Edexcel IGCSE Biology · Spec 3.39
Antibiotic Resistance
How antibiotic resistance develops in bacteria through natural selection, and how to reduce it.
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Antibiotic Resistance
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Explained
Antibiotic resistance is natural selection, happening quickly
Nothing new is invented when bacteria become resistant. A chance mutation makes one bacterium resistant before the antibiotic arrives. The antibiotic then kills the others, that one survives, and it reproduces. Because bacteria divide so rapidly, what would take thousands of years in a larger organism happens in days.
The sequence, in the order that earns marks
- A random mutation occurs in the DNA of one bacterium, making it resistant.
- The antibiotic is used, and kills the non resistant bacteria.
- The resistant bacterium survives, because the antibiotic has no effect on it.
- It reproduces, passing the resistance to its offspring.
- Over time the resistant strain becomes the common one.
The order matters. The mutation comes first, by chance, and the antibiotic then acts as the selection pressure. It does not cause the mutation.
The mistake that changes the whole meaning
Bacteria do not become resistant because they were exposed to an antibiotic and adapted to it. Individual bacteria do not change. What changes is the proportion of resistant ones in the population, because the others have been killed.
Saying that bacteria "learn to fight the antibiotic" or "get used to it" describes something that does not happen, and it is treated as wrong rather than as loose wording.
What examiners say about this topic
Principal examiner reports for Edexcel International GCSE Biology record a specific confusion here. A few candidates confused resistance with immunity, and incorrectly referred to bacteria becoming immune to antibiotics and producing antibodies.
Bacteria do not have an immune system and do not make antibodies. Antibodies are produced by your white blood cells against bacteria. Using immunity language in this answer does not simply lose style marks, it describes the wrong organism doing the wrong thing.
The reports also describe a full mark answer as containing the ideas of mutation, survival and reproduction. Three ideas, three marks. If a question here is worth three, check that all three are present before moving on.
Slowing it down
Doctors should prescribe antibiotics only when they are needed, and not for viral infections, since antibiotics have no effect on viruses at all.
Patients should finish the full course. Stopping early leaves behind the bacteria that were hardest to kill, which are precisely the ones you would least want to survive and reproduce.
Agricultural use matters too, because resistance developing in animals can spread to bacteria that infect people. That point is often the one that lifts an answer from adequate to complete.
MRSA
A superbug is a bacterium resistant to many antibiotics, and MRSA is the example to name. It is difficult to treat precisely because the usual options have already been selected against, which makes it a practical demonstration of everything above.
Spec 3.39
What you need to know
- Explain how antibiotic resistance develops
- Link it to natural selection
- Describe how to slow it down
Active recall
Quick check
Answer each question before opening the answer.
How do bacteria become resistant to an antibiotic?
A random mutation makes some bacteria resistant; the antibiotic kills the non-resistant ones, so the resistant survive and reproduce (natural selection).
How can we slow the spread of antibiotic resistance?
Only use antibiotics when necessary and always finish the full course.
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