This change interested the scientists because Veillonella is one of the few microbes known to metabolize lactate as its preferred fuel. Lactate, a form of lactic acid, is created by working muscles and can cross into the blood and from there, into the gut. When microbes metabolize lactate, they break it down into several substances, including one called propionate, that can influence blood-sugar metabolism, oxygen consumption and inflammation, including in muscles. And scientists found genetic evidence now that the increase in Veillonella in runners after the marathon was pumping up the production of propionate in their guts.

So, could it be, the scientists wondered, that the extra Veillonella and propionate prompted by the strenuous exercise might create an environment inside runners that would enable them to run better and longer, thanks to improved fuel metabolism and lower inflammation? To delve into that question, the scientists, in the final part of the study, infused mice with Veillonella from one of the runners or a placebo and, separately, with propionate or a placebo and had them all run. The animals that received either the bacteria or propionate ran significantly longer than the other mice before tiring.

Given these findings, the researchers “are optimistic” that athletes and inactive people might likewise benefit from popping a little supplemental Veillonella, says Aleksandar Kostic, an assistant professor of microbiology at Harvard Medical School and the Joslin Diabetes Center and the study’s senior author. He, like several of his co-authors, has equity in a new company that plans to offer such supplements. (The current study was funded by the Wyss Institute for Biologically Inspired Engineering at Harvard and the National Institutes of Health, with no contributions from the new company.)

“What excites me is the idea that this might help people who find exercise difficult to increase their exercise capacity,” Dr. Kostic says, while also potentially improving the performance of already-fit athletes

But so many questions remain unanswered. Primary among them is whether the preliminary results from mice can be replicated in people and whether the guts of people who are not runners will respond like those of athletes; Veillonella might work differently in the bodies of sedentary people, for example, or simply die out once it is in the gut. People also may not understand that the supplement, as currently imagined by the scientists, is meant to prop up and not replace exercise. Finally, the researchers did not look into whether levels of lactate actually increased in runners following their race or whether racers’ baseline levels of Veillonella were associated with faster or slower finishing times, so much remains to be learned about how the bacteria functions.