When the forces of evolution took over an experimental strain of bacteria, it derailed an experiment that researchers thought they were conducting, but led to something much more profound instead. The researchers used a colony of mice raised in a large plastic bubble, called an isolator, that was completely sterile, lacking even a single bacterium. They introduced a single type of bacteria into the mouse colony, but it mutated quickly into different types, making new bacteria that were hardier inside of the mice than the original bacterium was.