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6.2.11Topic 6 · Immunity, Infection and Forensics

Limiting Antibiotic Resistance — Edexcel A Level Biology A (SNAB) 9BI0 Revision Notes & Topical Questions

5 min read 9 parts Lesson 18 of 19 1 quiz

📚 Edexcel A Level Biology A (SNAB) 9BI0 Hub / Topical Revision
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Lesson summary

6.2.11 Limiting Antibiotic Resistance — Edexcel A Level Biology A (SNAB) 9BI0 notes: Explain how antibiotic resistance arises and spreads by natural selection and evaluate the strategies used to limit it.Within any large bacterial population, random mutations occasionally produce…

  • selection pressure
  • vertical transmission
  • horizontal gene transfer
  • Multidrug-resistant tuberculosis
  • Exam-style question 1
  • Exam-style question 2

Section 1 of 9How resistance arises

Within any large bacterial population, random mutations occasionally produce an allele that gives resistance to an antibiotic.

Such mutations are present before the antibiotic is used.
Selection pressure
When the antibiotic is applied, it acts as a selection pressure: susceptible bacteria die or fail to multiply, but resistant ones survive and reproduce, passing on the allele by vertical transmission.
Because bacteria divide in as little as 20 minutes, the resistant population grows very rapidly.

Resistance genes are often found on plasmids, which can be transferred to other bacteria, even of different species, by conjugation through pili. This horizontal gene transfer spreads resistance quickly. Resistance mechanisms include enzymes that destroy the antibiotic (beta-lactamases that break the penicillin ring), changes in the target site, reduced uptake, and pumps that expel the drug.

Section 2 of 9Examples of resistant bacteria

Examples of resistant bacteria
MRSA
(methicillin-resistant Staphylococcus aureus) is a hospital-acquired infection that is resistant to many beta-lactam antibiotics and spreads on hands, equipment and surfaces. Patients with wounds or weak immune systems are at risk.
Multidrug-resistant tuberculosis
Mycobacterium tuberculosis becomes resistant when patients do not take the full course, and the treatment then takes longer and is more costly.
Clostridioides difficile infections, which become serious when broad-spectrum antibiotics reduce the normal gut flora.

Section 3 of 9Strategies to limit resistance

StrategyReason
Do not prescribe antibiotics for viral infections or minor infections that clear unaidedReduces unnecessary exposure and so selection pressure
Complete the full courseEnsures all bacteria, including partly resistant ones, are killed, so survivors do not breed
Use narrow-spectrum antibiotics identified by sensitivity testingTargets only the pathogen and preserves normal flora
Use combinations of antibioticsA bacterium is unlikely to have resistance to all of them at once
Rotate or restrict last-line antibioticsKeeps them effective for serious cases
Limit use in farming as growth promoters and routine preventionReduces resistance in animal populations and in the food chain
Hygiene in hospitals: handwashing, barrier nursing, screening, isolation of infected patientsStops the spread of resistant strains
VaccinationPrevents infections, so fewer antibiotics are needed
Develop new antibiotics and alternatives such as bacteriophagesProvides new treatments, though development is slow and costly

Section 4 of 9Evaluating the measures

Each strategy has limits.

1
Patients may stop taking tablets when they feel better, so the full-course advice may not be followed; some argue that shorter courses may be adequate for some infections, and research continues.
2
Pharmaceutical companies have little commercial incentive to develop drugs that will be used rarely.
3
In many countries antibiotics are available without prescription.
4
Because bacteria cross borders by travel and trade, international cooperation is needed.
5
Surveillance of resistance patterns and rapid diagnostic tests help doctors select the right drug.

Section 5 of 9Links

This topic combines natural selection (Topic 5), bacterial structure and plasmids, and the action of antibiotics (previous lesson). Use the sequence mutation, selection pressure, survival and reproduction, increase in allele frequency in any explanation, and give particular reasons for each control measure.

Section 6 of 9Worked data question

Suppose a hospital records that 1200 patients were given a particular antibiotic in a year, and 150 of their bacterial infections were resistant.

1
Resistance = 150 / 1200 × 100 = 12.5%.
2
If the next year the use rose to 2400 patients and 480 infections were resistant, resistance = 20%.
3
The relative increase in the percentage is (20 − 12.5) / 12.5 × 100 = 60%.
4
Conclude that greater use is associated with a higher proportion of resistance, which supports selection pressure as an explanation, but add that correlation does not prove cause; other factors such as hygiene and patient type should be controlled.

A well-planned response to resistance combines prevention (vaccination, hygiene), careful prescribing (stewardship), surveillance and research. Students who give a balanced evaluation, with advantages, disadvantages and an overall judgement, achieve the highest marks.

Section 7 of 9Exam-style questions

  1. Explain how a population of bacteria can become resistant to an antibiotic. [5]5 marks

  2. Explain why patients are advised to complete a course of antibiotics even when they feel better. [3]3 marks

  3. Evaluate the use of antibiotics in farming to promote growth of animals. [3]3 marks

Section 8 of 9Key terms

TermMeaning
Antibiotic resistanceThe ability of bacteria to survive exposure to an antibiotic.
Selection pressureAn environmental factor that favours certain phenotypes.
Horizontal gene transferTransfer of genes between organisms other than from parent to offspring.
PlasmidA small circular DNA that can carry resistance genes.
MRSAMethicillin-resistant Staphylococcus aureus.
ConjugationTransfer of a plasmid between bacteria through a pilus.
Beta-lactamaseAn enzyme that breaks down penicillin-type antibiotics.
Sensitivity testingTesting which antibiotics are effective against a bacterial sample.

Section 9 of 9Exam technique

1
Saying the bacteria 'become used to' the antibiotic or the body becomes resistant. The bacteria population evolves; the human does not.
2
Forgetting that the mutation exists before the antibiotic is applied.
3
Writing that taking antibiotics for viruses is harmless. It does not help and it adds selection pressure.
4
Describing only individual patient behaviour. Hospital hygiene, farming practice and policy also matter.

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