Exam Tips: Antibiotics and Resistance

The word "mutation" is non-negotiable: Every mark scheme for antibiotic resistance questions includes "mutation" as a required term. Resistance does not arise from "adaptation," "evolution," or "change" alone — it specifically arises from random mutation. Without this word, you will lose marks regardless of how good the rest of your answer is.

Bacteria become resistant, not people: This distinction is explicitly tested. "The patient became resistant to the antibiotic" is wrong and would lose marks. "The bacteria in the patient became resistant" is correct. The antibiotic affected the bacterial population through natural selection; the human patient's physiology is irrelevant to resistance.

Natural selection sequence must be in order: Variation (mutation) → Selection pressure (antibiotic) → Differential survival (resistant survive) → Reproduction → Population change. If you give these steps in the wrong order or skip one, you will lose marks. Practice writing this sequence until it is automatic.

Disc diffusion — big zone means effective: A large zone of inhibition means the antibiotic diffused far from the disc and killed bacteria over a wide area. A small zone means limited effectiveness. No zone on control disc validates the experiment (the disc itself has no effect). Calculate area using pi x r squared for higher marks.

Link reduction strategies to mechanism: Each strategy reduces resistance by reducing selection pressure or slowing spread. "Complete the course" kills even partially resistant bacteria before they can dominate. "Only prescribe for bacterial infections" means fewer bacteria are exposed to antibiotics, so fewer are selected for resistance. Always explain the mechanism, not just state the strategy.

MRSA is the go-to example: MRSA (Methicillin-Resistant Staphylococcus aureus) is the AQA-favourite named example of antibiotic resistance. Mention it in extended answers to demonstrate specific knowledge. A resistant strain that now requires last-resort antibiotics — showing the real-world consequence of resistance evolution.