antibacterials 2 a Flashcards
(46 cards)
how do antibacterials get into bacteria (gram +ve)?
a have active peptide transporters
– dipeptide, tripeptide and OligoPePtide transporters (Opp)
– transport most hydrophilic antibacterials directly into the cytoplasm
– hydrophobic antibacterials can use passive diffusion
how do antibacterials get into gram negative bacteria?
Gram negative bacteria have Outer Membrane Proteins (porins, Omp) that act as
molecular sieves
– e.g. main porins in E. coli: OmpC and OmpF – allow hydrophilic molecules up to about 600 Da (1.1 – 1.2 nm diameter) to pass into periplasmic space
who targets the cell wall?
b-lactams
vancomycin
teicoplanin
isoniazid
what are b-lactam antibiotics?
Broad spectrum bactericidal activity against – both G+ve (except aztreonam) and G-ve bacteria
what are the characteristics of b-lactam antibiotics?
time dependent action
good distribution:
- increased with greater lipophilicity
-remember polarity
must have low PAE
largely renal excretion
why do we usually not use b-lactams for UTIs?
largely renal excreted
– not often used for UTIs to reduce increase in resistance
* treat with other suitable agents, if possible
* can be used for complicated UTIs, if necessary
what is the penicillin SAR?
- Amide and carboxylic acid are involved in binding
- Carboxylic acid binds as the carboxylate ion (pKa ~ 2.5 – 3.0)
- Mechanism of action involves the b-lactam ring
- Activity related to b-lactam ring strain
(subject to stability factors) - Bicyclic system increases b-lactam ring strain
- Not much variation in structure is possible
- Variations are limited to the side chain (R)
what are some areas for developements for b-lactams?
1* to increase chemical stability for oral administration (stomach acid)
* addition of electron withdrawing groups (R)
* prodrugs (-CO2H)
2* to increase resistance to b-lactamases
* addition of bulky groups (R)
3* to increase the range of activity
* addition of ionisable groups (R)
* addition of ureido groups (R)
what happens to b-lactam structure at physiological ph?
All have >99.9% ionised ‘acid’ at physiological pH
what is the MOA of b-lactams?
Act upon synthesis of bacterial cell membrane
b-Lactams interfere with peptidoglycan (components of call wall) formation through their interaction with the transpeptidase enzyme [penicillin binding proteins (PBPs)]
what is the consequence of b-lactams inhibiting the formation of cross-links in cell wall?
– reduces integrity and strength, eventually leads to cell lysis / death
* high osmotic pressure inside bacterial cells
* when cell wall weakened, easily bursts
what is prokaryotic cell wall composed of?
peptidoglycan
– polymer
– sugar units (NAG/NAM)
– cross-linked by peptide uni
what are 3 reasons for potential acid sensitivity?
- (1) Ring strain – acid-catalysed ring opening relieves ring of β-lactam ring
- (2) Highly reactive β-lactam carbonyl group – resonance stabilisation is not possible for β-lactam ring
- (3) Influence of the acyl side chain (neighbouring group participation)
what is a solution to acid sensitivity?
- β-lactam ring is vital for antibacterial activity and only the third factor can be
addressed
– reduce amount of neighbouring group participation
– use electron-withdrawing group in side chain to draw electrons away from the
carbonyl oxygen and reduce its nucleophilicity
– stability improvements can allow oral administration
what are the 4 mechanisms of resistance to B-lactam agents?
- Most resistance to b-lactam antibiotics due to specific enzymes
– b-lactamases
– break open b-lactam ring, which is essential to activity
– b-lactamase resistance to penicillin first seen in 1940, before clinical use started! - Modification of the target protein (PBP)
– most common in G+ve bacteria, e.g. Haemophilus and Neisseria strains
– less common in G-ve bacteria - Limited uptake into the bacterial cell, especially G-ve bacteria
– polar molecules, so poor passive diffusion, use porins
– mutations to porins reduce uptake and confer resistance - Increased efflux pump activity
– up-regulation of efflux pumps causes multi-drug resistance - affects many antibiotics (e.g. fluroquinolones, aminoglycosides)
what are the 4 classes b-lactamases are divided into?
– active-site serine β-lactamases (SBLs; classes A, C and D)
– zinc-dependent or metallo-β-lactamases (MBLs; class B
how do extended spectrum b-lactamases (ESBLs) work?
- hydrolyse even third generation cephalosporins, but not usually carbapenems
how do carbapenemases work?
- confer resistance to carbapenemases, alongside almost all clinical b-lactam
antibiotics
how do metallo b-lactamases work?
- broad range of b-lactam substrates, including carbapenems (but not aztreonam)
what are b-lactamases in gram +ve bacteria?
Beta-lactamases are inducible in G+ve bacteria – produced in response to presence of b-lactam antibiotic
what are b-lactamases in gram -ve bacteria?
constitutive in G-ve bacteria – they produce the enzyme all the time, whether or not beta-lactam antibiotics are present
– but, only if the G-ve bacterium has the b-lactamase gene
– located in periplasmic space
how do b-lactamases work?
Generally, have a similar mechanism of action, cleaving the b-lactam ring leading to inactivation of b-lactam antibiotics
how are SBLS related to PBPs?
– share a similar SER-based amino acid sequence
– Use serine as the nucleophile to hydrolyse β-lactams
how do MBLs work?
MBLs (class B) have a different hydrolytic mechanism
– use a metal-activated water nucleophile to to hydrolyse β-lactams
