1. بسم الله الرحمن الرحيم 1 1 1

2. Department Head of Microbiology
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
Inhibition of cell wall synthesis
Inhibition of cross-linking (transpeptidation) of peptidoglycan
-Penicillins
Cephalosporins
Imipenem
Aztreonam,
Vancomycin
Inhibition of other steps in peptidoglycan synthesis
- Cycloserine
- Bacitracin
Antifungal activity inhibition of β-glucan synthesis
- Caspofungin
Inhibition of protein synthesis
Action on 50S ribosomal subunit
- Chloramphenicol
erythromycin
clindamycin
linezolid
Action on 30S ribosomal subunit
-Tetracyclines
- Aminoglycosides

7. Antimicrobial Drugs: Mechanism of Action: Introduction
Mechanism of Action of Important Antibacterial and Antifungal Drugs
Mechanism of Action
Drugs
Inhibition of nucleic acid synthesis
Inhibition of nucleotide synthesis
Sulfonamides, trimethoprim
Inhibition of DNA synthesis
Quinolones, e.g., ciprofloxacin
Inhibition of mRNA synthesis
Rifampin
Alteration of cell membrane function
Antibacterial activity
Polymyxin, daptomycin
Antifungal activity
Amphotericin B, nystatin, terbinafine, azoles, e.g., itraconazole
Other mechanisms of action
1. Antibacterial activity
Isoniazid, metronidazole, ethambutol, pyrazinamide
2. Antifungal activity
Griseofulvin, pentamidine

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

39. 30S ribosomal subunit :Tetracyclines
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. 30S ribosomal subunit :Tetracyclines
Antimicrobial Drugs: Mechanism of Action: Inhibition of Protein Synthesis
30S ribosomal subunit :Tetracyclines

41. 30S ribosomal subunit :Tetracyclines
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. 50S ribosomal subunit: Chloramphenicol
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. 50S ribosomal subunit: Erythromycin
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. 50S ribosomal subunit: Clindamycin
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. Sulfonamides and trimethoprim
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. Sulfonamides and trimethoprim
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

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

52. Antimicrobial Drugs: Mechanism of Action: Inhibition of Protein Synthesis
Rifampin
Rifampin inhibits RNA synthesis in bacteria by blocking RNA polymerase that synthesizes mRNA.
Rifampin is typically used in combination with other drugs because there is high rate of mutation of RNA polymerase gene, which results in rapid resistance to drug.

53. Antimicrobial Drugs: Mechanism of Action: Alteration of Cell Membrane Function
Antifungal drugs predominate in this category.
These drugs have selective toxicity because fungal cell membranes contain ergosterol, whereas human cell membranes have cholesterol.
Bacteria, with exception of mycoplasma, do not have sterols in their membranes and therefore are resistant to these drugs.

54. Antimicrobial Drugs: Mechanism of Action: Alteration of Cell Membrane Function
Amphotericin B
Amphotericin B disrupts fungal cell membranes by binding at site of ergosterol in membrane.
It is used to treat most serious systemic fungal diseases
It has significant side effects, especially on kidney.

55. Antimicrobial Drugs: Mechanism of Action: Alteration of Cell Membrane Function
Azoles
Azoles are antifungal drugs that inhibit ergosterol synthesis.
The azole family includes drugs such as:
- Ketoconazole
- fluconazole
- Itraconazole
- clotrimazole.
They are useful in treatment of systemic, skin & mucous membrane infections.

56. Antimicrobial Drugs: Mechanism of Action: Additional Drug Mechanisms
Isoniazid
Isoniazid inhibits synthesis of mycolic acid—long-chain fatty acid found in cell wall of mycobacteria.
Isoniazid is prodrug that requires bacterial peroxidase (catalase) to activate isoniazid to metabolite that inhibits mycolic acid synthesis.
Isoniazid is most important drug used in treatment of tuberculosis & other mycobacterial diseases.

57. Antimicrobial Drugs: Mechanism of Action: Additional Drug Mechanisms
Metronidazole
Metronidazole is effective against anaerobic bacteria & certain protozoa
it acts as electron sink, taking away the electrons that organisms need to survive.
It also forms toxic intermediates that damage DNA.

58. Antimicrobial Drugs: Chemoprophylaxis
Antimicrobial drugs are used to prevent infectious diseases as well as to treat them.
Chemoprophylactic drugs are given primarily in three circumstances:
to prevent surgical wound infections
to prevent opportunistic infections in immuno-compromised patients
to prevent infections in those known to be exposed to pathogens that cause serious infectious diseases.

59. Antimicrobial Drugs: Probiotics
In contrast to chemical antibiotics, probiotics are live, nonpathogenic bacteria that may be effective in treatment or prevention of certain human diseases.
The suggested basis for possible beneficial effect lies in:
providing colonization resistance by which nonpathogen excludes pathogen from binding sites on mucosa
enhancing immune response against pathogen
reducing inflammatory response against pathogen.

60. Antimicrobial Drugs: Probiotics
Oral administration of live Lactobacillus rhamnosus strain GG significantly reduces number of cases of nosocomial diarrhea in young children.
Yeast Saccharomyces boulardii reduces risk of antibiotic- associated diarrhea caused by Clostridium difficile.
Adverse effects are few; however, serious complications have arisen in highly immunosuppressed patients and in patients with indwelling vascular catheters.

61. Antimicrobial Drugs:
Probiotics

62. Antimicrobial Drugs:
Probiotics