Figure 3.

Chemical structures of different classes of penicillins. Examples of different generations of penicillins are shown. Beta-lactam ring, the common feature of all classes, is highlighted in brown and the corresponding chemical substitute on the side chain is color-coded: blue, penicillin G (benzylpenicillin class, 1st generation); yellow, methicillin (2nd generation); green, ampicillin (aminopenicillin class, 3rd generation); orange, carbenicillin (carboxypenicillin class, 4th generation); purple, azlocillin (ureidopenicillin class, 4th generation).