If you have been on any social media platform as of late, you may have come across wellness influencers promoting gut health powders, probiotic gummies, or sparkling sodas infused with “30 billion CFUs”. Everything promises to ‘balance your microbiome,’ ‘boost your immunity,’ and ‘heal your gut’ - sometimes all before the end of the day. With all this hype, you might hope that these products can be real solutions to improving your gastrointestinal tract. But for all the clever slogans and packaging, there still lacks the key part of information that may make these probiotics beneficial: it’s not what you take, but also when you take it.
Before diving deeper into the study, let’s take a step back and examine the context of host-microbe interactions in the gut. Our GI tract is home to trillions of microbes, and while this might sound scary, most of them are harmless or even beneficial. These relationships fall under a spectrum:
- Mutualism- both host and microbe benefit (like Bifidobacteria helping digest milk sugars and training the immune system)
- Commensalism- the microbe benefits without harming or helping the host
- Parasitism- the microbe harms the host (what we’d call a pathogen)
A recent study by Shao et al. challenges the timing at which gut microbes are most helpful. In the case of one bacterium, Bifidobacterium, it turns out that showing up early may be the key to shaping a healthy gut - when a specific strain of Bifidobacterium gets to the gut first, it doesn’t just help, it takes over. B. breve is one of the earliest bacterial species to colonize the infant gut. Shaped like a Y (Fig. 1), they thrive in low-oxygen environments and play several crucial roles: digesting complex sugars in breast milk, strengthening the gut barrier, and helping train the immune system. It's essentially laying the blueprint for a healthy digestive and immune foundation.
Figure 1. From Brevicillin.it. Bifidobacterium breve under the microscope. Their unique structure helps them thrive in low-oxygen environments and adhere to the intestinal lining.
Shao et al. gathered information about the real-world microbiomes of over 700 human infants. By comparing babies born vaginally to those delivered via C-section, they found clear differences in who got which microbes first. Vaginally delivered infants were more likely to be colonized by B. breve, while C-section babies were often dominated by Enterococcus faecalis, a less desirable microbe. This kind of observational data helped set the stage for their next experiment.
Researchers conducted their experiment and followed over 1,200 neonatal mice and administered bacteria to find out how their gut microbiomes assembled over the first few weeks of life. They examined how the gut bacteria organized themselves, and found that the neonatal mice’s guts formed into one of three “community states,” each dominated by a different microbe: B. longum, B. breve, or E. faecalis. You can see this visually in the PCoA plot (Fig. 2), where each dot represents the gut microbial community of one mouse. The closer two dots are, the more similar those communities are.
Figure 2. From Shao et al., Nature Microbiology, 2024. PCoA plot showing early gut microbiomes falling into 3 distinct groups based on which bacteria take hold first- B. breve (blue cluster), E. faecalis (purple cluster), and B. longum (orange cluster).
The neonatal guts clustered into distinct “states” based on which bacteria dominated early in life. Those in the B. breve cluster tend to have healthier, more stable microbial communities, while those dominated by E. faecalis are more likely to experience instability and inflammation down the road. Early colonization sets the tone. The authors refer to this as a microbial “priority effect” - a concept borrowed from ecology, where the species that colonize an environment first often determine what comes next. If B. breve is among the first microbes to establish in a baby’s gut, it tends to stay and thrive. Should something else get there first, especially something like E. faecalis, B. breve may never get the chance.
The importance of timing comes up again in Figure 3, which shows the percentage of mice that stayed in the same microbial state they started with. Infants whose guts started in the B. breve-dominated state tended to stay that way. Those in the E. faecalis state? Not so much. Their microbiomes were more likely to shift, and often too late to avoid the early influence of harmful bacteria.
Figure 3. From Shao et al., Nature Microbiology, 2024. Gut microbial states show different levels of stability over time. Infants whose guts were colonized early by B. breve were more likely to maintain that stable, beneficial microbiome, while those dominated by E. faecalis had more unstable, shifting microbiomes.
This kind of early microbial instability is more than just messy. Scientists are convinced that the first few weeks and months of microbial exposure influence long-term immune function, inflammation, and even metabolic health. In other words, the microbes you meet first don’t just help digest food; they may help train your immune system, set an inflammatory basis, and establish your baseline health for years to come.
Another key to B. breve’s success lies in its ability to digest human milk. Human milk contains a unique set of sugars called human milk oligosaccharides (HMOs), which infants can’t digest on their own - but certain strains of B. breve can. Researchers used PCR and whole-genome sequencing to identify these strains and see how they performed in the gut. The strains equipped to digest 2’-fucosyllactose (2’-FL) thrived and crowded out pathogens. Figure 4 shows this effect clearly: only the “right” B. breve strain with the proper genes outcompetes harmful microbes. These strains thrived in the gut, crowding out harmful bacteria and establishing a strong microbial presence early on.
Figure 4. From Shao et al., Nature Microbiology, 2024. Only certain B. breve strains can digest 2’-FL. These strains carry the fucosidase enzyme genes (GH29/95) and outcompete pathogens in the gut. Strains lacking these genes fail to grow, just like the strains of B. longum, and certainly E. faecalis.
I’m sure you’re familiar with PCR, especially its use during the COVID-19 pandemic. In this study, the researchers use it similarly to identify and amplify specific genes in the bacteria. By using PCR, they were able to amplify DNA from the neonatal fecal samples to quantify the abundance of specific strains, monitor colonization, and compare how genetically equipped B. breve held up against pathogens. From there, researchers used whole-genome sequencing to compare the genetic content of different B. breve strains– especially examining for genes that encode enzymes capable of breaking down 2’FL.They found that mice that were introduced to the B. breve strains with the genes to digest 2’FL not only had higher levels of that strain in their gut but also showed significantly less colonization by harmful pathogens like Klebsiella pneumoniae. The B. breve strains were effectively establishing colonization resistance. But it gets better: the presence of B. breve didn’t just crowd out pathogens, but they primed the immune system. The researchers found that B. breve colonization triggered increased production of Reg3γ. This molecule helps defend against unwanted microbes, acting as a natural defense mechanism. So, not only does B. breve rapidly colonize the gut and physically prevent harmful microbes from establishing camp, but B. breve also helps stimulate immune cell development and encourages the gut to produce protective molecules, shaping the host’s defense.
This study doesn’t argue that B. breve is a magic cure or that everyone needs to buy this supplement. What it does show is that when the right strain of B. breve arrives at the right time—in this case, during the earliest stages of life— it can become a foundational part of a healthy gut microbiome. That said, it’s worth noting this study used mice raised in a controlled, germ-free environment, which is a far cry from a real baby gut filled with all sorts of microbes, formula, and maybe the occasional dog lick. Researchers didn’t look at long-term effects (it takes about 2 years to solidify our microbiome state), so we still don’t know if these stable microbiomes lead to better overall health. But still, it’s a big step toward understanding how probiotics might work when the right strain is delivered at the right time. Future studies can use these results to dive deeper into what happens when B. breve doesn’t arrive on time. Can probiotics be timed like vaccines? Can we reprogram our microbiome that may have gotten off on the wrong foot?
So yes, researchers concluded that B. breve may be one of the earliest builders of a healthy gut, but the study is also a reminder of how complex we are. It has offered us a nuanced look at probiotic science; timing might matter just as much as the type of microbe itself. In this day and age of commercial probiotics, we should move past the idea that more is better. Instead, we should be asking better questions: “What strain is this?”, “When should it be introduced?”, “What does it need to thrive?”, and “Could understanding microbial timing help us shape healthier humans from day one?”
About the Author:
Katie Rha ’25 is a double major in Biology and Psychology & Education from Queens, NY. After graduation, she plans to work as a certified nursing assistant as she prepares for a career as a physician assistant. Outside the classroom, Katie has been a member of the MHC basketball team, volunteers at Holyoke Medical Center, and teaches students at an elementary school in Northampton. In her free time, she enjoys trail running, practicing yoga, and reading.
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