Some target modifications require only one or a few mutations (e.g., resistance to fluoroquinolones), yet others, such as protection of the ribosome by methylation of rRNA, require new enzymes....

Some target modifications require only one or a few mutations (e.g., resistance to fluoroquinolones), yet others, such as protection of the ribosome by methylation of rRNA, require new enzymes. Explain the difference.

Why does mutation to resistance occur within a short time in some cases (e.g., erythromycin or penicillin) and only over a period of decades in other cases (e.g., vancomycin)?