Which bacterium was Alexander Fleming studying when he noticed penicillin’s antibacterial effect?

Staphylococcus aureus was the bacterium Alexander Fleming was studying when he noticed penicillin’s antibacterial effect.

In 1928, Alexander Fleming observed that a mold contaminant on a culture plate had created a clear zone where nearby S. aureus cells failed to grow. He identified the mold as a Penicillium species and called the antibacterial substance penicillin. The observation became a foundation of modern antibiotic medicine.

S. aureus is a Gram-positive, round bacterium that commonly grows in grape-like clusters. It can live harmlessly on skin or in the nose, but some strains cause wound infections, pneumonia, bloodstream infections, and other disease. The bacterium’s ability to develop antibiotic resistance has made strains such as MRSA an important public-health concern.

Fleming did not discover that S. aureus causes every infection associated with penicillin. He observed its inhibition in culture. Penicillin development into a widely usable medicine required later work by researchers including Howard Florey, Ernst Chain, and their colleagues.