VANCOUVER, CANADA—The life of the lab mouse is a far cry from the world of a hungry child in Africa. But a roomful of mice in St. Louis, Missouri, may offer a surprising explanation for why some starving children develop a form of malnutrition called kwashiorkor, in which their bellies and faces become bloated, but others fare better. The rodents have revealed that variations in gut bacteria may be at the heart of this disparity—and may also explain why some malnourished children respond better to nutritional supplements than others.
Malnutrition is the leading cause of childhood mortality worldwide. In Malawi, a hot spot for severe malnutrition, nutrition scientist Mark Manary of Washington University in St. Louis (WUSTL) is running a long-term study of twins to learn how best to deal with malnutrition. He has followed 317 pairs from birth through age 3. In this group, one-third regularly do not get enough to eat, and 2.5% develop the swollen bellies and faces indicative of kwashiorkor. Although the twins live together in close quarters and eat the same food, half the time only one twin suffers from malnutrition. Manary has been puzzled by this and recently involved WUSTL microbiologists to find out if variation in bacteria living in the guts of these twins could account for the difference.
WUSTL's Michelle Smith couldn't easily study the bacteria in the children themselves, so she used germ-free mice as surrogates. From birth, these mice were kept in special air-tight chambers and fed treated foods to ensure that they are not exposed to any microbes. The researchers collected and froze stool samples from a set of twins, one who had kwashiorkor and one who did not. They fed one twin's samples to one group of germ-free mice and the other to a second group to "humanize" each mouse's gut to be like one of the twins. After checking to make sure the children's set of bacteria had taken hold, Smith manipulated the diets of the two groups of mice, checking for changes in the gut bacteria and weighing the rodents every few days.
For 3 weeks, the mice ate a typical Malawian diet, which consists primarily of ground corn flour with a smattering of vegetables and meat. Then for 2 weeks, the mice feasted on a mixture of milk powder, peanut butter, sugar, vegetable oil, and vitamins—a so-called ready-to-use therapeutic food for malnutrition that packs in eight times as many calories as Malawian fare. Finally, the mice went back to the corn flour mash for two more weeks. The researchers isolated and sequenced DNA from the mouse droppings, studying a gene often used to distinguish bacterial species to find out what gut bugs were present. They also looked at the total repertoire of genes in the sampled DNA to assess how well the microbial community was working.
The mice with the bacteria from the child with kwashiorkor lost, then gained and lost again, more weight than the mice with bacteria from the healthy twin. The species composition of those bacteria from the kwashiorkor child fluctuated more with changes in diet, as did the makeup and abundance of bacterial enzymes, Smith reported here 9 March at the International Human Microbiome Congress. The bacterial community from the healthy child remained fairly stable in species composition and enzyme profile. For example, Smith saw a significant change in 237 enzymes in the profile of the healthy mice, but 505 enzymes were affected in the kwashiorkor mice.
In this pilot study, Smith looked at just microbes from just one pair of twins. But the data are supported by a parallel study that looked directly at the gut bacteria in 10 sets of twins in response to food supplements. Smith found that 42 enzymes changed in the healthy twin, on average, compared with 339 in the kwashiorkor twin. She plans to study more sets of humanized mice to extend these findings.
The mice findings suggest that the makeup of the gut bacteria is important in the response to starvation, says Andrew Serazin, a program officer at the Bill & Melinda Gates Foundation, which funded the work. One possibility to remedy this might be to provide a "probiotic" supplement that would encourage starvation-resistant microbes to thrive in the gut. But "the simple approach of just giving a food source or a bacterial species may not be enough. You might have to do both," adds Serazin.
Germ-free mice have "a lot of potential to [help researchers] figure out what is the optimal way to give calories and nutrients back" and to test the potential of probiotics, says microbiologist Martin Blaser of New York University.
The work also underscores the importance of studying the microbial communities associated with the human body, a field that is still in its infancy, adds immunologist Bhagirath Singhof the University of Western Ontario in London, Canada. "Nobody expected that in this time frame that we would have [this] potential impact on human health."
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