Antimicrobials & Resistance History 1908, Paul Ehrlich - Arsenic compound Arsphenamine 1929, Alexander Fleming - Discovery of Penicillin 1935, Gerhard Domag - Discovery of the red dye Prontosil (sulfonamide) 1944, Selman Waksman - Isolation of streptomycin from Streptomyces griseus 1
Bacteriostatic vs Bacteriocidal Bacteriostatic - Inhibit the growth of bacteria removed by body defenses - eg. tetracycline, sulfonamides, and chloramphenicol Bactericidal - Rapidly kill the target bacteria - eg. penicillin, streptomycin, and cephalosporins Growth Normal growth Bacteriostatic Bactericidal time MIC Minimum Inhibitory Concentration (MIC) - The minimum concentration of the drug necessary to inhibit the growth of bacteria - The most common test High Antibiotic concentration Low Bacteria 1 More resistant Bacteria 2 2
Spectrum Narrow spectrum: Some bacteria Gram-positive Gram-negative Broad spectrum: Both Gram-positive and Gram-negative Gram-positive? Gram-negative? Some bacteria? Antibiotic Targets Selective Toxicity - Antibiotics should be toxic for bacteria not for the host - Antibiotic targets should be present uniquely in bacteria Cell wall : β-lactam antibiotics (penicillins, cephalosporins) glycopeptides (vancomycin) Membrane: polypeptides (polymyxin) DNA RNA Protein DNA gyrase: quinolones, novobiocin DNA synthesis: sulphonamides DNA structure: metronidazole RNA-polymerase: rifampins Protein synthesis: -30S: aminoglycosides, tetracyclines -50S: chloramphenicol, macrolides, lincosamides 3
Inhibition of cell wall synthesis :Peptidoglycan synthesis Gram-positive cell wall Gram-negative cell wall Inhibition of cell wall synthesis :Peptidoglycan synthesis N-acetyl amino sugars Peptide 4
Inhibition of cell wall synthesis Beta lactams (Penicillins) - The beta lactam ring is structurally similar to the substrate (Dalanyl-D-alanine) of transpeptidase enzymes - The transpeptidase bind to penicillin, they are often referred to as penicillin binding proteins (PBP) -Weakening the peptdidoglycan bacterium bursts Inhibition of cell wall synthesis Cephalosporins - Action mechanism is similar to penicillins - Advantages: Resistance to penicillinase; not as allergenic as penicillin; broad spectrum of activity cephalosporin Bacitracin - A polypeptide isolated from Bacillus subtilis that interact with the bacterial cell wall Vancomycin - Prevent the synthesis of peptidoglycan - Used to treat serious staphylococcal infections in humans 5
Cell membrane inhibitors Generally, these compounds are more toxic Polymyxins - Binds to the outer surface of the cell membranes and disrupt the structure - Active against Gram-negatives but limitedly used for topical applications O antigen - Core - LipidA - Cationic Polypeptide + - O antigen Core Lipid A Targeting DNA Quinolones - Inhibit DNA gyrase A and topoisomerase IV Selectively block DNA synthesis - Totally synthetic antimicrobials Novobiocin -Inhibit DNA gyrase B (cf. Quinolones DNA gyrase A) Metronidazole - Its reduction by ferredoxin generates toxic compounds (free radicals) DNA damage 6
Targeting RNA Rifampin - Bactericidal - Bind to DNA-dependent RNA polymerase of bacteria inhibit RNA synthesis - No effect on the eukaryotic enzyme Inhibition of growth by analogues Sulfonamides - Bacteria require para aminobenzoic acid (PABA) to form folic acid cf. Eukaryotes can transport folates via membrane transport proteins - The sulfa drugs are analogues of PABA and compete with it DNA synthesis DHF 7
Targeting Protein Synthesis Ribosomes - Prokaryotes: 70S ribosomes (30S + 50S subunits) - Eukaryotes: 80S ribosomes (40S +60S subunits) Targeting 30S ribosomal subunit 1) Aminoglycoside antimicrobials - Streptomycin, neomycin, kanamycin, gentamicin, spectinomycin 2) Tetracycline - Widely used in veterinary medicine - Staining of calcified tissues (teeth and bones), a problem in human medicine Targeting Protein Synthesis Targeting 50S ribosomal subunit 1) Chloramphenicol - A low percentage of humans develop a severe and fatal anemia if treated with this drug; Strictly prohibited in food producing animals 2) Florfenicol - A structural analog of chloramphenicol w/o the same side effects 3) Marolide - Broad spectrum may eliminate much of the normal flora in the intestines 4) Lincosamide - Lyncomycin: Commonly used in feed in the US - Clindamycin: More commonly used in human medicine 8
Antibiotic Targets Cell wall : β-lactam antibiotics (penicillins, cephalosporins) glycopeptides (vancomycin) Membrane: polypeptides (polymyxin) DNA RNA Protein DNA gyrase: quinolones, novobiocin DNA synthesis: sulphonamides DNA structure: metronidazole RNA-polymerase: rifampins Protein synthesis: -30S: aminoglycosides, tetracyclines -50S: chloramphenicol, macrolides, lincosamides Antibiotic Resistance Meticillin-resistant S. aureus Vancomycin-resistant enterococci (VRE) Multidrug-resistant Gram-negatives 9
Overall Resistance Mechanisms Hydrophobic antibiotics Hydrophilic antibiotics Horizontal transfer Porin Pump Spread of resistance genes Horizontal Gene Transfer Nature Reviews Microbiology, 2006. 4:36-45 10
Development of Antimicrobials Slow-down of antibiotic discovery Fischbach&Walsh, Science, 2009.325, 89-93. Nature Reviews Microbiology, 2006. 4:36-45 Antibiotics as a Growth Promoter Intestine Growth Promotion & Disease Prevention 11
Antibiotics as a Growth Promoter Intestine R R R 80% of the antibiotics used in the US are used in agriculture Jan 21, 2002 12
Antibiotics as a Growth Promoter Macrolide resistance in Campylobacter coli from pigs in Denmark Ban Ban of AGP has increased the antibiotic use for therapeutics (eg. France and Netherland) Antimicrobial Susceptibility Tests Disk diffusion (Kirby-Bauer) test Lawn of test bacterium 13
Antimicrobial Susceptibility Tests Epsilometer test (Etest) Broth Dilution Dilution Susceptibility Tests High Antibiotic concentration Low Agar Dilution High Antibiotic concentration Low 14
Take-Home Message Thank you! Let s stretch! 15