antibiotics Flashcards
Examples of antimicrobial agents
- Quinine for malaria
○ Used since 1600s
○ Malaria parasite now become resistant- Mercury for syphilis
It worked but was banned because it is highly toxic for hosts
- Mercury for syphilis
What happened to reduce burden of infectious diseases
- Antibiotics - treat
- Vaccines - treat
- Improvement in sanitation - the most important
Prevent
antibiotic definition
any substance produced by a microorganism that is antagonistic to the growth of other microorganisms in high dilution
General properties of antimicrobial agent
Selective toxicity
- Antimicrobial Agenets needs to target biochemical process that occurs in the pathogen, but preferably not the host
- E.g. penicillin targets the biosynthesis of peptidoglycan in bacterial cell walls
○ No peptidoglycan biosynthesis pathway in mammalian cells
Therefore Penicillin doe not affect host cells
narrow spectrum
Effective only against a limited number of bacteria
broad spectrum
Effective only against a many different types of bacteria
Natural products
○ Fermentation of fungi or bacteria to produce these agents
E.g. penicillin, polyenes, aminoglycosides
Semisynthetic products
○ Chemically modified derivatives of natural products
E.g. b- lactams, cephalosporins
Synthetic products
○ Completely chemically synthesised
E.g. oxazolidinones quinolones
Bacteriostatic agents
○ Stop bacterial growth, not kill
Allow host defences to overcome infection
Bactericidal agents
Kill targeted bacterial agents
Effect of Antimicrobial Agents on target bacteria
Antimicrobials may be bactericidal for one organism and bacteriostatic for another
Antimicrobial targets
- Macromolecules (mostly enzymes) that are unique to the microbial cell or divergent from human
- Metabolic processes that can be bypassed in humans but not in pathogens
○ E.g. Folate incorporation from dietary sources
§ Necessary for pathogens can be turned off
Normal activity of the target must be limiting for microbial replication or virulence
- Metabolic processes that can be bypassed in humans but not in pathogens
Inhibitors of cell wall biosynthesis
- Peptidoglycan
○ Key structural component of bacterial cell walls
○ Synthesis is complicated process to produce
§ Basic monomer
§ Extend into chains
Crosslink the chains into mesh-like structure
Targeting penicillin- binding proteins (PBPs)
- PBPs play multiple roles in cell wall synthesis and maintenance of peptidoglycan
○ Translgycosylation (wall synthesis for growth and septation)
○ Transpeptidation (crosslinking and remodelling)
○ Peptide cleavage ( control of crosslinking, insertion of new strands)- Critical part of process is recognition of the D-ala-D-ala sequence of the MurNac-GlcNac pentapeptide
- Interfering with this recognition disrupts the cell wall synthesis
- PBPs are major antibacterial targets
- The b-lactam ring mimics the structure of the D-Ala-D-Ala link and bind to the same place in the PBPs (the active site), disrupting the crosslinking process
b-lactams inactivate PBPs.
B-lactam antibiotics
- B-lactams inactivate PBP by binding to active site, blocking access for the natural substrate
- B-lactams have varied affinities for different PBPs and different effects
- Bacteriocidal
- Resistance through production of
○ PBP mutations
○ b-lactamases - Broad spectrum
Oral delivery
Mode of action for b-lactam antibiotics
- New peptidoglycan material is inserted into old wall at specific points
- Blocked peptidoglycan crosslinking induces futile cycle of turnover and deregulates activities = burst
○ Elongated –> bulge formation –> lysis (bursts open)
Under-crosslinked PG provides less support against turgor or osmotic pressure
- Blocked peptidoglycan crosslinking induces futile cycle of turnover and deregulates activities = burst
Glycopeptide antibiotics
- Vancomycin
- The drug will bind to D-ala-D-ala dipeptide
○ Inhibiting Translgycosylation and transpeptidation - Bactericidal
- Gram+ve (due to permeability)
Most used in infections b-lactam resistant gram+ves
- The drug will bind to D-ala-D-ala dipeptide
Inhibitors of protein synthesis
Antimicrobial targeting bacterial ribosomes - Antibiotics that bind to 30S subunit ○ Amino acyl-tRNA blocker ○ Decoding disruptors - Antibiotics that bind to 50S subunit ○ Peptidyl transferase inhibitors Exit tunnel blockers
30s subunits antibiotics
tetracyclines
aminoglycosides
Tetracyclines
○ 4-ringed structures
○ Prevent entry of amino acyl-tRNA into A-site
○ Bacteriostatic, broad spectrum
○ Active against intracellular pathogens
Resistance through efflux or ribosomal protection proteins
aminoglycosides
○ Cyclohexane ring and amino sugars
○ Primary active against G-ve (aerobic)
○ Interfere with proof-reading process for incoming aa-tRNA
§ Premature termination or proteins with aa substitutions
○ Bactericidal
Resistance through target mutations
50s subunit antibiotics
macrolides
oxazolidinones
macrolides
○ Contain 12-24 carbon lactone rink linked to sugars
○ Bind reversibly to 23S rRNA in 50S subunit (peptidyl transferase domain)
○ Disrupt movement of tRNA from A site to P site (inhibiting peptide chain elongation)
○ Bacteriostatic
○ Mainly g+ve, broad spectrum
○ Enter in eukaryotic cells (e.g. macrophages), active against intracellular pathogens like Legionella, mycoplasma and chlamydia
○ Resistance mechanisms
§ Target modification (e.g. RNA methylation)
§ Efflux
Enzymatic modification
Oxazolidinones
○ Synthetic ○ Bind to rRNA on A side of peptidyl transferase centre (PTC) of the ribosome ○ Bacteriostatic ○ G+v (M. tuberculosis) spectrum Resistance through tRNA mutations
DNA replication inhibitors
Fluoroquinolones
Fluoroquinolones
- synthetic
- Quinolone ring
- Bind to gyrase and topoisomerases blocking formation of complex with nicked DNA
- Blockage of DNA replication and repair
- Bactericidal, broad spectrum
- UT and GIT infections (anthrax, legionellosis, pneumonia)
Resistance through GyrA and gyrB mutations (expression of gyrase-binding proteins)
Transcription inhibitors
Bind to DNA-dependent RNA polymerase and blocks synthesis of mRNA
Rifamycins
Fidaxomicin
Rifamycins
-semisynthetic
- bactericidal, relatively broad spectrum
- well absorbed orally and distributed throughout the body
- can cross BBB
- reserved for:
mycobacterial infections
prophylaxis of meningococcal and haemophilus close contacts
fidaxomicin
natural
not oral
new antibiotic for C. difficile infections
Metabolic antagonists
Sulphonamides and trimethoprim
Sulphonamides and trimethoprim
○ Broad spectrum and bacteriostatic
○ Selective
§ Humans obtain folic acid from diet
Resistance mediated by target mutations
Antiviral agents
- Target key steps in viral life cycle ○ Target viral attachment ○ Block egress into host cell ○ Block uncoating ○ Block nucleic acid synthesis ○ Block late protein synthesis Block release
Antifungal agents
- Target Ergosterol synthesis in cell wall ○ fluconazole - Bind to ergosterol ○ Amphotericin B - Glucan synthesis inhibitors ○ Caspofungin - Thymidine analogues Flucytosine
Antimicrobial resistance
- R and D into new antibiotics has declined
- Community-acquired resistant infections on the rise
Slow creeping (need more doses than before)
- Community-acquired resistant infections on the rise
Definitions of Antimicrobial resistance
If the antibiotic does not hinder or kill the bacteria at the approved dosage = resistant
Intrinsic resistance
○ Natural makeup causes the antibiotic to not function § Lack susceptible target § Target protection mechanism § Impermeable to agent Pre-existing modifying enzyme
Acquired resistance
○ Spontaneous mutation
Acquisition of resistance gene
Tolerance
Bacteria still hindered but do not lose viability and recover after antibiotic removal
How do they become resistant
- Chromosomal mutation produces drug resistant target, allowed to multiply while the ‘normal’ bacteria dies off until the mutated bacteria is the dominant bacteria.
- Resistance genes transferred via plasmids that spread from cell to cell faster than cells themselves divide and spread
Resistance genes on transposons (can jump to plasmid or chromosome) either way the population is resistant
- Resistance genes transferred via plasmids that spread from cell to cell faster than cells themselves divide and spread
Acquired resistance by mutation
- Spontaneous, rare
○ Because DNA areas are relatively rare in bacteria
○ Usually resistant to a single class of antibiotics
○ Alters antimicrobial target site, a modifying enzyme
○ Only compounds that look and work similarly will also be resistant- Single target mutation may be enough
Some need a series of mutations
- Single target mutation may be enough
Plasmid-mediated resistant
- Source of resistance is often “immunity” genes in antibiotic producing organisms
- “Resistome” gene pool in environment
- Horizontal gene transfer (HGT) of multiple resistance genes to unrelated antibiotic families
Can cross species barrier
Modification of antibiotic molecule
- Inactivate drug by chemically cleaving it
- B-lactamases
○ cleave penicillins and cephalosporins
○ Example TEM-3
§ Extended spectrum B-lactamase (ESBL) that hydrolyse 3rd gen cephalosporins
§ ESBL Gram-ve bacteria area major problem - Inactivate drug by adding chemicals to it
○ Aminoglycoside modifying enzymes
§ Chemical modification of aminoglycosides
§ Many types of enzymes
Make the molecule less effective by adding groups to it
- B-lactamases
Modify antibiotic transport
- Decreased cell wall permeability
○ Altered porin or Omp proteins leads to decreased uptake of b-lactam antibiotics in some bacteria- increased efflux from the cell
May resistant bacteria have acquired specific efflux pump mechanisms that sense when the antibiotic is in the cell and pump them out
- increased efflux from the cell
Prevention of antibiotic binding
- Alteration of target site to reduce susceptibility to antibiotic
○ Usually an enzyme that is bound to the antibiotic but with an altered target site cant bind as well not as effective
○ Example mutation of the erythromycin ribosomal methylase (Erm) target site
§ Erythromycin, acts by blocking protein synthesis by binding to 23S RNA in 50S ribosomal subunit
§ ERM enzymes methylate A2058 residue of 23S RNA, reducing ability to bind to ribosomal target- Acquisition of genes encoding enzymes that alter target
○ Example vancomycin resistance
§ Vancomycin binds to D-Ala-D-Ala
Resistance involves acquisition of gene encoding enzymes that alter D-Ala-D-Ala to D-Ala-D-Lac, reducing binding affinity
- Acquisition of genes encoding enzymes that alter target
