Farmers sometimes add antibiotics to animal feed to promote growth and prevent disease in healthy livestock. Scientists are concerned that this practice contributes to the spread of antibiotic-resistant bacteria. Explain how the widespread use of antibiotics in livestock farming could lead to the development of antibiotic-resistant bacteria, and explain why the presence of such bacteria is a serious concern for human health.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Antibiotics are used routinely in livestock farming across many countries. A significant proportion of the total antibiotics produced globally are given to farm animals, not humans.
Model answer (5 marks)
Antibiotics kill bacteria that are not resistant, leaving only those with a resistance‑conferring mutation alive.
Some bacteria already possess a random mutation that gives them resistance; these survive the antibiotic treatment.
The surviving resistant bacteria reproduce, passing the resistance trait to their offspring – natural selection favours the resistant strain.
Over many generations the proportion of resistant bacteria in the herd rises.
If these resistant bacteria spread to humans, infections caused by them cannot be treated with the usual antibiotics, making disease management difficult and increasing the risk of serious illness.
Some bacteria already possess a random mutation that gives them resistance; these survive the antibiotic treatment.
The surviving resistant bacteria reproduce, passing the resistance trait to their offspring – natural selection favours the resistant strain.
Over many generations the proportion of resistant bacteria in the herd rises.
If these resistant bacteria spread to humans, infections caused by them cannot be treated with the usual antibiotics, making disease management difficult and increasing the risk of serious illness.
Examiner tips
- Use the exact wording from the mark scheme – e.g. ‘kill non‑resistant bacteria’, ‘random mutation’, ‘natural selection’, ‘over many generations’, ‘cannot be treated effectively’.
- Show the causal chain clearly – each point should lead logically to the next.
- Keep the answer concise – 5 marks, so one sentence per point is ideal.
Common mistakes
- Mixing up ‘mutation’ with ‘acquisition of resistance genes’ – both are acceptable but the mark scheme emphasises random mutation.
- Failing to mention that the antibiotic is no longer effective in humans – the key concern is treatment failure.
- Writing a long paragraph that obscures the five distinct points – examiners look for clear, separate statements.
Mark scheme (5 marks)
- Antibiotics kill non-resistant bacteria / bacteria that are not resistant to the antibiotic are killed
- Some bacteria have a random mutation that gives resistance / resistant bacteria naturally occur in the population by chance
- Resistant bacteria survive and reproduce, passing on resistance to offspring / natural selection favours resistant bacteria
- Over time / over many generations, the proportion of resistant bacteria in the population increases
- Antibiotic-resistant bacteria could spread to humans and infections caused by these bacteria cannot be treated effectively with antibiotics / the antibiotic will no longer work against these bacteria in humans
Key terms in this question
antibiotic · antibiotic-resistant bacteria
Related
- All Edexcel A-Level Biology A: Salters-Nuffield (9BN0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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