What was Alexander Fleming’s key observation that led to the discovery of penicillin?

A mould (Penicillium) inhibiting Staphylococci growth

The correct answer is Option (2) - A mould (Penicillium) inhibiting Staphylococci growth

Antibiotics produced by microbes are regarded as one of the most significant discoveries of the twentieth century and have greatly contributed towards the welfare of the human society. Anti is a Greek word that means ‘against’, and bio means ‘life’, together they mean ‘against life’ (in the context of disease causing organisms); whereas with reference to human beings, they are ‘pro life’ and not against.

Antibiotics are chemical substances, which are produced by some microbes and can kill or retard the growth of other (disease-causing) microbes. Penicillin was the first antibiotic to be discovered, and it was a chance discovery. Alexander Fleming while working on Staphylococci bacteria, once observed a mould growing in one of his unwashed culture plates around which Staphylococci could not grow.

He found out that it was due to a chemical produced by the mould and he named it Penicillin after the mould Penicillium notatum. However, its full potential as an effective antibiotic was established much later by Ernest Chain and Howard Florey.

This antibiotic was extensively used to treat American soldiers wounded in World War II. Fleming, Chain and Florey were awarded the Nobel Prize in 1945, for this discovery.