January 11 – Gut Check

Today’s factismal: Most scientific studies modify or refute earlier ones.

It is no secret that science isn’t perfect. Ask any scientist and she’ll tell you that most of her work consists of checking work done by other scientists to see if it was done right. Most of the time, it is (which is why most research isn’t published; it really isn’t very interesting).  But sometimes the scientist will discover that the earlier work was wrong. It may have been done incorrectly or it may have been described wrong or it may just have been a lucky fluke.  But for whatever reason, there are many times when what we thought we knew turns out to be not so right.

For example, let’s consider the tried-and-true statement that there are ten times more microbes living on your body than there are cells in your body; that is, that you are outnumbered in your own body! The basis for this statement came from the realization that instead of being just “human cells”, we all have a collection of bacteria, funguses, and even archae that live in or on us; collectively, they are called “microbes” because they are “tiny {micro} lives {bios}”. If you are a typical person, then 1,000 species of microbe live in your mouth. 440 more species live on your forearm, along with another 1,500 or so in your gut and 150 that thrive behind your ears. These species are so specialized and localized that the microbes on your left hand aren’t the same as the ones on your right! All in all, more than 10,000 different species of microbe live on or in the typical person.

In order to estimate how many microbes call you home, Thomas D. Luckey counted the number of critters in “intestinal fluid” (basically poop and other crap from the rectum – how would you like to be the TA assigned to collect specimens for him?). On average, he found that there were about 100 billion microbes per gram. Given that the average adult human has about a kilogram of crap floating around in their intestines, that works out to be 100 trillion microbes per person. At about the same time, another medical researcher estimated that the typical adult human has some 10 trillion cells. Add the two bits of information together and a ratio was born – microbes out number us ten to one.

But not so fast! A group of researchers has been looking at other researchers who have counted up intestinal flora (as they are euphemistically called) and those who have counted up human cells, and they found something interesting. More recent counts for intestinal flora put the number at about 39 trillion and the number of human cells at closer to thirty trillion. (What’s a factor of three among friends?) WHen you use the more recent numbers, it turns out that the number of microbes is about equal to the number of human cells in your body. Indeed, the number is close enough that the researchers half-jokingly suggested that you could have more human cells one your morning “movement” is complete!

Now this may sound like silly potty humor to you, but to a researcher it is serious (if smelly) business. That’s because we are still discovering exactly how a person’s collection of microbes (what wonks call a “personal biome”) affects their health. Can diabetes be cured by changing a person’s intestinal flora? Could an imbalance in microbes cause heart disease? Is it possible that a person’s microbes could make him more susceptible to stroke? Can you cure diseases by doing a microbe transplant? As you might guess, there are a lot of scientists who are very interested in learning the answers to these questions.

One group that’s working on this problem is the Understanding Human Oral Health at Stanford and UCSF. They are seeking people to participate in their projects that will link the biome and other factors to dental health. To learn more, swish over to:

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