Lesson 5 of 5 / Infectious diseases in humans
Antibiotics and resistance
Why can repeated antibiotic exposure leave more resistant bacteria?
I can state that antibiotics act against bacteria, not viruses, and explain how misuse and overuse favour resistance.
I can state that antibiotics kill bacteria and are ineffective against viruses.Syllabus
Syllabus K224 / K225, Topic 7(f). State that antibiotics kill bacteria and are ineffective against viruses.
I can explain how misuse and overuse can accelerate antibiotic resistance.Syllabus
Syllabus K224 / K225, Topic 7(g). Explain how misuse and overuse can accelerate antibiotic 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 kill bacteria or stop them growing. They act on parts that only bacteria have, such as the bacterial cell wall and bacterial ribosomes. Viruses have no cell wall and no ribosomes of their own; they multiply inside our cells. So antibiotics do not work against viruses such as flu.
In a population of bacteria, some may be more resistant to an antibiotic than others. Resistant bacteria can survive a dose that kills the others.
The resistant bacteria that survive multiply and pass resistance on to their offspring. So, over time, more and more of the population is resistant. The antibiotic does not make bacteria become resistant; it kills the others and leaves the resistant ones.
Using antibiotics wrongly speeds this up. Taking antibiotics for a virus kills no virus but gives resistant bacteria an advantage. Stopping the course early, when you feel better, leaves the more resistant bacteria alive to multiply. Take antibiotics exactly as prescribed, finish the full course, and never share them.
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
What best explains resistance becoming more common after antibiotic exposure?
Show the answer
Resistant bacteria survive more successfully and reproduce.
Unequal survival and subsequent reproduction increase the resistant proportion.
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?