AQA AS Level Biology Paper 1, June 2025: Question 9
10 marks · Medium difficulty · Short Answer
Answer questions on the evolution of antibiotic resistance in Staphylococcus aureus, including naming the type of selection, explaining allele frequency increase, and calculating the rise in MRSA cases per 100,000.
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Question text
09 Read the following passage.
Penicillin-resistant Staphylococcus aureus (PRSA) bacteria have evolved by
natural selection. From 1941, penicillin was used extensively to treat
S. aureus infections. The first case of PRSA was reported in 1942. By the late
1960s, more than 80% of S. aureus bacteria were resistant to penicillin.
Many antibiotics have a beta-lactam ring in their chemical structure, which 5
inhibits the synthesis of bacterial cell walls. PRSA have an allele of a gene
blaZ that codes for an enzyme called beta lactamase. This enzyme hydrolyses
beta-lactam rings in antibiotics such as penicillin.
The evolution of PRSA resulted in the development of new antibiotics, such as
methicillin in 1960. However, by 1961, methicillin-resistant S. aureus (MRSA) 10
were identified. In 2022, 675 cases of MRSA were reported in England.
However, in 2023, the number of cases of MRSA rose to 787. Patients in
hospitals are at greater risk of MRSA infections than the general population.
Use the information in the passage and your own knowledge to answer the
following questions.
09.1 Penicillin-resistant Staphylococcus aureus (PRSA) have evolved by natural selection
(lines 1–2).
Name the type of natural selection.
[1 mark]
09.2 By the late 1960s, more than 80% of S. aureus were resistant to penicillin (lines 3–4).
Explain why.
[3 marks]
09.3 Many antibiotics have a beta-lactam ring in their chemical structure, which inhibits
synthesis of bacterial cell walls (lines 5–6).
*26Suggest* two reasons why this makes infections caused by PRSA difficult to treat.
[2 marks]
09 4 In 2022, the population of England was 5.644 ×107
.
In 2023, the population of England was 5.672 ×107
Use information in lines 10–12 to calculate the increase in MRSA cases per 100 000
population.
Show your working.
[2 marks]
Increase in MRSA cases per 100 000 population28
09.5 Patients in hospitals are at greater risk from MRSA infections than the general
population (lines 12–13).
Suggest and explain two reasons why.
[2 marks]
Mark scheme
Show the mark scheme
Question Marking Guidance Mark Comments
Directional; 1
09.1 (1 x
AO1)
1. (Use of) penicillin/antibiotic creates selection
pressure
OR
Penicillin/antibiotic kills susceptible/non-resistant 3
S. aureus/bacteria; (1 x
09.2
AO1, 2
2 and 3. Reject ‘gene’
2. (Resistant S. aureus/bacteria) are more likely to x AO2)
for ‘allele’ once
survive and reproduce to pass on
blaZ/penicillin-resistance/beta lactamase allele;
3. Increased frequency of (blaZ) allele (in s
offspring/next generation);
1. (PRSA) can be resistant to many antibiotics
1. Accept ‘most’ for
many
OR
Many antibiotics destroyed (by PRSA)
OR
2 max
Many antibiotics ineffective (against PRSA);
09.3 (2 x
AO2)
2. Bacteria would be able to synthesise cell walls
and continue reproducing;
3. New/different antibiotics needed
OR
No new antibiotics in recent years;
Correct answer, of 0.19 / 0.2 = 2 marks;;
Incorrect but shows 1.19596 and 1.3875 (accept
any correct rounding) = 1 mark 2
09.4 (2 x 17
OR AO2)
Incorrect but shows 675 ÷ (5.644 ×107 ÷ 100 000)
and 787 ÷ (5.672 ×107 ÷ 100 000) = 1 mark;
1. Weaker immune systems so more susceptible
OR
More likely to be infected as close contact with
large numbers of people; 2 max
09.5 (2 x
AO2)
2. More likely to be infected as close contact with
large numbers of people;
3. More MRSA sufferers so more likely to come
into contact with it;
How to answer it
Natural Selection & Antibiotic Resistance in Staphylococcus aureus
This question assesses fundamental biological concepts from Genetics, Variation, and Interdependence:
- Types of Natural Selection: Identifying directional selection acting against non-resistant phenotypes.
- Evolutionary Mechanism: Explaining allele frequency changes using correct selection terminology (selection pressure, differential survival, reproduction, allele inheritance).
- Bacterial Structure & Mode of Action: Applying passage details to explain why broad-spectrum resistance targeting beta-lactam rings makes bacterial infections hard to treat.
- Standardised Rate Calculations: Handling standard form (×10⁷) to calculate and compare rates per 100 000 population.
- Transmission & Susceptibility: Evaluating healthcare environments and patient vulnerability to opportunistic hospital-acquired pathogens.
Type of Natural Selection
Identifying the evolutionary mechanism acting on bacterial populations
✅ Correct Answer
Directional (selection)
💡 Key Knowledge
Directional selection occurs when environmental conditions change (e.g. introducing penicillin). Individuals with an extreme phenotype (resistance) possess a selective advantage, causing the mean of the population to shift towards that extreme over generations.
❌ Common Errors
- Writing stabilising selection (which maintains intermediate phenotypes in an unchanging environment).
- Writing disruptive selection (which favours both phenotypic extremes).
Increase in Penicillin Resistance
Explaining the rapid rise of resistant alleles by natural selection
✅ Correct Answer Structure
- Selection pressure: Penicillin/antibiotic creates a selection pressure OR kills non-resistant / susceptible S. aureus bacteria.
- Differential survival & reproduction: Resistant bacteria with the blaZ / resistance allele survive, reproduce, and pass on this allele to offspring.
- Allele frequency: Frequency of the resistant / blaZ allele increases in the population over generations.
🧠 Exam Technique: "Gene" vs "Allele"
Always write allele, not gene! The mark scheme specifically states: "Reject 'gene' for 'allele' once".
Bacteria do not "develop resistance to survive"; mutations occur randomly, and the antibiotic acts as the environmental filter selecting for pre-existing mutant alleles.
❌ Common Errors & Mark Losses
- Stating that bacteria "become immune" to antibiotics (bacteria do not have immune systems).
- Claiming that penicillin "causes" the mutation to occur.
- Failing to explicitly state that the frequency of the allele increases in the next generation.
Difficulty in Treating PRSA Infections
Application of passage details regarding beta-lactam rings
✅ Correct Answers (Any two of the following)
- PRSA is resistant to many / most antibiotics OR many antibiotics are hydrolysed / destroyed / ineffective against PRSA.
- Bacteria can still synthesise cell walls and continue to reproduce / divide.
- New or alternative antibiotics are required (which are scarce or have not been developed recently).
💡 Key Knowledge
Penicillins and related cephalosporins contain a beta-lactam ring that binds transpeptidase enzymes, preventing peptidoglycan wall cross-linking. Because the blaZ gene produces beta-lactamase, it inactivates all beta-lactam-based drugs, severely limiting treatment options.
🧠 Exam Technique
The prompt specifically references lines 5–6: "Many antibiotics have a beta-lactam ring...". This is your clue that PRSA produces an enzyme breaking this common structure, meaning resistance extends across an entire class of antibiotics, not just penicillin alone.
Rate Calculation: Increase in MRSA Cases
Standardising epidemiological data per 100 000 population
📐 Step-by-Step Calculation
Rate 2022 = (675 ÷ 56 440 000) × 100 000 = 675 ÷ 564.4 = 1.19596...
Rate 2023 = (787 ÷ 56 720 000) × 100 000 = 787 ÷ 567.2 = 1.38751...
0.19 (or 0.2)
• Correct final answer of 0.19 or 0.2 gives 2 marks automatically.
• Working showing intermediate values 1.19596 and 1.3875 scores 1 mark if final subtraction is incorrect.
❌ Common Calculation Traps
- Finding simple case increase: Merely doing 787 - 675 = 112 ignores the population change and per 100 000 conversion (scores 0).
- Power of ten errors: Misinterpreting 5.644 × 10⁷ as 5,644,000 instead of 56,440,000.
- Premature rounding: Rounding 1.196 and 1.388 too early can introduce truncation errors in tight margin questions.
Hospital Infection Vulnerability
Suggest and explain why hospitalised patients are at higher risk
✅ Correct Answer Pairs (Need Suggestion + Explanation)
Any two complete pairs from:
- Vulnerability: Patients have weaker immune systems / open wounds / surgery sites so they are more susceptible to opportunistic infection.
- Transmission: Patients are in close contact with medical staff / many visitors so they are more likely to pick up transmitted bacteria.
- Pathogen density: Higher concentration of infected patients / MRSA carriers in a confined area so greater chance of transmission.
🧠 Exam Technique: "Suggest and Explain"
Notice the command words: Suggest AND explain. A naked suggestion (e.g. "patients are sick") will score zero. You must link the condition to the outcome with connecting phrases like "so / therefore / because...".
❌ Common Errors
- Omitting the link: Writing "hospitals have ill people" without connecting to weakened immunity or increased infection risk.
- Confusing virus transmission with bacterial antibiotic resistance.
- Vague statements like "hospitals are dirty" (unacceptable scientific terminology).
Topics
Biology · Practical skills · 3.4 Genetic information, variation and relationships between organisms · 3.2 Cells · Data analysis
Question and mark scheme from the AQA AS Level Biology examination, Paper 1, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.