Fleming noted penicillin's usefulness for isolating which influenza-linked bacillus?

Fleming noted penicillin’s usefulness for isolating Pfeiffer’s bacillus, the bacterium historically associated with influenza.

Alexander Fleming presented his findings in February 1929 in a talk titled “A medium for the isolation of Pfeiffer’s bacillus.” Penicillin inhibited many unwanted bacteria in a culture while allowing penicillin-resistant organisms to remain, making it useful as a selective laboratory tool.

“Pfeiffer’s bacillus” was the older name for Haemophilus influenzae, named after its association with influenza during an era when the disease’s viral cause was not yet understood. Fleming’s own sinus infection experiments failed partly because H. influenzae is not susceptible to penicillin.

A common mix-up is to treat Fleming’s discovery as an immediately successful medicine. His 1929 work mainly described antibacterial activity and laboratory applications; the Oxford team led by Howard Florey and Ernst Chain later developed methods to purify and test penicillin therapeutically.