Enterococcus, by poisoning its rivals, was saving the worms. This change depended entirely on the presence of Staph. When King exposed 15 generations of worms to Enterococcus alone, the mildly harmful bacterium became slightly more harmful. “On its own, it’s a little bit of a parasite,” says King. “But when it interacts with this much more virulent organism, it shifts along the continuum to be much more beneficial.