9 Fascinating Facts About The Microbiome, The Trillions Of Microbes Hidden Inside Us

The still-mysterious ecosystem of bacteria living inside you determines an incredible amount about your health–and your personality.

“Bacteria are living things. They are not trying to hurt you. They are not trying to help you. They are just trying to stay alive, like the rest of us.”


So begins the American Museum of Natural History’s newest exhibit, “The Secret World Inside You,” which explores the incredible, invisible, and bountiful lifeforms that call the human body their home. Today, scientists call this collection of microorganisms the “human microbiome.”

The exhibit is packed with information to help visitors grasp the new scientific understanding that is emerging around the importance of each person’s unique, individual microbiome to their health.

After spending years co-curating the exhibit, Susan Perkins, a researcher at the museum, felt her own identity challenged: “I do not see myself as a lone entity anymore. I have crossed that line. I am an ecosystem,” she said at a preview of the exhibit in November.


Below are just a few of the interesting things you can learn at the exhibit. Prepare for some mind-blowing knowledge, and certainly go visit if you can and see for yourself (Or come on Tuesday, November 10, to Fast Company’s Innovation Festival, where we will co-host an evening of appreciation of all things fermented at AMNH.)


Numbering anywhere from 30 trillion to 100s of trillions, there are more microorganisms in your body than there are stars in the Milky Way. Today, they’re often compared to an extra “organ,” since together they weigh three pounds–about the same as your brain–and seem to perform countless functions in our body, from helping our digestion to affecting our mood, of which researchers are only just now starting to grasp the scope. Microbial genes outnumber our own human DNA by a ratio of about 100:1.


In the womb, babies are in a sterile zone with no microbiome at all. Most babies pick up their first dose of microbes as they go through the birth canal–a crucial part of our development since our early microbes shape our immune, digestive systems and even our brains. Even the bacteria babies need to digest breast milk–Lactobacillus–is usually picked up in the mother’s birth canal, before their first drink.


What happens with C-section deliveries? These babies often acquire gut bacteria from the doctor’s hands or surrounding environment. This may put their guts off-balance: C-section babies have slightly higher rates of asthma, food allergies, gut infections, and delayed immune system development.

One day, C-section babies may receive probiotics to make up for the ones they haven’t received. (In other animals, there’s evidence that spraying newborn chicks with 29 healthy probiotic bacteria can reduce salmonella infections.) For now, however, the exhibit notes that vaginal births may be preferable, though obviously only in cases where a C-section is not medically necessary.


Breast milk contains several types of bacteria passed to infants, and also nourishes other bacteria–about 10% of the nutrients in all breast milk can’t be digested by humans at all. Breast milk also contains chemical “decoys” that mimic place where disease-causing microbes attack–so pathogens stick to milk molecules rather than a baby’s cells.


No one knows exactly why babies with colic cry, but some have fewer helpful bacteria acquired from breast milk and more of a kind of bacteria–Proteobacteria–which is less healthy and can cause gas. Rebalancing their bacteria has been shown to be a cure for colic.


Our skin microbiomes reveal differences about us. Some make sense: Kids under 12 have fewer oil-loving bacteria than people over 12. Dog owners have different and more diverse skin microbiomes than non-dog owners–and they may share a microbial profile with their pets. And people who wear antiperspirant have about 50 times fewer bacteria under their arms than people who just use soap.

Other differences are less intuitive. Women’s hands have more lactic acid bacteria–a kind similar to those used to make yogurt, pickles, and wine. Men have more Corynebacterium cells, similar to those used to make the flavoring MSG. People in the U.S. have different skin microbes than those outside of it. Even your left and right hands offer different microbial environments.



More than 1,000 kinds of bacteria live in the human mouth, and around 100 to 200 species. Most do no harm, or actively protect from infection. But the right conditions can also cause tooth decay, bad breath, and gum disease. Different bacteria dominate your mouth during night and day–the ones on your tongue produce the stinky “sulfurous” chemicals at night when food is scarce and your mouth dries out (and saliva isn’t around to wash them away).


Your immune system must be trained not to attack everything. Exposure to certain microbes has been shown to reduce chances of getting allergies or asthma. For example, children who pick up bacteria such as Lactobacillus johnsonii from dust or the family dog have lower rates of asthma or lung allergies. In addition, mice with peanut allergies become less sensitive when given certain strains of common gut bacteria–soon bacteria might help prevent or treat human allergies, too.


For both people and other wildlife, gut microbiomes also provide nutrients that we’d lack from our food. Some people in New Guinea, for example, can live on a diet of 90% sweet potatoes, because their gut microbiomes are evolved to produce the protein missing from their meals.


Microbes also help us digest foods as adults. While most of us can’t digest seaweed, people in Japan can, thanks to microbes they’ve acquired. Similarly, our bodies produce 20 enzymes for digesting carbs–but a single bacterium in our gut makes an additional 260 more.


What causes obesity and why is it rising in the U.S.? There are likely many interconnected reasons, including diet, lifestyle, and exposure to chemicals. One other leading theory is our microbiome. Studies have found there are differences in the microbiomes in lean and obese people (and in lean and obese mice). In one incredible study, one group of sterile mice was given the microbes of obese people. The other got microbes from lean people. The mice with obese people’s microbes gained more weight from the same food.

Antibiotics make animals gain weight (that’s why we feed them in low doses to farm animals). Do they make humans gain weight too? This isn’t yet known, but there’s some evidence of correlation. The U.S. states with the highest obesity rates–like Mississippi and West Virginia–also take the most antibiotics. But it’s also possible that obese people in these states just happen to get sick more often.



Your gut and brain talk to each other all the time (hence, the phrase a “gut feeling,” and the digestive troubles many of us get when we’re anxious). Consider this: Your gut contains 500 million neurons connected to the brain through the Vagus nerve. It also produces about 80% of the body’s serotonin, the key hormone that regulates mood, sleep, and memory (and the key hormone manipulated by many anti-depressants). This may be why gut disorders often come with depression, and certain brain disorders, like autism, often come with digestive trouble.

Gut microbes can also change behavior. In an experiment with mice, anxious mice–those who hesitated for minutes before exploring a new space–were separated from mice that did not hold back. Yet amazingly, exchanging their gut microbes altered their behavior: The previously cautious group jumped right away to explore, and the bold group gave pause.


About the author

Jessica Leber is a staff editor and writer for Fast Company's Co.Exist. Previously, she was a business reporter for MIT’s Technology Review and an environmental reporter at ClimateWire