This hidden immaturity matters because our microbes help us to harvest energy from our diet. If they’re not doing that effectively, their hosts might suffer, especially when their diet offers very little energy to begin with. “Most people think about development from the perspective of our human cells and organs, but there’s another facet to it–our gut microbiota, an organ composed of microbes,” says Gordon. “It gives us a more transcendent view of human developmental biology.”
Now, following similar work in Bangladesh, Gordon’s graduate student Laura Blanton studied the changing microbiomes of 60 healthy Malawian infants, and devised an algorithm that calculated their microbiological age based on the species in their guts. She then applied the algorithm to another group of 259 babies, and found that their microbiological age scores at 12 months predicted how much weight they had put on by 18 months. That’s a sign that the microbes are influencing the babies’ growth, rather than merely reacting to them.
The team found another such sign by transplanting stool samples from Malawian infants into sterile baby mice. Even though all the rodents ate the same food, those that received microbes from an underweight infant put on less weight and developed weaker bones than those which received a healthy baby’s microbiome.
So, if certain gut microbes can stymie a baby’s growth, perhaps others can speed things up? To find out, Blanton simply implanted mice with microbes from either a healthy infant or an underweight one, and housed them together in the same cages. Mice willingly eat each other’s poop, and so regularly bombard their own microbiomes with those of cage-mates. And in these “Battles of the Microbiomes”, the healthy communities came out on top, invading and displacing the immature ones.
The team then identified five species of bacteria that were especially successful colonizers, and had a particularly strong influence on their host’s growth. When mice ate this quintet, even if they had been previously loaded with microbiomes from malnourished children, they gained normal amounts of weight. Thanks to the incoming bacteria, they were better at converting amino acids in their diet into flesh and muscle, rather than simply breaking down those nutrients for energy.
“This underscores the critical importance of early life gut microbiome composition and its capacity to transform dietary components into molecules that significantly influence childhood development,” says Susan Lynch from the University of California, San Francisco. And Fergus Shanahan from University College Cork agrees: “This is a beautiful piece of work,” he says. “It’s ground-breaking in showing how elements of the microbiota influence their host’s health and well-being.”
Of course, none of this explains why the microbiomes of malnourished infants stall in their development in the first place. There are many possible reasons including antibiotic exposures, gut diseases, and poor diets, which vary from person to person. Still, Gordon’s team has now found similar patterns in malnourished children from both Bangladesh and Malawi—two countries with very different cultural and dietary traditions. “That gives us hope that there are basic themes in normal gut microbiota development that operate across children,” he says. “That may allow us to devise interventions that affect large groups of individuals.”