Michael Byrne explains what happens:
It’s tempting to look at bacteria as a kind of binary realm, with “good” and the “bad” sorts that have good and bad impacts on health, when it’s really not that easy. Our own personal bacterial flora might help keep harmful bacteria at bay through competitive pressure, but the goodness of these tiny helpers is less a function of benevolence than geography – set them loose elsewhere in the body, beyond the inner-outside of the digestive tract, and very bad things will happen. A different set of bad things awaits a host with just a bit too much or too little friendly bacteria, ranging from cancer to inflammatory bowel diseases. While it’s possible to live without gut flora, such an existence portends a wide variety of troubles.
One fascinating aspect of this would-be dualism is how the bacteria that we provisionally know as friendly and harmful interact with each other. We know well enough that our gut flora help us out with immunity and keeping virulent bacterial invaders at bay, but it’s hardly because of some secret intraspecies armistice. A study out [last] week in the journal PLOS ONE examines the response of gut flora (in mice) to colitis-causing bacterial infection elsewhere in the body, finding that our own personal colonies of helper bacteria get sick themselves in a very real sense. It’s an observation that paves the way for not just better understandings of bacterial interrelationships, but also “early warning” tools for diseases.
The Dish 