Lesson 6 of 6 / Infectious diseases
Antibiotics and resistance
Why can repeated antibiotic exposure leave more resistant bacteria?
I can explain antibiotic targets and how selection increases the proportion of resistant bacteria.
I can explain antibiotic action on bacteria and lack of viral targets.Syllabus
Syllabus K325, Topic 9(h). Explain antibiotic action on bacteria and lack of viral targets.
I can explain how antibiotic misuse/overuse accelerates resistance.Syllabus
Syllabus K325, Topic 9(i). Explain how antibiotic misuse/overuse accelerates resistance.
About 6 min
Make a guess. You are not marked.
You have a cold, which is caused by a virus. Will antibiotics help you recover faster?
Show the answer
Antibiotics act on bacteria, not viruses.
Antibiotics target bacterial cell walls and ribosomes. Viruses have neither, so antibiotics do nothing to them.
The key idea
Antibiotics act on susceptible bacteria. Exposure can select resistant bacteria, which survive and reproduce; the bacteria become resistant, not the person.
Before exposure: this illustrative population contains susceptible bacteria (S) and one resistant bacterium (R). Variation exists before this selection event.
S / tan: susceptible to this antibiotic. R / green: resistant. Original simplified model; real survival and resistance patterns vary. Resetting the stage only revisits the diagram.
A virus lacks the bacterial targets of antibiotics. That is different from a bacterium carrying resistance. Unnecessary antibiotic use can select resistant bacteria without treating a viral infection.
Explanation
Antibiotics interfere with bacterial processes, such as making the bacterial cell wall or making proteins on bacterial ribosomes. Viruses have no cell wall and no ribosomes of their own, and they reproduce inside host cells using the host cell's ribosomes and enzymes, so an antibiotic has nothing in a virus to act on. Depending on the drug and bacterium, an antibiotic can kill bacteria or stop their growth, but it does not treat viral infections such as influenza.
A bacterial population can contain variation. Some bacteria may have a genetic change that makes them resistant to a particular antibiotic. Resistance can also spread between bacteria through transferred genetic material. A diagram showing existing variation is a model of selection, not a claim that every population starts with the same resistant fraction.
During antibiotic exposure, susceptible bacteria are more likely to be killed or inhibited. Resistant bacteria survive more successfully. They reproduce and pass on resistance, so the resistant proportion can increase. The antibiotic does not make all bacteria deliberately adapt because they need to survive.
Misuse and overuse add unnecessary selection pressure and can accelerate the emergence and spread of resistance. Antibiotics should be used according to the relevant prescription, rather than shared or used for viral illness. An antibiotic being ineffective against a virus is different from a bacterium being resistant to that antibiotic. A common misuse is not completing the full course: stopping when the symptoms go leaves the more resistant bacteria alive, and they multiply. Other misuses are taking antibiotics for viral infections such as colds and flu, and sharing leftover antibiotics.
Step by step
- 1
Start with variation
Identify susceptible and resistant bacteria before the selection event in the model.
- 2
Explain survival
The antibiotic removes or inhibits susceptible bacteria more strongly.
- 3
Explain the population change
Survivors reproduce, increasing the proportion carrying resistance.
Worked example
A changing proportion
In an illustrative model, 9 of 10 bacteria are susceptible and 1 is resistant. The susceptible bacteria are removed; the resistant bacterium reproduces. Why does resistance become more common?
One way to explain it
The antibiotic selects for the resistant bacterium by removing susceptible competitors. Its descendants inherit resistance, so resistant bacteria make up a larger proportion of the later population.
Why this answer works
- Resistance is present before this selection event.
- Explain unequal survival.
- Compare proportions, not only total bacterial numbers.
Watch out for this
A student says: "Taking antibiotics makes the person resistant, so all antibiotics stop working." What is wrong with this?
Show the answer
Resistance is a property of bacteria to particular antibiotics. Antibiotic use selects bacteria; it does not make the person an antibiotic-resistant organism.
Check your understanding
Which explanation accounts for resistance increasing after antibiotic exposure?
Show the answer
Resistant bacteria survive more successfully and reproduce, passing on resistance.
Differential survival and reproduction change the bacterial population.
Exam-style questions
Check your understanding
Invented data: in a hospital, 2% of one bacterial species was resistant to an antibiotic in 2015 and 30% in 2025, after heavy use. Which is the best explanation?
Show the answer
The antibiotic killed non-resistant bacteria, so resistant ones reproduced more
Resistant bacteria survived each course and passed on resistance, so their share rose.
Quick recall
Why can repeated antibiotic exposure leave more resistant bacteria?