What causes antibiotic resistance?

Bacteria resist a drug when the bacteria change in some way. The change may protect the bacteria from the drug's effects or limit the drug's access to the bacteria. Or the change may cause the bacteria to change the drug or destroy it.

Bacteria that survive an antibiotic treatment can multiply and pass on resistant properties. Also, some bacteria can pass on their drug-resistant properties to other bacteria. This is similar to them passing along tips to help each other survive.

The fact that bacteria develop resistance to a drug is normal and expected. But the way that drugs are used affects how quickly and to what degree resistance occurs.