INF1 - F. BACTERIAL WALL SYNTHESIS AND TARGETING BY ANTIBIOTICS-COVERED Flashcards
what is the structure of peptidoglycan monomer
2 amino sugars
- NAG
- NAM
- NAG-NAM joined into polymer chains via a glycosidic bond
- 5 amino acids (pentane-tide) attached to NAM, including 2 terminal D-alanines but one D-alanine is cleaved during crosslinking assembly of cell wall
(some gram +ve bacteria - 5 glycine amino acids added to NAM-pentapeptide aswell) - polymer chains interlocked via the NAM-tetrapeptides either directly or via an extra sequence of approximately 5 glycine amino acids
what is the process of cross-linking peptidoglycan chains called
transpeptidation
(amount of cross-linking varies)
peptidoglycan biosynthesis in cytoplasm of cell
- NAM synthesised from NAG by addition of lactic acid derived from phosphoenolpyruvate (PEP) and UDP used as an energy source, losing inorganic phosphate in process (attached at all times)
Enolpyruvate transferase aids this
Fosfomycin blocks this - 3 amino acids added to NAM:
L-alanine
D-glutamic acid and either
L-lysine (gram +ve)
mDap - diaminopimelic (gram -ve)
Position 3 is a diamino for cross-linking - L-alanine converted to D-alanine by a racemes enzyme. 2 D-alanines added by a ligase enzyme to form the NAM-pentapeptide
1 D-alanine cleaved off as an energy source to drive cross-linking between polymer chains
D-cycloserine blocks D-alanine addition
transport of peptidoglycan across cell membrane via lipid carrier
- NAM pentapeptide passed to a lipid carrier bactoprenol in cell membrane
Bacitracin blocks this - no longer licensed in UK - NAG added to NAM-pentapeptide forming NAG-NAM-pentapeptide
(gram +ve: pentaglycine bridge may beg added to pentapeptide) - lipid carrier binds to NAG-NAM-pentapeptide and takes it across cell membrane
- after delivering NAG-NAM-pentapeptide across cell membrane, carrier is recycled back to pick up more NAG-NAM monomers
insertion and cross-linking of peptidoglycan in cell wall
- NAG-NAM pentapeptide arrives at cell wall and linked to peptidoglycan polymer by enzyme peptidoglycan glycosyltransferases located on outer membrane of cytoplasmic membrane, building the polymer chains
Vancomycin and Teicoplanin bind to D-alanyl-D-alanine dipeptide which blocks glycosidic bond formation - Transpeptidase (in outer membrane of cell membrane) cross-links pentapeptides. Terminal D-alanine is removed, energy generated used to cross-link new peptidoglycan monomer to third amino cid of existing cross-linked peptidoglycan. Gram +ve: pentaglycine sequence forms cross-bridge
Beta-lactams (pencillins, cephalosporins, monobactams, carbapenems) block transpeptidase and hence cross-linking
what activity do most of antibiotics that block cell wall display
Bacteriocidal - kill bacterium
Those that block translation may be bacteriostatic - stop bacteria growing but still alive effectively
Allows time for immune system to clear up the living bacteria
how does fosfomycin work
- PEP analogue that inhibits enzymes that convert NAG to NAM
- needs to be taken into bacterial cytoplasm
- active uptake via transporters to acquire nutrients eg-glucose
- especially effective against gram -ve bacteria that infect urinary tract including E. coli as it is excreted unchanged in urine ie - not metabolised by body
- antibacterial synergy with beta-lactams, aminoglycosides (target protein synthesis) and fluoroquinolines (target DNA synthesis)
- resistance due to altered transporter activity/reduced uptake/mutation in enolpyruvate transferase
how does D-cycloserine work
- acts as a structural homologue for D-alanine inhibiting:
racemase that converts L-alanine to D form
ligase that couples D-alanyl-D-alanine dipeptide - used as a 2nd line antibiotic for treatment of TB
- resistance due to over expression of alanine racemase
how do glycopeptides (vancomycin and teicoplanin) work
- affect polymerisation of peptidoglycan, bactericidal
- transglycosylase assembles linear glycol chains in cell wall by transferring growing glycan chain to disaccharide peptidoglycan precursor on lipid carrier as it crosses cell membrane
- glycopeptides block this by binding to D-alanyl-D-alanine dipeptide which prevents binding and function of TG-ase
- resistance due to DNA encoding enzymes that catalyse formation of D-Ala-D-lactacte instead of D-Ala-D-Ala so it no longer binds
- Vancomycin = complex tricyclic glycopeptide. Large so can’t penetrate outer membrane of gram -ve bacteria and not effect against them.
Treats gram +ve infections - MRSA and C. difficile
how do beta-lactams work
- bactericidal
- inhibit transpeptidase enzymes (PBP) involved in cross-linking
- mimic D-alanyl-D-alanine residues (where enzyme binds for cross-linking so antibiotic occupies active site and D…residues can’t bind)
- beta-lactam ring broken, enzyme inactivated
- destabilised cell wall causing cell wall to become leaky, osmotic burst
what are the 4 subclasses of beta-lactams
- penicillins
- cephalosporins
- monobactams
- carbapenems
all have different chemical substituents attached to ring
main side effects are associated with allergic reactions (rashes to anaphylaxis, sickness, diarrhoea)
what determines if an antibiotic is broad-spectrum for narrow-spectrum
properties of antibiotic and its uptake by bacteria
what type of antibiotics pass through OM of gram -ve bacteria more easier
hydrophilic agents due to hydrophilic fatty acids
what spectrum of activity do hydrophilic agents have
broad spectrum
ampicillin, amoxicillin, piperacillin
what spectrum of activity do hydrophobic agents have
narrow spectrum
flucloxacillin, penicillin
where do penicillins come from
naturally occurring from moulds penicillium notatum and penicillium chrysogenum
what is the biggest cause of resistance to beta-lactams
beta-lactamases
what structure do cephlasporins have
beta-lactam ring but more accessible and better targeted to actual site of infection via 2 side chains (R1 and R2) which alter stability, PKs, resistance breakdown
what type of activity does cephlasporins display
broad-spectrum
treats septicaemia, meningitis, UTIs
why were carbapenems developed
to avoid issues of developing reistance to penicillins and cephlasporins (4 drugs)
what is the structure of carbapenems
derivatives of penicillin and cephalosporin where S atom is replaced with a C atom (R1R2 R3)
*stable against most beta-lactamases, not effective against MRSA
what activity do cephlasporins display
broad spectrum
treat septicaemia and severe HCAI
reserved for severe infections
what is the only drug in monobactams
aztreonam
what are monobactams active against
gram-ve bacteria
useful for those with penicillin allergy
what bacteria are resistant to aztreonam
gram -ve bacteria with extended-spectrum beta-lactamases (super-enzymes)
what is antibiotic resistance encoded by
chromosomal or acquired (extrinsic) genes
what type of resistance does gram +ve bacteria show
- chromosomal mutation in a transpeptidase encoding gene
- stops transpeptidases ability to bind to particular beta-lactam
- or by acquisition of a gene encoding a transpeptidase with low affinity for the beta-lactam
where is resistance associated with beta-lactamases encoded
chromosome or plasmid
where are beta-lactamases located
periplasmic space
what decoy agents are used in antibiotics for beta-lactamases
clavulanic acid
