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In this article:
- Wherever we are and whatever we do, the fact is that we’re never alone. We’re accompanied by trillions of microorganisms living in and on our bodies.
- It may seem icky, but these bacteria, fungi, viruses, and micro-animals play a role in maintaining our health, and so much of what’s going on in these microbiomes are still being studied by researchers.
- These tiny companions have been with us — and fighting off infections for us — from the moment we were born, and it’s a relationship that’s been around for as long as humans have been around, too.
In science class, we learned that we’re made up of cells, which make up tissues, which in turn make up organs. Different organs function within systems, and these systems are what make our bodies work every day. Sometimes these body parts might get sick, and that’s where doctors come in to help out our immune systems. The end.
But that’s barely half of the story — literally. Alongside the trillions of cells that make up our bodies are trillions of non-human living things, too.
In fact, the fascinating (and slightly icky) truth of the matter is that we’re never truly alone, as we live in symbiosis with countless invisible microorganisms living in and on us. Most of these microbes are one-celled organisms like bacteria and fungi, but we’re also home to micro-animals, which I definitely didn’t learn about in school.
We often talk about microorganisms in the context of disease — the plague, after all, is caused by a bacterium called Yersinia pestis — but these microorganisms can be good for us, too.
When microorganisms live in or on our bodies without causing harm, this is called colonization, and is perhaps the only positive use for the word (because human colonizers are awful then and now). In contrast, when microorganisms cause harm, we call it an infection.
So what exactly are the tiny forms of life colonizing our bodies, and where do they hang out? And, perhaps most interestingly, how did they get there?
The What
The microorganisms in and on our bodies are collectively called the human microbiome, though these usually just refer to the single-celled microorganisms that call us home. Alongside the microbiota in and on us are micro-animals, which are exactly what they sound like: Animals that are too small to see with the naked eye.
Bacteria
As one of the first forms of life on Earth, bacteria are present almost everywhere, so it makes sense that they make up the biggest portion of the human microbiome.
Scientists have long tried to figure out just how many there are of them in our bodies, but the latest studies suggest that there are approximately 1.3 bacteria cells for every one human cell. That’s right — your body is actually a bit more bacteria than it is human.
Often just a few micrometers in length, bacteria come in many different species of varying shapes and sizes across different parts of our bodies. Some areas of the body, like the mouth and our gut, play host to more diverse bacteria colonies as compared to others, like the skin.
But the type and number of bacteria present can also vary greatly from person to person, according to fascinating findings by The Human Microbiome Project, a research initiative by the US National Institutes of Health.
Fungi
Like bacteria, fungi tend to be something we only hear about when we’re sick: We get fungal infections on our feet, under our nails, and in sensitive areas like the vagina and buttocks.
But just as fungi can exist as both the icky stuff in our shower and the delicious mushrooms we add to our soups and pastas, the fungi in our bodies can be as good as they can be bad, too.
Studies conducted by the National Human Genome Research Institute have found that different types of fungi live on different parts of the body.
While our heads and torsos serve as home to fungi belonging to the genus Malassezia, our feet tend to be home to the most kinds of fungi. Our heels alone play host to 80 species, our toes are the domain of 60 species, and our toenails are home sweet home to 40 fungi species.
Described by researchers as “families” of fungi, these microorganisms work together to promote bodily functions and defend against outside pathogens, or bad microorganisms.
Viruses
By now, you might be sensing a pattern: There’s bad bacteria and good bacteria, bad fungi and good fungi, and these exist in the millions in and on us. The same is true in terms of viruses.
Among mice whose gut bacteria were wiped out by antibiotics, viruses can come in to save the day, too.
Of course, we’re kind of in the middle of a deadly pandemic caused by a virus, so the existence of good viruses can be a little hard to believe at the moment. But viruses, too, can play a role in keeping us healthy.
For example, bacteriophages — often shortened to “phages” — are a type of virus that infect and kill bad bacteria in the mucus membranes of our respiratory, digestive, and reproductive tracts. The mucus itself is there to protect cells from invading bacteria, and the phages living in it help it do so.
Scientists have since taken inspiration from this and genetically engineered individual stages of phages to help treat bacterial infections when antibiotics aren’t able to help.
There are also some types of viruses that fight off worse viruses. It’s not exactly altruism on their part, as they also need to get rid of competitive viruses so they can last longer. For instance, the Pegivirus C, sometimes called the GBV-C, has been linked to improved survival rates among HIV-positive individuals, as it slows down the progression of HIV.
Another study from the American Society for Microbiology found that among mice whose gut bacteria were wiped out by antibiotics, viruses can come in to save the day, too. Without the good bacteria that fight off gut infections, mice were still protected by noroviruses.
Micro-Animals
Micro-animals are typically excluded from the umbrella term of human microbiota, but they’re definitely living in and on us.
They’re called micro-animals because unlike the types of microorganisms listed above, they’re not unicellular. In other words, they’re made up of different kinds of cells — much like other animals are, us included.
But not a lot is known about these micro-animals outside of the facts that a) they exist, and b) some of them live on and off of us.
For example, Demodex mites — specifically, the D.folliculorum and the D.brevis — live on our face. They live on the tiny hair follicles on our nose, cheeks, forehead, chin, and eyelids. (However, they’ve also been found in other places where we grow hair. For that, I suppose you can use your imagination.)
Most of their days are spent head-down in our hair follicles to eat, though some scientists disagree as to what.
“We joke that the mites come out to party, because they have sex on your face at night.”
Some say they eat the sebum (or natural oils) that our glands release, while others argue that they may also be eating the actual cells on our follicles, as well as the bacteria that live on them, which can help us fight off infection.
At night, the Demodex mites crawl out of our hair follicles and mate right there on our faces. “We joke that the mites come out to party, because they have sex on your face at night,” shares researcher Holly Menninger.
The Where
Different places in the world offer distinct kinds of environments for various ecosystems to thrive — whether it’s a chilly tundra or a lush rainforest. Think of your body as kind of similar: Different locations provide unique conditions for microorganisms.
On the Skin
Our skin’s main function is to prevent the invasion of pathogens or the kinds of microbes that cause infections. But the skin is also home to good microbes that can either be relatively permanent or transient.
Most of these are bacteria, the kind and number of which can vary widely by skin region, level of moisture, and the amount of sebum. The bacteria on our skin help prevent “bad” bacteria from invading our bodies and making us sick.
In contrast, fungi tend to be fairly consistent in terms of composition no matter where they’re found in the body.
Sometimes, however, even these “good” microbiota can turn bad. For example, cuts on the skin can get infected by bacteria that live on our skin, and this is a problem because they don’t belong inside your body.
In the Nose
The environment within your nose is called the nasal microbiome, and is home to a complex community of tiny living things that help us stay healthy.
On average, we breathe around 7 or 8 liters of air per minute, and this air contains bacteria that we can breathe in, too. However, little is known about the other kinds of living things that might be residing in our noses, like viruses and fungi.
In the Mouth
The mouth — or what scientists call the oral cavity — is second only to the gut when it comes to the number and diversity of tiny living things that reside in its different niches, like the teeth, the throat, the floor of the mouth, and the tongue, among others.
Oral bacteria is something we’re most familiar with, but the oral microbiome is also home to fungi, viruses, and other microorganisms like protozoa and archaea, which tend to be less researched. The number and kinds of microorganisms in our mouths depend on a lot of things, like our diet, the changes in pH, and the interactions of microorganisms within our mouths.
Many of these microorganisms don’t cause harm, and can even fight off bad microorganisms. They can also help regulate digestion and metabolism.
However, sometimes the bacteria in the oral microbiome can also be harmful. For example, some bacteria can stick to the surface of the teeth and, without proper tooth brushing, they can produce plaque and acids that can cause tooth decay.
Intestinal microbiota provide vitamins we can’t produce ourselves, like Vitamins B12 and K, while also helping with digestion and fighting off pathogens.
In the Gut
Our intestines are home to the largest and most diverse groups of microorganisms, which is why they are the focus of plenty of research about bacteria. Intestinal microbiota provide vitamins we can’t produce ourselves, like Vitamins B12 and K, while also helping with digestion and fighting off pathogens.
Scientists estimate that over 1,000 different kinds of bacteria live in a healthy gut, and low numbers of these can make us more prone to getting sick. Interestingly, some types of gut bacteria can make us more susceptible to becoming obese.
Many of the bacteriophages mentioned above call our guts home, too. It is estimated that there are over 100,000 different kinds of viruses in our intestinal microbiome.
In the Vagina
For those assigned female at birth, the vagina is an especially dynamic microbiome that undergoes changes throughout the menstrual cycle and across our lifetimes.
However, a healthy vaginal microbiome tends to be one that is dominated by Lactobacillus bacteria, which fights off bad bacteria by producing hydrogen peroxide and lactic acid. Without it, we can get diagnosed with bacterial vaginosis or candidiasis, more commonly known as a yeast infection.
The Lactobacilli in our mothers’ vaginas are some of the first tiny living things to move in.
The How
So the burning question now is: How do all these microorganisms find their way to all these body parts?
Research suggests that colonization begins at birth. We welcome our first microorganisms during delivery and, for those born via cesarean section, from skin-to-skin contact after birth. The Lactobacilli in our mothers’ vaginas are some of the first tiny living things to move in, and they’re followed by other types of bacteria we might get from the nurses and medical staff, as well as in the air around us.
When we begin to eat, microbes in our diet start making a home for themselves in our bodies, too. Babies crawl, put things in their mouths, and lick toys — and all these activities introduce microorganisms into the body. At around the three-year mark, our microbiota becomes more or less stable all the way into adulthood.
There Are Microorganisms in and on Us, and That’s Okay
“A lot of people think, ‘Oh my god, that’s disgusting,’” says Dr. David Relman, a microbiologist at Stanford University. “I think, ‘Wow, this is really reassuring’ because it tells us that we have friends that we didn’t know we had.”
Indeed, studies done across the different areas of the human microbiome suggest that the relationship our bodies have with these microorganisms date back for millennia. They aren’t there by accident: We give them a home, and they help us survive.
Of course, the knowledge that so many living things are in and on us shouldn’t change much about our daily lives. We’re still the same microorganism-filled people on the same microorganism-filled earth, but perhaps, with just a deeper understanding of how we’re all just a little bit human — the rest of us is microbiota.
