Antimicrobial Therapy

1. Antimicrobial Therapy • Chapter 10

2. History of Antimicrobials • 1600s • Quinine for malaria • Emetine for amebiasis (Entamoeba histolytica) • 1900-1910 • Arsphenamines for syphilis • 1935 • Sulfonamides - broadly active • 1940 • Penicillin - substantially more active than sulfa drugs • Originally discovered in 1929 by Alexander Fleming (Scottish) • Nobel Prize, 1945 • Knighted, 1944 • Produced by fungus Penicillium chrysogenum

3. Mechanisms of Action of Anitmicrobial Drugs • Selective toxicity • Antimicrobials must be toxic to the microbe, but not to the host • Unfortunately, no such antibiotic exists • Mechanisms of action • Cell wall synthesis inhibitors • Cell membrane inhibitors • Protein synthesis inhibitors • Nucleic acid synthesis inhibitors • Metabolic Pathways

4. Cell Wall Inhibitors • Cell wall • Outer layer of bacterial cell • Barrier to outside • Maintains osmotic pressure • Peptidoglycan (polymer) • Polysaccharide and cross-linked peptides (transpeptidation) • N-acetylglucosamine (NAG) • N-acetylmuramic acid* (NAM) • *Only found in bacteria • Synthesis of peptidoglycan layer is performed by several enzymes • Gram+ have substantially thicker peptidoglycan layer

5. Cell Wall Inhibitors • Penicillin and Cephalosporin • Highly insoluble in natural form • Usually converted to a salt to increase solubility • Contains a β-lactam ring that interferes with cell wall synthesis • Penicillin is first bound by cellular penicillin binding receptors (PBP) • This binding interferes with transpeptidation reaction • This prevents peptidoglycan synthesis

6. Cell Wall Inhibitors Semisynthetic penicillins

7. Cell Membrane Function Inhibitors • The cell membrane is a biochemically-rich compartment • Polymyxins • Contain detergent-like (amphipathic) cyclic peptides • These damage membranes containing phosphatidylethanolamine • Novobiocin - inhibits teichoic acid synthesis • Ionophores - disrupt ion transport • Discharge membrane potential • Disrupts oxidative phosphorylation

8. Protein Synthesis Inhibitors

9. Protein Synthesis Inhibitors • Most interfere with ribosomes • By preventing ribosome function, polypeptide synthesis is inhibited • Compounds • Aminoglycosides (e.g., streptomycin) • Bind to 30S subunit • Interferes with initiation complex • mRNA localization to P site • fMet tRNA • Incorrect amino acid is incorporated into polypeptide • Tetracyclines • Bind to 30S subunit • Prevents IF3 binding • No tRNA binding

10. Protein Synthesis Inhibitors • Others • Macrolides - initiation complex, translocation • Azalides - initiation complex, translocation • Ketolides - initiation complex, translocation • Lincomycins - initiation complex, translocation • Glycylcyclines - Tet analogs; bind with higher affinity • Chloramphenicol - Inhibits peptidyl transferase • Streptogramins - Irreversible binding to 50S subunit; unknown mechanism • Oxazolidinones - Inhibit fMet tRNA binding to P site

11. Nucleic Acid Synthesis Inhibitors • Types • DNA/RNA polymerase inhibitors • Base analogs • Rifampin • Binds with high affinity to β subunit of DNA-dependent RNA polymerase • Prevents RNA synthesis • Quinolones - inhibit bacterial DNA gyrase • Sulfonamides • Structural homologs of p-aminobenzoic acid (PABA) • PABA is required for folic acid synthesis by dihydropteroate synthetase (DHPS) • Folic acid is a nucleotide precursor • Sulfa compounds compete with PABA for the active site of DHPS

12. DHPS Nucleic Acid Synthesis Inhibitors

13. Resistance to Antimicrobial Drugs • Mechanisms of resistance • Enzymes that cleave or otherwise inactivate antibiotics • β-lactamases • Changes in bacterial permeabilities • Prevents entry of antibiotic into cell • Mutation in target molecule • Alter binding characteristics of the antibiotics • Alteration of metabolic pathways • Some resistant bacteria can acquire PABA from the environment • Molecular pumps (efflux systems) • Secretion systems that export antibiotics faster than the rate of import

14. Nongenetic Origins of Drug Resistance • Low replication rates • Antibiotic is metabolized or neutralized before it act • Mycobacteria spp. • Alteration of cellular physiology • Bacterial L forms are cell wall-free • Streptococcus spp., Treponema spp., Bacillius spp., others • Colonization of sites where antibiotics cannot reach • Gentamicin cannot enter cells • Salmonella are thus resistant to gentamicin

15. Genetic Origins of Drug Resistance • Chromosomal Resistance • Genes that regulate susceptibility • Often found in enzymes, rRNA and secretion system genes • Mutations in RNApol render it resistant to the effects of rifampin • Efflux pumps with specificity for antibiotics • Found in all bacteria • All possess large hydrophobic cavity for binding antibiotics Five efflux pumps (“antiporters”) that regulate antibiotic resistance (Paulsen, 2003)

16. Genetic Origins of Drug Resistance • Extrachromosomal Resistance • Often account for interspecies acquisition of resistance • Contribute to multi-drug resistance (MDR) • Genetic elements are: • Plasmids • Transposons • Conjugation • Transduction • Transformation

17. Drug Resistance

18. Antimicrobial Activity In Vivo • Drug-Pathogen Relationships • Environment • State of metabolic activity: slow-growing or dormant bacteria less susceptible • Distribution of drug: CNS is often exclusionary • Location of organisms: Some drugs do not enter host cells • Interfering substances: pH, damaged tissues, etc. • Concentration • Absorption: some cannot be taken orally • Distribution: some accumulate in certain tissues • Variability of concentration: peaks and troughs • Postantibiotic effect: delayed regrowth of bacteria

19. Antimicrobial Activity In Vivo • Host-Pathogen Relationships • Alteration of tissue response • Suppression of microbe can reduce inflammatory responses • Alteration of immune response • Prevention of autoimmune antibodies (e.g., rheumatic fever) • Alteration of microbial flora • Expansion of harmful flora (e.g., C. difficile)

20. Clinical Use of Antibiotics • Selection of appropriate antibiotic • Accurate diagnosis is critical • Susceptibility testing should be performed if: • Isolated microbe is often antibiotic resistant • Infection would likely be fatal if incorrect drug is selected • Need rapidly bactericidal activity (e.g., endocarditis) • Susceptibility testing is often performed with antibiotic discs • A large zone of clearance suggest sensitivity

21. Minimal Inhibitory Concentration • The MIC determines the dose of antibiotic necessary to kill or retard bacteria • It is usually done as a tube test (i.e., liquid phase) • Serial dilutions of an antibiotic is made, then a defined number of bacteria are added to the tubes • Tubes are read the following day (or days) for the endpoint

22. Minimal Inhibitory Concentration

23. Dangers of Indiscriminate Use • In some countries antibiotics are available OTC • This has led to the emergence of antibiotic resistance • Often the wrong antibiotic is used • The full regimen is not completed • Hypersensitivities (e.g., penicillin anaphylaxis) • Hepatotoxicity • Changes in normal flora

24. Antimicrobial Chemoprophylaxis • Exposure to specific pathogens (e.g., N. meningitidis) • Health-related susceptibilities • Heart disease/valve replacement • Respiratory disease (e.g., influenza, measles) • Recurrent urinary tract infections • Opportunistic infections • Post surgery • Disinfectants • Medical devices (e.g., catheters)

25. Antifungal Drugs

26. Antiprotozoal and Antihelminth Drugs

27. Antiprotozoal and Antihelminth Drugs

28. Toxic Side Effects • Penicillins: Hypersensitivity • Cephalosporins: Hypersensitivity, nephritis, hemolytic anemia • Tetracyclines: Discoloring of teeth • Chloramphenicol: Disruption of RBC production • Erythromycins: Hepatitis • Vancomycin: Deafness, leukopenia, renal damage • Sulfonamides: Hemolytic anemia, bone marrow depression