Lecture 21- Antibiotic Resistance Flashcards
(16 cards)
B-lactams mode of action recap
Penicillin beta-lactate ring mimics D-Ala-D-Ala peptide bond
Penicillin will bind and irreversibly block the active site of PBP (enzyme cannot tell the difference)
Side chain linking = blocked cells will ultimately lyse (autolysin) - keeps cutting holes but PBPs cannot fill the holes. Peptidoglycan loses structural integrity
Beta lactams only kill actively growing cells
Greater effects on gram + cells
Glycopeptides- another peptidoglycan target
Most important member= vancomycin
Inhibitors of cell wall biosynthesis; just like beta lactam
Binds to D-Ala-D-Ala
Blocks transhlycosylation and transpeptidation (PBP cannot get to substrate)
No cross linking of peptidoglycan subunits -> cell lysis
Narrow spectrum= only active against some gram + bacteria and staphylococcus aureus, enterococci and clostridia
Toxic to humans, with some strong side effects
Last resort; Antibiotics used to treat MRSA
Inhibiting protein synthesis; aminoglycosides
Broad spectrum= not anaerobes; pass through cell membrane using O2
Targets the 30S subunit of the bacterial ribosome by binding to the 16s rRNA
Aminoglycoside binding results in;
- structural change of the 30S subunit
- amino acid proof reading errors
Truncated (short and faulty) and aberrant (correct length- wrong AA) proteins
Usually bactericidal
Bacterial antibiotics- tetracyclines
Also inhibits protein synthesis by interfering with the 30S subunit and binding to 16S rRNA
Inhibits binding of tRNAs to A site
- considered bacteriostatic
- characterised by a napthacene ring system (tetra and cycline = 4 rings) e.g. tetracycline
Easy to modify due to lots of side groups of the napthacene ring
Very broad spectrum= inhibits almost all gram + and gram - bacteria
- important in human medicine, veterinary medicine and as growth promoters for live stock
Intrinsic resistance
- Different cell wall structure;
- if antibiotic targets cell wall e..g penicillin - Efflux pumps and membrane permeability;
- some bacteria have natural efflux pumps that pump out certain antimicrobials - Enzymatic activity;
- some bacteria naturally produce beta-lactamases that break down B-lactams - Lack of target; or no target;
- mycoplasma bacteria have no cell wall= no peptidoglycan
- outer membrane makes peptidoglycan less accessible in gram- bacteria
Temporary resistance + Acquired
Cells are temporarily in a physiological state that renders them ‘resistant’ endospores, dormant cells in biofilms
Acquired; renders a formerly sensitive bacterium resistant e.g. through spontaneous mutation/ picking up resistance determinants
Bacterial biofilm
Aggregates of cells attached to a surface and/or each other, that are surrounded by an extracellular polymeric matrix
EPS- defining structure - key roles in;
- structure, antimicrobial protection, retain water/nutrients, virulence factor, adhesion, genetic transfer and communication
Spread of antibiotic resistance
Most resistance= encoded by genes e.g alternative pathways/targets
Horizontal gene transfer; common in bacteria;
- transposons (jumping genes move within/between bacterial genome)
- plasmids
- pathogenicity islands
Resistance can be spread between different bacterial genus and species
Plasmids and mobile genetic elements = collect different resistance genes; multidrug resistance
Horizontal gene transfer as a resistance mechanism
Transduction = uses a bacteriophage as a vehicle to carry DNA from donor > recipient
Conjugation = bacterial appendages form and direct contact allows DNA exchange
Transformation = bacteria take up free DNA from the environment
4 mechanisms of acquired antibiotic resistance
- Modify the target
- spontaneous mutations
- use alternative genes/pathways
Cellular target different= antibiotic no longer effective - Degrade the antibiotic;
- hydrolytic enzymes (beta-lactamases) - destroy the drug before it destroys the bacterial cell - Modify the antibiotic;
- modifying enzymes - adds an acetate group= may no longer bind to target - Export the antibiotic;
- active transporters; efflux pumps
- pump the antibiotic out of the cell before it can accumulate
Vancomycin resistance
Developing an alternative biosynthetic pathway;
Synthesis of alternative PG precursors
Alternative ending to D-Ala-D-Ala e.g. D-Ala-D-Lactase
Vancomycin cannot bind to new ending- can no longer block PBP activity = bacterial cell survives and infection continues
Exporting the antibiotic
Can work by proton motive force, Na+ antiport or ATP hydrolysis
- located in the cytoplasmic membrane to pump drugs out of the cytoplasm
Gram - bacteria;
Drug needs to be removed from the periplasm
^ pump capable of this = resistance-nodulation-division (RND) pumps
^ e.g. of tripartite efflux systems
- uses proton motive force- not ATP
Problems with antibiotic discovery
1- science
= keep picking up same classes of compounds
- Economics
- very high costs of research and development + clinical trials
- long and complex approval process
- short treatment times
Cefiderocol
New semi-synthetic antibiotic
Effective against gram - bacteria
Antibiotic targets
Cell wall synthesis;
- B-lactams e.g. penicillin + vancomycin inhibit peptidoglycan synthesis -> weakens bacterial cells + causes cell lysis
Cell membrane;
Daptomycin = disrupts the bacterial membrane causing leakage of cellular contents + death
Protein synthesis;
Antibiotics like; tetracyclines, macrolides, aminoglycosides and linezolid= inhibit bacterial ribosomes which differ from human ribosomes = prevent protein production
DNA/RNA synthesis;
Fluroquinolones and rifampicin = inhibit enzymes involved in DNA replication and transcription = blocking bacterial production
Folate synthesis; metabolic pathways;
Sulphonamides and trimethroprim= block the synthesis of folic acid necessary for nucleic acid production. Humans do not synthesise folic acid = selective target
Antibiotic resistance
Efflux pumps; actively expel antibiotics; e.g. tetracyclines and macrolides= preventing accumulation
Enzymatic inactivation; produces enzymes like B-Lactamases that degrade antibiotics
Target modification; alter the antibiotic binding site; ribosome/ DNA gyrase= preventing drug action
Bypass mechanisms; use alternate pathways/enzymes such as modifying folic acid synthesis enzymes to resist sulphonamides or trimethoprim
