The threat of drug-resistant bacteria grows more pressing with every year. These microbes can shrug off the most potent antibiotics, including some drugs of last resort. Some bacteria have become resistant to all of our available drugs. Scary stuff, but bacteria don’t have to resist antibiotics to defy them. There is another way—a much simpler, very common, and largely unappreciated one.
The vast majority of antibiotics work by killing bacteria that are actively growing and multiplying. Think of the antibiotics as wrenches thrown into the midst of whirring machines. If the machines are off—their cogs still, their motors silent—the wrenches have no effect. So it is with bacteria. By simply doing nothing, and entering a dormant or extremely slow-growing state, they can survive. They can keep their heads down until the antibiotic has diffused away and the danger has passed.
This strategy is called tolerance, and the cells that practice it are called persisters. It’s very different from resistance. Resistant microbes have special genetic tools that allow them to flout antibiotics: They build pumps that expel the drugs, or enzymes that destroy them. Persisters have no such innate tricks; if they start growing again in the presence of an antibiotic, they’d die. It’s their behavior that saves them—and that behavior is common to many microbes. Hit a colony with antibiotics and most will die, but a small fraction will persist. When the drugs disappear, they can rebound, which is partly why many infections are so hard to treat.