Living past 100 years is a rare feat for a human. New research suggests that centenarians might have their gut microbiomes to thank, at least in part.

A study of Japanese people who have lived over a century found they harbor distinct groups of bacteria in their gut, which generate unique compounds and might even ward off infections and other environmental stressors.

Centenarians, in general, are less susceptible to age-related chronic illness and somehow capable of thwarting infectious diseases. Now it seems the gut microbiome – the billions of microorganisms living in our digestive tract – could help to explain why.

We might think that the secrets to a long life are written in our genes. But genetics accounts for less than 30 percent of longevity, which leaves the door wide open to many other factors that change over time, including diet, relationships, and now, possibly, gut bacteria.

The study recruited 160 centenarians from across Japan who had racked up an average age of 107 years, and compared the bacterial communities found in their fecal samples to the gut bugs of another 112 elderly people in their late 80s, and also to those of 47 younger folks.

The researchers, led by microbiologist Yuko Sato from Keio University School of Medicine in Tokyo, were looking for differences in their gut microbiomes – that is, the types of bacterial species present, and the sorts of compounds they produced.

Previous studies of centenarians from Sardinia in Italy have found that people aged over 100 years had a higher diversity of core microbiota species living in their intestines than young people and the elderly.

The allure of studying the gut microbiome is that by figuring what a healthy one looks like, scientists might be able to find ways to shift bacterial communities or correct their imbalances to prevent disease and improve health in other people.

As attractive as that idea sounds, the gut microbiome is a big beast that's mighty complex but also very sensitive. Studies have shown that changes in diet can rapidly alter the composition of gut microbes in a matter of days, by promoting certain species over others.

That said, centenarians are an extreme example of healthy aging, so these people must be doing something right to live so long.

While some of the centenarians in this new Japanese study showed typical signs of aging, such as low-level inflammation, "the majority of centenarians were free of chronic diseases such as obesity, diabetes, hypertension, and cancer," the researchers write in their paper.

The total bile acid load was no different between the groups, but centenarians had a unique mix of bile acids, thanks to a handful of bacterial strains capable of synthesizing them.

On further inspection, one group of bugs stood out: Odoribacteraceae strains, which reliably produced a type of bile acid called isoallo-lithocholic acid (isoalloLCA).

"Notably, both fecal acid profile and [microbial] community type of centenarians were generally stable between longitudinal samples," collected over the course of 1-2 years, the researchers write.

But the study didn't account for other lifestyle factors, such as diet, so we can't say what exactly contributed to their unique microbiome profile.

In further experiments, Sato and colleagues showed that isoalloLCA could inhibit the growth of lab-cultured Clostridium difficile, a common gut bug that causes severe diarrhea and serious inflammation of the colon.

And when mice infected with C. difficile were dosed with Odoribacteraceae, the bile-producing strains identified in the centenarians, the treatment reduced the amount of C. difficile shed in mouse droppings below detectable levels, which suggests it helped them ward off the infection.

"To our knowledge, isoalloLCA is one of the most potent antimicrobial agents selective against gram-positive microbes, including multidrug-resistant pathogens," the researchers conclude in their paper, going so far as to suggest that isoalloLCA-producing species, such as Odoribacteraceae, could be used to help maintain good gut health into old age.

While these lab experiments do help to explain how the bacterial species found in centenarians could offer some protection against infections via bile acids, other experts are more cautious about the results.

"Like many studies that seek to implicate specific microbiome signatures with particular conditions in humans, as yet the work mostly reveals correlations rather than causality," explains research physiologist Kim Barrett, at UC San Diego, who was not involved in the study.

On the other hand, bile acids are emerging as a new class of intestinal hormones that appear to do more than just aid fat digestion and absorption, Barrett says.

"It is certainly conceivable that manipulating concentrations of specific bile acids, whether microbial or by giving them directly, could exert health benefits," she says.

However, probiotic treatments containing live bacteria thought to confer health benefits have had variable results in research thus far, so loads more research is needed before you can expect any bacteria-packed pill for longevity.

Plus, there are plenty of other things that have been linked to long life which we can all try to do, from staying social to reducing stress and - you guessed it - eating well.

The research was published in Nature.