Mechanism of action of the beta lactam antibiotics (penicillins and cephalosporins)
The beta-lactam antibiotics can kill susceptible bacteria. Although knowledge of the mechanism of this action is incomplete, numerous researchers have supplied information that allows understanding of the basic phenomenon (seeGhuysen, 1991; Bayles, 2000).
The cell walls of bacteria are essential for their normal growth and development. Peptidoglycan is a heteropolymeric component of the cell wall that provides rigid mechanical stability by virtue of its highly cross-linked latticework structure . In gram-positive microorganisms, the cell wall is 50 to 100 molecules thick, but it is only 1 or 2 molecules thick in gram-negative bacteria . The peptidoglycan is composed of glycan chains, which are linear strands of two alternating amino sugars (N-acetylglucosamine and N-acetylmuramic acid) that are cross-linked by peptide chains.
The biosynthesis of the peptidoglycan involves about 30 bacterial enzymes and may be considered in three stages. The first stage, precursor formation, takes place in the cytoplasm. The product, uridine diphosphate (UDP)-acetylmuramyl-pentapeptide, accumulates in cells when subsequent synthetic stages are inhibited. The last reaction in the synthesis of this compound is the addition of a dipeptide, D-alanyl-D-alanine. Synthesis of the dipeptide involves prior racemization of L-alanine and condensation catalyzed by D-alanyl-D-alanine synthetase. D-Cycloserine is a structural analog of D-alanine and acts as a competitive inhibitor of both the racemase and the synthetase .
During reactions of the second stage, UDP-acetylmuramyl-pentapeptide and UDP-acetylglucosamine are linked (with the release of the uridine nucleotides) to form a long polymer.
The third and final stage involves completion of the cross-link. This is accomplished by a transpeptidation reaction that occurs outside the cell membrane. The transpeptidase itself is membrane-bound. The terminal glycine residue of the pentaglycine bridge is linked to the fourth residue of the pentapeptide (D-alanine), releasing the fifth residue (also D-alanine) . It is this last step in peptidoglycan synthesis that is inhibited by the b-lactam antibiotics and glycopeptide antibiotics such as vancomycin (by a different mechanism than the b-lactams; ). Stereomodels reveal that the conformation of penicillin is very similar to that of D-alanyl-D-alanine. The transpeptidase probably is acylated by penicillin; that is, penicilloyl enzyme apparently is formed, with cleavage of the ¾ CO¾N ¾ bond of the b-lactam ring.
Although inhibition of the transpeptidase just described is demonstrably important, there are additional, related targets for the actions of penicillins and cephalosporins; these are collectively termed penicillin-binding proteins (PBPs). All bacteria have several such entities; for example, S. aureus has four PBPs, whereas Escherichia coli has at least seven. The PBPs vary in their affinities for different b-lactam antibiotics, although the interactions eventually become covalent. The higher-molecular-weight PBPs of E. coli (PBPs 1a and 1b) include the transpeptidases responsible for synthesis of the peptidoglycan. Other PBPs in E. coli include those that are necessary for maintenance of the rodlike shape of the bacterium and for septum formation at division. Inhibition of the transpeptidases causes spheroplast formation and rapid lysis. However, inhibition of the activities of other PBPs may cause delayed lysis (PBP 2) or the production of long, filamentous forms of the bacterium (PBP 3). The lethality of penicillin for bacteria appears to involve both lytic and nonlytic mechanisms. Penicillin’s disruption of the balance between PBP-mediated peptidoglycan assembly and murein hydrolase activity results in autolysis. Nonlytic killing by penicillin may involve holin-like proteins in the bacterial membrane that collapse the membrane potential (Bayles, 2000).
Watch a related animation on a cell wall synthesis inhibitor: vancomycin.