hewwo :3
Cool topic you're learning about. Antibiotic resistance occurs because of the fact that bacteria change in response to the use of medicaments, such as penicillin. The problem about antibiotic resistance, an international health crisis, is that people OVERDOSE in medicaments. Antibiotics are known to kill weak and sensitive bacteria, but some resistant bacteria grow and multiplicate, since they know how the medicament works.
Therefore, I would say that bacteria that are not initially killed by the antibiotic, they go to reproduce passing the resistance on to offspring. Antibiotic resistance is a big example when it comes to natural selection.