Behold yourself, for a moment, as an organism. A trillion cells stuck together, arrayed into tissues and organs and harnessed by your DNA to the elemental goals of survival and propagation. But is that all? An electron microscope would reveal that you are teeming with other life-forms. Any part of your body that comes into contact with the outside world-your skin, mouth, nose and (especially) digestive tract-is home to bacteria, fungi and protozoa that outnumber the cells you call your own by 10, or perhaps a hundred, to one.

Their ancestors began colonizing you the moment you came into the world, inches from the least sanitary part of your mother’s body, and their descendants will have their final feast on your corpse, and join you in death. There are thousands of different species, found in combinations “as unique as our DNA or our fingerprints,” says Stanford biologist David Relman, who is investigating the complex web of interactions microbes maintain with our digestive, immune and nervous systems. Where do you leave off, and they begin? Microbes, Relman holds, are “a part of who we are.”

Relman is a leader in rethinking our relationship to bacteria, which for most of the last century was dominated by the paradigm of Total Warfare. “It’s awful the way we treat our microbes,” he says, not intending a joke; “people still think the only good microbe is a dead one.” We try to kill them off with antibiotics and hand sanitizers. But bacteria never surrender; if there were one salmonella left in the world, doubling every 30 minutes, it would take less than a week to give everyone alive diarrhea. In the early years of antibiotics, doctors dreamed of eliminating infectious disease. Instead, a new paper in The Journal of the American Medical Association reports on the prevalence of Methicillinresistant Staphylococcus aureus (MRSA), which was responsible for almost 19,000 deaths in the United States in 2005-about twice as many as previously thought, and more than AIDS. Elizabeth Bancroft, a leading epidemiologist, called this finding “astounding.”

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