Introduction to Antimicrobial PDF

Introduction to Antimicrobial Dr. Ng Woei Kean PhD, Molecular Medicine  Learning Outcome At the end of the lecture, student should be able to:  Explain the terms selective toxicity, bactericidal, bacteriostatic, minimum inhibitory concentration (...

Introduction to Antimicrobial Dr. Ng Woei Kean PhD, Molecular Medicine  Learning Outcome At the end of the lecture, student should be able to:  Explain the terms selective toxicity, bactericidal, bacteriostatic, minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC).  Classify antibiotics according to their mechanism of action on bacteria with suitable examples.  Describe the mechanisms by which organisms develop drug resistance. NWK 2 The History Ancient Egyptians used moldy bread to treat wounds. NWK 3  Ernst Chain & Howard Florey – mass production of penicillin 1940 Gerhard Domagk & Ernest Fourneau – discovery of prontosil, a red dye, (contains sulfonamides), Alexander Fleming – inhibits growth of Gram-positive discovery of mold Penicillium bacteria to inhibit growth of bacteria 1910 Paul Ehrlich – mechanism of chemical substances binding to tissues NWK 4 NWK 5 “Doctors could now cure disease, and this was astonishing, most of all to the doctors themselves.” Lewis Thomas NWK 6 Definition Selective toxicity The ability of an antimicrobial agent to attack the target microorganism without causing significant damage to the host. Penicillin - toxic dosage level vs therapeutic dosage level. NWK 7 Bacteriostatic Antimicrobial activity that inhibits growth but does not kill the organisms. Bactericidal Antimicrobial activity that not only inhibits growth but is lethal to bacteria. NWK 8 Minimum Inhibitory Concentration (MIC) The lowest concentration (expressed as mg/L or μg/μL) of an antimicrobial agent that inhibits the visible in-vitro growth of microorganisms. Minimum Bactericidal Concentration (MBC) The lowest concentration of antibiotics that kills 99.9% of the inoculum. NWK 9  Bacteria + MIC antibiotic Concentration of antibiotic (սg/ml): 8 4 2 1 0.5 0.25 MBC Antibiotic-free agars NWK 10 Mechanism of Action of Antibiotics NWK 11  1. Inhibition of Cell Wall Synthesis Peptidoglycan is an important component of bacterial cell wall. NWK 12 Some antibiotics contain a chemical structure called a β-lactam ring, which attaches to the enzymes that cross-link peptidoglycans and thus, interfering the cell wall synthesis. NWK 13  Instead of binding to the enzyme that cross-links peptidoglycans, vancomycin binding directly to the terminal amino acids of the peptide side chains – preventing peptidoglycan NWK cross-linking. 14  2. Disruption of Cell Membrane Certain polypeptide antibiotics, such as polymyxins, act as detergents to distort bacterial cell membranes & alter their permeability, resulting in loss of essential cytoplasmic components and bacterial death. NWK 15 3. Inhibition of Protein Synthesis Aminoglycoside antibiotics, such as streptomycin, act on the 30S portion of bacterial ribosomes by interfering with the accurate reading (translation) of the mRNA message. NWK 16 Chloramphenicol and erythromycin act on the 50S portion of bacterial ribosomes, inhibiting the formation of the growing polypeptide NWK 17 4. Inhibition of Nucleic Acid Synthesis Quinolones – inhibiting DNA topoisomerase & DNA gyrase thus, interfering DNA replication. Rifampin – blocking RNA synthesis by binding to RNA polymerase. NWK 18 Folate inhibitors, such as sulfonamides - interfere with synthesis of folic acid by bacteria and thus inhibiting nucleic acid synthesis. NWK 19 NWK 20 Antimicrobial Resistance Mechanisms of Resistance: 1. Alteration of membrane permeability 2. Alterations of an antimicrobial target 3. Inactivation of antimicrobial NWK 21  1. Alteration of membrane permeability Change in the membrane transport system or pores in the membrane, so an antimicrobial agent can no longer cross the membrane. Pseudomonas aeruginosa develops resistance to imipenem through loss of the outer membrane protein which is required for imipenem penetration. NWK 22 Some bacteria have energy-dependent efflux mechanisms that pump either tetracyclines or fluoroquinolones from the cell. Example: carbapenem resistance in Pseudomonas aeruginosa. NWK 23  2. Alterations of an Antimicrobial Target A target alteration can be mediated by mutations or enzymatic modifications generating a protein (modified target) with reduced or null affinity for the antibiotics. NWK 24 Alteration in transpeptidases (also called penicillin-binding proteins, PBP) causes penicillin resistance in Staph. aureus (methicillin-resistant Staphylococcus aureus, MRSA). NWK 25 Vancomycin-resistant enterococci have enzyme systems that substitute an amino acid in the terminal position of the peptidoglycan side chain. Vancomycin does not bind to the alternate amino acid, and these strains are resistant. NWK 26 3. Inactivation of Antimicrobial Bacteria can acquire or develop resistance to antibiotics through the activity of enzymes which hydrolyse or chemically modify the antibiotics preventing their binding to the target. NWK 27  β-lactamase Several β-lactamases exist in various bacteria; they are capable of breaking the β-lactam ring in penicillins and some cephalosporins. Similar enzymes that can destroy various aminoglycosides and chloramphenicol have been found in certain Gram-negative bacteria. NWK 28  Modifying enzyme Bacteria produce modifying enzyme to chemically modify antimicrobial agent, particularly aminoglycosides, by acetylate, adenylate, or phosphorylate hydroxyl or amino groups on the aminoglycoside molecule. NWK 29 Summary NWK 30  Thank You References: 1. Black et al., Microbiology: Principles and Explorations, 9th Edition. 2. Ryan et al., Sherris Medical Microbiology, 7th Edition. 3. Sanseverino, I., Navarro Cuenca, A., Loos, R., Marinov, D. and Lettieri, T., 2018. State of the Art on the Contribution of Water to Antimicrobial Resistance. Publications Office of the European Union. NWK 31