A lactam is a chemical ring. A β-lactam is a four-molecule ring (three carbons and one nitrogen) and is the nucleus of the penicillin molecule. The penicillin nucleus is shown in Figure 18-3. Modifications to the five-membered ring are the basis on which cephalosporins and carbapenems (two other β-lactam antibiotics) were derived.

Carbapenems differ structurally from penicillins in that the five-membered ring that is attached to the β-lactam ring contains a carbon atom instead of a sulfur atom. The name carbapenem comes from carbon and penicillin. Figure 18-6 shows the core structure of a carbapenem but does not show the side chains.

Glycopeptides inhibit cell wall synthesis by attaching to the end of the peptidoglycan precursor units (a short four– or five–amino acid sequence called the d-alanyl-d-alanine [D-ALA-D-ALA] terminus) that are required to be laid down into the matrix. This step is catalyzed by transglycosylase. Because the glycopeptide binds to the end of the precursor, the precursor is not released from the carrier and thus peptidoglycan synthesis stops (Figure 18-7).

Fluoroquinolones bind to and inhibit DNA gyrase (also called topoisomerase II) and topoisomerase IV. Fluoroquinolones inhibit DNA gyrase in gram-negative organisms and topoisomerase IV in gram-positive organisms (Figure 18-8).

As the name suggests, fluoroquinolones possess a fluorine ion. They all contain two six-membered rings. Ciprofloxacin is shown in the diagram (Figure 18-9). Earlier generations of fluoroquinolones were not fluorinated and were simply called quinolones. They are infrequently used now.