Microcosm by Carl Zimmer Page A

may be only thirty feet, but it’s about 7 million times the length of
E. coli.
If you dived into the ocean in Los Angeles and swam 7 million body lengths, you could cross the Pacific.
    As
E. coli
drifts through the human gut, its hook-tipped hairs snag on the intestinal walls. A gentle flow of food is enough to detach the hooks, allowing the microbe to roll along. But if the flow becomes strong, the hairs begin to grip stubbornly to the wall. It just so happens that the hairs bring
E. coli
to a halt exactly in the place in the large intestine that suits it best, where food flows by at top speed. The warmth of the gut triggers it to make proteins it can use to harvest iron, to break down sugar, and to weld together amino acids. It begins to feed and thrive, at least for a few days.
    As
E. coli
grows and multiplies, it prepares the way for its own downfall. It uses up much of the oxygen in the intestines and alters their chemistry by releasing carbon dioxide and other wastes. It creates a new habitat that other species of microbes can invade and dominate. This ecosystem
E. coli
helps to build in our bodies is spectacular. It can reach a population of 100 trillion, outnumbering the cells of our body ten to one. Scientists estimate that a thousand species of microbes can coexist in a single human gut, which means that if you were to make a list of all the genes in your body, the vast majority of them would not be human.
    As other species prosper,
E. coli
dwindles away until it makes up just one-tenth of 1 percent of the population of gut microbes. It becomes prey to viruses and predatory protozoans. It must compete with other microbes for food. But it also comes to depend on other species of microbes for food. As its host grows older and gives up milk, the gut starts to fill with starches and other complex sugars that
E. coli
can’t break down. It’s like going to a restaurant and having your waiter suddenly switch your chocolate mousse with a bowl of hay.
E. coli
must now wait for other species of bacteria to break down complex sugars so it can feed on their waste.
    Yet even as a minor scavenger,
E. coli
may be able to repay the other microbes for their services. Some research suggests that by clearing away simple sugars, scavengers like
E. coli
may allow other microbes to break down complex sugars more quickly.
E. coli
also continues to snatch up what little oxygen accumulates in the gut from time to time. By keeping the level of oxygen at a steady low,
E. coli
makes the gut reliably comfortable for the vast majority of resident microbes. Cradled in this ecological web,
E. coli
colonies will grow in the human gut for the host’s entire lifetime. As many as thirty different strains may live there at any moment. It is a very rare person who is ever
E. coli
free.
    Here is another way in which we are like
E. coli:
we, too, depend on our microbial jungle. We need bacteria to break down many of the carbohydrates in our food. Our microbial passengers synthesize some of the vitamins and amino acids we need. They help control the calories that flow from our food to our bodies. A change in the bacteria in your gut may change your weight. Intestinal microbes also ward off diseases, a fact that has led doctors to feed premature infants protective strains of
E. coli.
The bacteria protect the gut by releasing chemicals that repel pathogens and by creating a tightly knit community that the pathogens simply can’t invade.
    It is difficult, in fact, to say exactly where these bacteria stop and our own immune systems begin. They help our immune systems manage a delicate balance between killing pathogens and not destroying our own tissues. Studies show that some strains of
E. coli
can cool down battle-frenzied immune cells. A healthy supply of
E. coli
may help ward off not just pathogens but autoimmune diseases such as colitis. Some scientists argue that our immune systems return the favor by stimulating the bacteria to form thick protective