Penicillins, like all β-lactam antibiotics, inhibit bacterial growth by interfering with the transpeptidation reaction of bacterial cell wall synthesis. The cell wall is a rigid outer layer that completely surrounds the cytoplasmic membrane (Figure 43–3), maintains cell integrity, and prevents cell lysis from high osmotic pressure. The cell wall is composed of a complex, cross-linked polymer of polysaccharides and peptides known as peptidoglycan. The polysaccharide contains alternating amino sugars, N-acetylglucosamine and N-acetylmuramic acid (Figure 43–4). A five-amino-acid peptide is linked to the N-acetylmuramic acid sugar. This peptide terminates in d-alanyl-d-alanine. Penicillin-binding protein (PBP, an enzyme) removes the terminal alanine in the process of forming a cross-link with a nearby peptide. Cross-links give the cell wall its rigidity. Beta-lactam antibiotics, structural analogs of the natural d-Ala-d-Ala substrate, covalently bind to the active site of PBPs. This binding inhibits the transpeptidation reaction (Figure 43–5) and halts peptidoglycan synthesis, and the cell dies. The exact mechanism of cell death is not completely understood, but autolysins are involved in addition to the disruption of cross linking of the cell wall. Beta-lactam antibiotics kill bacterial cells only when they are actively growing and synthesizing cell wall.
Resistance to penicillins and other β-lactams is due to one of four general mechanisms: (1) inactivation of antibiotic by β-lactamase, (2) modification of target PBPs, (3) impaired penetration of drug to target PBPs, and (4) antibiotic efflux. Beta-lactamase production is the most common mechanism of resistance. Hundreds of different β-lactamases have been identified. Some, such as
FIGURE 43–3 A highly simplified diagram of the cell envelope of a Gram-negative bacterium. The outer membrane, a lipid bilayer, is present in Gram-negative but not Gram-positive organisms. It is penetrated by porins, proteins that form channels providing hydrophilic access to the cytoplasmic membrane. The peptidoglycan layer is unique to bacteria and is much thicker in Gram-positive organisms than in Gram-negative ones. Together, the outer membrane and the peptidoglycan layer constitute the cell wall. Penicillin-binding proteins (PBPs) are membrane proteins that cross-link peptidoglycan. Beta-lactamases, if present, reside in the periplasmic space or on the outer surface of the cytoplasmic membrane, where they may destroy β-lactam antibiotics that penetrate the outer membrane.
FIGURE 43–4 The transpeptidation reaction in Staphylococcus aureus that is inhibited by β-lactam antibiotics. The cell wall of Gram-positive bacteria is made up of long peptidoglycan polymer chains consisting of the alternating aminohexoses N-acetylglucosamine (G) and N-acetylmuramic acid (M) with pentapeptide side chains linked (in S aureus) by pentaglycine bridges. The exact composition of the side chains varies among species. The diagram illustrates small segments of two such polymer chains and their amino acid side chains. These linear polymers must be cross-linked by transpeptidation of the side chains at the points indicated by the asterisk to achieve the strength necessary for cell viability.