Q.How was penicillin discovered?

Penicillin was discovered by Alexander Fleming in 1928 through a chance observation. While studying the bacterium Staphylococcus, Fleming noticed that a mould, identified as Penicillium notatum, had contaminated one of his culture plates. Around this mould, the bacterial colonies had been killed — a clear zone of inhibition was visible. …

The story of penicillin begins not in a laboratory full of gleaming equipment, but in a cluttered, untidy workspace at St. Mary’s Hospital in London. In 1928, Dr. Alexander Fleming, a Scottish bacteriologist, was studying the bacteria Staphylococcus aureus. He had a habit of leaving stacks of petri dishes on his bench before going on holiday — a practice that would change medicine forever.

When Fleming returned from his vacation, he began sorting through the old dishes. Many were contaminated with mould, which was common enough. But one dish caught his eye. Around a patch of greenish mould, the Staphylococcus colonies had turned transparent and appeared to be dissolving. The mould was creating a clear, bacteria-free zone around itself. Fleming did not throw the dish away. Instead, he recognised that something in the mould was actively killing the bacteria.

Fleming identified the mould as belonging to the genus Penicillium (later named Penicillium notatum). He grew more of it in a broth and found that the liquid, which he called “mould juice,” could kill a wide range of disease-causing bacteria, even when diluted many times. He published his findings in 1929, but he could not purify the active substance — which he named penicillin — or keep it stable long enough for use in humans. For over a decade, penicillin remained a laboratory curiosity.

A chronological milestone table, rather than a single narrative.