It’s a product of natural selection – in a population of bacteria there will be some that have genes that let them resist being killed by antibiotics. If antibiotics are used on that population, the resistant bacteria will survive and be able to pass the resistance genes on and form a resistance population. There’s also a phenomenon called “horizontal gene transfer”, where bacteria are able to share genes between each other which means that bacteria can acquire resistance from their neighbours.
Exactly as David said. Usually a bacteria with a resistance gene will be at a disadvantage because of that gene – it will cost the bacteria energy, or the gene won’t be quite as good as the normal non-resistant version of the gene. But then when there’s loads of antibiotics in the bacteria’s environment then the resistance gene means that the bacteria is more likely to survive, and bacteria without the resistance gene will be killed. So very quickly all of the bacteria have the resistance gene – and have become superbugs. The more we use antibiotics in medicine and on our farms, the more this will happen.
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