Say Hello to the Bugs in Your Gut

Whenever you eat, even if it's just a bowl of cereal while standing at the kitchen counter, you're feasting with trillions of your closest compadres. Bacteria, fungi and other microbes share the bounty as food churns through the vital inner tube that makes up your gut. It isn't a one-way relationship. These microscopic critters are essential contributors to our good health. They break down toxins, manufacture some vitamins and essential amino acids, educate the immune system and form a barrier against infective invaders. A provocative new avenue of research suggests that the makeup of microbes in the gut may influence our weight, too. If true, this could provide new strategies for weight control.

Which species of microbes live in the gut and what they do in there are just two of the many key questions that scientists are asking about this largely unexplored realm. "The landscape of the human gut is truly terra incognita," says Jeffrey Gordon, a genome scientist at Washington University in St. Louis whose research team is spearheading this effort. "The menagerie of microscopic organisms living there acts like an organ that carries out functions that we humans have not had to evolve."

The early work on our gut microbiota (loosely translated from Latin as "community of tiny living things") is challenging our notion of what it means to be human. From an early age, the human body is home to a huge but ever-changing community of microbes: for each one of our cells, there are 10 microbial cells in or on the body. Most of them live in the intestines; the bulk of the rest inhabit the mouth, esophagus, stomach, upper airway, skin and vagina. No one knows how many different species coexist inside the human gut. In the first comprehensive census of the gut, David Relman and his colleagues at Stanford University quit counting after they hit 395 different species in three healthy subjects. The real cast of characters will almost certainly number in the thousands.

Curiously, we don't start life with such a microbial partnership. A developing baby floats in sterile amniotic fluid, protected from bumps—and bugs. That isolation ends during the baby's trip through the birth canal, which is a haven for bacteria. The baby picks up microbes on his or her skin; some get into the mouth. From then on, helpful microbes somehow convince the immune system that they mean no harm. They settle down in hospitable regions, and crowd out those that can't compete.

At first, the microbes in a baby's body resemble those in the mother. Over time, the community takes on its own identity, nudged this way and that by the child's genes, the environment and the unceasing flow of new microbes from food, beverages and unwashed hands. Eventually, an individual's gut microbiota becomes as unique as a fingerprint.

The gut is composed of the small and large intestine. Stretched out, it's as long as a schoolbus. Flatten out the millions of fingerlike projections that line its sides and it would easily cover a tennis court. The small intestine is where much of the food you eat is broken down into simple sugars, fats and amino acids. These are all small enough to be shuttled across the lining of the intestine and into nearby blood vessels. Fiber from fruits, vegetables and grains, along with other indigestible material, passes largely unchanged into the large intestine, also known as the colon.