Antibiotics Flashcards
What is the target of beta-lactams?
Bacterial cell wall
Why are non-proliferating bacterial cells insensitive to beta-lactams?
Auto-Lysins must be active
How do beta-lactams perform their function?
Ser of transpeptidase enzyme hydrolyses strained 4-membered ring carbonyl of beta-lactam forming covalent penicilloyl enzyme that is slow to hydrolyse. Autolytics induce lytic death.
Vancomycin
Targets bacterial cell wall biosynthesis. Binds to pentapeptidyl tails in PG repeating sequence terminating in D-Ala4 -D-Ala5 so transpeptidase enzyme cannot attack. Also reduces accessibility of transglycolase enzyme.
How do tetracyclins work? Bacteriostatic/bactericidal? Specificity?
Bind to 30S ribosome. Inhibit the entry of aminoacyl-tRNA into the A-site of the 70S ribosome. Bacteriostatic. Broad spectrum.
Tetracycline
Binds to 16s rRNA of 30S. Causes electrostatic interactions between oxygens of internucleotide phosphodiester links via its Mg ion. Prevents rotation of aminoacyl-tRNA into A-site and causes premature release of aminoacyl-tRNA. Broad spectrum.
How do aminoglycosides work? Specificity?
Target protein synthesis. Bind to 30S subunit and freeze pre-initiation complex (30S, mRNA, fmet-tRNA) so no further initiation can occur. Slows current protein synthesis and induces misreading of mRNA. Narrow spectrum - active uptake into gram(-).
How do macrolides work?
Target 50S subunit and inhibit peptide translocation.
Erythromycin
Binds to 23s rRNA in 50S subunit (entrance of polypeptide exit tunnel). Allow 6-8 oligopeptidyl-tRNA build-up before elongation is blocked and terminated. Narrow spectrum and short elimination half life.
Chloramphenicol
Binds to 50S peptidyl-transferase centre preventing transfer from the P-site to the A-site. Broad spectrum, limited in use due to bone marrow suppression (serious toxicity).
Fusidic acid
Inhibits EF G (GTP dependent factor required for peptide translocation). Narrow spectrum (Gram(+))
How do quinolones work?
Topo II isomerase inhibitor. Bind to intermediate state in catalysis stabilising the normally transient cleavage in DNA. Cleaved complex accumulates. DNA repair machinery may also be recruited that upon failure initiates cell death pathway.
How do aminocoumarins work?
Topo II isomerase inhibitors. Target GyrB DNA binding domain
Fluoroquinolones e.g. Ciprofloxacin
Synthetic topo II isomerase inhibitors. Broad spectrum. Bactericidal. Used for urinary tract infections.
Name an inhibitor of RNA synthesis
Rifampin
Rifampin
RNA synthesis inhibitor. Binds to beta-subunit of DNA-dependent RNA polymerase blocks RNA transport tunnel blocking the elongating RNA chain. Does not block synthesis already in process. Used in TB
Daunomycin
Interacts with ds DNA. Planar molecule that intercalates between nucleobases of ds DNA. Core becomes longer, DNA same length so ds DNA unwinds. Major/minor grooves change.
How do bleomycins work?
Interact with ds DNA. Metal chelating glycopeptide antibiotics. Interaction between O2 and bound iron generates superoxide and hydroxyl radicals causing single and double stranded breaks in DNA. Toxic to gram(+) and mammalian cells.
Aziridines
Interacts with ds DNA. Alkylating agent. Crosslinks G bases between 2 strands preferentially at GC positions. Prevents strand separation during DNA replication and transcription.
What are the main targets of antibiotic action?
Cell wall biosynthesis. Protein synthesis. DNA replication, repair and expression. Folic acid synthesis. Membrane.
Why is folic acid synthesis a good target for antibiotics?
Only bacteria must synthesise the folate skeleton de novo. DHPS is absent from humans and DHFR has enough differences to enable selective inhibition.
Trimethoprim
Folic acid synthesis inhibitor. Inhibits DHFR.
Sulfamethoxazole
Folic acid synthesis inhibitor. Blocks DHPS.
Co-trimoxazole
Combination drug of trimethoprim and Sulfamethoxazole. Targets DHPS and DHFR.
Valinomycin
Targets bacterial membrane. Ionophoretic capacity. Transports K+ ions. Dissipates membrane potential and proton motive force.
Colistin
Targets bacterial cell membrane. Cyclic peptide antibiotic. Binds to gram(-) cell envelope and makes it leaky to ions.
Nystatin, amphotericin B
Target fungal membrane. Form ion channels in membrane as they recognise ergosterol.
What resistance mechanisms are there to beta-lactams?
Enzymes that hydrolyse strained beta-lactam ring. A, C, D inhibitors are active site serine enzymes. B inhibitors direct a water molecule via zinc to add to the strained ring.
What resistance mechanisms are there to aminoglycosides?
Resistant bacteria covalently modify specificity conferring OH and NH2 groups and interfere with recognition by the 16S rRNA. Adenylation, acetylation and phosphorylation reduce drug affinity for target.
MRSA
Resistance to methicillin in Staphylococcus aureus. Acquisition of mecA gene encoding a bifunctional transglycolase/transpeptidase enzyme insensitive to beta-lactams. Fem auxiliary gene that adds pentaglycyl bridge to PG strands before cross linking. Better substrate for bifunctional enzyme.
Vancomycin resistance in Entericoccus faecalis
Common in patients with compromised immune system. VanA/VanB change the vancomycin binding site from N-Acyl-D-Ala-D-Ala to N-acyl-D-Ala-D-Lactate causing a 1000 fold decrease in affinity.
Resistance to Sulfamethoxazole and trimethoprim
Change from sensitive DHPS and DHFR to insensitive proteins due to expression from resistance plasmids. Very high levels of expression of sensitive DHPS and DHFR so antibiotics cannot block them all.
What are the 3 main mechanisms of antibiotic resistance?
Drug transport, target modification, enzymatic inactivation of drugs.
How can drug transport affect resistance?
Reduced uptake e.g. Mutations in outer membrane porins and active uptake systems in the cytoplasmic membrane.
Active efflux systems e.g. Acquisition of transport genes on plasmids or transposons.
