With the discovery of penicillin, the first naturally-occurring antibiotic, in 1928, and its mass production in the early 1940s, infections with Staphylococcus aureus were mostly treatable, without any major complications. However, clinicians soon observed the emergence of penicillin-resistant strains of S. aureus, which was largely driven by bacterial expression of beta-lactamases. Methicillin, a novel penicillin analogue that was resistant to beta-lactamases, was introduced in 1959, and was initially effective against penicillin-resistant S. aureus strains. However, this success was short-lived, as the first methicillin-resistant S. aureus (MRSA) strain had been identified in the lab by 1961, and cases of MRSA were first observed in the clinic in 1968. Initially, MRSA strains only occurred in small and local outbreaks across the US and Europe, but they can now be found around the world.