WE ALL KNOW that bacteria and other microbes can make us sick. Now, it turns out, they might also make us fat. Recent research comparing the "gut flora" of overweight and slender mice -- as well as people -- showed that the microorganism count varied consistently in the two groups. Plump individuals have more of one kind, leaner types have more of another. The cause-and-effect relationship, if one exists, is not at all clear. Maybe the bacteria are an indicator of fatness but not its cause (like owning a lot of elastic-waist pants), or maybe having those microbes means obese people metabolize food differently than lean people, giving them distressingly more bang for their caloric buck.
Either way, these findings don't do much to improve the reputation of microbes. Just saying "bacteria" and "fat" in the same sentence is enough to reinforce a million toothpaste commercials: Bacteria are bad. . In fact, humans and microbes have a deep, seamless relationship -- for good and ill, but mostly for good.
For starters, we are not just occasionally dusted or doused in bacteria; we wallow in them, with more than 2,000 different kinds swarming in our insides, on our skin and hair, inside our noses and mouths. Most cause us no harm -- fewer than 100 types of bacteria cause disease -- and rather than fighting them off tooth and nail, we can wallow at will.
The key research isn't about fat or even \o7E. coli\f7 but basic health. Exposure to bacteria appears to calibrate our immune systems to distinguish harmless (or beneficial) co-inhabitants of the planet from threatening pathogens.
Scientists are increasingly convinced that the lack of "immune stimulation" -- a lack of exposure to microbes -- in childhood can cause problems with allergies and asthma later in life, turning innocuous substances such as pollen or peanuts into deadly killers.
The need for immune stimulation starts early. A group of Norwegian researchers looked for a connection between children's sensitivities to eggs, fish or nuts and whether they had been born vaginally or via cesarean section.
Childbirth, in case you hadn't noticed, is a rather messy process, and along with an introduction to air, light and exclamations of joy, a newborn receives a hearty dose of bacteria to colonize his or her intestinal tract during the passage through the birth canal. Babies removed surgically from their mothers miss out on this germ-laden opportunity; they acquire their intestinal menagerie more slowly than vaginally delivered children.
The research found that when a mother was allergic to eggs, fish or nuts, her child was seven times more likely to share that allergy if he or she had been born by C-section.
Other work has found that exposure to microorganisms via pets, siblings and a rural lifestyle can mean a lower incidence of allergies and asthma in older children. Such ailments are much more rare in less-developed countries, even when pollution levels are high. And although parents may fret about colds their children catch in day care, having more colds as a youngster seems to mean less asthma later in life. This is not to say that sickness is good but that a life without microbes -- even the ones that cause disease -- can cause more problems than it solves.
Other research indicates even deeper connections between our health and our microbes. Margaret McFall-Ngai of the University of Wisconsin suggests that the complex immune system of vertebrates, including humans, may owe its ability to remember previous encounters with diseases -- and therefore trigger antibodies to fight them -- to the need for dealing with the vast array of microbes we live with. She theorizes that such "memory" is important primarily in developing a way to manage the masses of microbes that can inhabit our bodies and that the immune benefits are just gravy.
The moral of the story is that our bodies and our lives are so enmeshed with microbes that it's futile, and potentially even harmful, to try to get rid of them with soaps and wipes and purifiers. So get over all those scare stories about the number of bacteria on your telephone or toilet seat. They belong there, for the most part, and you would miss them if they were gone.