Antibiotics, Antibacterials & Resistance mechanisms Flashcards
What are the general mechanisms of antimicrobial resistance?
- drug target alteration
- drug target overexpression
- reduction of intracellular accumulation of the drug (efflux pump)
- alternate structures and metabolites
- activation of cellular stress pathways
Definition antibacterial
agents selective acting against prokaryotes
Definition antibiotics
antibacterials produced by a living organism
Name the first antibiotics/antibacterials
- antibiotic: Pyocyanase (1890)
- antibacterial: Salvarsan (1910)
- ß-lactam: penicillin (1928)
- Sulfadrug: Prontosil (1932, antibacterial)
What makes an ideal antibacterial?
- high specificity
- bactericidal
- high therapeutic index (ratio between lethal (animal)/toxic (human) and therapeutic dose)
- various routes of administration
- good absorptionsfähiges and distribution
- no resistances
Descriminate between broad and narrow spectrum antibacterials
broad: active against gram- and gram+
narrow: limited activity and are primarily useful against particular species
Describe the molecular structure of the different antibacterial classes
- ß-lactams: ß-lactation ring
- aminoglycosides: aminomodified sugars and different sidechains
- sulfonamide: amide with sulfure instead of oxygen in main chain
- glycopeptides: glycosylated cyclic or polycyclic peptides
- quinolones:
ß-lactams
- interfere with cell wall synthesis
- lactam is a cyclic amide
- structural analog to natural D-Ala-D-Ala -> covalent inhibition of DD-Transpeptidase (PBP) -> weakens cell wall
- ~50 different in use
- all are bacteriocidal and non toxic + relatively inexpensive
What do autolysins do?
- membrane-associated enzymes that break bonds between/within peptidoglycans
- important for cell wall turnover and shape
- trigger cell lyses in combination with inhibited cell wall synthesis
Explain the downside of antibiotic treatment
the use of antibiotics/antibacterial can lead to selection of resistances, already present in some microorganisms of the population, especially if the drugs are not used in a high enough dose
Resistances against ß-lactams
- ß-lactamase:
- hydrolysis of ß-lactam ring -> compound is inactivated
- sensitive bacteria are not fast enough in cleaving the ß-lactam ring if ß-lactamases are present - Mutation in PBP
- amino-acid replacement in the active site -> diminished affinity for ß-lactam
- MRSA encode for special PBP-2 variant PBP-2A -> no efficient shut down- PBP-2 and PBP-2A cooperate in transpeptidation and transglycosydation
Penicillin alternative?
- Cephalosporins
- same mode of action
- broader spectrum than penicillins
- more resistant to ß-lactamases
- instead of Thiazolidine (5) next to ß-lactam ring they have a Dihydrothiazine (6) ring
What can treatment with ß-lactams improve?
- ß-lactamase inhibitors in combination therapy
- other classes of antibiotics
Glycopeptides
- molecules produced by Actinobacteria
- glycosylated cyclic/polycyclic peptides
- last resort in MRSA and MDR Strep. pneu.
- only against gram+
Resistances against glycopeptides
vanA-type vancomycin resistance:
- last peptide switches -> D-Lac instead of D-Ala -> no binding of vancomycin
- S. aureus acquired gene horizontally from VRE
Fosfomycin
- broad-spectrum antibiotic
- cell entry through glycerophosphate transporter
- inhibits first enzyme of cell wall biogenesis/peptidoglycan biosynthesis -> MurA
- alkylating cysteine residue in active site
Resistance to fosfomycin
- in S. aureus
- fosfomycin modification (phosphorylation) -> FomA
- fosfomycin hydrolysis (FosA, FosB, FosX)
- modification of MurA
- MurA overexpression
Tetracyclines
- Naphtacene core
- broad-spectrum bacteriostatic antibiotic
- primary binding site (Tet-1) overlaps with a-tRNA in A-site, interaction mainly with 16S rRNA
- secondary binding sites (Tet-2) also found
Resistances to Tetracyclines
TetA/TetR:
- efflux pump, TetA is gene, TetR is regulator
- like lac-operon TetA is only transcribed when TetR is bound to Tet and doesn‘t inhibit TetA promotor
also ribosomal protection, degradation and rRNA mutations
Aminoglycosides
- mainly against gram- bacteria
- bactericidal
- natural and synthetic
- suffix: -mycin -> from Streptomyces
- suffix: -micin -> from Micromonospora
- often used as last resort
Aminoglycosides - mode of action
- bind to A-site (incorrect proofreading) or between A- and P-site (blocking of translocation) of ribosome
- ultimately they lead to mistranslated and therefore misfolded proteins
- misfolded proteins in membrane disrupts cell membrane integrity -> even more drug uptake
Resistances against Aminoglycosides
- rRNA-modification by Methyltransferase
- (point-)mutations of ribosomal proteins (S12 peptide) -> no aminoglycoside binding
Macrolides
- lactone ring bonded to sugar
- 20% of used antibiotics are macrolides
- Erythromycin is broad-spectrum antibiotic targeting 50S subunit of ribosome
Macrolides - mode of action
- partial inhibition of protein synthesis -> leads to imbalance in protein synthesis
-> imbalance in proteosome disrupts metabolic function at all levels - block of ribosomal peptide exit channel
Resistances to Macrolides
- mutation in L4 and L22 -> L22 is flexible -> exit tunnel big enough for peptide
- efflux pumps (Mef & Msr)
- ## modifications at 23S rRNA
Glycopeptides - mode of action
- bind to D-Ala-D-Ala dipeptide and block PBP crosslinking activity -> steric inhibition
Tetracyclines - mode of action
- steric hindering of incoming t-RNA
- prokaryotic specificity: uptake via Omp (outer membrane proteins) only possible in prokaryotes; Tet is bound to metal at pH 7
Quinolones
- all bactericidal
- broad-spectrum
- small hydrophilic molecules -> easily diffuse through membranes
Quinolone target
DNA-gyrase (topoisomerase type II) and topoisomerase IV
What do topoisomerases do?
- twist cyclic DNA to make them more compact
Lk = Tw + Wr - type I: produce transient nicks (single strand) -> Lk change by 1 or n
- type II: produce transient breaks (double strand) -> Lk change by +/- 2
- ATP-dependent; DNA gyrase and Topo IV
Resistances to Quinolones
- modifications of quinolone binding site of target
- transferable plasmid-borne resistance: gene qnr expresses a pentapeptide repeat protein which blocks quinolone binding site without inhibition
- efflux proteins
Rifampicin
- bactericidal
- primarily targets gram+ (some gram-)
- in combination used to treat TB and MRSA
Rifampicin - mode of action
- binds 2-5 nucleotides away from catalytic center of polymerase -> blocks elongation
- binding only possible if no nucleotides already occupy the space
Resistances to Rifampicin
- spontaneous mutation in gene expressing ß-subunit of RNA-Pol -> loses affinity
- transferable plasmid: enzyme that inactivates rifampicin by transferring an ADP-ribosyl
Sulfonamides
- against Staph., Strep., and E. coli
- rarely (3%) can have severe side effects (17% fatality)
- where the first produced antibiotics
Sulfonamides - mode of action
- inhibition of folic acid (Vit B9) synthesis by substrate competition
- analog of p-Aminobenzoic acid (PABA) -> part of folic acid synthesis
- humans don‘t have that synthesis pathway (no own production of folic acid/Vit B9)
Resistances to Sulfonamides
- mutational changes of target enzyme dihydropteroate -> limited binding
- natural resistances due to lack or overexpression of dihydrofolate reductase
- development of alternative pathway -> active uptake
Name the ways bacteria can acquire resistances
- Transformation (free DNA)
- Transduction (phage)
- Conjugation (plasmid transfer)
- Transposition
