Caution: Killing Germs May Be Hazardous to Your Health

The task is complicated, in part because some bacteria seem to be both good and bad. The best-known is Helicobacter pylori, a microbe that has evolved to live in the acid environment of the stomach. It survives by burrowing into the stomach's mucous lining and secreting enzymes that reduce acidity. Nobel laureates Barry Marshall and Robin Warren showed it could cause gastric ulcers and stomach cancer. But then further studies discovered that infection with H. pylori was protective against esophageal reflux and cancer of the esophagus, and may also reduce the incidence of asthma. H. pylori, which is spread in drinking water and direct contact among family members, was virtually universal a few generations ago but is now on the verge of extinction in the developed world. The result is fewer ulcers and stomach cancer, but more cancer of the esophagus—which is increasing faster than any other form of cancer in America—more asthma, and … what else? We don't know. "H. pylori has colonized our guts since before humans migrated out of Africa," says Blaser. "You can't get rid of it and not expect consequences."

Blaser questions whether eliminating H. pylori is a good idea. Someday, conceivably, we might intentionally inoculate children with a bioengineered version of H. pylori that keeps its benefits without running the risk of stomach cancer. There is already a burgeoning market for "probiotics," bacteria with supposed health benefits, either in pill form or as food. Consumers last year slurped down more than $100 million worth of Dannon's Activia, a yogurt containing what the Web site impressively calls "billions" of beneficial microbes in every container. The microbes are a strain of Bifidobacterium animalis, which helps improve what advertisers delicately call "regularity," a fact Dannon has underscored by rechristening the species with its trademarked name "Bifidus regularis." Other products contain Lactobacillus casei, which is supposed to stimulate production of infection-fighting lymphocytes. Many others on the market are untested and of dubious value. Labels that claim ANTIBIOTIC RESISTANT ought to be considered a warning, not a boast. Bacteria swap genetic material among themselves, and the last thing you want to do is introduce a resistant strain, even of a beneficial microbe, into your body.

And there's one more thing that microbes can do, perhaps the most remarkable of all. Mycobacterium vaccae, a soil microbe found in East Africa that has powerful effects on the immune system, was tested at the University of Bristol as a cancer therapy. The results were equivocal, but researchers made the startling observation that patients receiving it felt better regardless of whether their cancer was actually improving. Neuroscientist Chris Lowry injected mice with it, and found, to his amazement, that it activated the serotonin receptors in the prefrontal cortex—in other words, it worked like an antidepressant, only without the side effects of insomnia and anxiety. Researchers believe M. vaccae works through the interleukin-10 pathway, although the precise mechanism is uncertain. But there is at least the tantalizing, if disconcerting, suggestion that microbes may be able to manipulate our happiness. Could the hygiene hypothesis help explain the rise in, of all things, depression? We're a long way from being able to say that, much less use that insight to treat people. But at least we are asking the right questions: not how to kill bacteria, but how to live with them.

With Matthew Philips, Raina Kelley and Karen Springen

© 2007