Medical Microbiology Antibacterial Agents and their mechanism of action Antimicrobial Agents Antibacterial agent - chemical that kills or inhibits the growth of microorganisms at a very low concentration • Protosil (sulfanilamide) in 1935 Antimicrobial agents produced by living organisms are called antibiotics • Fleming’s discovery of penicillin Antibiotics Important Properties: • Selective toxicity • Broad-spectrum vs. narrow-spectrum drugs BACTERICIDAL AGENTS Beta lactams (penicillins, cephalosporins, imipenem) Trimethoprim/sulfamethoxazole Vancomycin Fluoroquinolones Aminoglycosides Targets of Antimicrobial Drugs Antibiotics have one of the following targets: • • • • • Inhibition of cell wall synthesis Disruption of the bacterial membrane Inhibition of protein synthesis Inhibition of nucleic acid synthesis Antimetabolites DRUGS INHIBITING CELL WALL SYNTHESIS Penicillins Cephalosporins Imipenem Vancomycin Fosfomycin β-lactams Inhibition of cell wall synthesis Inhibition of cell wall synthesis • Beta-lactams (penicillins, cephalosporins, etc.) inhibit the enzymes required to build peptidoglycan • structure of the cell wall - Barrel analogy • Penicillin inhibits the formation of the peptide links DRUGS INHIBITING CELL WALL SYNTHESIS Penicillins Cephalosporins Imipenem Vancomycin Fosfomycin β-lactams Dividing Bacteria Division Growth Plus penicillin Emerging Spheroplast Growth site Spheroplast www.uccs.edu/ Penicillin Binding Proteins Transpeptidases Penicillin Carboxypeptidases Endopeptidases BETA LACTAMASE INHIBITORS COMBINATIONS WITH BETA LACTAMASE INHIBITORS Penicillin plus a beta lactamase inhibitor. Inhibition of cell wall synthesis • Vancomycin – only effective against Gram + • Isoniazid, ethambutol, and others are only effective against Mycobacteria E nz y m e NAG-NAM D-ALA PeptidoglycanTransglycosylase L-GLU LYS VAN D-ALA D-ALA Vancomycin is in a class of medications called glycopeptide antibiotics. It works by killing bacteria in the intestines. RESISTANCE TO BETA LACTAMS Penicillinase Increased production of betalactamase (penicillinase) enzymes. Beta lactamases Damage to Membranes Damage to Membranes – Polymixin antibiotics • Only used topically because of toxicity Surface active amphipathic agents.Interact strongly with phospholipids and disrupt the structure of cell membranes DAPTOMYCIN Depolarizes the cell membrane Inhibition of protein synthesis Inhibition of protein synthesis • Aminoglycosides – bind to the 30S sub-unit and cause: • Misreading of mRNA • Early termination of translation • Tetracyclines – bind to the 30S subunit and block tRNA binding ANTIBIOTICS INHIBITING PROTEIN SYNTHESIS Macrolides Clindamycin Linezolid Streptogramins Chloramphenicol Tetracyclines Aminoglycosides ANTIBIOTICS INHIBITING PROTEIN SYNTHESIS Macrolides Clindamycin Linezolid Streptogramins Chloramphenicol Tetracyclines Aminoglycosides Procaryotic Ribosome 50S 30S 70S-M.W.2,500,000 Eucaryotic Ribosome 60S 40S 80S--M.W. 4,200,000 Antibiotics binding to the 50S ribosomal subunit and inhibiting protein synthesis Erythromycin and other macrolides Chloramphenicol Linezolid Streptogramins Antibiotics binding to the 30S ribosomal subunit and inhibiting protein synthesis Aminoglycosides Tetracyclines CLEanS TAg Macrolides (Erythromycin, Azithromycin and Clarithromycin) Nascent polypeptide chain A 50S MACROLIDES Transferase site aa mRNA template P 30S TRANSLOCATION CHLORAMPHENICOL Chloramphenicol Nascent polypeptide chain 50S A Transferase site aa mRNA template P 30S Mechanism of action of Chloramphenicol INITIATION STREPTOGRAMINS Quinupristin/Dalfopristin (30:70) Nascent polypeptide chain 50S DALFOPRISTIN A QUINUPRISTIN (MACROLIDE) Transferase site aa mRNA template P 30S INITIATION AMINOGLYCOSIDES Bind irreversibly to the 30S subunit. Exact mechanism of cell death is unknown. Postantibiotic effect. 50S A aa Nascent polypeptide chain Transferase site mRNA template P 30S Tetracycline INHIBITION OF MITOCHONDRIAL PROTEIN SYNTHESIS Mitochondrial ribosome resembles bacterial ribosome. May account for some toxic effects (e.g. chloramphenicol, linezolid). RESISTANCE Alterations in ribosomal proteins (e.g. macrolides). Decreased permeability to the antibiotic. TETRACYCLINE RESISTANCE ATP Tetracycline Inhibition of nucleic acid synthesis Inhibition of nucleic acid synthesis • Quinolones – inhibit DNA replication enzymes (gyrase) • Ciprofloxicin and others • Rifampin – disrupts transcription Antimetabolites Antimetabolites: • Sulfonamides and trimethoprim – inhibit folic acid synthesis • broad-spectrum effectiveness ANTIBIOTICS ACTING AS ANTIMETABOLITES Sulfonamides Trimethoprim plus sulfamethoxazole FOLIC ACID BIOSYNTHESIS DIHYDROPTERIDINE 2 ATP PYROPHOSPHATE DERIVATIVE Dihydropteroate 2HN Synthetase DIHYDROPTEROIC ACID Glutamic Acid DIHYDROFOLIC ACID COOH 2HN SO2NH2 TRIMETHOPRIMSULFAMETHOXAZOLE OCH3 2HN CH OCH3 2 OCH3 80 mg TRIMETHOPRIM 2HN SO2NH N O 400 mg SULFAMETHOXAZOLE CH3 PABA Dihydropteroate Synthetase + Pteridine SULFONAMIDE DIHYDROPTEROIC ACID Dihydrofolate Synthetase DIHYROFOLIC ACID Dihydrofolate Reductase TRIMETHOPRIM TETRAHYDROFOLIC ACID Advantages of sulfonamidetrimethoprim combination SULFONAMIDERESISTANCE Results from multiple mechanisms. Altered dihydropteroate synthetase. Cross-resistance among all sulfonamides. ANTIBIOTICS ACTING AS ANTIMETABOLITES Sulfonamides Trimethoprim plus sulfamethoxazole FOLIC ACID BIOSYNTHESIS DIHYDROPTERIDINE 2 ATP PYROPHOSPHATE DERIVATIVE Dihydropteroate 2HN Synthetase DIHYDROPTEROIC ACID Glutamic Acid DIHYDROFOLIC ACID COOH 2HN SO2NH2 TRIMETHOPRIMSULFAMETHOXAZOLE OCH3 2HN CH OCH3 2 OCH3 80 mg TRIMETHOPRIM 2HN SO2NH N O 400 mg SULFAMETHOXAZOLE CH3 PABA Dihydropteroate Synthetase + Pteridine SULFONAMIDE DIHYDROPTEROIC ACID Dihydrofolate Synthetase DIHYROFOLIC ACID Dihydrofolate Reductase TRIMETHOPRIM TETRAHYDROFOLIC ACID SULFONAMIDERESISTANCE Results from multiple mechanisms. Altered dihydropteroate synthetase. Cross-resistance among all sulfonamides. ANTIBIOTICS AFFECTING NUCLEIC ACID SYNTHESIS. Fluoroquinolones Metronidazole Rifampin FLUOROQUINOLONES FLUOROQUINOLONES Gyrase (Topoisomerase I)-older quinolones Topoisomerase IV-3rd and 4th gen quinolones. FQ RESISTANCE Changes in gyrase and topoisomerase Increased efflux Metronidazole Metronidazole Ferredoxin reduced Short lived intermediates DNA RNA Protein Other targets Inactive end products Inactive End Products Mechanism of action of metronidazole on an anaerobic organism RIFAMPIN BACTERIOSTATIC AGENTS Sulfonamides Drugs inhibiting protein synthesis except aminoglycosides (macrolides, chloramphenicol, tetracyclines etc). Antibiotic Resistance Mechanisms of resistance: • destroy the drug • Beta-lactamase - enzyme that destroys penicillins and other beta-lactams • alter the drug’s target site • increase drug elimination • become impermeable to the drug Antibiotic Resistance Acquisition of resistance: • spontaneous mutation • transfer of plasmids containing a resistance gene Antibiotic Resistance Human practices leading to resistance: • overuse of antibiotics • use of antibiotics by immunosuppressed individuals • Failure of patients to follow a prescribed drug treatment • use of antibiotics in animal feed Drug Targets What would be a problem with drugs used to fight fungal infections? How about those used to fight viral infections?
