a treatment for C. difficle infection, in which a pathogenic microbial species has overtaken the gut, but can be out-competed by transplanted species. It would not be a great leap to adapt this to the treatment of aging.
The authors of today's open access paper are much in favor of stool banking as a way to establish material for transplantation, sampling a young individual's microbiome and storing it for later. This seems to me to have the same issues as stem cell banking, in that over a time span of decades it is highly unlikely that technology will remain static. Producing a youthful microbiome for transplant, either by screening out unwanted species from young donors, or by manufacturing to order, should be an everyday occurrence not so very long from now. Screening donors could be implemented now, given a sufficiently motivated industry.
Industrial advances have been associated with large-scale changes in the human gut microbiome and a higher incidence of complex human diseases. Rewilding the human gut microbiome by transplanting the whole gut microbial community from donors in nonindustrial societies may result in a dramatic mismatch between our industrial environment/lifestyles and the ancestral microbiome.
Emerging studies suggest that stool banking and autologous fecal microbiota transplantation (FMT), using the recipients' own stool samples collected at a younger age when they are disease-free, may be a better - or at least an alternative - solution. This leads to the idea of rejuvenating the human gut microbiome. The conceptual similarity between stool banking for autologous FMT and cord blood banking for an autologous transplant implies the potential for rejuvenating the human gut microbiome.
Here we propose rejuvenating the human gut microbiome by stool banking and autologous fecal microbiota transplantation, that is, collecting the hosts' stool samples at a younger age when they are at optimal health, and cryopreserving the samples in a stool bank for the hosts' own future use. In this article we discuss the motivation, applications, feasibility, and challenges of this solution. Basic research in cataloging, characterizing, and even engineering individual microbes (or well-defined consortia of them) and their functions (or metabolic fuels/products) is still a very promising solution to restoring a healthy gut microbiota. However, considering the daunting complexity of the human gut microbiota, both bottom-up mechanistic approaches and top-down systems approaches (based on FMT) will be needed.