Patients should complete their course of antibiotics so all bacteria are…

The idea

Antibiotics kill bacteria, but within a population some bacteria may carry random mutations that make them less affected by the drug. If a patient stops treatment early, when they feel better, the least susceptible bacteria are more likely to survive. These survivors reproduce rapidly by binary fission, passing on the advantageous allele, so the population becomes dominated by resistant bacteria. Completing the prescribed course maintains the antibiotic concentration for long enough to kill even those bacteria with partial resistance, reducing the chance that any survive to mutate further or transfer resistance genes.

For example: A patient with a throat infection may feel well after three days, but stopping a seven-day course can leave resistant survivors that later cause an infection that the same antibiotic cannot treat.

What earns the marks

• Antibiotics kill bacteria but do not kill viruses, so they are only prescribed for bacterial infections.
• Random mutations can arise in bacterial populations and some mutations reduce the effect of an antibiotic.
• Stopping a course early allows bacteria that are less susceptible to survive and reproduce by binary fission.
• Surviving resistant bacteria pass on the advantageous allele to their offspring, increasing the proportion of resistant bacteria in the population.
• Completing the full course keeps the antibiotic at an effective concentration for long enough to kill bacteria that are only partially resistant.
• Reducing the number of surviving bacteria lowers the chance of further mutation and of resistance spreading to other bacteria.

Common mistakes and how to put them right

• Thinking that the human body becomes resistant to antibiotics.

  Put it right: Bacteria become resistant through mutation and natural selection.

• Believing that antibiotics kill viruses.

  Put it right: Antibiotics target bacterial structures and processes, so they are ineffective against viruses.

• Assuming that stopping early is safe once symptoms improve.

  Put it right: The full prescribed course is needed to reduce surviving bacteria.

Exam tips

• Use the terms mutation, natural selection, survival, reproduction and allele when explaining how resistance develops.
• Link each stage of the explanation to the consequence for the patient or population, such as treatment failure or spread of resistance.
• If asked to evaluate a patient's decision, state the decision, give the biological consequence and reach a clear judgement.