How do bacteria become resistant to antibiotics?

Bacteria become resistant to antibiotics by evolution, fast-forwarded. Among billions of bacteria, a few carry a random genetic change that happens to help them survive a drug. When an antibiotic kills off their neighbours, those few survive and multiply, and soon the whole population is resistant. Biologists call the drug's effect selective pressure: it doesn't create resistance, it picks out the microbes that already have it.

One common misunderstanding is that people become resistant to antibiotics. They don't. Resistance belongs to the microbe, not the person it infects. And bacteria have a second trick that animals lack: they can swap genes. A resistance gene that appears in one bacterium can be passed to others, even of a different species, and even from a harmless microbe to a dangerous one.

The danger was spotted almost as soon as antibiotics existed. Alexander Fleming, who discovered penicillin, noticed early on that bacteria became resistant when too little of the drug was used or for too short a time. In his Nobel lecture he warned that if penicillin could be bought in shops, people might underdose themselves and teach their microbes to resist. Around then, the first clinical case of penicillin resistance was reported. Today the WHO counts antimicrobial resistance among the top global threats to health: bacterial resistance was directly responsible for 1.27 million deaths in 2019.