Doctors found that the percentage of Staphylococcus bacteria resistant to the antibiotic penicillin increased from 5% to 80% over 20 years.
Explain how the population of bacteria became resistant to penicillin. [6]
- (random) mutation produced an allele / gene for resistance (before penicillin was used)(1)
- variation: some bacteria were resistant and some were not(1)
- penicillin / antibiotic kills the non-resistant bacteria(1)
- resistant bacteria survive (natural selection / survival of the fittest) and reproduce; allow less competition for resistant bacteria(1)
- pass on the resistance allele / gene to offspring; reject 'pass on immunity'(1)
- over many generations the proportion / percentage of resistant bacteria increases (from 5% to 80%); accept bacteria reproduce rapidly / by binary fission so the allele spreads quickly; accept overuse / misuse of antibiotics increases selection(1)
- Answer: Mutation creates resistant allele; penicillin kills non-resistant bacteria; resistant survivors reproduce and pass on allele; proportion rises over generations.
- Examiner tip: Bacteria do not 'become immune' or 'adapt' because of the antibiotic — the resistance allele arises by random mutation, and penicillin only selects the bacteria that already carry it.