بسم الله الرحمن الرحيم

1. بسم الله الرحمن الرحيم

2. Antimicrobial Drugs: Mechanism of Action Dr. Manal El Said Department Head of Microbiology

3. Antimicrobial Drugs: Mechanism of Action: Introduction • There are four major sites in bacterial cell that serve as basis for action of effective drugs: • Cell wall • Ribosomes • Nucleic acids • Cell membrane

4. Antimicrobial Drugs: Mechanism of Action: Introduction

5. Antimicrobial Drugs: Mechanism of Action: Introduction

6. Antimicrobial Drugs: Mechanism of Action: Introduction

7. Antimicrobial Drugs: Mechanism of Action: Introduction

8. Antimicrobial Drugs: Mechanism of Action: Introduction • Selective toxicity: • It is selective inhibition of growth of microorganism without damage to host. • It is achieved by exploiting differences between metabolism & structure of microorganism & human cells. • Penicillins & cephalosporins are effective antibacterial agents prevent synthesis of peptidoglycan • inhibiting growth of bacterial (not human cells).

9. Antimicrobial Drugs: Mechanism of Action: Introduction • Broad-spectrum antibiotics are active against several types of microorganisms • e.g., tetracyclines are active against many gram-negative rods, chlamydiae, mycoplasmas, & rickettsiae. • Narrow-spectrum antibiotics are active against one or very few types, • e.g., vancomycin is used against certain gram-positive cocci, staphylococci & enterococci.

10. Antimicrobial Drugs: Mechanism of Action: Introduction • Bactericidal drug kills bacteria • Bacteriostatic drug inhibits their growth but does not kill them • -Bacteria can grow again when drug is withdrawn • -Host defense mechanisms, such as phagocytosis, are required to kill bacteria.

11. Antimicrobial Drugs: Mechanism of Action: Introduction

12. Antimicrobial Drugs: Mechanism of Action: Introduction • Bactericidal drugs are useful in certain infections: • Life-threatening • Patients whose polymorphonuclear leukocyte count is below 500/μL • Endocarditis, in which phagocytosis is limited by fibrinous network of vegetations & bacteriostatic drugs do not effect cure.

13. Inhibition of Cell Wall Synthesis

14. Antimicrobial Drugs: Mechanism of Action: Inhibition of Cell Wall Synthesis Penicillins • Penicillins & cephalosporins act by inhibiting transpeptidases (penicillin-binding proteins, PBP) enzymes that cross-link peptidoglycan. • Several important bacteria, e.g., Streptococcus pneumoniae, manifest resistance to penicillins based on mutations in genes encoding PBP.

15. Antimicrobial Drugs: Mechanism of Action: Inhibition of Cell Wall Synthesis Penicillins

16. Antimicrobial Drugs: Mechanism of Action: Inhibition of Cell Wall Synthesis Penicillins Gram-negative Gram-positive

17. Antimicrobial Drugs: Mechanism of Action: Inhibition of Cell Wall Synthesis Penicillins mecA Gene & Production of Altered Penicillin Binding Protein. Altered penicillin binding protein (PBP2a) resists binding of beta-lactam antimicrobial, but maintains function of cross-lining bacterial cell wall components. Abbreviations: PBP = penicillin binding protein; SCC = staphylococcal chromosomal cassette Role of Penicillin Binding Protein in Cross-Linking of Bacterial Cell Wall Subunits

18. Antimicrobial Drugs: Mechanism of Action: Inhibition of Cell Wall Synthesis Penicillins • Exposure to penicillins activates autolytic enzymes degrade bacteria. • If these autolytic enzyme are not activated, e.g., in certain strains of Staphylococcus aureus, bacteria are not killed & strain is said to be tolerant.

19. Antimicrobial Drugs: Mechanism of Action: Inhibition of Cell Wall Synthesis Penicillins

20. Antimicrobial Drugs: Mechanism of Action: Inhibition of Cell Wall Synthesis Penicillins • Penicillins kill bacteria when they are growing more active during log phase of bacterial growth. • Penicillins & cephalosporins are β -lactam drugs, i.e., intact β -lactam ring is required for activity. • β –lactamases (penicillinases & cephalosporinases) cleave β -lactam ring & inactivate drug.

21. Antimicrobial Drugs: Mechanism of Action: Inhibition of Cell Wall Synthesis Penicillins • Modification of side chain adjacent to β -lactam ring endows these drugs with new properties: • - Expanded activity against gram-negative rods • - Ability to be taken orally • - Protection against degradation by β-lactamases.

22. Antimicrobial Drugs: Mechanism of Action: Inhibition of Cell Wall Synthesis Penicillins • Penicillin G is available in three main forms: • Aqueous penicillin G (metabolized most rapidly). • Procaine penicillin G (penicillin G is conjugated to procaine) & metabolized more slowly & is less painful when injected intramuscularly (procaine acts as anesthetic). • Benzathine penicillin G (penicillin G is conjugated to benzathine) & metabolized very slowly .

23. Antimicrobial Drugs: Mechanism of Action: Inhibition of Cell Wall Synthesis Penicillins • Benzylpenicillin has three disadvantages have been overcome by chemical modification of side chain. • Limited effectiveness against many gram-negative rods (due to inability of drug to penetrate outer membrane of organism) • Hydrolysis by gastric acids & not be taken orally • Hydrolysis is prevented addition of oxygen (penicillin V) or amino group (ampicillin) As activity against gram-negative bacteria increases, activity against gram-positive bacteria decreases.

24. Antimicrobial Drugs: Mechanism of Action: Inhibition of Cell Wall Synthesis Penicillins

25. Antimicrobial Drugs: Mechanism of Action: Inhibition of Cell Wall Synthesis Penicillins 3) inactivation by β-lactamases. It can be blocked by: -Modification of side chain with addition of large aromatic rings containing bulky methyl or ethyl groups (methicillin, oxacillin, nafcillin). -Inhibitors such as clavulanic acid & sulbactam(structural analogues of penicillin that have little antibacterial activity but bind strongly to β-lactamases & protect penicillin).

26. Antimicrobial Drugs: Mechanism of Action: Inhibition of Cell Wall Synthesis Penicillins methicillin oxacillin nafcillin

27. Antimicrobial Drugs: Mechanism of Action: Inhibition of Cell Wall Synthesis Cephalosporins • Cephalosporins have six-membered ring adjacent to β-lactam ring & are substituted in two places on 7-aminocephalosporanic acid nucleus (penicillins have five-membered ring & are substituted in only one place). • First-generation cephalosporins are active against gram-positive cocci • Second, third, & fourth generations have expanded coverage against gram-negative rods.

28. Antimicrobial Drugs: Mechanism of Action: Inhibition of Cell Wall Synthesis Cephalosporins

29. Antimicrobial Drugs: Mechanism of Action: Inhibition of Cell Wall Synthesis Carbapenems • Carbapenems are β-lactam drugs that has different structure from penicillins & cephalosporins. • Imipenem has widest spectrum of activity & excellent bactericidal activity against : • -Gram-positive • -Gram-negative (including extended-spectrum β-Lactamases resistant to all penicillins & cephalosporins) • - Anaerobic bacteria

30. Antimicrobial Drugs: Mechanism of Action: Inhibition of Cell Wall Synthesis Carbapenems

31. Antimicrobial Drugs: Mechanism of Action: Inhibition of Cell Wall Synthesis Carbapenems • Imipenem is prescribed in combination with cilastatin, which is inhibitor of dehydropeptidase, kidney enzyme that inactivates imipenem. • Imipenem is not inactivated by most β-lactamases (carbapenemases have emerged). • Two other carbapenems, ertapenem & meropenem, are available.

32. Antimicrobial Drugs: Mechanism of Action: Inhibition of Cell Wall Synthesis Vancomycin • Vancomycin is glycopeptide, i.e., it is not β-lactam drug • its mode of action is very similar to that of penicillins & cephalosporins, i.e., it inhibits transpeptidases.

33. Antimicrobial Drugs: Mechanism of Action: Inhibition of Cell Wall Synthesis Vancomycin

34. Antimicrobial Drugs: Mechanism of Action: Inhibition of Cell Wall Synthesis Caspofungin • Caspofungin is lipopeptide that inhibits fungal cell wall synthesis by blocking synthesis of β-glucan, polysaccharide component of cell wall.

35. Inhibition of Protein Synthesis

36. Antimicrobial Drugs: Mechanism of Action: Inhibition of Protein Synthesis • Antibiotics act at level of 30S ribosomal subunit: • - Aminoglycosides • - Tetracyclines • Antibiotics act at level of 50S ribosomal subunit: • - Chloramphenicol • - erythromycins • - clindamycin

37. Antimicrobial Drugs: Mechanism of Action: Inhibition of Protein Synthesis 30S ribosomal subunit :Aminoglycosides • Aminoglycosides inhibit bacterial protein synthesis by binding to 30S subunit, which blocks initiation complex. • No peptide bonds are formed & no polysomes are made. • Aminoglycosides are family of drugs that includes: • - gentamicin • - tobramycin • - streptomycin

38. Antimicrobial Drugs: Mechanism of Action: Inhibition of Protein Synthesis 30S ribosomal subunit :Aminoglycosides

39. Antimicrobial Drugs: Mechanism of Action: Inhibition of Protein Synthesis 30S ribosomal subunit :Tetracyclines • Tetracyclines inhibit bacterial protein synthesis by blocking binding of aminoacyl t-RNA to 30S ribosomal subunit. • Tetracyclines are family of drugs; doxycycline is used most often.

40. Antimicrobial Drugs: Mechanism of Action: Inhibition of Protein Synthesis 30S ribosomal subunit :Tetracyclines

41. Antimicrobial Drugs: Mechanism of Action: Inhibition of Protein Synthesis 30S ribosomal subunit :Tetracyclines The tetracyclines (tetracycline, doxycycline, demeclocycline, minocycline )block bacterial translation by binding reversibly to the 30S subunit and distorting it in such a way that the anticodons of the charged tRNAs cannot align properly with the codons of the mRNA.

42. Antimicrobial Drugs: Mechanism of Action: Inhibition of Protein Synthesis 50S ribosomal subunit: Chloramphenicol • Chloramphenicol inhibits bacterial protein synthesis by blocking peptidyl transferase, enzyme that adds new amino acid to growing polypeptide. • Chloramphenicol can cause bone marrow suppression.

43. Antimicrobial Drugs: Mechanism of Action: Inhibition of Protein Synthesis 50S ribosomal subunit: Erythromycin • Erythromycin inhibits bacterial protein synthesis by blocking release of t-RNA after it has delivered its amino acid to growing polypeptide. • Erythromycin is member of macrolide family of drugs that includes azithromycin & clarithromycin.

44. Antimicrobial Drugs: Mechanism of Action: Inhibition of Protein Synthesis 50S ribosomal subunit: Clindamycin • Clindamycin binds to same site on ribosome as does erythromycin & is thought to act in same manner. • It is effective against many anaerobic bacteria. • Clindamycin is one of antibiotics that predisposes to pseudomembranous colitis caused by Clostridium difficile & is infrequently used.

45. Inhibition of Nucleic Acid Synthesis

46. Antimicrobial Drugs: Mechanism of Action:Inhibition of Nucleic Acid Synthesis • Sulfonamides & trimethoprim inhibit nucleotide synthesis, • Quinolones inhibit DNA synthesis • Rifampin inhibits RNA synthesis.

47. Antimicrobial Drugs: Mechanism of Action: Inhibition of Protein Synthesis Sulfonamides and trimethoprim • Sulfonamides & trimethoprim inhibit synthesis of tetrahydrofolic acid—main donor of methyl groups that are required to synthesize adenine, guanine, & thymine. • Sulfonamides are structural analogues of p-aminobenzoic acid, which is component of folic acid.

48. Antimicrobial Drugs: Mechanism of Action: Inhibition of Protein Synthesis Sulfonamides and trimethoprim • Trimethoprim inhibits dihydrofolate reductase—enzyme that reduces dihydrofolic acid to tetrahydrofolic acid. • Combination of sulfamethoxazole & trimethoprim is used because bacteria resistant to one drug will be inhibited by other.

49. Antimicrobial Drugs: Mechanism of Action: Inhibition of Protein Synthesis Quinolones • Quinolones inhibit DNA synthesis in bacteria by blocking DNA gyrase (topoisomerase)- enzyme that unwinds DNA strands so that they can be replicated. • Quinolones are family of drugs that includes: • - ciprofloxacin, • - ofloxacin, • -levofloxacin.

50. Antimicrobial Drugs: Mechanism of Action: Inhibition of Protein Synthesis Quinolones