Not having healthy food to eat isn't the only cause of malnutrition in young children — the types of microbes living in a child’s gut appear to play a big role, too. At least, that’s what two studies in mice suggest. If the findings hold true, they may indicate that probiotics should play a role in malnutrition efforts.

People’s gut bacteria develops as they grow — some strains become more prominent, others less so. This is a normal part of aging. But some people have bacterial colonies that don’t change as they age. Instead, their gut bacteria looks like it belongs to someone younger than they are. Two studies today suggest that may contribute to malnutrition.

Having an immature community of gut bacteria was found to lead to stunted growth, even while eating an otherwise healthy diet. In one study, mice were given the gut bacteria — or microbiome — of different young children in Malawi. The researchers, from the Washington University School of Medicine, then fed the mice a typical Malawian diet. Despite their shared diet, the mice with gut bacteria from malnourished children had "pronounced" growth issues, including issues with body mass and skeletal features. The mice with gut bacteria from healthy children grew much larger, according to results published in Science.

A second study, also published in Science, looks at how different microbiomes respond to diets with few nutrients. A research team from University of Lyon fed a low-nutrient diet to mice with both a typical mixture of gut bacteria and to mice with no gut bacteria at all. Those without any bacteria were smaller and gained less weight, suggesting that the other mice's gut bacteria was able to promote growth even when presented with a lack of nutrients.

Humans serve as the habitat for more than 10,000 species of microbes, according to the Human Microbe Project. What kinds of bacteria live on you may also play a significant role in your health: changes in gut microbiomes have been linked to asthma, mental health issues, and obesity. Taken together, today’s studies build out our knowledge of how the microbiome interacts with food — and begins to suggest a plan of action for creating a healthier one in people who are malnourished.

"These are seminal studies that advance our knowledge of the role of the gut microbiota in growth and malnutrition in children," says Honorine Ward, a professor of medicine at Tufts University School of Medicine, who was not involved in the studies. Ward says that these findings imply that altering "gut microbiota with specific bacterial species or 'probiotics' may be useful in preventing or treating childhood malnutrition."

Foods and supplements have already been developed with the goal of treating malnutrition. But these foods still don't restore a person's microbiome to a healthy state, says Jeffrey Gordon, a director at Washington University’s Center for Genome Sciences & Systems Biology and co-author of one of the papers. That means malnutrition may never be fully treated without taking bacteria into account, he says. "Children are walking around with a persistent abnormality," Gordon says.

Of course, the next step toward solving that is determining which strains of bacteria promote a healthy microbiome. Both research groups looked into that, too. Laura Blanton, a postdoc research associate at the Washington University School of Medicine and lead author on the Malawian gut study says her group ran additional studies "trying to narrow down bit by bit what the responsible bacteria might be to make this difference in growth happen." Blanton looked for bacteria that not only promoted growth in mice, but was able to effectively do so even when added to the gut of a mouse that already had bacteria linked to malnutrition. Ultimately, her paper identifies two successful species of bacteria: Ruminococcus gnavus and Clostridium symbiosum.

Researchers at Lyon used a different approach. Rather than working with bacteria taken from the human gut, that team turned to a strain of bacteria known to promote growth in fruit flies even when they receive a low-nutrient diet. One strain of Lactobacillus plantarum stood out, with the study's corresponding author, François Leulier, saying that it reduced the harmful effects of a poor diet "as efficiently as more complex intestinal microbial communities" — at least, it did so in mice with only one strain of bacteria in their gut, he adds.

Even so, Leulier believes his team's work is "not far" from being applied to humans. Next they need to study whether the bacteria they identified can help mice even when their gut is filled with bacteria linked to malnutrition, instead of having no bacterial competition. "As soon as this validation is obtained, this is the 'Go' for moving to the clinic," he says.

On the other hand, Gordon suggests there's more to do before approaching clinical trials. "It's important to emphasize this is the beginning of a journey identifying [which bacteria strains] have growth promoting effects," he says. Gordon’s lab is also exploring which foods can be used to promote healthy gut bacteria — those foods, he says, need to be both affordable and a good cultural fit for areas struggling with malnutrition. (A separate study out of Gordon's lab, being published today in Cell, moves in that direction by identifying a specific element of breastmilk that can help promote growth once healthy bacteria is already in place.)

Leulier sees the two new studies working toward a solution. Together, he says, they open up "exciting" possibilities for the development of "microbial intervention procedures" that work alongside "re-nutrition solutions to treat the devastating syndromes caused by infant malnutrition."