Transcript Slide 1
Antimicrobials 1: Origins and modes of action Dr Fiona Walsh Objectives of lecture • Antibiotic discovery • Time-line of currently prescribed antibiotics • General principles of antimicrobial agents • How antibiotics inhibit or kill bacteria • Introduction to all antibiotic classes Definitions • Antibiotic is a naturally occurring substance that inhibits or kills bacteria • Antibacterial is a natural, semi-synthetic or synthetic substance that inhibits bacteria • Antimicrobial agent is a natural, semisynthetic or synthetic substance that inhibits microbes Antibiotic discovery 19th Century Louis Pasteur Robert Koch Identified bacteria as causative agent of disease. (Germ theory) Now know what is causing disease, need to find out how to stop it. 1877 Pasteur Soil bacteria injected into animals made Anthrax harmless 1888 de Freudenreich Isolated product from bacteria with antibacterial properties. Toxic and unstable. Antibiotic discovery 20th Century Erhlich Worked with dyes and arsenicals worked against Trypanosomes, very toxic. 1st antibacterial, only cured syphilis. Domagk Research on dyes. 1st synthetic antibacterial in clinical use. Prontosil cured streptococcus diseases in animals. Active component: sulphonamide group attached to dye. Toxic. Sulphonamide derivatives still used. Less toxic. Antibiotic discovery 20th Century Fleming and serendipity (1928) Plates left on bench over weekend. Staphylococcus colonies lysed/killed. Fungi beside Staphylococcus. Hypothesis: Fungi lysed Staph. Unable to purify in large quantities. No animal or human tests performed. Antibiotic discovery 20th Century Florey, Chain Purified the penicillin from the fungus. and Heatley (1939) 1940s (World War II) European and US cooperation led to increased scale production of penicillin. Antibiotic discovery 20th century Waksman (1943) Isolated streptomycin from soil bacteria Streptomyces. Effective against Mycobacterium tuberculosis and gram negatives. Toxic antibiotic. Used until 1950s when isoniazid used due to shorter course of therapy. 1928 Discovery Penicillin 1932 Sulphonamides 1939 Gramicidin 1930 1940 1942 1943 1945 1947 1949 1948 1952 Streptomycin & Bacitracin Cephalosporins Chloramphenicol & Chlorotetracycline Neomycin Trimethoprim Oxytetracycline Erythromycin 1956 1957 Vancomycin Kanamycin 1961 1963 1964 1966 1967 1968 Nalidixic acid Gentamicin 1971 1972 Tobramycin Cephamycins & Minocycline Fluoroquinolones Oxazolidinones 1950 1960 Clindamycin 1970 1980s 1990s General Principles • Selective toxicity – The essential property of an antimicrobial drug that equips it for systemic use in treating infections is selective toxicity – Drug must inhibit microorganism at lower concentrations than those that produce toxic effects in humans – No antibiotic is completely safe General Principles • Oral and Parental – Oral antibiotics must be able to survive stomach acid – Advantage: Ease and reduced cost – Disadvantage: Circuitous route, antibiotic passes to lower bowel – Parental antibiotics given by i.v. – Advantage: Direct route to site of infection – Disadvantage: Increased cost and need for qualified staff General Principles • Half-Lives – The length of time it takes for the activity of the drug to reduce by half – Short half lives require frequent dosing – Old antibiotics have short half lives – New antibiotics may have half lives up to 33 hours General Principles • Broad and Narrow spectrum antimicrobials – Broad spectrum antibiotics inhibit a wide range of bacteria – Narrow spectrum antibiotics inhibit a narrow range of bacteria – Broad spectrum desirable if infecting organism not yet identified – Narrow spectrum preferable when organism has been identified General Principles • Bactericidal or bacteriostatic action – Bactericidal antibiotics kill bacteria – Bacteriostatic antibiotics inhibit the bacterial growth – Bacteriostatic antibiotics may work as well as bactericidal antibiotics if they sufficiently arrest the bacterial growth to enable the immune system to eliminate the bacteria General Principles • Combinations of antibiotics – Some antibiotics work better together than alone – Combining 2 or more drugs may be required to prevent the emergence of resistance e.g. tuberculosis – Combinations should not be given when 1 drug would suffice • Antagonistic effects • No ability to adjust 1 drug concentration Modes of action Antimicrobial agents inhibit 5 essential bacterial processes: 1. 2. 3. 4. 5. Protein synthesis Folic acid synthesis DNA synthesis RNA synthesis Cell wall synthesis 1. Protein synthesis inhibitors Protein synthesis DNA mRNA transcription Protein translation Ribosome is a protein factory in bacteria takes mRNA in and produces proteins from them. Bacterial ribosome has 2 parts: – – 30S binds to mRNA to translate mRNA into amino acids, which form proteins 50S required for peptide elongation 3 phases from mRNA to protein – Initiation – Elongation – Termination Protein synthesis inhibitors – Aminoglycosides – Macrolides/Ketolides – Tetracyclines – Lincomycins – Chloramphenicol – Oxazolidinones Protein synthesis inhibitors • Bind irreversibly to ribosome • Ribosome cannot bind to mRNA to form amino acid chains (30S) or elongate the chains to form proteins (50S) • Disruptive effect on many essential bacterial functions leading to cell death 2. Folic acid synthesis inhibitors pterdine + para-amino benzoic acid Dihydropteroate synthetase dihydropterate Sulphamethoxazole (Sulphonamides) Structural analogues of PABA dihydrofolate Dihydrofolate reductase tetrahydrofolate Trimethoprim (Diaminopyrimidines) Binding DNA/RNA Reasons for combining Trimethoprim and Sulphonamides • There is synergy between the two drugs - the combined effect is greater that the expected sum of their activities • Individually the drugs are bacteriostatic; however, in combination they are bactericidal • The use of two drugs will delay the emergence of resistance 3. DNA synthesis inhibitors • Enzymes required for DNA replication • Topoisomerase II (DNA gyrase): GyrA and GyrB • Topoisomerase IV: ParC and ParE • Quinolones interact/bind to the topoisomerases, which stops DNA replication e.g. nalidixic acid, ciprofloxacin Action of fluoroquinolones GyrA/ GyrB DNA ParC/ ParE DNA gyrase Topoisomerase IV Quinolones Cell death DNA synthesis inhibitors • Metronidazole – Nitro group is reduced by bacterial enzyme – Produces short-lived, highly cytotoxic free radicals that disrupt the DNA – Similar effect to UV radiation on cell DNA 4. RNA synthesis inhibitors • Rifampicin • Forms a stable complex with bacterial DNA-dependent RNA polymerase • Prevents chain initiation process of DNA transcription • Mammalian RNA synthesis not affected as RNA polymerase is much less sensitive to rifampicin 5. Cell wall synthesis inhibitors – Vancomycin – Bacitracin – β-lactams • • • • Penicillins Cephalosporins Carbapenems Monobactams – β-lactamase inhibitors • Clavulanic acid • Sulbactam • Tazobactam Action of Cell wall synthesis inhibitors N-acetyl-glucosamine (NAG) Phospho-enol pyruvate Peptidoglycan formation 1. Building Blocks N-acetyl-muramic acid (NAMA) L-alanine D-glutamic acid L-lysine NAMA L-ala-D-glu-L-lys D-ala-D-ala NAMA L-ala-D-glu-L-lys-D-ala-D-ala D-ala L-ala Action of Cell wall synthesis inhibitors NAMA Lipid carrier Bacitracin inhibits L-ala-D-glu-L-lys-D-ala-D-ala NAG NAMA - NAG L-ala-D-glu-L-lys-D-ala-D-ala 5 gly Phospholipid NAMA - NAG L-ala-D-glu-L-lys-D-ala-D-ala 5 gly Vancomycin &Teicoplanin binds, prevents enzyme polymerisation Action of Cell wall synthesis inhibitors Polymerisation NAMA NAG NAMA NAG NAMA NAG NAMA NAG NAMA L-ala L-ala L-ala L-ala L-ala D-glu D-glu D-glu D-glu D-glu L-lys 5 gly L-lys 5 gly L-lys 5 gly L-lys 5 gly L-lys D-ala D-ala D-ala D-ala D-ala D-ala D-ala D-ala D-ala D-ala NAG 5 gly Action of Cell wall synthesis inhibitors Transpeptidation NAMA NAMA NAG NAG NAMA L-ala L-ala L-ala D-glu D-glu D-glu D-ala L-lys D-ala D-ala D-ala D-ala L-lys D-ala L-lys 5 gly NAG 5 gly NAG D-ala L-lys D-ala D-ala D-ala L-lys D-ala L-lys 5 gly D-ala D-glu D-glu D-glu L-ala L-ala L-ala NAMA NAG NAMA NAG NAMA b-lactams resemble Dala-D-ala, bind to enzyme, inhibit crosslinking Penicillin Binding Proteins Enzymes involved in cell wall formation • Reseal cell as new peptidoglycan layers added • Penicillins bind to PBPs block enzyme cross-linking chains • Weak cell wall • Build up osmotic pressure • Lysis Keynote points • Recent history of antibiotic discovery • General principles of antibiotic action • 5 modes of action • Examples of each