Methicillin resistantStaphylococcus aureusemerged long before the introduction of methicillin in to clinical practice

The spread of drug-resistant bacterial pathogens pose a major threat to global health. It is widely recognised that the widespread use of antibiotics has generated selective pressures that have driven the emergence of resistant strains. Methicillin-resistantStaphylococcus aureus(MRSA) was first observed in 1960, less than one year after the introduction of this second generation β-lactam antibiotic into clinical practice. Epidemiological evidence has always suggested that resistance arose around this period, when themecAgene encoding methicillin resistance carried on an SCCmecelement, was horizontally transferred to an intrinsically sensitive strain ofS. aureus. Whole genome sequencing a collection of the very first MRSA isolates allowed us to reconstruct the evolutionary history of the archetypal MRSA. Bayesian phylogenetic reconstruction was applied to infer the time point at which this early MRSA lineage arose and when SCCmecwas acquired. MRSA emerged in the mid 1940s, following the acquisition of an ancestral type I SCCmecelement, some fourteen years prior to the first therapeutic use of methicillin. Methicillin use was not the original driving factor in the evolution of MRSA as previously thought. Rather it was the widespread use of first generation β-lactams such as penicillin in the years prior to the introduction of methicillin, which selected forS. aureusstrains carrying themecAdeterminant. Crucially this highlights how new drugs, introduced to circumvent known resistance mechanisms, can be rendered ineffective by unrecognised adaptations in the bacterial population due to the historic selective landscape created by the widespread use of other antibiotics.