Some things we know about good bacteria, besides the generalization that they help to counteract pathogens:

Good bacteria can break down certain foods, such as plant starches, that we cannot digest on our own. "This enables us to extract more energy from what we consume," says Jeffrey I. Gordon, director of the Center for Genome Sciences at Washington University School of Medicine in St. Louis. (Similarly, cows can digest cellulose thanks to the good bacteria that live in their rumens.)

Good bacteria promote the storage of energy as fat. According to Gordon, this raises the possibility that "an individual's predisposition to obesity or leanness may be partly determined by the composition of the microbes living in the gut."

Good bacteria help shape our postnatal development. For example, they help to form our intestinal blood vessels, through which we absorb nutrients.

Good bacteria synthesize vitamin K and other vitamins that we cannot generate on our own. They break down carcinogens. They also may influence the metabolism of drugs.

Good bacteria increase the rate at which the cells of the intestinal lining renew themselves, ridding us of damaged cells that could bring on gastrointestinal cancer.

The good bacteria that infants acquire from their mothers and from the general environment at birth "educate the newborns' immune systems," says Gordon. "This appears to reduce allergic responses."

Each human carries a different set of bacteria, and its composition varies along the length of the gut. Some of these bacteria are permanent residents; others are transient "tourists," just passing through.

—Maya Pines

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Reprinted from the HHMI Bulletin,
Winter 2005, pages 26–30.
©2005 Howard Hughes Medical Institute