Doctors give antibiotics to patients to kill bacteria in their bodies. How has the overuse of antibiotics led to the evolution of antibiotic-resistant bacteria?
This is due to 'survival of the fittest'.
Antibiotics hardly ever kill the entire population of bacteria it targets. Overuse of antibiotics is an environmental pressure causing the strongest bacteria to survive. These bacteria have developed mechanisms over the years that makes them less sensitive/ insensitive to the antibiotics.
Bacteria that survive will pass this trait on to the next generation, which will be a fully resistant generation for a specific type of antibiotics. Interestingly, bacteria can also exchange genes with other bacteria in their population, so without reproducing! Sometimes they can even exchange these genes with other species.
By often prescribing wide spectrum antibiotics, the pressure on the bacteria increases, also increasing the opportunities for the bacteria to select the best mechanisms for resistance. We should therefore be cautious with the (ab)use of antibiotics.