The MRSA bacterium that plagues many hospitals had evolved resistance to the antibiotic methicillin before the drug was even discovered.
Matthew Holden at the University of St Andrews in Scotland and his colleagues collected 209 samples of methicillin-resistant Staphylococcus aureus (MRSA) that were isolated in the United Kingdom and Denmark between 1960 and 1989. By sequencing their genomes and determining the rate at which the different strains had evolved, the researchers determined that the bacteria had acquired the methicillin-resistance gene, mecA, in the mid-1940s. Methicillin did not enter widespread clinical use in Britain until 1959, and resistance was first observed in 1960.
The researchers suggest that the use of earlier drugs such as penicillin, which are part of the same broad class of beta-lactam antibiotics, inadvertently selected for S. aureus strains carrying the mecA gene.