Antimicrobial Agents

1. LECTURES IN MICROBIOLOGY Antimicrobial Agents Sofronio Agustin Professor LESSON 8

2. Lesson 8 Topics • Antimicrobial Therapy • Selective Toxicity • Survey of Antimicrobial Agents • Microbial Drug Resistance • Drug-Host Interaction

3. The Ideal Antimicrobial Drug

4. Selective Toxicity • An ideal in chemotherapy that an antimicrobial drug kills only pathogenic microbes without harming the host. • Historically, reminiscent of the “magic bullet” of Paul Ehrlich.

5. Terms in Chemotherapy • Chemotherapy - use of drugs to treat diseases. • Antimicrobials - any drug used in treating infectious diseases. • Antibiotics - substances produced by some microbes that inhibit or kill other microbes. • Synthetic drugs - antimicrobial compounds synthesized in the laboratory.

6. Historical Note in Chemotherapy • 1928 – Alexander Fleming discovered penicillin from Penicilliumnotatum. • 1940 – Howard Florey and Ernst Chain performed first clinical trials of penicillin.

7. Antibiotics • Naturally occurring • Metabolic products of bacteria and fungi • Reduce competition for nutrients and space • Examples: Bacteria- Streptomyces, Bacillus Molds -Penicillium, Cephalosporium

8. Antimicrobial Activity • Narrow-spectrum • Broad-spectrum • Bactericidal • Bacteriostatic

9. Antimicrobial Activity

10. Modes of Action Primary target sites of antimicrobial drugs in bacterial cells.

11. Cell Wall Active Agents • Bactericidal • Penicillin and Cephalosporins – binds and blocks peptidases involved in cross-linking the glycan molecules. • Vancomycin – prevents peptidoglycan elongation • Cycloserine – inhibits the formation of the basic peptidoglycan subunits

12. Cell Wall Active Agents Antibiotics weaken the cell wall and cause the cell to lyse.

13. Cell Wall Active Agents Penicillins and cephalosporins destroy the peptidoglycan layer by disrupting the peptide cross bridges.

14. Cell Wall Active Agents Penicillin • Natural penicillins • Semi-synthetic penicillins • Molecular Structure • Thiazolidine ring • Beta-lactamring • Variable side chain (R group)

15. Penicillins

16. Penicillinase

17. Penicillins • Penicillinase-resistant penicillins • Extended-spectrum penicillins • Penicillins + -lactamase inhibitors • Carbapenems • Monobactam

18. Penicillins • Bactericidal • Narrow spectrum. • Used to treat: • Streptococcal • Staphylococcal • Meningococcal, and • Spirochaete infections.

19. Cephalosporins • Derived from Cephalosporium acremonium • Beta lactam antibiotic like penicillin • Main ring different from penicillin • 2 sites for R groups

20. Cephalosporins • Inhibit cell wall synthesis • Broad-spectrum or extended spectrum antibiotic • 2nd, 3rd, 4th generations more effective against Gram-negatives

21. Cephalosporins Different R groups allow for versatility and improved effectiveness of cephalosporins.

22. Polypeptide Antibiotics Bacitracin • Topical application • Effective against Gram-positives Vancomycin • Glycopeptide • Important "last line" against antibiotic resistant S. aureus • Hinders peptidoglycan elongation

23. Mycolic Acid Inhibitors • Antimycobacterial antibiotics • Isoniazid (INH) - inhibits mycolic acid synthesis • Ethambutol - inhibits incorporation of mycolic acid into cell wall

24. Inhibition of Protein Synthesis Various antibiotics and their sites of protein synthesis inhibition on the prokaryotic ribosome.

25. Inhibitors of Protein Synthesis Aminoglycosides • Broad-spectrum antibiotics • Changes shape of 30S subunit • Treatment of bubonic plague,STD, and Gram-negative infections • Examples: Streptomycin, neomycin, gentamycin

26. Aminoglycoside Structure Amino sugars and a six-carbon ring (aminocyclitol) in Streptomycin.

27. Streptomyces Streptomyces sp. synthesizes many antibiotics such as: aminoglycosides, tetracycline, chloramphenicol, and erythromycin.

28. Tetracycline • Broad spectrum • Interferes with tRNA attachment • Treat intracellular infections • Risk to pregnant women Chemical Structure of Tetracycline

29. Chloramphenicol • Broad-spectrum • Binds 50S subunit, inhibits peptide bond formation • Cheap synthetic • Treat typhoid fever • Side effects: Aplastic anemia Nitrobenzene ring of chloramphenicol

30. Erythromycin • A macrolide • Bactericidal • Binds 50s, prevents translocation • Gram positives • Side effects: GI disturbance Lactone ring of erythromycin

31. Streptogramins • A combination drug of quinopristin and dalfopristin • Bactericidal • Binds 50s, inhibits translation • Affect Gram-positives Example: Synercid

32. Oxazolidinones • Bactericidal • Binds 50S, prevents formation of 70S ribosome • Affect Gram-positives Example: Linezolid

33. Injury to Cell Membrane • Polymyxins • Interact with membrane phospholipids • Topical • Combined with Bacitracin and Neomycin as over-the counter antibiotic • Amphotericin B • Anit-fungal agent • Forms complexes with sterols in the membrane • Causes cytoplasmic leakage • Can affect human cell membranes (toxicity)

34. Nucleic Acid Synthesis Inhibitors Rifamycin • Inhibits RNA synthesis • Anti-tuberculosis drug Quinolones and fluoroquinolones • inhibits DNA unwinding enzymes (gyrases) • Urinary tract infections • Ciprofloxacin

35. Nucleic Acid Synthesis Inhibitors Chloroquine • binds and cross-links the double helix • anti-malarial Quinolones - e.g. Cirpofloxacin • inhibits DNA unwinding enzymes (gyrases) Azidothymidine (AZT) • Antiviral • Analogs of purines and pyrimidines

36. Sulfa Drugs • Analogs of important metabolites (folic acid) • Competitive enzyme inhibition • Prevents the metabolism of DNA, RNA, and amino acid • Examples: Sulfonamides, and trimethoprim

37. Sulfa Drugs Sulfonamides compete with PABA for the active site on the enzyme.

38. Sulfonamides Attachment of different R groups to the main structural nucleus affords versatility of sulfonamides.

39. Sulfonamides • Synthetic drug derived from dyes (Prontosil of Domagk) • Synergistic combination as Trimethoprim/Sulfamethoxazole • Treatment of pneumonia in AIDS patients

40. Antifungal Drugs (a) Polyenes (b) Azoles (c) Fluorocytosine

41. Antifungal Drugs Amphotericin B • Polyene derivative • Affects sterols in fungal membrane • Causes cytoplasmic leakage • Can affect human cell membranes (nephrotoxicity) • For systemic fungal infections

42. Antifungal Drugs • Azoles- Miconazole, Triazoles • Inhibit ergosterol synthesis • For cutaneous fungal infections

43. Antifungal Drugs Echinocandins • Inhibit synthesis of -glucan, cell wall component in yeasts • Used against Candida and Pneumocystis infections

44. Antifungal Drugs Fluorocytosine (5-FC) • Cytosine analog, interferes with RNA synthesis • Used in serious systemic fungal infections • For Amphotericin B resistant fungi

45. Antifungal Drugs Pentamidine isothionate • May bind DNA • For Pneumocystis infections Griseofulvin • Inhibition of microtubules (mitosis) • For superficial mycoses Tolnaftate • Action unknown • For Athlete’s foot

46. Antiprotozoal Drugs Chloroquine • Inhibits DNA synthesis • For Malaria Metronidazole • Damages DNA • For Entamoeba, Trichomonas infections

47. Antihelminthic Drugs Niclosamide • Prevents ATP generation • For Tapeworms Praziquantel • Alters membrane permeability • For Flatworms Pyrantel pamoate • Neuromuscular block • Intestinal roundworms

48. Antihelminthic Drugs Mebendazole • Inhibits nutrient absorption • For intestinal roundworms Ivermectin • Paralyzes worm • For intestinal roundworms

49. Antiviral Drugs • Few antiviral drugs available • Selective toxicity difficult - viruses are intracellular in host cells • Targets in viral replication cycle: -Entry -Nucleic acid synthesis -Assembly and release • Interferons – natural or artificial

50. Antiviral Drugs