When bacteria start building cities, we’re in trouble. The normally free-floating cells can gather in large numbers and secrete a slimy matrix that they live within. These communities are called biofilms, and they grow wherever there is a surface to support them. Hospital catheters are prime real estate, but they’ll settle on everything from plumbing to oil refineries to ship hulls.

Within a biofilm, bacteria are extraordinarily durable. Antibacterial chemicals have a tough time reaching them within their slimy fortress. Even if they do, there’s always a batch of dormant cells that can persist through a chemical onslaught and restart the community. They’re involved in the majority of persistent hospital infections, and it’s easy to see why. You could bleach