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This is scientific American 60 second science, I’m Christopher Intagliata.

Perhaps the chief poster child of antibiotic resistance is methicillin-resistant Staphylococcus aureus, or MRSA. The bacterium is impervious to a suite of antibiotics, and can cause blood infections, pneumonia…even death.

And you’d assume that it developed its namesake resistance to methicillin…by being exposed to methicillin.

But that doesn’t seem to be the case. Instead, the culprit for resistance appears to be an earlier, and chemically related antibiotic: penicillin.

“We think it’s a very early use of penicillin that forced the strains to pick up these mechanisms.” Matthew Holden, a molecular microbiologist at the University of Saint Andrews in Scotland.

Holden and his team analyzed the genomes of freeze-dried strains of MRSA bacteria, from the 1960s through the ’80s.

“In effect what we were doing was sort of genomic archaeology, in looking at the genomes, and comparing the variation and using that information to effectively reconstruct the evolutionary histories.”

What they found was that the Staph bacteria seem to have acquired the methicillin-resistance gene in the mid-1940s—about 15 years before methicillin even hit the market. And they determined that it was the widespread use of penicillin that led to that adaptation.

The results are in the journal Genome Biology.

Methicillin was introduced in the U.K. in 1959. Less than a year later, resistance was first reported—resistance that it appears now was already baked into the staph strains.

Looking ahead, Holden says we’d do well to vigilantly monitor the genetics of circulating strains—to find out which bugs may be armed to battle our newest antibiotics as soon as they’re developed.

Thanks for listening for scientific American 60 second science, I’m Christopher Intagliata.