Beta-lactam antibiotics: from antibiosis to resistance and bacteriology

Abstract

This review focuses on the era of antibiosis that led to a better understanding of bacterial morphology, in particular the cell wall component peptidoglycan. This is an effort to take readers on a tour de force from the concept of antibiosis, to the serendipity of antibiotics, evolution of beta-lactam development, and the molecular biology of antibiotic resistance. These areas of research have culminated in a deeper understanding of microbiology, particularly in the area of bacterial cell wall synthesis and recycling. In spite of this knowledge, which has enabled design of new even more effective therapeutics to combat bacterial infection and has provided new research tools, antibiotic resistance remains a worldwide health care problem.

Fig. 1

Antibiotic hall of fame. Courtesy of Nobel e-Museum (http://www.nobel.se/medicine/laureates/1945/index.html).

Fig. 2

Molecular structure of 6-aminopenicillic acid, 7-aminopenicillic acid and D-alanine D-alanine

Fig. 3

Schematic representation of the (A) S. aureus and (B) Escherichia coli murein monomers. (A) In S. aureus, N-acetylglucosamine and N-acetylmuramic acid form the backbone of the murein. It is attached to pentapeptides containing L-alanine, D-glutamc acid, L-lysine and D-alanyl-D-alanine. The two pentapeptides are linked with a penta-glycine cross bridge. (B) In Escherichia coli, the murein backbone is composed of the same sugar moieties as S. aureus, N-acetylglucosamine and N-acetylmuramic acid. The backbone is attached to pentapeptides containing L-alanine, D-glutamic acid, L-diaminopimelic acid and D-alanyl-D-alanine. Cross-linking occurs between diaminopimelic acid and D-alanine. The D-alanines in red boxes are usually liberated upon transpeptidation.

Fig. 4

The biosynthetic pathway of bacterial murein. The pathway initiates with committing steps, generating UDP-acetylmuramic acid (UDP-MurNAc). L-alanine, D-glutamic acid, lysine or diaminophenolic acid, and D-alanyl-D-alanine are enzymatically added to UDP-MurNAc in a stepwise manner. The formation of UDP-acetylmuramyl-pentapeptide (UDP-MurNAc-pentapeptide) occurs in the cytoplasm. Then, the UDP-MurNAc-pentapeptide is transferred to the cytoplasmic membrane where two lipid carriers (Lipid I and Lipid II) are involved in the joining of UDP-MurNAc-pentapeptide and UDP-acetylglucosamine (UDP-GlcNAc) to form a linear murein with protruding short peptide chains. Finally, the linear murein chains are cross-linked with each other and to pre-existing murein. A few chemical compounds are known to block this pathway. β-lactams are known to inhibit the cross-linking, whereas bacitracin inhibits the recycling of undecaprenyl. Moenomycin and vancomycin inhibit the function of Lipid II that carries the disaccharide peptide monomer units.

Fig. 5

Structure of the active site of DD-carboxypeptidase and β-lactamase. A. The X-ray crystal structure of E. coli PBP5 (240) and Bacillus licheniformis BS3 beta-lactamase (241). The conserved SerXXLys, Ser[Tyr]XAsn, and Lys[His]Thr[Ser]Gly motifs (242) are colored in yellow, orange and green, respectively. The catalytic serine of the SerXXLys motif is colored in red.

Fig. 6

Mechanism of β-lactamase induction in Enterobacteriaceae. In the absence of β-lactam antibiotics, the murein is constantly being re-modeled for cell elongation and cell division. The breakdown products, muramyl peptides, are released into the periplasm and transported to the cytoplasm via AmpG. These muramyl peptides are then processed by ampD for the murein recycling. In this instance, the ampC gene is kept at basal-level by a suppressed AmpR transcriptional regulator. In the presence of β-lactam antibiotics, the homeostasis is disrupted because these antibiotics target the murein resulting in an excessive breakdown of the murein and an accumulation of the muramyl peptides both in the periplasm and cytoplasm. However, the AmpD protein is the limiting factor. An accumulation of the muramyl peptides renders the activation of AmpR, which then induces the production of AmpC β-lactamase. It is not clear which of the muramyl products directly interact with AmpR. AmpE has been implicated in the transport of free pentapeptide to the periplasm (40). The transport of the muramyl peptides into the periplasm may involve AmpE or some yet to be unidentified protein. Abbreviations, CM, IM, PP, PG and OM refer to cytoplasm, inner membrane, periplasm, peptidoglycan and outer membrane, respectively.