Which disease-causing organism became resistant to methicillin soon after the antibiotic was introduced in 1959?

Staphylococcus aureus became resistant to methicillin soon after the antibiotic was introduced in 1959.

Methicillin-resistant Staphylococcus aureus, commonly abbreviated MRSA, was first reported in 1960, only about a year after methicillin entered clinical use. Methicillin had been developed to treat staphylococci that were resistant to penicillin, but bacterial populations quickly included strains able to survive the newer drug.

Resistance evolves because antibiotic exposure creates strong selection: susceptible bacteria are killed or inhibited, while bacteria with resistance mechanisms survive and reproduce. In S. aureus, the mecA gene produces an altered penicillin-binding protein that has low affinity for methicillin and related beta-lactam antibiotics.

Antibiotics do not cause bacteria to deliberately adapt. Random genetic changes and gene acquisition create variation, and treatment changes which variants leave descendants. MRSA can spread in hospitals, communities, and livestock settings, making infection prevention and appropriate antibiotic use important public-health measures.