What bacterium did Fleming name Micrococcus lysodeikticus?

The bacterium Fleming named Micrococcus lysodeikticus is now known as Micrococcus luteus. The older name refers to the organism's striking susceptibility to lysozyme, the antibacterial enzyme Fleming discovered in 1922.

Fleming noticed the bacterium while investigating nasal secretions from a patient with a cold. The organism formed small colonies and was rapidly broken down, or lysed, when exposed to material in the secretion. He concluded that an enzyme was responsible and coined “lysozyme” for it. He used the bacterium as a particularly useful test organism for studying that activity.

Micrococcus lysodeikticus remained in scientific use for decades, but taxonomic work reassigned the organism to the existing species Micrococcus luteus in 1972. The name luteus means “yellow” in Latin, referring to the bright yellow colonies the bacterium typically produces.

This is not the same bacterium as Staphylococcus aureus, the organism associated with Fleming's later penicillin discovery. The two episodes involved different experiments and different bacterial contexts.