Penicillin-resistance genes are transferred from resistant strains to nonresistant strains, leading to the evolution of penicillin resistance due to long generation times and underuse of penicillin in recent years.
Penicillin is a natural derivative of the fungus Penicillium. Bacteria respond more quickly to other antibiotics because they are designed to kill specific types of bacteria and are not broad-spectrum like penicillin.
Through the use of newer antibiotics such as ampicillin or amoxicillin, penicillin is prescribed more rarely. Through disuse of penicillin, bacteria developed resistance to that particular antibiotic.
Resistance is caused by evolution by natural selection through overuse of penicillin, as well as the rapid transfer of penicillin-resistance genes within and among bacterial species.