What color was the mould Fleming found contaminating his culture plate?

The mould contaminating Fleming’s culture plate was blue-green. Its striking colour helped distinguish the fungal growth from the bacterial colonies Fleming had been studying.

When Alexander Fleming returned to his laboratory at St Mary’s Hospital in September 1928, he and Daniel Merlin Pryce found one plate with an open lid and a blue-green mould colony. Around the mould was a clear zone where the Staphylococcus aureus bacteria had failed to grow. Farther away, the bacterial colonies appeared normal.

The colour alone did not prove that the mould was producing an antibiotic. Fleming photographed the plate, isolated a sample, and grew the organism in fresh cultures. Tests showed that the mould released a substance active against several Gram-positive bacteria, including staphylococci and streptococci. Fleming eventually called that substance penicillin.

Descriptions of the fungus have changed as mycologists reclassified it. It was initially assigned names including Penicillium rubrum and Penicillium notatum; modern work identifies Fleming’s strain as Penicillium rubens. Thus, “blue-green” describes the visible mould colony, not the chemical colour of penicillin itself.