Which bacterium causing gonorrhoea did Fleming find was affected by penicillin?

The bacterium causing gonorrhoea that Fleming found was affected by penicillin was Neisseria gonorrhoeae.

Fleming tested penicillin’s activity against numerous organisms after observing that a Penicillium mould inhibited staphylococci. Penicillin worked especially well against many Gram-positive bacteria, but Fleming also recorded activity against N. gonorrhoeae, an important Gram-negative exception.

The finding mattered because gonorrhoea is caused by a specific bacterium, not by a virus or fungus. Neisseria gonorrhoeae is a diplococcus, meaning its cells typically appear in pairs under a microscope. The species name is the canonical scientific answer; “the gonococcus” is a common shorthand.

A modern complication is antibiotic resistance. Penicillin was once an effective treatment for gonorrhoea, but resistant strains became widespread, so contemporary therapy relies on other antibiotics and public-health guidance rather than Fleming’s original drug.