This is Scientific American's 60 Second Science.

I'm Christopher Intagiata.

Perhaps the chief poster child of antibiotic resistance

is methicillin-resistant Staphylococcus oreus, or MRSA.

The bacterium's impervious to a suite of antibiotics and it 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 was around that then forced the strains to pick up these mechanisms.

Matthew Holden, a molecular microbiologist at the University of St Andrews in Scotland.

Holden and his team analyzed the genomes of freeze-dried strains of MRSA bacteria

from the 1960s to 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 history as what they found was that the staff bacteria

seemed 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 UK in 1959.

...