Bacteria Outnumber Cells in Human Body

The human body contains 20 times more microbes than it does cells. In fact, a visitor from outer space might think the human race is just one big chain of microbe hotels.

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DEBBIE ELLIOTT, host:

Now we know our audience isn't as big as the World Cup's, but we have a nice audience and we appreciate every one of you, and you'll be happy to know we have just figured out a way to increase our listenership by a factor of, oh, 100 trillion, without adding anybody new. Here's our correspondent Robert Krulwich to explain how.

ROBERT KRULWICH reporting:

If I said to you, who are you? Well, if your name's Melanie - she works here in the office - then she'd probably say...

MELANIE (NPR Employee): I'm Melanie.

KRULWICH: Right? There's no arguing. Most people think of themselves as themselves. However, there's another way to think about who you are, because you are also a collection of cells.

MELANIE: Yes.

KRULWICH: You know how many cells?

MELANIE: No.

KRULWICH: Well, as an adult human, you are, with some margin of error, composed of a hundred trillion cells.

MELANIE: Wow, that's a lot.

(Soundbite of laughter)

KRULWICH: Well, it may seem like a lot. Now, here's the paradox. Inside this hundred trillion sack of cells that is you, you, Melanie and everybody listening to this...

MELANIE: Yeah.

KRULWICH: ...we all of us have houseguests in our bodies, little creatures, bacteria. You know them from the plaque in your mouth. Though here's their favorite place inside you, says microbiologist Glenn Gibson of the University of Reading in Britain.

Professor GLENN GIBSON (University of Reading): If you put your hand just around the area of your bellybutton, as it's called, or we call it that in England.

(Soundbite of laughter)

KRULWICH: We call it that, too.

Prof. GIBSON: Your navel.

(Soundbite of laughter)

KRULWICH: Yes.

Prof. GIBSON: Put your forehand on that.

KRULWICH: And just under your bellybutton, in the large intestine below, you will find the largest gathering of your bacteria. But with their own bodies, their own genetic material, they're not you. They're just living in you.

Prof. GIBSON: And they're just sitting there waiting for whatever you eat, and whatever you eat will feed them, and they will grow, and they'll grow to pretty huge numbers.

KRULWICH: So Professor Gibson says in every gram, that's every 30th of an ounce of your large intestine...

Prof. GIBSON: There's a 10 to the power 12 bacteria.

KRULWICH: That's a hundred billion bacteria.

Prof. GIBSON: That's the number of bacteria in every single gram.

KRULWICH: And when you add up all the grams that make up you, with bacteria, without bacteria, it turns out that right now you are playing host to many trillions of bacteria.

Prof. GIBSON: Trillions and trillions, yeah. So it's huge, huge numbers indeed.

KRULWICH: So many that it turns out that we have more bacteria living inside us than we have cells.

Prof. GIBSON: So each human, if you take that literally, is 20 times more microbes than they are mammal.

KRULWICH: Whoa.

(Soundbite of laughter)

Prof. GIBSON: In terms of number.

KRULWICH: If I were to count the number of cells that make up me, that would be, say, a hundred trillion cells.

Prof. GIBSON: Mm-hmm.

KRULWICH: But if I were count the number of microbes that are living in me, like in the hotel of me, it would be 10 times more?

Prof. GIBSON: Ten to 20 times. Yeah, 20 times. Twenty if you're lucky.

(Soundbite of laughter)

KRULWICH: So they must be very small.

Prof. GIBSON: Tiny.

KRULWICH: So if a bacteria is smaller than a cell, that means if an intelligent alien were to land on, say, you, Melanie, it could either say, well, there's Melanie. Or it could say, there's a lot of bacteria living inside something called Melanie, like, kind of like a hotel. Which do you think?

MELANIE: You mean, like a hotel full of bacteria? Or...

KRULWICH: Or you. Which was the truer picture of you?

MELANIE: Yeah. I would say me.

KRULWICH: But an intelligent alien, says Prof. Gibson, could see it differently. After all, if it was just looking at Melanie and counting what's there, there is more bacteria there than there is Melanie, in Melanie. Something to ponder, whether you're an alien or Melanie.

(Soundbite of laughter)

Robert Krulwich, NPR News in New York.

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