A LITTLE LIGHTBULB MOMENT
Antimicrobial mechanisms and resistance
MRSA uses an altered PBP
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The wrench changes shape and ignores the drug.
The idea behind the cue
MRSA commonly carries mecA, which encodes PBP2a with low affinity for many beta-lactams. The drug still reaches the bacterium, but the target has changed enough that binding is poor and cell-wall synthesis continues.
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Short & simple
The wrench changes shape and ignores the drug.
Acronym
MPD — Mischievous Pancakes Daydream.
Story
A locksmith swaps the old lock for a new one-shaped wrong. The antibiotic key no longer fits, so the wall crew keeps working even while the medicine is present.
Connect the pieces
- M (Mischievous) → mecA: MRSA commonly carries mecA, which encodes PBP2a.
- P (Pancakes) → PBP2a: PBP2a has low affinity for many beta-lactams.
- D (Daydream) → Drug-entry: The drug can still enter, but the target has changed enough that cell-wall synthesis continues.
- Mischievous · mecA: MRSA commonly carries mecA, which encodes PBP2a.
- Pancakes · PBP2a: PBP2a has low affinity for many beta-lactams.
- Daydream · Drug-entry: The drug can still enter, but the target has changed enough that cell-wall synthesis continues.
Know the limits
This explains methicillin resistance in staphylococci; other organisms may use different PBP changes.
Further reading · OpenStax Pharmacology for Nurses · foundational mechanisms ↗Check your understanding
What is the classic mechanism of methicillin resistance in Staphylococcus aureus?
A. Conversion of folate to tetrahydrofolate
B. Efflux of the drug from the outer membrane
C. Blocking bacterial RNA polymerase
D. Production of an altered penicillin-binding protein
Reveal explanation
Production of an altered penicillin-binding protein
MRSA resistance is classically due to mecA-encoded PBP2a with reduced affinity for beta-lactams.