Science Puzzle
Why Do Antibiotics Stop Working?
When penicillin came into wide use in the 1940s, it cured infections that had killed people for centuries. Within a few years, doctors were finding bacteria that it no longer affected.
Nobody had changed the drug, and bacteria can't think, so they can't have "learned" to fight it. Where did the resistant bacteria come from?
The Answer
They were there all along, or appeared by chance. The antibiotic didn't create them; it picked them out.
A single infection can involve billions of bacteria, and they aren't all identical. Every time a bacterium copies its DNA there's a small chance of a mistake, a mutation. Bacteria can also swap little loops of DNA with each other. So in a big enough population, a few will happen to carry a gene that helps them survive the drug: a pump that throws it out of the cell, say, or an enzyme that breaks it apart.
The antibiotic kills everything else. The few survivors suddenly have the place to themselves, and in good conditions bacteria can divide every 20 minutes or so. Within days the whole population is made of their descendants; every one of them carries the resistance gene. Nothing learned anything: the drug simply removed the competition.
Alexander Fleming, who discovered penicillin, warned about this in 1945, when he collected his Nobel Prize. It's why doctors try to prescribe antibiotics only when they're really needed. Every dose gives the resistant few an advantage. Resistant infections are now one of the biggest threats in medicine.
The principle: Natural selection. When a population varies, anything that kills most of it leaves behind the individuals best able to survive, and their offspring take over.