Resistance doesn't switch off when the drug stops
- The idea
- Predict
- See it
- Why
- Check
The idea
“Once the antibiotic is gone, the bacteria don't need their resistance, so they switch back.”
Taking the drug away sounds like it should undo what the drug did. Resistance is a change in the DNA, though, and a cell doesn't lose it just because the drug is gone.
The short version
The mistake: Once the antibiotic is gone, the bacteria don't need their resistance, so they switch back.
What's actually true: Resistant cells stay resistant and pass it on. Their share falls only if resistance has a cost, and then gradually.
- Resistance comes from a mutation, a change in the DNA. A resistant cell passes it to its offspring whether the drug is around or not.
- Removing the drug removes the selection that favored resistance. Nothing makes a cell undo its mutation.
- Resistance often has a fitness cost, like slower growth. Then sensitive cells slowly outgrow resistant ones, over many generations.
- Resistant cells can also pick up new mutations that shrink the cost. Then resistance can stay common for a long time.
On the exam: Ask what resistance costs. No cost means it stays. A cost means a slow fade over many generations.