The Body Politic

Feature / by Courtney Humphries /

The deep symbiosis between bacteria and their human hosts is forcing scientists to ask: Are we organisms or living ecosystems?

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Few people are more familiar with life’s interdependence and the blurriness of its distinctions than microbiologists. The recent metagenomic studies have revealed a daunting amount of diversity in microbial life, with none of the clear divisions we’re used to in the “macro” world. Among bacteria, the entire concept of species breaks down; it’s difficult for scientists to even categorize what they are seeing. Microbes offer a picture of life that is fluid and ever changing.

To come to terms with this diversity, microbiologists are today relinquishing the desire to name names. When studying a community, they no longer focus on developing a roster of who is there; instead, they ask what kinds of genes are present and what their functions are. In the human microbiome, which species we harbor may be less important than what they are doing.

William Karasov, a physiologist and ecologist at University of Wisconsin–Madison, believes that the consequences of this new approach will be profound. “We’ve all been trained to think of ourselves as human,” he says. Bacteria have been considered only as the source of infections, or as something benign living in the body. But now, he says, it appears that “we are so interconnected with our microbes that anything studied before could have a microbial component that we hadn’t thought about.” It will take a major cultural shift, says Karasov, for nonmicrobiologists who study the human body to begin to take microorganisms seriously as a part of the system.

Equally challenging, though in a different respect, will be changing long-held ideas about ourselves as independent individuals. How do we make sense of this suddenly crowded self? David Relman suggests that how well you come to terms with symbiosis “depends on how comfortable you are with not being alone.” A body that is a habitat and a continuously evolving system is not something most of us consider; the sense of a singular, continuous self is a prerequisite for sanity, at least in Western psychology. A symbiotic perspective depends on a willingness to see yourself as the product of evolutionary timescales. After all, our cells carry an ancient stamp of symbiosis in the form of mitochondria. These energy-producing organelles are the vestiges of
symbiotic bacteria that migrated into cells long ago. Even those parts of us we consider human are part bacterial. “In some ways, we’re an amalgam and a continuously evolving collective,” Relman says.

He also believes that we might have something to gain by embracing our bacterial side. Bacteria are often dismissed as simpler, less sophisticated, and less worthy of our consideration. “We put a lot of weight on a life form’s ability to think independently,” Relman says, but microbes have achieved fantastic evolutionary success by operating on a very different principle. Microbial communities are filled with examples of self-sacrifice for the benefit of the larger colony. They form physically close communities in which some cells exist solely to provide structural support or protection for others. This “intertwining of fate,” as Relman puts it, is something that humans could consider more seriously in the dynamics of their own societies, instead of focusing so keenly on individual identity and success.

Perhaps we could learn a lesson in fluidity from our symbionts. Science is always challenging us to let go of treasured categories and divisions. The theory of evolution, for instance, forced us to see species as points along a shared history, rather than as fixed identities. Symbiosis goes a step further by showing us how species are linked by more than history; they are living together in a continuous, interconnected now.

When scientists in 1977 first discovered life in the deep-sea hydrothermal vents, including gigantic tubeworms living in scalding-hot water filled with hydrogen sulfide, they could not explain it. Until then, all life was thought to derive its energy from the sun, but this habitat was far from any light. Then scientists found that the worms harbored symbiotic bacteria, which fed on hydrogen sulfide, turning this poison into something usable by other life forms. The discovery underscored the fact that life as we know it is built upon microbes, whether we look in the deepest oceans or our own intestines. We once had the luxury of ignoring the diminutive members of our bodies and other ecosystems. Now the blinders are off. 

Originally published April 14, 2009

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