Antimicrobial Compounds and Modes of Action

• What are antimicrobials?


  Compounds that target essential biological processes or unique features of microbes

  
  

  Targets must be sufficiently different from the host to reduce potential off-target activity and side effects (toxicity)
• What is important for antimicrobials to exhibit?

  Selective toxicity

  Which is the ability of drug to kill or inhibit pathogen while damaging host as little as possible
• What does it mean if an antimicrobial has a broad/narrow spectrum?

  Broad- affects many different species of organism

  
  

  Narrow- specific to one particular group

  
• What is the difference between antibiotics and antimicrobials?

  Antibioticsare natural products produced by microbes that inhibit or kill other microbes i.e. penicillin.

  Antimicrobialsare synthetic formulations (sometimes based on molecular structure of natural products)
• How is antimicrobial activity determined?

  Minimum Inhibitory Concentration (MIC)-lowest concentration of substance that inhibits growth of microbe

  Minimum Lethal Concentration (MLC)-lowest concentration of substance that kills microbe
• What does it mean if antimicrobials are static/cidal towards the microbe?

  Static- prevents growth of the microbe so immune defence can kill it (i.e. fungistatic)

  Cidal- kills microbe directly (i.e. bactericidal)

  E.g bacteriostatic and bacteriocidal
• What is the general mechanism of action of antibacterials?

  Antibacterials target essential bacterial processes including synthesis of cell wall, protein synthesis and plasma membrane
• What is the structure of peptidoglycan?

  -Peptidoglycan is a major component of bacterial cells walls providing rigidity and strength.

  
  

  -The structure of which is a polymer of alternating sugars:

  
  

  N-acetyl glucosamine (NAG) and N-acetyl muramic acid (NAM)

  
  

  NAG-NAM sugar polymers are cross linked by short peptide chains.

  
  

  -Peptidoglycan is not found in humans, therefore it is a good target.

  
• How can you inhibit cell wall synthesis?

  Can inhibit NAG and NAM synthesis or cross-linking

  
  

  Only affects actively growing cells

  
  

  New cell walls are severely weakened

  
  

  Internal turgor pressure causes cell lysis and death
• What is the main mechanism of action of penicillins?

  Penicillins inhibit thebacterial transpeptidase enzymeused to crosslink peptidoglycan chains required for cell wall strength and rigidity

  Insufficient crosslinking between NAG/NAM chains weakens bacterial cell wall strength eventually causing cell lysis

  
• What is vancomycin active against?

  Gram positive bacteria (and poor activity against gram negative bacteria)
• What is the main mechanism of action of vancomycin?

  Vancomycin prevents assembly ofNAG/NAM polymersby forming hydrogen bonds with theterminal D-alanyl-D-alanine moietiesof NAM/NAG andprevents polymer cross-linking

  Insufficient NAG/NAM polymerisation and crosslinkingseverelyweakens bacterial cell walls

  
• What is another target for antibacterials?

  Plasma membrane (phospholipid bilayers)
• What is daptomycin active against?

  Gram positive bacteria
• What is the main mechanism of action of daptomycin?

  Daptomycin inserts into the bacterial plasma membrane where it oligomerises to form pores that increase membrane permeability

  
  

  Binding to the membrane requires phosphatidylglycerol and pore formation induces membrane depolarisation, leading to cell death

  
• What subunits make up the different ribosomes?

  Eukaryotic: 80S (40S & 60S)

  
  

  Prokaryotic: 70S (30S & 50S)

  
• How can protein synthesis be inhibited?

  Inhibiting peptide (amide) bond formation

  
  

  Inhibiting tRNA recruitment

  
  

  Inducing mistranslation of mRNA
• What is the main mechanism of action of erythromycin?

  Erythromycin binds to the 50S ribosomal subunit inhibiting aminoacyl translocation

  
  

  Transfer of tRNA from the acceptor "A" site on the ribosome to the peptidyl "P" site is prevented, halting protein synthesis
• What is the main mechanism of action of tetracycline?

  Tetracycline binds to the 30S ribosomal subunit preventing recruitment of charged aminoacyl-tRNA molecules to the acceptor "A" site on the ribosome

  
  

  Blocking tRNA recruitment prevents the incorporation of new amino acids into the growing peptide chain

  
• What is the main mechanism of action of kanamycin?

  Kanamycin interacts with the 30S ribosomal subunit, inhibiting ribosomal translocation & promoting mistranslation of mRNA transcripts

  
  

  This leads to the production of functionally defective proteins
• Summarise the different mechanisms of actions of the different antimicrobials.

  
• What are fungi?


  Eukaryotic microbes

  e.g.Candida albicans, Histoplasma capsulatum, Aspergillus fumigatus, Cryptococcus neoformans
• What are antifungals also known as?

  Antimycotics.

  
  

  Antimycotics can either be fungicidal or fungistatic
• What is the general mechanism of action of antifungals?

  Weakening of either plasma membrane or cell wall causing cell lysis or inhibition of protein/DNA synthesis

  
  

  Fungal cell wall = thick

  
  

  Plasma membrane = thin

  
• Describe the structure of the fungal plasma membrane and cell wall

  Plasma membrane includes: Ergosterol, B1,3 glucan synthase and the outer face of the membrane has Chitin.

  Cell wall:B1, 3 & B1, 6 Glucans and mannoproteins

  
• What are 3 main targets of antifungals?

  Plasma membrane -Azores, Polyenes, Allylamines, Morpholinescan interfere with the membrane

  Cell wall -Echinocandinscan interfere with cell wall

  Protein/DNA synthesis -Molecular analoguescan interfere with protein and DNA synthesis

  
• What is ergosterol?


  Ergosterol is an importantsterol (third lipid class)found in fungal plasma membranes and not in mammalian plasma membranes

  
• What happens when fungi plasma membranes don't have ergosterol?

  Without ergosterol, the fungal plasma membrane is severely weakened

  
  

  Ergosterol, and ergosterol biosynthesis is therefore an attractive target for the development of antifungals
• What is the fungal ergosterol biosynthesis pathway?

  Starts off with Acteyl CoA and goes through a series of reactions to form Ergosterol.

  
  

  ERG11 is an important gene in this pathway

  
• What are azoles?


  Synthetic antifungals with broad spectrum of activity.

  
  

  They're fungistatic agents and there are many different kinds of antifungal azoles
• What is the mechanism of action of azoles?

  They’reenzyme inhibitors

  Azoles inhibitlanosterol 14α-demethylase (ERG11)required to convert lanosterol to ergosterol

  
• What were the 1st generation azoles?

  Imidazoles:

  -imidazole ring present.-excellent spectrum of activity– but the azole ring can be degraded in vivo

  Examples of these azoles include:clotrimazole & miconazole

  
• What were the 2nd generation azoles?

  Also based on imidazole structures

  BUT

  modified so increasedhydrophilicity& now suitable for oral administration

  There are still issues with degradation & stability.

  Example of this azole includesketoconazole

  
• What are the 3nd generation azoles?

  Based on1,2,4-triazolestructures & are suitable for oral administration

  Good hydrophilicity

  Triazole ring is far less susceptible to degradation in vivo

  Examples of these azoles includefluconazole&voriconazole

  
• What are polyenes?


  Macrocyclic polyunsaturated compounds.

  which have alternate single double carbon-carbon bonds and it's anamphipathicmolecule (both hydrophobic+hydrophilic)

  Examples includenystatinandamphotericin B

  
• What are the problems with polyenes?

  Limited solubility and issues with toxicity (kidney)
• What is the mechanism of action of polyenes?

  Polyenes bind to ergosterol, disrupting the osmotic integrity of the fungal plasma membrane.

  
  

  This causes leakage of intracellular ions and components from the cell.
• How can the toxicity of polyenes be reduced?

  Altering formulation:

  Amphotericin B is packaged in a lipid-based delivery system so there's a reduced interaction with host membranes=reduced nephrotoxicity

  
• How does amphotericin B work?

  Binds to ergosterol, altering cell membrane permeability and causing cell death.

  
  

  Creates a 'pore' and material is lost from the cell

  
• Whereabouts in the pathway of ergosterol biosynthesis can antifungals target?

  We can use Allylamines, Azoles, Morpholines and polyenes (which bind directly to ergosterol and disrupt the membrane)