Inside Job: Teams of Microbes Pull Strings in the Human Body

Article excerpt


You are surrounded, grossly out-numbered and being manipulated from within.

Teeming masses of bacteria are in your mouth, on your skin, up your nose and on the surface of your eye, in your stomach, deep in your bowels and well, just about everywhere. In fact, the number of bacterial cells you harbor exceeds the count of your own body's cells by 10-to-1.

But don't be too hasty in reaching for the disinfectant. You can't wash these microbes away. Nor should you. They are--for the most part--friendly. So friendly that many scientists now view humans as conglomerate superorganisms composed of thousands of species. Scientists have dubbed this internal flora the "microbiome," a nod to the little ecosystems that have blossomed in the body throughout human evolution.

These microbes are no mere hitchhikers. They're hard at work cleaning up your insides and pumping out compounds that have all kinds of effects on health, development and perhaps even some behavior, emerging evidence suggests.

While humans are definitely in a relationship with microbes, the status of that relationship is probably best described as "It's complicated." On the positive side, studies show that intestinal bacteria help to digest food, provide key vitamins and even feed cells lining the intestines. Friendly microbes in the gut and vagina and on the skin can protect against infections from disease-causing bacteria and educate the immune system. Some bacteria in the mouth even help prevent tooth decay.

Other bacteria may turn out to be "frenemies." These otherwise friendly microbes might break your heart and even control your brain. New experiments--mostly with mice--are uncovering secrets about how bacteria beguile, coax and outright manipulate their hosts, including humans.

And just as scientists are learning what these microbes are capable of, it's starting to look like clean living is breaking up some of the healthy friendships between people and microbes, contributing to disease. Unfriending a bacterial buddy, even one that is sometimes disruptive, can have unforeseen and potentially unpleasant side effects.

Whether they're helping or hurting, these trillions of tiny passengers are here to stay, so new research is mapping their preferred human habitats and figuring out what they do. Ultimately, understanding how bacteria operate inside their human hosts might reveal ways for humans to manipulate their own microbiomes to prevent or treat disease.

Meet your microbiome

Researchers are just beginning to compile a Who's Who of human-inhabiting microbes (SN: 12/6/08, p. 11; SN Online: 11/5/09). But even when bacteria are identified, it's often not clear which are do-gooders and which are troublemakers.

"We've moved away from saying 'What are healthy bacteria?' to 'What are normal bacteria?'" says Julie Segre of the National Human Genome Research Institute in Bethesda, Md. Segre is one of the researchers taking inventory of the bacteria that grow on skin as part of the National Institutes of Health's Human Microbiome Project. "Having acne--is it healthy? I don't know, but it's normal," she says. The same goes for dandruff and other common microbe-related skin problems.

It may take a shift in the numbers of microbes in a mix to cause illness. Skewed microbial mixes have been fingered as contributors to obesity (SN: 6/17/06, p. 373) and high cholesterol. How much fat gets into the liver may also depend on the blend of bacteria in a person's intestines, researchers at the University of North Carolina at Chapel Hill and colleagues reported in the March Gastroenterology.

In a study of bacteria in habiting healthy women's vaginas, Jacques Ravel of the University of Maryland School of Medicine in Baltimore and his colleagues found that each woman had one of five major communities of microorganisms. Four of the communities were dominated by types of Lactobacillus, bacteria like those found in yogurt that are well-known for making infection-fighting lactic acid, the researchers reported in the March 15 Proceedings of the National Academy of Sciences. …