Antibiotics; Inhibitors of Cell Wall Synthesis

1. LECTURE 22: Antibiotics; Inhibitors of Cell Wall Synthesis Microbiology and Virology; 3 Credit hours Atta-ur-Rahman School of Applied Biosciences (ASAB) National University of Sciences and Technology (NUST)

2. Antibiotics • Antibiotics are agents that are "selectively" toxic for bacteria (either killing them [bactericidal] or inhibiting their growth [bacteriostatic]) without harm to the patient. • Antibiotics work most efficiently in conjunction with an active immune system to kill infecting bacteria in the host. • The minimal inhibitory concentration (MIC) refers to the lowest concentration of an antibiotic that stops visible growth.

3. Components of Cell Wall

4. Peptidoglycan • Peptidoglycan consists of polysaccharide backbone consisting of N-acetyl muramic acid and N-acetyl glucosamine with peptide side chains containing D- and L- amino acids and in some instances diaminopimelic acid. • The side chains are cross-linked by peptide bridges by Penicillin binding protein (PBP).

7. Synthesis of Cell Wall • The precursor subunit (muramylpentapeptide attached to uridinediphosphate, UDP) is synthesized in the cytoplasm and passed to the cell membrane. • The subunit is moved enzymatically to a lipid carrier (undecaprenol) and built into a completed subunit (disaccharide pentapeptide with attached bridge peptide). • The completed subunits are then exported to the cell wall. • After release of the monomer the undecaprenol is recirculated in the cell membrane and used again.

8. Synthesis of Cell Wall • The glycan backbones of the existing cell wall is enzymatically broken (by autolysins) to allow insertion of the newly synthesized subunit. • Cross-linking of the peptide side-chain of the inserted subunit to the existing chain then occurs enzymatically (penicillin binding proteins).

11. Synthesis of Lipopolysaccharide • Lipid A is assembled in the cell membrane and the core sugars attached sequentially. • O-antigen subunits are independently synthesized (on a lipid carrier as in peptidoglycan synthesis). • The fully synthesized O-antigen is then attached to the lipid A-core (generating lipopolysaccharide) in the cell membrane before insertion into the outer membrane.

12. Antibiotics; Mechanism

13. 1- Cycloserine; Inhibition of peptidoglycan synthesis • The terminal two amino acids of a peptide side chain of peptidoglycan are unusual amino acids (D-alanine as opposed to its isomer L-alanine). • The antibiotic cycloserine is an analog of D-alanine and interferes with enzymatic conversion of L-alanine to D-alanine in the cytoplasm. • Thus, subsequent synthesis of peptidoglycan cannot occur.

14. Cycloserine; Antibiotic

15. 2- Vancomycin; Inhibition of peptidoglycan synthesis • Cross-linking of the peptide occurs in the cell wall. • During this process D-alanine is enzymatically excised from the end of a pre-existing peptide side chain allowing it to be cross-linked (by a new peptide bond) to the recently synthesized peptidoglycan subunit. • Vancomycin binds to D-alanine-D-alanine thus sterically inhibits transpeptidation (cross-linking).

16. Vancomycin

17. 3- β-Lactam; Inhibition of peptidoglycan synthesis • The beta lactam antibiotics include penicillins (e.g. ampicillin), cephalosporins and monobactams and carbapenems. • They bind to and inhibit enzymes (penicillin binding proteins) involved in the transpeptidation (cross-linking) of peptidoglycan. • These antibiotics have in common the four membered lactam ring. • Attached to the lactam, penicillins have an additional five membered ring and cephalosporins a six membered ring. • Monobactams consist of the lactam ring alone and display antibiotic activity.

18. β-Lactam Antibiotics Penicillin nucleus Benzyl penicillin Monobactamnucleus Ampicillin Aztreonm Carbapenemnucleus Imipenem

19. 4- Bacitracin; Inhibition of peptidoglycan synthesis • The peptidoglycan subunit is passed across the cytoplasmic membrane attached to undecaprenoldiphosphate. • After the nascent peptidoglycan monomer leaves the carrier on reaching the cell wall, the undecaprenoldiphosphate is dephosphorylated to its monophosphate form. • Bacitracin inhibits the dephosphorylation reaction and in the absence of monophosphorylated carrier peptidoglycan subunit synthesis stops. 

20. Bacitracin

21. 5- Polymyxin B ; Inhibition of peptidoglycan synthesis • It is derived from the bacterium Bacillus polymyxa. • Polymyxin B binds to the lipid A portion of lipopolysaccharide and also to phospholipids. However, it binds preferentially to lipid A. This disrupts the outer membrane of Gram negative bacteria. • Since the cell membrane is not exposed in Gram positive bacteria polymyxin has little activity against them. • This drug is toxic to human cells, since it can also lyze eukaryotic membranes; this explains its limited clinical use.