Figure 1. Alexander Fleming's own photograph of a penicillin mold. The staphylococci around the mold were obviously destroyed. (Courtesy of the Alexander Fleming Museum, London, UK.)

Figure 2. Structure of penicillin G. Enzymatic cleavage of penicillin G (green arrow) yields 6-aminopenicillanic acid, which can be linked to residues to give ampicillin or methicillin. Hydrolysis of penicillin by (beta-) β-lactamase (black arrow) leads to products with no antibiotic activity. (Diagram: Anne Kemmling, Goettingen, Germany.)

Figure 3. Antibiotic susceptibility tests. (a) Different strains are streaked on an agar plate, then the antibiotic to be tested is placed in the central hole. Strains E and F are insensitive, strains A and B are most sensitive. (b) Disks impregnated with different antibiotics are placed on an agar plate seeded with a certain bacterial species and incubated overnight. Result: the antibiotic on disk b exhibits the highest inhibitory effect. There is no inhibition by the antibiotic on disk d. (Drawing: Anne Kemmling, Goettingen, Germany.)