Beta-lactam antibiotics (penicillin, methicillin, oxacillin) kill bacteria by binding the active site of penicillin-binding proteins (PBPs) — transpeptidase enzymes that crosslink peptide bridges between glycan strands to build the peptidoglycan sacculus, the mesh that keeps the cell from bursting under osmotic pressure.
MRSA carries the mobile genetic element SCCmec with the mecA gene, encoding an alternative transpeptidase, PBP2a, whose active site has drastically lower affinity for beta-lactams. Per exposure cycle, the probability a given PBP is knocked out by the drug is modelled as:
P(inhibit) = 1 − e^(−k · C)
k(normal PBP) ≈ 0.8 (high affinity)
k(PBP2a) ≈ 0.0008 (~1000× lower affinity)
C = antibiotic concentration, in multiples of the MIC
Wall integrity evolves from the balance between PBP-driven crosslink synthesis and constant autolysin-mediated remodelling:
crosslink fraction = (1−r)·[1−P(inhibit)_normal] + r·[1−P(inhibit)_PBP2a]
dIntegrity/dt = k_synth · crosslink_fraction − k_autolysin
r = mecA / PBP2a expression fraction
At high antibiotic concentration with r = 0 (MSSA), nearly every normal PBP is inhibited, crosslink fraction collapses, autolysins outpace synthesis, and the wall integrity falls to zero — lysis. With r near 1 (MRSA), most transpeptidase activity runs through PBP2a, which the drug barely touches, so crosslinking continues and the cell survives even at concentrations many multiples above the MIC for a susceptible strain. This is exactly why methicillin/oxacillin — and every other beta-lactam except the newest anti-MRSA cephalosporins — fail against MRSA, and why vancomycin (a different, cell-wall-precursor-binding mechanism) became the historical fallback.
- Concentration slider — beta-lactam dose in multiples of the MIC for a fully susceptible strain.
- mecA / PBP2a slider — fraction of transpeptidase activity running through the resistant PBP2a instead of native PBPs (heteroresistant populations sit between 0 and 100%).
- Presets — MSSA loads r = 0%, MRSA loads r = 95%.
- Green vertical rods are intact peptide crosslinks; red/dim rods are sites currently uninhibited-by-drug but not yet crosslinked, or missing entirely under strong inhibition. Small tumbling particles are antibiotic molecules, scaled to concentration.