Here is the full answer:

The short answer: S. aureus already beat penicillin years before MRSA was identified. Methicillin was specifically invented to replace penicillin once resistance to it became widespread - so when resistance to that drug emerged, the name stuck.

1940s-1950s - Penicillin resistance comes first S. aureus became resistant to penicillin rapidly after penicillin entered clinical use in the 1940s. The resistance mechanism was simple: the bacteria acquired a beta-lactamase (penicillinase) enzyme that breaks the beta-lactam ring of penicillin, destroying it before it can act. By the late 1950s, penicillin-resistant S. aureus was rampant in hospitals.

1959 - Methicillin is invented specifically to fight penicillin-resistant staph Methicillin was a semi-synthetic, beta-lactamase-stable penicillin. It was engineered with bulky side chains that shielded its beta-lactam ring from being cleaved by the penicillinase enzymes. It was the drug that actually worked against penicillin-resistant S. aureus.

1961 - MRSA is born, and named Just two years after methicillin's introduction, British professor Patricia Jevons at Colindale Laboratories identified strains of S. aureus that were now resistant even to methicillin. This was the new, more alarming level of resistance - so researchers named it after the drug it had just defeated: methicillin-resistant S. aureus.

By that point, calling it "penicillin-resistant S. aureus" would have been unremarkable - nearly all hospital S. aureus was already penicillin-resistant. The discovery worth naming was resistance to this newer, supposedly penicillinase-proof drug.

This is why MRSA is so clinically significant. Penicillin resistance is just enzyme-mediated destruction. MRSA resistance via PBP2a confers resistance to virtually all beta-lactams - penicillins, cephalosporins, carbapenems - because the altered target has very low affinity for the entire drug class.

Methicillin was withdrawn from clinical use in the 1980s (it caused interstitial nephritis), replaced by oxacillin and flucloxacillin. Technically, labs now test with oxacillin or detect the mecA gene directly. Some call it oxacillin-resistant S. aureus (ORSA), but MRSA had become so entrenched in clinical and public language that it never changed - it is a historical artifact of how the strain was first named in 1961.