National Geographic : 2013 Jan
normally lives in the gut and helps us digest milk, becomes more abundant in the vagina, expos- ing the baby to the bacterium, perhaps to help prepare the way for digesting breast milk. Our bodies also host some pretty shifty charac- ters. At any one time about a third of us harbor in our nostrils Staphylococcus aureus, a normally benign bacterium that can turn virulent. Usually competition from other members of the nostril community appears to keep this bacterium un- der control. But S. aureus can get nasty, especially when it ventures into other environments. In the skin it can cause everything from an occasional pimple to a life-threatening infection. Under cer- tain conditions, the individual bacteria coalesce into a filmy mass that acts as a united front, invading new tissues and even infecting intra- venous catheters and other hospital equipment. Superbug strains of S. aureus can cause lethal infections such as toxic shock syndrome or nec- rotizing fasciitis—flesh-eating disease. What makes these strains so dangerous is their resistance to antibiotics, those miracles of modern medicine that since the middle of the past century have saved millions of lives. The more we learn about our microbiota, however, the more we realize how easy it is for helpful mi- crobes to get caught in the line of fire between an antibiotic and its intended target. Some 10 to 40 percent of children who are given a broad- spectrum antibiotic develop antibiotic-associated diarrhea, because their gut microbiota have been disturbed. The widespread use of antibiotics early in life may have more profound effects over time. The stomach microbe Helicobacter pylori has long been known to provoke ulcers in some people but in most serves the useful function of regulat- ing immune cells in the stomach. Martin Blaser, a microbiologist at New York University who has studied H. pylori for decades, notes that an ever shrinking share of adults is populated with the microbe, partly because of repeated high doses of antibiotics during childhood. Blaser believes the diminished presence of the bacteria and the rise in asthma in American youth might be related. So should we treat our wheezing children with a healthy dose of H. pylori? It’s often more complicated than that. As we learn more about the relationships between ourselves and our microbes—and their own complex relation- ships with one another—scientists are coming to see the microbiome the way ecologists have long viewed an ecosystem: not as a collection of species but as a dynamic environment, de- fined by the multitude of interactions among its constituents. This should mean greater care in the use of antibiotics and, increasingly, targeted probiotic treatments that don’t just temporarily boost the numbers of one microbe or another but that shore up the whole population so that our health is improved. “We know how to dis- turb a community,” says Katherine Lemon, a microbiome researcher at the Forsyth Institute in Cambridge, Massachusetts, and a clinician at Boston Children’s Hospital. “What we need to learn is how to coax it back into a healthy state.” This perspective on our relationship with microbes—as fellow travelers to be cared for and managed to our benefit—is a far cry from my day-job view of them as killers to be hunted down and eradicated before they can spread. Both views are valid, of course. We should never let our guard down against the threat of infec- tious pathogens. But as we continue to explore the microbial world, our fear of the invisible be- ings around us, and in us, should be tempered with respect for what we are learning about them—and a rush of excitement for what re- mains to be discovered. j All told, the microbes in your body can Weigh aS much aS or more than your brain— about three pounds in an average adult.