In 1928, Sir Alexander Fleming observed a culture of mold and discovered that the antibacterial substance was not produced by all molds, only by certain strains of Penicillium, namely, Penicillium notatum. Although he could not isolate it, he named the active substance “penicillin.”

The evidence of the first culture, which he photographed, indicated that he observed lysis, the weakening and destruction of bacteria—as in his lysozyme studies.