Human milk oligosaccharides (HMOs) are a diverse group of carbohydrates found in breastmilk. Because the HMOs can’t be used by the infant directly for energy, scientists believe their purpose is to stimulate the development of a healthy gut microbiome. During the first year of life, an infant’s gut is dominated by Bifidobacteria, in particular B. infantis, and B. bifidum. In a recent publication scientists measured the difference in HMO utilization between these bugs, and discovered they have very different and important strategies for HMO utilization. The results were published in Nature Scientific Reports.
The scientists first isolated multiple strains of each species, B. infantis and B. bifidum, from the feces of newborn infants. They then attempted to culture each strain alone in a mixture of HMOs from breastmilk, as well as the individual HMOs alone. They learned that the each B. infantum strain could grow on pooled HMOs, but interestingly some of the B. bifidum strains could not grow alone on HMOs. When the bacteria were cultured with mucins (containing sialic acid or fucose, as previously discussed on this blog) none of the B. infantis could grow, whereas most of the B. bifidum could. This implies that B. infantis alone cannot utilize fucose or sialic acid, but rather needs the help of other bugs to break these down to utilize them. After, the scientists looked at the regulation of different genes during the culturing experiments. From these results they determined that B. infantum transports the HMOs inside the cell before breaking them down for energy. B. bifidum, on the other hand, breaks the oligosaccharides down extracellularly before taking up smaller, simpler sugars.
All together we see that there is a complex assemblage of bugs in the guts of infants that all rely on one another for energy and metabolism. The breastmilk cocktail of HMOs itself is so complicated that it almost necessitates the interdependent communities to grow. Overall, this creates a robust and resilient microbiome that prevents pathogens from taking hold and protects the infant during his or her most vulnerable years.