Antimicrobial Compounds & Modes of Action Flashcards
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 general mechanism of action of antibacterials?
Antibacterials target essential bacterial processes including synthesis of cell wall, protein synthesis and plasma membrane.
What is the difference between antibiotics and antimicrobials?
Antibiotics are natural products produced by microbes that inhibit or kill other microbes i.e. penicillin.
Antimicrobials are 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 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?
Bacteriocidal β-lactam antibiotics – many different types. Originally derived from Penicillium fungi
Penicillins inhibit the bacterial transpeptidase enzyme used 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?
glycopeptide antibiotic and Bacteriocidal.
Vancomycin prevents assembly of NAG/NAM polymers by forming hydrogen bonds with the terminal D-alanyl-D-alanine moieties of NAM/NAG and prevents polymer cross-linking
Insufficient NAG/NAM polymerisation and crosslinking severely weakens bacterial cell walls
What is the main mechanism of action of daptomycin?
lipopeptide antibiotic. 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 is daptomycin active against?
Gram positive bacteria
What is another target for antibacterials?
Plasma membrane (phospholipid bilayers)
Selectively permeable to biological molecules
Encapsulates the cytoplasm – surrounded by cell wall
Antibacterial target
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?
Bacteriostatic and Macrolide antibiotic.
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?
Broad spectrum polyketide antibiotic and bacteriostatic.
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?
Aminoglycoside antibiotic and Bacteriocidal.
Kanamycin interacts with the 30S ribosomal subunit, inhibiting ribosomal translocation & promoting mistranslation of mRNA transcripts
This leads to the production of functionally defective proteins
What are antifungals also known as?
Antimycotics.
Antimycotics can either be fungicidal or fungistatic
What are fungi?
Eukaryotic microbes
e.g. Candida albicans, Histoplasma capsulatum, Aspergillus fumigatus, Cryptococcus neoformans
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, Morpholines can interfere with the membrane
Cell wall - Echinocandins can interfere with cell wall
Protein/DNA synthesis - Molecular analogues can interfere with protein and DNA synthesis
What is ergosterol?
Ergosterol is an important sterol (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 is the mechanism of action of azoles?
They’re enzyme inhibitors
Azoles inhibit lanosterol 14 α-demethylase (ERG11) required to convert lanosterol to ergosterol
What are azoles?
Synthetic antifungals with broad spectrum of activity.
They’re fungistatic agents and there are many different kinds of antifungal azoles
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 increased hydrophilicity & now suitable for oral administration
There are still issues with degradation & stability.
Example of this azole includes ketoconazole
What are the 3nd generation azoles?
Based on 1,2,4-triazole structures & are suitable for oral administration
Good hydrophilicity
Triazole ring is far less susceptible to degradation in vivo
Examples of these azoles include fluconazole & voriconazole
What are polyenes?
Macrocyclic polyunsaturated compounds.
which have alternate single double carbon-carbon bonds and it’s an amphipathic molecule (both hydrophobic+hydrophilic)
Examples include nystatin and amphotericin 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).
What are echinocandins?
-Caspofungin
-Micafungin
-Anidulafungin
These attack the fungal cell wa
Describe caspofungin and what it’s used for.
It’s a cyclic glycopeptides that attacks the fungal cell wall and must be used intravenously.
It’s very potent against some but not all fungi.
Against species of Candida, it’s fungicidal, but against species of Aspergillus, it’s fungistatic
What is the mechanism of action of echinocandins?
Echinocandins inhibit β-glucan synthase required for the production of β-1,3 glucans (cell wall).
What are molecular analogues?
Molecules designed to look like other molecules
e.g. 5-fluorocytosine (5-FC) is a pyrimidine analogue so it resembles pyrimidine
What is the mechanism of action of 5-FC?
5-FC inhibits protein and DNA synthesis
How does 5-FC work?
- 5-FC crosses cell membrane via cytosine permease, into the fungus
- 5-FC is converted to 5-Fluorouracil via cytosine deaminase.
2 different things happen after step 2.
- 5-Fluorouracil is converted into 5-fluorouridine-TP which is incorporated into RNA and this causes inhibition of protein synthesis.
- 5-Fluorouracil converted into 5-Fluorodeoxyuridine-MP which causes inhibition of thymidylate synthetase = inhibition of DNA synthesis
What is important to consider when giving treatments like antibacterials or antifungals?
When considering combinational treatment- order of treatment can be critical
e.g. giving tetracycline & penicillin:
Tetracycline- bacteriostatic (inhibits bacterial growth)
whilst penicillin- bacteriocidal (kills bacteria)
Penicillin requires that bacteria are actively growing to be active but there would be no growth (due to tetracycline) so penicillin would be ineffective
What are some examples of viruses and how do they replicate?
Retroviruses, picornaviruses, herpesviruses and pox viruses.
They use many host functions to replicate which makes viral selectivity a major challenge
How must antivirals work?
Antiviral agents must either block viral entry into or block the exit from the cell or be active inside the host cell.
Antivirals have many side effects
When are antivirals most efficacious?
When the virus is replicating
What are DNA polymerase inhibitors?
Molecular analogues that interfere with DNA replication
(e.g. trifluridine- pyrimidine analogue and acyclovir- purine analogue)
What are the different classes of antivirals?
Some are inhibitors, some prevent cell fusions etc
What is the mechanism of action of DNA polymerase inhibitors?
DNA polymerase inhibitors are incorporated into DNA during normal replication
The lack of a 3’ hydroxyl group prevents the incorporation of the next nucleotide (chain termination)
What are reverse transcription inhibitors?
They’re active against retroviruses
The most common type of this drug is nucleoside analogue reverse transcriptase inhibitors (NRTI)
Examples include tenofovir & azidothymidine (AZT)
They inhibit the reverse transcription of viral RNA genome (RNA–->DNA)
How do protease inhibitors work?
Inhibit assembly of new virus by blocking protease enzyme.
so they prevent release of mature viral proteins from polyproteins during viral life cycle
Examples include nelfinovir & ritonavir
What is HAART?
Highly active antiretroviral therapy:
Combinational therapy, typically with at least three drugs (NRTIs and protease inhibitors).
This induces a powerful multifaceted attack employing different mechanisms of activity,
Leading to a simultaneous blockade of multiple steps essential for viral activity and reproduction.
