You can thank intestinal bugs for the change in a weaning baby’s diaper contents. Research shows that beneficial or “healthy” bacteria living in babies’ guts thrive on unique sugars found in breast milk, then wane once the diet switches to solid food.
According to a recent UC Davis study, the genetic sequence of one of these bugs provides clues for understanding how breast milk nurtures the win-win relationship between healthy bacteria and babies.
Breast-fed infants have lower incidences of diarrhea, diabetes and asthma compared to formula-fed babies. Premature infants that cannot breast feed are particularly vulnerable to a deadly intestinal disease during their hospital stay.
“We think that premature babies, often living in intensive care units for months, are colonized by unhealthy hospital bacteria rather than by healthy bacteria that they would ordinarily get from close contact with their mothers,” said Charles Bevins, a professor of microbiology and immunology at UC Davis who was not involved in the study, in an e-mail interview.
Interestingly, human milk contains a complex mix of sugars called oligosaccharides (HMOs) that are useless to an infant but are invaluable to a species of healthy bacteria called B. infantis that flourishes in a breast-fed baby’s intestines.
“This got us to thinking we should sequence the genome of this [species] to see if there’s anything unusual in that genome … that explains why they’re able to grow on these unusual sugars while others are not,” said David Mills, senior study author and a professor of viticulture and enology at UC Davis.
The researchers found all the genes needed for digesting HMOs clustered within the same region of the B. infantis genome while related species that are prevalent in the adult intestine lack this unique genetic arsenal. Each gene encodes a protein that cleaves a specific chemical bond between the component sugars of HMOS.
The proximity of all these genes to each other suggests that they function and evolved together, and may have even come into the host a long time ago as a single unit, Mills said.
“In a sense, the mother is delivering milk to the infant with the idea that these HMOs are going to enrich the right bugs inside the infant that will keep the infant protected during this critical stage [of growth],” he said.
Carlito Lebrilla, a professor of chemistry, identified the HMOs that B. infantis consumes.
“We elucidated the structures of the [HMOs] that allowed us to match to the specific enzymes that degrade them for consumption,” he said in an e-mail interview.
Researchers also confirmed that the genes are turned on when B. infantis is grown on HMOs in the lab, and identified the metabolic pathway that breaks down and converts HMOs into important biochemical building blocks like fatty acids.
In addition to those involved in HMO digestion, researchers identified genes that help the bug stick to the host infant’s intestines. Another gene cluster is involved in capturing and recycling the waste product urea to produce sufficient dietary nitrogen that a growing infant could not otherwise obtain from the proteins in milk.
“The study provides a model for understanding what [and how] other bacteria … utilize milk, [to] help us better understand the evolutionary processes that influence the composition and the function of infant-associated bacteria,” said lead author David Sela, a microbiology graduate student, in an e-mail interview.
The study is part of a larger UC Davis project called the Milk Bioactives Consortium to characterize and understand the function of breast milk components in infant health.
This goal is important for research to improve the health of infants who cannot breast feed, such as formula-fed and premature infants, Mills explained.
“If we understand how milk nourishes and protects the infant, including enriching the right organisms, we can help prompt that process perhaps with premature infants and … put some of these microbes into their foods,” he said.
Mills, Lebrilla and other faculty recently received funding from the National Institute of Health to evaluate the efficacy of HMOs and live healthy bacteria as dietary supplements to prevent intestinal disease in premature infants at the UC Davis Children’s Hospital.
ELAINE HSIA can be reached at firstname.lastname@example.org.