CAMBRIDGE, MA -- There are good bacteria and there are bad bacteria -- and sometimes both coexist within the same species.
Take, for instance, Pseudomonas aeruginosa, a microbe common in soil and water. This bacterium has been found to colonize medical equipment in hospitals and clinics around the world, and can live benignly in healthy individuals. But the bug poses a risk to those with weakened immune systems, and can turn pathogenic in patients with cystic fibrosis, forming thick layers of mucus in the lungs.