What is already known on this topic
Human breastmilk contains sugars that promote the growth of specific gut microbes, including certain species of bifidobacteria. Studies have shown that bifidobacteria are prevalent in the gut of breastfed infants, and their depletion in early life has been associated with a higher risk of allergy and asthma in childhood. However, how bifidobacteria benefit infants has so far been unclear.

What this research adds
By analyzing fecal samples from nearly 60 Danish babies, researchers have found that the types of bifidobacteria growing in the infants’ guts and using breastmilk sugars contain an enzyme that helps the bacteria to produce small molecules called aromatic lactic acids. These molecules are thought to have a beneficial effect on health by strengthening the gut barrier, protecting against infections, and influencing host metabolism.

Conclusions
The findings suggest that bifidobacteria associated with breastmilk provide health benefits in early life.

Breastfeeding is touted as providing many health benefits to infants, but what exactly happens in the gut of breastfed babies is poorly understood. Now, researchers have found that bifidobacteria associated with breastmilk produce small molecules that may improve immune function.

The findings, published in Nature Microbiology, suggest that some bifidobacteria provide health benefits in early life. “The results of the study are useful for supporting measures aimed at helping children develop a balanced gut microbiota, which supports a well-functioning immune system,” says study co-senior author Henrik Roager at the National Food Institute, Technical University of Denmark. “Such measures include supporting breastfeeding and developing new types of infant formula and probiotics that promote the presence of these bifidobacteria in life early.”

Human breastmilk contains sugars that promote the growth of specific gut microbes, including certain species of bifidobacteria. Studies have shown that bifidobacteria are prevalent in the gut of breastfed infants, and their depletion in early life has been associated with a higher risk of allergy and asthma in childhood. However, how bifidobacteria benefit infants has so far been unclear.

To address this question, Roager, Tine Licht and their colleagues analyzed fecal samples from 59 healthy Danish babies.

Metabolite production

The researchers found that stool samples from breastfed infants contained high levels of certain molecules called aromatic lactic acids. Bifidobacteria, but no other bacterial species, were associated with high fecal concentrations of aromatic lactic acids, which suggests that Bifidobacterium species produce aromatic lactic acids in the infant gut.

When grown in a lab dish, some bifidobacteria — including B. bifidum, B. breve, and B. longum — produced aromatic lactic acids, whereas other Bifidobacterium species, such as B. adolescentis, B. animalis, and B. dentium, produced only low amounts of these metabolites.

Studies on Lactobacillus bacteria have suggested that an enzyme called aromatic lactate dehydrogenase is responsible for the production of aromatic lactic acids. So, the researchers searched for all available lactate dehydrogenase genes in bifidobacteria. They found that breastmilk-promoted Bifidobacterium species had a lactate dehydrogenase gene responsible for the production of aromatic lactic acids.

Infant health

Aromatic lactic acids are thought to have a beneficial effect on health by strengthening the gut barrier, protecting against infections, and influencing host metabolism. Further experiments showed that some aromatic lactic acids can modulate immune function, and measurements of these metabolites in urine samples of infants indicated that aromatic lactic acids are absorbed from the gut into the blood circulation. This suggests that the metabolites may have systemic effects, the researchers say.

Future work should investigate the relationship between these metabolites and the development of the infant immune system. “We do not know if there is a window where this effect of breastfeeding is particularly important in order to achieve the optimal effect on the immune system,” says study first author Martin Laursen. “It is interesting and relevant to investigate this further.”

The researchers hope that their findings will spur more research on the role of aromatic lactic acids in preventing disease and help to develop new ways of strengthening the immune systems in early life.

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Giorgia Guglielmi is a freelance science writer based in Cambridge, Massachusetts. She received a PhD in Biology from the European Molecular Biology Laboratory and a Master’s in Science Writing from the Massachusetts Institute of Technology.