Human Microbial Diversity

Human Microbial Diversity

Every person may have a very different mix of microbes living inside them.


Your bacterial profile. I'm Bob Hirshon and this is Science Update.

We all have trillions of benign microbes living on or in us. But scientists are just starting to realize how different these personal microbe communities are. The latest comes from University of Colorado at Boulder microbial ecologist Rob Knight and his colleagues. They analyzed the microbial populations of eighteen anatomical sites on nine different people. It was already known that different sites on the same person harbor different microbes.

What was really surprising is when you control for all of those differences, essentially every site on the body is highly variable between different people.

About 80 to 90 percent variable, in fact. They even found that one person's skin bacteria wouldn't grow on another person. Knight says our unique microbial profile may be an overlooked influence on our health. I'm Bob Hirshon for AAAS, the Science Society.

Making Sense of the Research

It might sound disgusting, but our bodies are crawling with bacteria and other microbes—about 100 trillion altogether, inside and out. They live on our skin, in our mouths, inside our intestine, and in countless other places. In most cases, these bacteria are not only harmless, but they actually benefit us. For example, the bacterium Lactobacillus acidophilus breaks down lactose, or milk sugar, in our food, helps form and digest other nutrients, and produces by-products that fend off other bacteria. 

Scientists have long known that people have different bacterial populations on different sites in the body. The bacteria in your nose, for example, would be very different from the bacteria in your belly button (yep, there are bacteria there too). They also knew that different people have different bacterial populations. In fact, a previous study by this team found 4,200 species of bacteria on 102 human hands, but identified only about five species shared by all 51 participants. This study sheds more light on how dramatically bacterial populations vary from person to person. 

Knight's team took bacterial samples from volunteers four times over a three-month period. Each sample was taken about an hour or two after the subjects showered. They took samples from 18 different sites on the skin, as well as the hair, ear canals, nostrils, mouth, and lower intestine. They typed the bacteria by isolating their microbial DNA.  

The researchers found surprising differences within individuals: for example, different populations on the left hand than the right hand. But they also found that one person's armpit (another site they tested) had a very different population than another's, and the same went for just about every site on the body. The differences between individuals went as high as 80 to 90 percent, meaning that 80 to 90 percent of the bacteria on Person A's right hand would be different from the bacteria on Person B's. 

The highest diversity skin sites were the forearms, palm, index finger, back of the knee, and sole of the foot. Bacterial populations varied the least inside the mouth, both between individuals and within the same person over time. Knight's team even tried sterilizing certain skin sites and transferring bacteria from another site on the same person, or a different person. Results varied, but in general, the new bacteria didn't last.

It's not clear what determines our bacterial profile: are we born predisposed to grow certain species, or are they determined by our environment, diet, age, or other factors? The findings have implications beyond just curiosity. For example, our unique bacterial profiles may be an unrecognized influence on our health. One person's stomach bacteria, for example, might make them unusually susceptible to ulcers (which are caused by bacteria) but especially resistant to parasites. In the future, it's possible that doctors may turn to our bacterial profile, just like our genes and family history, to better understand our health.

Now try and answer these questions:

  1. Where do bacteria live in the body? What purpose do they serve?
  2. How do bacterial populations differ within a person? Between people?
  3. Why do you think the researchers took samples from each person four times over a three-month period, instead of just once? Why compare one person's body part to another's, rather than the total bacterial population of the whole body?
  4. Why might our personal bacterial population matter?

You may want to check out the November 20, 2009 Science Update Podcast to hear further information about this Science Update and the other programs for that week. This podcast's topics include: ancient insect pollinators, your body's unique microbes, and a marine creature that could fix broken bones. Also: does chewing gum really take years to digest?

Going Further

For Educators

For more information on this topic, check out the related resources below.

Related Resources

The Ecology of Your Skin 1: Bacteria that Live on the Skin
9-12 |
The Ecology of Your Skin 2: The Microbial World Is an Olfactory World
9-12 |
The Ecology of Your Skin 3: The Body Food Connection
9-12 | Hands-On

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