How can the use of antibiotics contribute to bacteria becoming resistant?

Bacterial DNA can spontaneously mutate, which means a change in the nucleotide reading frame. By chance, a mutation may cause the bacteria to prevent being killed by the antibiotic. For example, by developing a 'pump' to expel the antibiotic. Whilst the use of antibiotics does not directly cause resistance, the increased use can encourage the persistence of resistant bacteria and eventually resistant strains. Therefore, it is advised that antibiotics are only used when necessary to treat a bacterial infection, and the entire course of medication should be taken.