On the Antibacterial Action of Cultures of a Penicillium, with Special Reference to their Use in the Isolation of B. influenzæ
Fleming reported that a mould of the genus Penicillium, which had contaminated a culture plate, produced a diffusible substance—which he named 'penicillin'—that inhibited the growth of many common pathogenic bacteria. He characterized the antibacterial spectrum and potency of the crude culture filtrate, showing it was strongly active against Gram-positive organisms (such as staphylococci and streptococci) while sparing certain Gram-negative bacilli. He proposed using the agent both as a selective culture medium for isolating Bacillus influenzae and as a relatively non-toxic antiseptic.