It sounds downright creepy at first. As human beings, each of us has 300–500 different species of bacteria living in a complex ecosystem within our guts. In fact, the bacteria living in our guts nearly outnumber our own cells.
But while the idea might sound creepy, these “bugs” in our intestines are actually quite important to our health and well-being, says Sara Campbell, assistant professor of kinesiology and health at Rutgers University. They aid in digestion, breaking down nutrients for us that we wouldn’t otherwise be able to access, and they also work in concert with our immune system to protect us from disease. “These tummy bugs are there to help us, and without them, we wouldn’t be able to survive,” explains Campbell.
Where do we get those bugs?
At a macro level, every human gut microbiome has a lot in common, says Hannah Holscher, assistant professor of food science and human nutrition at the University of Illinois at Urbana-Champaign. We all tend to have the same five basic phyla (categories) of bacteria in our guts. But at a micro level, there are substantial differences from one person to the next, since we each carry different strains of microbes within those five broad phyla. “It’s somewhat similar to our fingerprints, in that you’ll have your own unique composition of gut microbiome,” says Holscher.
The specific composition of a person’s gut bacteria is influenced right from birth. As a baby passes out of its mother’s body, the baby is colonized with bacteria from the mother, which passes into the infant’s gut. (Importantly, the gut microbiomes of babies born via C-section are typically not as rich as those born vaginally.) A baby’s earliest feeding experiences also have an impact. Those who are breastfed have markedly different gut microbiomes than babies who are formula fed. And antibiotic use, especially in the first few years of life, also plays a role. Two Finnish studies published in 2016 both showed that when children received antibiotics early in life—especially the broad-spectrum antibiotics commonly used to treat respiratory infections—their gut microbiomes developed at a slower rate and showed less diversity than peers who hadn’t taken antibiotics