We compulsively wash our hands, spray our countertops and grimace when someone sneezes near us—in fact, we do everything we can to avoid unnecessary encounters with the germ world. But the truth is we are practically walking petri dishes, rife with bacterial colonies from our skin to the deepest recesses of our guts.

All the bacteria living inside you would fill a half-gallon jug; there are 10 times more bacterial cells in your body than human cells, according to Carolyn Bohach, a microbiologist at the University of Idaho (U.I.), along with other estimates from scientific studies. (Despite their vast numbers, bacteria don't take up that much space because bacteria are far smaller than human cells.) Although that sounds pretty gross, it's actually a very good thing.

The infestation begins at birth: Babies ingest mouthfuls of bacteria during birthing and pick up plenty more from their mother's skin and milk—during breast-feeding, the mammary glands become colonized with bacteria. "Our interaction with our mother is the biggest burst of microbes that we get," says Gary Huffnagle, a microbiologist and internist at the University of Michigan at Ann Arbor. And that's just for starters: Throughout our lives, we consume bacteria in our food and water "and who knows where else," Huffnagle says.

Starting in the mouth, nose or other orifices, these microbes travel through the esophagus, stomach and / or intestines—locations where most of them set up camp. Although there are estimated to be more than 500 species living at any one time in an adult intestine, the majority belong to two phyla, the Firmicutes (which include Streptococcus, Clostridium and Staphylococcus), and the Bacteroidetes (which include Flavobacterium).

For a long time, scientists assumed that these bacteria, despite their numbers, neither did us much harm nor much good. But in the past decade or so, researchers have changed their tune.

For one thing, bacteria produce chemicals that help us harness energy and nutrients from our food, Huffnagle explains. Germ-free rodents have to consume nearly a third more calories than normal rodents to maintain their body weight, and when the same animals were later given a dose of bacteria, their body fat levels spiked, even if they didn't eat any more than they had before.

Intestinal bacteria also appear to keep our immune systems healthy. Several studies suggest that microbes regulate the population and density of intestinal immune cells by aiding in the development of gut-associated lymphoid tissues that mediate a variety of immune functions.

The bacteria also appear to influence the function of immune cells like dendritic cells, T cells and B cells, although scientists don't know the precise mechanisms yet. And one chemical released by the bacterium Bacteroides fragilis is capable of directing how the developing immune system matures.

Further, probiotics—dietary supplements containing potentially beneficial microbes—have been shown to boost immunity. Not only do gut bacteria "help protect against other disease-causing bacteria that might come from your food and water," Huffnagle says, "they truly represent another arm of the immune system."

7 Comments

1. 1. DrBeckerMA 11:43 PM 12/2/07

   Could too germ free environments in infancy lead to obesity in later life?

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2. 2. Galois 03:51 PM 12/5/07

   I didn't get that from the article. Thought only that the body was less efficient at extracting nutrients without the foreign bacteria so the body needed to consume more.

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3. 3. mwenner 06:31 PM 12/5/07

   There's more information about the link between gut microbes and fat storage in these two papers:
   
   F. Backhed, R.E. Ley, J.L. Sonnenburg, D.A. Peterson and J.I. Gordon, Host-bacterial mutualism in the human intestine, Science 307 (2005), pp. 19151920
   
   F. Backhed, H. Ding, T. Wang, L.V. Hooper, G.Y. Koh, A. Nagy, C.F. Semenkovich and J.I. Gordon, The gut microbiota as an environmental factor that regulates fat storage, Proc Natl Acad Sci USA 101 (2004), pp. 1571815723.

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4. 4. MarcBennett 06:34 AM 2/1/08

   So... it seems that humans definately adapt and evolve to changing environmental and biological stresses. But, if our cells and bodies are modified for the benefit of the bacteria we play host to, especially when one considers the effects on human genes; It seems the real questions are where do they end and we begin? What is making the decisions and who are we? And, is it at all possible to clearly understand the course of human evolution?

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5. 5. Andira 09:54 PM 10/5/09

   Perhaps there is a glimpse of an explanation here why some people can eat more than others and nevertheless remain slim. This is so obvious that only experts deny it. The other lesson would be that in addition to exercise, physiological and mental, preventive and curative medicine will benefit from strategies for checking for faults and repairing them in the human eco-system.

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6. 6. Pejaroon in reply to MarcBennett 02:19 PM 2/5/11

   I applaud your thinking mind! It sounds as if you are searching out the answer to one of life's ultimate questions: Where do we come from? But your questioning searches deeper than that... What have you come up with so far?

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7. 7. CLove in reply to DrBeckerMA 07:46 PM 2/25/12

   A baby is born, most often one of two ways, vaginally to a mother who has an unhealthy flora due to use of anti-biotics over the course of her life (birth control pills also contribute to gut dysbiosis) or via c-section, during which mothers are given I.V. anti-biotics (many vaginal births get I.V. anti-biotics too.) Now, you've got an infant under one of these conditions plus a mother who probably wont nurse (at least long). Imbalanced gut flora and formula fed until you add solids. In America, solids = french fries, wheat products, corn products, and soy products. The American diet is a dysbiosis inducing feed lot. It gets even better for the kids. They're going to get sick so you know they're going to be exposed to anti-biotics early and often, enabling the bad flora's growth and worsening the dysbiosis symptoms.
   
   Obesity is only one thing to worry about.
   
   The only source I've found for a reasonable approach to this problem has been the GAPS Diet. Don't let the 'P' fool you, the diet is for anyone with any gut dysbiosis (so everyone) and the author is writing a new book that will address the physiological diseases. http://gapsdiet.com/Home_Page.html
   
   Once, healthy, humans should be able to live well and healthy with traditional food preparation techniques a al Weston A. Price Foundation. http://www.westonaprice.org/
   
   I'm not associated with either of these 'groups', but I'm pretty impressed with what I'm learning about both.

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