Scott LaFee, Contributor

EACH OF us is a singular crowd - a walking, talking, air-breathing, waste-producing assemblage of microbial organisms. Your body consists of 100 trillion cells, give or take, but they're in the minority. Each human is also home to, among other things, an estimated 1,000 trillion individual bacteria, or 10 times more microbes than human cells.

What does this mean? Well, for one thing, it means that you're not alone. Ever. But more importantly, it means you're alive - and well.

"People like to think of themselves as exclusively human," said Dr. Jeffrey Gordon, a molecular biologist at Washington University in St. Louis, "but we're really a composite of many species, an amalgamation of human and microbial attributes. To fully understand what it means to be human, we need to embrace, explore and explain what microbes mean to us."

Because they cause disease and bodily harm, bacterial pathogens such as Vibrio cholerae and Mycobacterium tuberculosis tend to grab most of our attention. But there are many more bacteria species that pose no threat to humans, and quite a few that are, in fact, symbiotic or mutually beneficial.

"These microbes provide humans with features we haven't had to evolve on our own, such as breaking down otherwise indigestible foods, detoxifying carcinogens, educating our immune system and generally keeping bad bugs out," said Gordon.

Mouth germs

Bacterial symbionts tend to be found in and on specific parts of our bodies: the intestines, the mouth, the nose and upper respiratory tract, our skin and the vaginal epithelium. The average person's mouth contains perhaps 25 species of bacteria, with a quarter-teaspoon of saliva holding up to 40 million individual cells.

But it's the intestines that are the real microbial zoo. Gordon says the human gut is the densest bacterial ecosystem known, home to perhaps 500 species and 10 trillion to 100 trillion individual microbes. That works out to about four pounds of bugs in an adult, but you don't want to lose the weight. Besides breaking down otherwise indigestible carbohydrates and helping absorb minerals like copper and iron, intestinal bacteria produce vital nutrients like vitamin K (necessary for clotting blood) and folic acid.

When you upset them by, say, taking antibiotics that can kill good bacteria along with the bad, intestinal microbes could cause you to be, well, upset, too.

Small comforts

In the womb, human babies are essentially germ-free, but that soon changes. Newborns pick up their first bugs passing through mom's birth canal then spend the rest of their lives acquiring more and different bacterial residents and visitors.

Doing so may, in fact, be a factor in how long you live. A 2003 study by Caltech scientists found that fruit flies exposed to bacteria in the first week of life lived 30 per cent longer than flies exposed to bacteria at midlife or not exposed at all. Some researchers suggest something similar may happen with people.

"Microbes in the human body is no accident," says Dr. Martin Blaser, a professor of internal medicine and microbiology at the New York University Medical Center. "They've been with us a long time. They've co-evolved with us. They've been naturally selected because they help keep us alive. They are indisputably important."

How important isn't completely understood. It's not known, for example, exactly how many types of bacteria inhabit humans. In February, Blaser and colleagues at NYU published a study that sampled forearm skin from six healthy volunteers. They counted 182 bacteria species in the samples, including eight new ones. Only four species were common to all six volunteers.

The full nature of our symbiotic relationship with bacteria is even more mysterious, not the least because it's so incredibly complicated. Bacteria that are benign in one person may be pathogenic in another. They might be beneficial - or at least harmless - in one part of the body, but problematic somewhere else.

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