Antimicrobial chemotherapy - agents and mechanisms of action Flashcards
When are antimicrobial agents therapeutically useful?
Useful if the target is;
- not present in man
- if microorganism has higher affinity for the drug than man
- most antibiotics in clinical usage are directed against bacterial cell wall synthesis, bacterial ps or bac nucleic acid synthesis
Selective toxicity?
- Must be highly effective against microbe but have minimal or no toxicity to humans
- In practice it is expressed y a drug’s therapeutic index (TI) - ratio of the toxic dose to the pt to the therapeutic dos to eliminate the infec
- larger the index = safer the drug is
What are antimicrobial agents? Types?
Can be narrow (either gram pos or neg) or broad spectrum (against gram pos and neg)
Control specific infecting organisms
Desirable characteristics of antibiotics?
- Wide spectrum activity and can destroy or inhibit many different species of pathogenic organisms
- Non-toxic to host
- No undesirable side effects
- Non-allergenic
- Reach body part with infec
- Not eliminate normal flora of host
- Cheap, easy to produce
How are antimicrobials classified?
By chemical structure, target site and according t whether they are bactericidal (kill) or bacteriostatic (inhibit growth)
What are the main targets for antimicrobials?
- Cell wall - peptidoglycan
- PS - ribosomes or enzymes
- Metabolic pathways
- DNA
- Membranes
- Enzymes
How to test for antibiotics?
Disc diffusion on agar - if bac sensitive to antibiotic = clear zones formed
How to test for antibiotics in liquid?
MIC/MBC test
MIC = minimal inhibitory conc = min antibiotic needed to inhibit bac growth = conc that stops growth = clear broth
MBC - minimal bactericidal conc = min. to kill bac = lowest conc where no colonies formed = clear plate
Add same amount of bac to each tube and decrease conc of antibiotic in each tube
Cell wall targets?
Peptidoglycan = unique, needs to be crosslinked = AA crosslinked
What are the main classes of agents that act against the cell wall?
- Beta-lactams - penicillins and cephalosporins
- Glycopeptides - vancomycin
- Cycloserine - inhibits alanine racemase
Beta-lactam antibiotics?
Bactericidal compounds, contain beta-lactam ring and inhibit normal cell wall formation
Beta-lactam ring can have diff structures attached;
- penicillins - 5 membered
- cephalosporins - six-membered
What do beta-lactam’s do?
Inhibit peptidoglycan formation
How does penicillin’s structure aid it’s function?
Mimics structure of D-ala-D-ala = inhibits formation of peptidoglycan cross-links in the bacterial cell wall by binding of the 4-membered beta-lactam ring of penicillin to the enzyme DD-transpeptidase
DD-transpeptidase cannot then catalyse formation of these crosslinks = cell death
Vancomycin?
Effective against Gram positive organisms
Binds to D-alanyl D-alanine dipeptide on side chain of newly synthesised peptidoglycan subunits, preventing them from being incorporated into cell wall by penicillin binding proteins (PBPs)
How do antibiotics inhibit protein synthesis?
- Aminoglycosides – bind to 30S subunit & cause misreading of genetic code
- Tetracyclines – inhibit binding of tRNA to mRNA – ribosome complex
- Erythromycin – binds to a molecule in 50S subunit blocking exit of nascent polypeptide chain
- Fusidic acid – ‘elongation factor G (EFG)’ – bacterial protein needed for translocation on bacterial ribosome after peptide bond formation during protein synthesis. Fucidic acid binds EFG preventing protein synthesis.
Aminoglycosides?
Bind to 30S subunit = misreading of genetic code
contain an aminocyclitol ring linked to a sugar
Effective against aerobes and facultative anaerobes
Not active against anaerobes
Why are aminogylcosides not effective against anaerobes?
Bacterial up-take requires oxygen- or nitrate-dependent electron transport
Problem with aminoglycosides?
Not absorbed from the gut
must be given intravenously or intramuscularly for systemic treatment
Side effects – nephrotoxicity, ototoxicity
Tetraclyclines?
Inhibit binding of tRNA to mRNA/ribosome complex
- Bacteriostatic compounds = all broad spectrum, penetrate mammalian cells = reach intracellular organisms, incorporated in developing bone and teeth
- Use restricted due to resitance
Macrolides?
- Bind to 50S subunit blocking exit of nascent polypetide chain
- Large cyclic molecules containing macrocyclic lactone ring - bacteriostatic
- Erythromycin most commonly used = e.g. for penicillin allergic pts - penetrates mammalian cells to reach intracellular organisms
What are the agents that affect DNA? How?
- Quinolones - Target site is DNA gyrase – DNA gyrase is a topoisomerase that relieves strain while DS DNA is being unwound by helicase.
- Rifamycins – inhibit DNA dependent RNA synthesis due to high affinity for bacterial RNA polymerase (poor affinity for mammalian)
- Metronidazole - disrupts DNA – only works when it is reduced and as this reaction only usually happens in anaerobic cells – effective against anaerobes
Nitroimidazoles?
Disrupt DNA
E.g metronidazole, tinadazole = antiparasitic and antibac properties
How does metronidazole work?
- Activated in cell by redox enzyme pyruvate-ferredoxin oxidoreductase
- In anaerobes, ferredoxin is an e- transporter molecule that reduces (gives electrons to) Metronidazole
- This single electron transfer reduces nitro group of met. creating highly reactive anion – disrupts DNA helix
intermediate is short-lived and decomposes - Metronidazole is active only against strictly anaerobic organisms
because only these can produce the low redox potential necessary to reduce the drug.
Which antibiotics interfere with metabolic pathways?
- Folic acid enzymes = needed for AA synthesis and PS
- E.g Sulfonamides = inhibit these enzymes
- Active against gram pos and neg
Problem with metabolic pathway antibiotics?
Resistance
Define antibiotic resistance
Organism that is not inhibited or killed by an antibacterial agent at concentrations of the drug achievable in the body after normal dosage.
How is antibiotic resistance developed?
Chromosomal mutation
Coded for by plasmid DNA
Plasmids are transmissible
Transposons can carry resistance genes and jump between chromosome and plasmid
How is antibiotic resistance transferred?
Cassettes of multiple resistance genes can be organised into genetic elements called integrons
Integrons contain gene for recomination enzyme to allow insertion
Mechanisms of antibiotic resistance?
- The target is structurally altered (by mutation)
Now has a lower affinity for the antibacterial
Pencillin-binding proteins
- The target is overproduced Dihydropteroate synthetase (bacterial target) and sulphonamide
- The drug is not activated
Aerobes and metronidazole - The drug is removed
Enzymic destruction
β-lactamase
3 modifying enzymes responsible for acquired aminoglycoside resistance - Efflux
Tetracyclines, quinolones - The drug cannot gain entry to the cell
outer-membrane barrier, lack of transport mechanism
Antivirals?
Few in number
Narrow spectrum
Virustatic
Need to interfere with viral machinery without effecting host
How do antivirals work?
1)Penetration/uncoating
Amantadine
Prevent fusion of viral envelope with cell membrane
2) Taking over cell machinery
- Transcription
Nucleoside analogues - Zidovudin, acyclovir
Zidovudin (AZT) acts as substrate for & inhibitors of viral reverse transcriptase
Acyclovir – inhibits HSV DNA polymerase
- Translation
anti-sense morpholinos (oligomer molecules that block target sequence in RNA by binding); ribozymes (cut viral RNA)
Post-translation inhibition of antivirals?
Post-translation inhibition;
- Protease inhibitors
- Protease cleaves viral polyproteins into structural proteins required for viral replication
- Protease inhibition – immature, defective viral particles - HAART (highly active antiretroviral therapy)
