Drugs, antibiotics and resistance
How antibiotics work, and why resistance develops.
Review these first
Learning objectives
What you need to be able to do
Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.
- 15.1.1Define a drug, describe the use of antibiotics, and explain the development of resistant bacteria.
5 minute read
Drugs, antibiotics and resistance
A drug is any substance taken into the body that modifies or affects chemical reactions in the body.
Antibiotics
Antibiotics kill or stop the growth of bacteria. They have no effect whatsoever on viruses, because viruses reproduce inside your own cells and lack the bacterial structures antibiotics attack. Taking antibiotics for a cold or flu is useless.
How resistance develops
This is a direct example of natural selection, and the sequence matters:
- Within a bacterial population there is variation — by chance, a few bacteria carry a mutation making them resistant.
- When antibiotic is used, the non-resistant bacteria are killed.
- The resistant ones survive and reproduce, passing on the resistance allele.
- Over time the whole population becomes resistant, and the antibiotic no longer works.
The crucial point: the antibiotic does not create resistance. The mutation was already there by chance; the antibiotic merely selects for it.
Reducing the problem
Only prescribe antibiotics when genuinely needed, complete the full course even after feeling better (so surviving bacteria are not left to breed), and avoid routine use in farming.
Misused drugs
Alcohol is a depressant: it slows reaction times, impairs judgement and self-control, and long-term use damages the liver. Heroin is highly addictive, and injecting with shared needles risks transmitting HIV.
Think of it like this
Antibiotic resistance is a sieve, not a factory. The antibiotic does not manufacture resistant bacteria — it removes everything that is not resistant, leaving the rare survivors to fill the space.
Worked examples
Method, step by step
Explain how a population of bacteria can become resistant to an antibiotic.
- 1Random mutation means a small number of bacteria in the population are already resistant.
- 2When the antibiotic is used, the non-resistant bacteria are killed.
- 3The resistant bacteria survive and reproduce, passing on the resistance allele.
- 4Over generations the proportion of resistant bacteria increases until the antibiotic is ineffective.
Random mutation produces a few resistant bacteria. The antibiotic kills the rest, so only the resistant survive and reproduce, and resistance spreads through the population by natural selection.
Common misconceptions
- Thinking bacteria "become resistant because of" the antibiotic. Mutation happens randomly beforehand; the antibiotic only selects for survivors.
- Believing antibiotics treat viral infections. They do not — this misunderstanding is a major driver of resistance.
- Thinking you can stop the course once you feel better. Stopping early leaves the hardier bacteria alive to multiply.
In the exam
- Answer resistance questions with the four-step natural-selection sequence: variation → selection pressure → survival and reproduction → allele frequency increases.
- Never write that bacteria "want" or "try" to become resistant. Mutation is random, and describing it as purposeful loses credit.