You’ve probably heard that our bodies are crawling with bacteria—some harmful, but many beneficial. Probiotics, fermented foods, and certain yogurts often promise to support the “good” bacteria in our gut, claiming to boost health and digestion. One fact you may have seen repeated in ads, articles, and websites is that humans have 10 times more bacterial cells than human cells. But is this statistic really accurate?
We asked four microbiology experts to break down the facts. Here’s what we discovered.
Where Are the Bacteria in Our Bodies—and What Are They Doing?
When people hear the word “bacteria,” they often think of infections. But most bacteria in and on our bodies are actually essential to our health. These helpful microbes are known as symbiotic bacteria, meaning they live in partnership with us, offering benefits in return for a place to live.
One key group is the Prevotella bacteria, which live in the digestive tract. They help break down complex carbohydrates like fiber—something our own digestive enzymes can’t do alone. In fact, the majority of our body’s bacteria live in the digestive system, particularly the stomach, small intestine, and colon.
Fun fact: over half of our stool is made up of bacteria. But microbes don’t just live in the gut—they’re also found on the skin, in the mouth, lungs, and other areas. Alongside bacteria, our bodies also host fungi, viruses, and other microorganisms. Together, this vast community is called the human microbiome, and it plays a vital role in maintaining our health.
Is It True We Have 10 Times More Bacteria Than Human Cells?
The widely shared “10-to-1” bacteria-to-human cell ratio dates back to a 1972 estimate by scientist Thomas Luckey. But as Professor Bruno Pot from VUB in Belgium points out, this figure was never intended to be a precise calculation. It became popularized over time but wasn’t backed by modern scientific methods.
Dr. Hannah Wardill, a microbiome researcher at the University of Adelaide, explains that newer studies have revised this number. “The current estimate is closer to 3:1, 2:1, or even a 1:1 ratio of microbes to human cells,” she says. “Even if it’s 1:1, it’s amazing to think that we’re as much microbe as we are human.”
How Do Scientists Calculate These Numbers?
Dr. Hanne Tytgat from Wageningen University in the Netherlands references a more recent and reliable study published in 2016. Researchers analyzed a “reference man” weighing 70 kilograms to estimate the total number of human cells—roughly 30 trillion—compared to about 38 trillion bacterial cells.
These estimates came from analyzing human tissues and fluids. For example, red blood cells make up around 97% of all human cells, so scientists measured red blood cell density in a small blood sample and scaled it up for the entire body. To count bacteria, they examined stool samples and estimated bacterial populations across the gut and other body parts.
Thanks to more advanced techniques and a better understanding of where bacteria live—especially in the colon—scientists now believe the original 10:1 figure was significantly overstated.
So, What’s the Takeaway?
Although the 10-to-1 ratio is outdated, the truth is still fascinating. On average, human and microbial cells are present in roughly equal numbers. This means that about 50% of your body is made up of microorganisms—a mix of bacteria, fungi, viruses, and more.
Our microbiome is like a hidden ecosystem that impacts everything from digestion to immunity. While we still have a lot to learn, one thing is clear: being human also means being microbial.
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