Our Gut is not an Isolated Island
When people live together, something invisible begins to happen. Over time, they exchange microbes, and the communities of bacteria living in their guts become more similar. Within a few months of sharing a home, the millions of bacteria in one person’s gut begin to overlap with another’s, drifting in both directions until two communities that were once distinct start to align. Studies tracking couples, families, and cohabitants have found this pattern repeatedly. In short, the gut bacteria living in you are not entirely yours. Many of them are shared.
This is interesting, but it is also crucial. For scientists, it adds an extra layer to how we should think about the human gut. We tend to picture each person’s gut as an isolated island, but it is better understood as part of a connected system. In nature, no system is truly isolated. Animals migrate between forests, carrying seeds and pollen with them. Rivers connect lakes to seas, transporting water, sediment, nutrients, and living organisms across great distances. The gut, it turns out, is no different.
What the Gut Microbiome is
The gut microbiome refers to the trillions of microbes, and their collective genetic material, living in your gastrointestinal tract, especially the large intestine. Most of them are bacteria, but they also include archaea, fungi, viruses, and protozoa. You can think of them as a small ecosystem in which every member plays a role. Some break down fibers that we cannot digest on our own. Some produce vitamins, and others release chemical signals that influence our immune system. Still others keep harmful microbes in check simply by occupying the space and resources those microbes would otherwise use.
Diet, antibiotics, age, and environment all shape the microbial communities in our guts, which is why no two people have the exact same microbiome. In a real sense, the microbes inside us are part of who we are. So, when I say that microbes are shared between people, I am saying that a living part of you is shared with the people you spend your life with.
Your Microbes are Social
I find it fascinating that people who live together share more gut bacteria than strangers do. Couples and families don’t just share a home; they also share tiny parts of their microbial world, and over time their microbiomes grow more alike. Researchers have even identified the sharing of particular varieties, or strains, of bacteria between parents and children in early life. And this sharing does not always depend on such close contact. Microbes can also travel indirectly, through surfaces we all touch, like doorknobs, bathroom fixtures, and shared utensils.
What this means is that the gut microbiome is not really a closed system. It is more like one community connected to many others. Your gut is one community, mine is another, and microbes travel between them. Microbiologists have a name for this web of connected communities: a metacommunity. Metacommunity is just a fancy phrase for “a community of communities”. Seeing the gut this way matters, because it means that any treatment which tries to add or remove bacteria or even fight an infection in the gut has to reckon with the fact that microbes are being shared in the first place.
The Trouble with Studying this in Real People
The truth is that we do not really know the details of how this sharing happens. How fast do gut bacteria actually move between people? When a new bacterium arrives in someone’s gut, does it stick around or get muscled out? And does the rate of exchange shift when someone in the network is taking antibiotics? These are the questions I keep coming back to.
They are difficult to answer in real humans, because there are so many variables and each person’s gut holds trillions of microbes. Your diet is different from mine. Our stress levels, our medications, our genetics, our habits, and the environments that shaped us are all different. Trying to isolate the effect of bacterial transmission against all that is a bit like trying to hear a whisper at a football match — there is simply too much noise. So, as scientists, we build a quieter version of the system in the lab.
A Tiny Society of Guts in the Lab
In our lab at the Rega Institute, KU Leuven, I work with a panel of seven bacterial species, chosen because together they capture most of the important jobs of a real gut microbiome. Some break down fibers our own enzymes cannot digest, some produce butyrate (a chemical that keeps the gut wall healthy), and others clean up the hydrogen gas that other bacteria leave behind. One of them is a strain of E. coli called LF82, which we microbiologists describe as a pathobiont — a microbe that lives peacefully in a healthy, balanced gut but can trigger inflammation when that balance is lost. In short, it is a troublemaker worth keeping an eye on.
We grow these seven species inside small bioreactors, and you can think of each bioreactor as a computer-controlled flask that stands in for a single gut. The bioreactors sit inside an anaerobic chamber, which is a sealed glove box with no oxygen because oxygen is lethal to most gut microbes. The large intestine, where most of our gut bacteria live, is itself an oxygenfree environment, and the chamber lets me reach in with my hands while keeping the inside as inhospitable to oxygen as the colon.
My main experiment is to connect these bioreactors in different patterns, so that the communities inside them are linked in different ways depending on the scenario I want to recreate. In one setup, every reactor stays isolated, with no sharing at all; this is my control, the lonely-island version of the microbiome. In another, all the reactors are connected to imitate a family, with a small amount of liquid moving back and forth between them to mimic the constant, low-level sharing that happens when people live together. In a third, one reactor starts out carrying E. coli LF82 and passes it to its neighbors. This is a stripped-down version of a hospital ward with a pathobiont inside it.
During the experiments, I track how the number of cells from each species changes over time. I also examine which bacteria successfully move between communities, how they compete or cooperate, and which chemicals they produce. This steady tracking is what will let me answer my research questions.
Eventually, all of this data will feed into a predictive mathematical model that estimates how the different species are likely to behave over time, and how the communities in each network might shift when conditions change. By combining experiments with prediction, we can better anticipate how microbial communities stay balanced, and what happens when that balance is disturbed by a species that is newly introduced or suddenly lost.
Why This Matters
This work matters for probiotics and microbiome therapies. One of the biggest problems with probiotics is that the bacteria we swallow usually fail to settle into the gut community. They pass straight through and leave the resident bacteria unchanged. We still do not fully understand why some microbes take hold while others bounce off, but the answer probably lies both in the nature of the incoming bacteria and in the community already living there. Treating the gut as a community within a network, rather than an empty vessel waiting to be filled, could change how we design these therapies.
It also matters for medicine more broadly. Hospital-acquired infections are a serious problem and understanding how a pathobiont like E. coli LF82 competes with other gut bacteria, both under normal conditions and under stress, such as a change in pH or a course of antibiotics could reveal new ways to stop unwanted bacteria from taking over the gut, even after they have entered the wider microbial network.
A Short Personal Note
This work speaks to something I already find fascinating about how interconnected the world is. As we go about our lives, we breathe the same air, touch the same surfaces as strangers, and handle banknotes that have passed through countless unknown hands. We are more porous, and more connected, than we usually admit. Seeing the same pattern play out in the gut feels to me like a small confirmation of how the world tends to work. In my PhD, I am trying to find out how these small societies of microbes and, by extension, the larger society we are all part of really work when their members can reach one another.
