Lesson 5 of 5 / Infectious diseases in humans
Antibiotics and resistance
Why can repeated antibiotic exposure leave more resistant bacteria?
In this lesson: State that antibiotics act against bacteria, not viruses, and explain how misuse and overuse favour resistance.
About 6 min
The key ideaAntibiotics 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 can kill susceptible bacteria; some instead inhibit their growth. They are ineffective against viruses, so an antibiotic does not treat the influenza virus. This distinction depends on the pathogen causing an illness, not on whether two illnesses share a symptom such as a cough.
A bacterial population can contain bacteria that differ in susceptibility to a particular antibiotic. Resistant bacteria can survive an exposure that kills susceptible bacteria. Not all bacteria start with the same resistance, and resistance to one antibiotic does not automatically mean resistance to every antibiotic.
During exposure, resistant bacteria survive more successfully. They reproduce and pass resistance to their descendants, so the resistant proportion of the population can increase. The antibiotic does not make all bacteria choose to become resistant.
Misuse and overuse create unnecessary opportunities for this selection and can accelerate the emergence and spread of resistant bacteria. Using antibiotics for viral infections can expose other bacteria to selection without treating the virus. Antibiotics should be used according to the relevant prescription, rather than shared or used for a viral illness.
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.
Is this true? "Taking antibiotics makes the person resistant, so all antibiotics stop working."
Resistance is a property of bacteria to particular antibiotics. Antibiotic use selects bacteria; it does not make the person an antibiotic-resistant organism.