Learning focus
Use precise biological vocabulary, interpret diagrams and data, explain mechanisms as linked sequences, and evaluate biological applications and environmental decisions.
Initial variation
A bacterial population may contain rare cells with a mutation that gives resistance. The antibiotic does not create the needed mutation; resistant variants exist before exposure or arise randomly.
Selection by antibiotic
When the antibiotic is used, susceptible bacteria are killed or prevented from reproducing. Resistant bacteria survive. Competition is reduced and survivors reproduce rapidly, passing resistance genes to descendants.
Development of a strain
After repeated selection, resistant bacteria form a larger proportion of the population and a resistant strain develops. MRSA is an important example. Treatment becomes more difficult because some antibiotics no longer work.
Minimising resistance
Antibiotics should be used only when essential and according to professional guidance. They do not treat viral infection. Infection control, hygiene and monitoring also reduce spread of resistant strains.
Practical or data skill
Interpret bacterial-growth data before and after antibiotic treatment and identify the selection pressure and surviving phenotype.
Examination tip
Resistance develops in the bacterial population, not in the patient’s body.
Review questions and suggested answers
Question 1
Did the antibiotic cause the useful mutation?
Suggested answer
No, mutation is random and resistant variants may already be present.
Question 2
Why do resistant bacteria become common?
Suggested answer
They survive treatment and reproduce.
Question 3
How can resistance be minimised?
Suggested answer
Use antibiotics only when essential and apply infection-control measures.
