A growing body of research suggests that the bacterial populations existing in symbiosis with your tissues play an important role in how metabolic processes interact to determine longevity. This is yet another good reason not to bet on metabolic engineering as the fast horse in the race to produce engineered longevity - the complexity is staggering.
Here's a little background reading on the topic of aging and your bacteria:
Generally, there is an age-related decline in the human gut titer of Bifidobacterium species, but the titer in healthy centenarians was previously reported to be comparable to that found in much younger people. We addressed whether elevated Bifidobacterium titers relate positively to immune function.
This study evaluated the immunoactivities of 2 Bifidobacterium strains (B adolescentis BBMN23 and B longum BBMN68) isolated from healthy centenarians in China.
In conclusion, ingestion of B. adolescentis BBMN23 and B. longum BBMN68 can enhance both innate and acquired immunity in healthy specific pathogen-free (SPF) mice and strains of bifidobacteria from healthy centenarians in Bama longevity villages in China may possess potentially valuable immunomodulatory properties.
A immune system in better shape should be enough to raise life expectancy in and of itself, as the immune system not only protects against pathogens, but also destroys damaging senescent cells and cancerous cells, amongst other jobs.
The whole point of a serious program of longevity research, of course, would be to make irrelevant the range of metabolic differences in human populations and the bacteria that inhabit us. These differences are only interesting because we don't yet have a method in hand for reliably adding multiple decades to healthy and maximum life spans - for all that we can clearly and in great detail envisage what that method looks like.