Oligosaccharides in breast-milk determine how bacteria establish in an infant’s gut

Infants’ gut bacteria strongly influence their health and development. New research conducted at the National Food Institute, Technical University of Denmark shows how the composition of the bacteria in the gut is determined by priority effects – a natural process in which the time and sequence of arrival of the bacterial species affects the structure of the microbial community.

“Our study shows that the decisive factors are not only which bacteria arrive first in the infant’s gut but also their ability to utilise the oligosaccharides in breast-milk. This emphasises the importance of breastfeeding for establishment of a healthy gut microbiome among infants,” explains the lead author, Martin Frederik Laursen, Associate Professor at the Technical University of Denmark, who conducted the research together with his colleague, Henrik Munch Roager, Associate Professor at the University of Copenhagen.

Much information lacking

The new study analysed the gut microbiomes of 25 breastfed infants over the first six months of their lives. In particular, the researchers focused on Bifidobacterium, which are early colonisers of the infant gut and especially thrive on the oligosaccharides in breast-milk.

“In a previous study, we found that Bifidobacterium, which are promoted by breastfeeding, can utilise the oligosaccharides in the breast-milk to grow and establish themselves in the intestine and that the quantity of these bacteria is very closely linked to the production of substances that are immunoregulating and therefore appear to positively influence the development of the infant’s immune system,” says Martin Frederik Laursen.

However, little is known about how dynamic the gut bacterial community is, and researchers often only have one sample from one point in time and then one sample a year later, and then they try to draw conclusions based on comparing between individuals or within individuals over large time spans.

“But information is lacking about what actually happens within an individual over shorter time spans. How stable are bacterial communities? The development especially early in life is extremely important because prior research indicates that the microbiome is highly dynamic in this early phase and we can learn how and why the microbiome changes when it is under establishment,” says Martin Frederik Laursen.

Determine change over time

The bacteria begin to colonise the infant’s gut at birth, and the composition is continuously determined by which bacteria the infant is exposed to, such as via the mother and other family members during infancy. Exposure is required for colonisation and establishment in the gut. 

"However, little is known about how and when this happens. In addition to the exposure itself, several factors determine whether the bacteria actually colonise and to what extent, such as the diet early in life."

Precisely for these reasons, the researchers were fascinated by the chance to observe bacterial populations within infants who are breastfed so that they could determine how they change over time.

"Our results showed that the infants’ gut is initially often dominated by very specific Bifidobacterium species, which only partly utilise the oligosaccharides in breast-milk, such as Bifidobacterium longum or Bifidobacterium breve,” explains Martin Frederik Laursen.

Strong declines in the unutilised oligosaccharides

However, the patterns changed in the infants over time.

“Interestingly, we found one species dominated in the first few weeks to months. And then, about half the children suddenly shifted to another species, Bifidobacterium infantis, which began to dominate and comprised 80–99% of the entire gut microbiome,” adds Martin Frederik Laursen.

The researchers linked the amount of B. infantis to the quantity of oligosaccharides that came from breastfeeding by measuring them in the infants’ stool samples.

“The quantity is relatively high to start with when the infants have other Bifidobacterium species, such as B. longum, but as soon as B. infantis establishes itself and starts to increase in numbers, the oligosaccharides remaining in faeces decline rapidly. B. infantis consumes the oligosaccharides very efficiently,” says Martin Frederik Laursen.

A strong selective pressure

The experiments thus indicate that B. infantis is extremely good at utilising the oligosaccharides from breast-milk and therefore has a great competitive advantage over the other species. The big question, however, was why B. infantis only outcompeted the other bacteria later.

“Our data and the literature show that B. infantis typically colonises later than the other species. The infants are often exposed to B. infantis later than they are exposed to the other species. Unlike many other species, B. infantis is not highly transferred from mother to child but likely more so between infants. That is probably why it colonises later, because they have to pick it up from other babies,” explains Martin Frederik Laursen.

The first Bifidobacterium species that enter the intestinal system have an advantage in establishing themselves first, simply because they arrive first – in other words a ‘priority effect’. However, if a new, later arriving species more efficiently consumes the oligosaccharides from breast-milk, this will become selected for.

“And that is exactly what we think is happening with B. infantis – it is simply selected for over time. It says something about how extremely selectively these oligosaccharides from breast-milk can work,” adds Martin Frederik Laursen.

Priority effects and selection

The researchers recreated the study’s observations through subsequent animal experiments to determine whether they had understood the system correctly.

“We used germ-free mice that have no intestinal bacteria at all, as a model for the infant, which initially also has no intestinal bacteria but then subsequently to birth is colonised by bacteria." 

To one group of mice the researchers first gave B. longum, which suboptimally uses the oligosaccharides, and then afterwards B. infantis, which efficiently uses them. 

"For another group, we then swapped the colonisation order. We repeated this entire set-up, but this time we gave all the mice oligosaccharides in the drinking-water to simulate breastfeeding,” explain Martin Frederik Laursen.

Ultimately determines the dominating species

After a few days, when the bacteria had established themselves, the researchers then revisited them to examine the power relationships.

“Without the oligosaccharides from the breast-milk, the Bifidobacterium species that arrived first dominated at the end of the experiment, which proves the priority effect. But when we instead gave the mice the oligosaccharides in their drinking-water – to simulate breastfeeding – the species that arrived first did not necessarily dominate in the end,” says Martin Frederik Laursen.

In the mice, too, the ability to utilise the oligosaccharides turned out to determine which species ultimately dominated. Similar to the infants, B. infantis dominated, regardless of whether it arrived first or last.

“So the strong selective pressure from the oligosaccharides ultimately determines which species dominates in the gut despite priority effects. This confirms our observations with the infants,” explains Martin Frederik Laursen.

Affects the immune system but in danger of extinction

The new studies are important for understanding the development of infants’ gut microbiota. They emphasise the importance of both the colonisation sequence and lactation.

“Our previous studies have shown that many of the Bifidobacterium species that consume the oligosaccharides produce immunoregulatory substances. Effective users of oligosaccharides – such as B. infantis – are associated with a lower incidence of atopic eczema and inflammatory conditions in the gut as well as improved immune system development and better growth among malnourished infants. So these bacteria seem to be important,” says Martin Frederik Laursen.

A few studies have given B. infantis to infants, and it appears to benefit immune development.

“This may constitute an evolution explanation of why we select for this species through breastfeeding. However, although B. infantis was most frequent in 11 of the 25 infants examined, it has a low prevalence in many Western populations, which raises concerns about its possible extinction,” he adds.

This is especially true in the United States, where infants have very few B. infantis, probably because it has been lost in the population, so that it is no longer transmitted from individual to individual.

“But several studies have given B. infantis to breastfed infants and shown that it is possible to reintroduce the bacteria into the ecosystem,” explains Martin Frederik Laursen.

Breast-feeding and probiotics

Together with other recent evidence from the field, the new research may have important implications for children’s health.

“Our observations highlight that promoting and supporting breastfeeding remains a key priority. This can be supplemented with early probiotic administration, such as B. infantis, to ensure effective intestinal colonisation with these important Bifidobacterium species in the infant’s early life – especially for the infants who for some reason lack these,” says Martin Frederik Laursen.

The researchers think that early treatment can be crucial, partly because B. infantis and other efficient milk oligosaccharide consumers seem to be selected for the infant gut through evolution and partly because very early life is extremely important in determining the individual’s health later in life.

“A lot happens in very early in life in relation to the development of the immune system, growth and neurodevelopment. Therefore, we would very much like to better understand exactly how the gut microbiome is established and develops early in life, to better understand the connections between gut bacteria and the development of diseases,” concludes Martin Frederik Laursen.