Lesson 6 of 6 / Infectious diseases
Antibiotics and resistance
Why can repeated antibiotic exposure leave more resistant bacteria?
In this lesson: Explain antibiotic targets and how selection increases the proportion of resistant bacteria.
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 interfere with bacterial processes, such as the synthesis of a cell wall or proteins. Depending on the drug and bacterium, this can kill bacteria or stop their growth. Viruses lack these bacterial structures and reproduce using host-cell machinery, so antibiotics do 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.
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.