Q.Describe how Alexander Fleming discovered penicillin, and name the fungus he observed producing an inhibitory substance around a bacterial colony.

[!TLDR] Fleming noticed Penicillium notatum mould inhibiting Staphylococcus growth on a contaminated culture plate in 1928, and named the substance penicillin.

[!ANSWER] Alexander Fleming observed that a Staphylococcus culture plate accidentally contaminated with the mould Penicillium notatum showed a clear zone of inhibited bacterial growth around the mould colony, leading him to identify and name penicillin.

In 1928, Alexander Fleming, while working with culture plates of the bacterium Staphylococcus, noticed that one plate had become accidentally contaminated with a mould. Around this mould colony, he observed a clear zone in which the surrounding bacterial growth had been inhibited -- the bacteria simply were not growing in the area close to the mould. Fleming identified the contaminating fungus as Penicillium notatum, and correctly reasoned that it must be releasing some substance into its surroundings capable of killing or inhibiting the nearby bacteria; he named this substance penicillin. This accidental but carefully observed finding became the starting point for the entire field of antibiotics, though Fleming himself was unable to purify or mass-produce the compound.

[!ANSWER] Fleming discovered penicillin in 1928 when he noticed Penicillium notatum mould inhibiting Staphylococcus growth on an accidentally contaminated culture plate.

Describe the discovery as the sequence it actually was: accidental contamination → observed inhibition zone → correct inference of a bacteria-inhibiting substance → naming it penicillin.

Students often say Fleming 'invented' or 'produced' penicillin -- he only discovered/observed the phenomenon and named the substance; purification and mass production came later, through Chain and Florey.