Antimicrobial resistance in bacteria is natural selection that you can follow step by step. Some bacteria happen to carry a resistance allele. When an antibiotic is present, non-resistant bacteria die and resistant ones survive, reproduce and pass on the allele, so the population becomes mainly resistant.
This lesson is science content only. It does not advise anyone about medicines, and health questions belong with a doctor or pharmacist. It belongs to disease and defence concepts and uses the same logic as variation and selection.
How do you explain it as natural selection?
Use five linked steps, in this order:
- Variation: a population of bacteria contains differences, and a few have a resistance allele that arose by mutation.
- Selection pressure: the antibiotic is present.
- Survival: non-resistant bacteria are killed, but resistant bacteria survive.
- Reproduction: the survivors reproduce and pass on the allele.
- Change: the next generations contain a higher proportion of resistant bacteria.
Each step needs its own sentence. If you skip step 1 or step 4, the explanation is incomplete.
Worked example
A culture starts with 1000 bacteria, and 2 of them carry a resistance allele. The numbers are invented for practice.
Question: An antibiotic kills every non-resistant bacterium. The survivors each divide once every hour. How many bacteria are there after 10 hours, and why are they all resistant?
Step 1, survivors: 1000 − 998 non-resistant = 2 survivors. Only the 2 resistant bacteria remain.
Step 2, doubling: 10 hours means 10 doublings, so multiply by 2 ten times. 2¹⁰ = 1024.
Step 3, total: 2 × 1024 = 2048. Check: 1024 × 2 = 2048.
Step 4, explain: every bacterium is a descendant of the two survivors, so each carries the resistance allele.
Answer: 2048 bacteria, all resistant, because only resistant bacteria survived and reproduced.
The mistake to watch for
A common slip is to say the individual bacteria changed in response to the drug.
Mistaken answer: “The bacteria got used to the antibiotic and became resistant.”
The student described a single organism adapting, which is not how selection works.
The correction is to say the resistance allele was already present in some bacteria by chance, and the antibiotic selected for them. The population changed because resistant bacteria survived and reproduced. No single bacterium decided or learned anything.
Check yourself
Try these, then open each answer.
1. Why does an antibiotic not work on a virus?
Show answer
Antibiotics act on bacteria, and viruses are not bacteria. So an antibiotic does not act against a viral infection.
2. A culture has 4 surviving resistant bacteria that double every hour. How many after 5 hours?
Show answer
5 doublings: 2⁵ = 32. So 4 × 32 = 128 bacteria. Check: 4 → 8 → 16 → 32 → 64 → 128.
3. Write the word that links mutation, antibiotic and survival in one phrase.
Show answer
Natural selection. Mutation provides the resistance allele, the antibiotic is the selection pressure, and the resistant bacteria survive and reproduce.
Where this leads next
Return to the topic overview to check the whole route, then test yourself with the mixed practice set. The idea here connects to variation and selection later in the course.
Students who find the selection argument hard to write fluently can practise it in online one-to-one Biology tuition.