Antibiotic Mechanisms of Action and Resistance

1. Antibiotic Mechanisms of Action and Resistance MLAB 2434 – Microbiology Keri Brophy-Martinez

2. Overview • Antimicrobial Therapy • Broad term for use of chemical compounds to treat diseases caused by microorganisms • Antimicrobial agents used to treat infections are directed by antimicrobial susceptibility testing (AST) • Targets specific body sites or specific characteristics of microbes

3. Agents • Antimycobacterials • Treat mycobacterial diseases • Antifungals • Treat fungal disease • Antiprotozoals • Tread protozoal disease • Antivirals • Treat viral disease • Antibiotics • Treat bacterial disease

4. Antibiotics • Antibiotics are naturally occurring substances produced by a fungus or bacteria • Used to treat bacterial infections • Alternate Forms • Synthetic • Totally manufactured or artificial • Semi-synthetic compounds • Naturally occurring substances that have been chemically altered

5. Definitions • Bacteriocidal • Kills the bacteria • Bacteriostatic • Inhibit microbial growth

6. Definitions (Cont’d) • Spectrum of activity • Range of susceptible organisms • Narrow-spectrum • Kill either Gram positive or Gram negative organisms • Organism specifc • Broad-spectrum • Kill both Gram positive and Gram negative organisms • Extensive • Empirical therapy • Initiation of therapy prior to organism ID

7. Definitions (Cont’d) • Additive Effects • Combining two antimicrobials causes twice the effect of the two drugs by themselves • Indifference • No effect of combining antimicrobial therapies • Synergy • Combined effect is greater than the two individual effects added together • Antagonism • One drug counteracts the other

8. Antimicrobial Agents:Factors to Consider • What is the targeted bacteria? • Where is it located? Can the antimicrobial reach that site in sufficient concentration? • Can the antimicrobial be retained in the body long enough to be effective? • What are the side effects? How is it excreted? • What is the cost?

9. Antimicrobial Categories • Mechanisms of action • Effects on Cell Wall Synthesis • Interruption of Cell Membrane Structure and Function • Inhibition of Protein Synthesis • Inhibition of Folate Synthesis • Interference with Nucleic Acid Metabolism

10. Effects on Cell Wall Synthesis • Cell wall protects the bacteria cytoplasmic membrance • Cell wall primarily composed of a peptidoglycan layer • Inactivating or interfering with enzymes that synthesize the cell wall can destroy the bacteria

11. β-Lactam Antibacterial Agents • Effect cell wall synthesis • Sizable portion of antibacterial agents used today • Includes penicillins, monobactams, and carbapenems, and cephalosporins

12. β-Lactam Antibacterial Agents: Overview • Bind specific enzymes known as penicillin-binding proteins (PBPs) • PBPs mediate peptidoglycan cross-linking • If PBPs are bound by the beta-lactam, the cross-linking of the cell wall is incomplete, results in cell death

13. β-Lactam Antibacterial Agents • Penicillins • Simple penicillins are effective against many streps, Neisseria, Pasteurella, and a number of anaerobes • Monobactams • Limited to aerobic Gram negative bacilli • Carbapenems • Broadest antimicrobial spectrum • Effective against gram positive and negative organisms, and anaerobes • Resistant to beta-lactamase • Cephalosporins • Classified by their spectrum of activity and are spoken of in terms of “generations”

14. Generations of Cephalosporins • First-generation • Have good GP and GN activity • Second-generation • Have better GN, and anerobes activity • Third-generation • Better with Enterobacteriaceae and Pseudomonas spp. • Fourth –generation • Effective against GNR that are resistant to 3rd generation cephalosporins • Fifth-generation • Spectrum of activity includes the 3rd and 4th generation

15. β-Lactam/β-Lactamase Inhibitors • Combination of a β-lactam and a β-lactamase inhibitor act in synergy • Bind to beta-lactamase produced by certain microbes • β-Lactamase Inhibitors • Offer no antibacterial activity by themselves • Examples include: clavulanic acid, sulbactam, tazobactam

16. Effects on Cell Wall Synthesis • Glycopeptides • Bind certain amino acids and inhibit enymes in the developing peptidoglycan layer • Vancomycin • Most clinically important • Effective against MRSA, other GP organisms, and organisms resistant to penicillin

17. Interruption of Cell Membrane Structure and Function • Damages the cytoplasmic membrane of the organism • Bacitracin • Prevents the addition of peptidogylcan to the cell wall • Disrupts the cell membrane • Primarily effective against GP organism • Because of toxicity, these are limited to topical medications (ex. Neosporin, etc.)

18. Interruption of Cell Membrane Structure and Function • Polymyxins • Bind to outer surface of cell membrane, affecting phospholoid • Leads to leakage of intracellular contents and cell death • Effective against gram negative bacteria

19. Inhibition of Protein Synthesis • These antimicrobials bind to ribosomal subunits • This binding is either irreversible, resulting in cell death(bactericidal), or reversible, resulting in bacteriostatic effects • Antibiotics • Aminoglycosides, tetracyclines, macrolides, clindamycin chloramphennicol, and oxazolidinone

20. Antibiotics of Protein Synthesis Inhibition • Aminoglycosides • Bactericidal • Used primarily against GN bacteria

21. Antibiotics of Protein Synthesis Inhibition • Tetracyclines • Bacteriostatic • Broad spectrum • Effective against GP and GN organisms • Tetracycline is NOT used in young children or in pregnancy, as it affects tooth and bone development

22. Antibiotics of Protein Synthesis Inhibition • Macrolides • Bacteriostatic • Broad spectrum • Effective against GP and some GN organisms, spirochetes, Mycoplasma, Legionella, and Chlamydia • Agents include: erythromycin, azithromycin, clarithromycin

23. Antibiotics of Protein Synthesis Inhibition • Clindamycin • Bacteriostatic • Excellent activity against aerobic GP organisms • Extremely potent against anaerobes • “D” test • Detects resistance to clindamycin based on past treatment with erythromycin

24. Antibiotics of Protein Synthesis Inhibition • Chloramphenicol • Bacteriostatic • Has broad activity but is extremely toxic • Oxazolidinone • Linezolid • Effective against MRSA, VRE, and mycobacteria

25. Inhibition of Folate Synthesis • Folic acid pathway provides essential precursor molecules for DNA synthesis • Antibiotics can block steps in this pathway resulting in cell death • Agents: sulfonamides, trimethoprim • Used in combination • Active against broad spectrum, including GP and GN organisms, except for P. aeruginosa

26. Interference with Nucleic Acid Metabolism • Interfere with either DNA or RNA metabolism • Inhibit enzymes required in the replication process • Agents: quinolones/fluoroquinolones, rifamycins

27. Antibiotics of Nucleic Acid Metabolism Interference • RNA Synthesis Interference • Rifampin • Mainly used for M. tuberculosis and M. avium complex • Has a broad spectrum of activity

28. Antibiotics of Nucleic Acid Metabolism Interference • DNA Synthesis Interference • Quinolones/Fluoroquinolones • Bactericidal • Used to treat GN organisms • Agents- ciprofloxacin, levofloxacin • Metronidzole • Activates under anaerobic conditions • Effective against anaerobes and protozoa, bacterial vaginosis • Nitrofurantoin • Used against GN and GN organisms • Concentrates well in urine

29. Mechanisms of Antimicrobial Resistance • Modify target • If target is altered, reduction or prevention of antimicrobial binding can occur • End result- antimicrobial is ineffective • How does the microbe modify the target? • Chromosomal mutations • Transposons • Plasmids

30. Mechanisms of Antimicrobial Resistance • Inactivation of Antimicrobial Agent • Genes of the microbe encode enzymes that convert active antimicrobial agents to an inactive form • Encoding of enzymes via chromosomal or plasmid-mediated genes • Example: beta-lactamase producing organisms

31. Mechanisms of Antimicrobial Resistance • Blockage of antimicrobial entry into the cell • Mechanisms • Decreased permeability • Decreased uptake • Increased ability to pump antimicrobial out of cell

32. References • Kiser, K. M., Payne, W. C., & Taff, T. A. (2011). Clinical Laboratory Microbiology: A Practical Approach . Upper Saddle River, NJ: Pearson Education. • Mahon, C. R., Lehman, D. C., & Manuselis, G. (2011). Textbook of Diagnostic Microbiology (4th ed.). Maryland Heights, MO: Saunders. • http://www.parn.org.pk/index_files/D.test.html