Lecture 19B - Antibiotics B Flashcards
What is tetrahydrofolate (THF)?
Cofactor essential for synthesis of precursors of DNA, RNA, protein, fatty acids and vitamins in bacteria and humans, and for N-formylmethionine (fMet) in bacteria (initiation of protein synthesis)
Precursor for THF?
dihydrofolic acid (DHF)
Precursor for DHF?
p-aminebenzoic acid (PABA)
Enzymes that convert PABA to DHF?
Pteridine synthetase and DHF synthetase
What is the difference in how bacteria synthesize DHF compared to mammals?
Mammalian cells use Folic Acid (Folate) obtained from food while bacteria synthesize it from PABA -> dihydropteroic acid -> DHF
What do sulfonamides target?
Structurally similar to
PABA – they are competitive
inhibitors of pteridine synthase in
bacteria
What do trimethoprim target?
Structurally similar to DHF – it acts as a competitive inhibitor of the microbial DHF reductase but not the mammalian enzyme
How does resistance develop against sulfonamides?
Mutation in pteridine synthetase such that it no longer binds but is still functional (immunity), some bacteria can make THF directly from folic acid like mammals (bypass)
How does resistance develop against trimethoprim?
Mutation in DHF reductase such that it no longer binds but is still functional (immunity), acquisition of a plasmid-encoded DHF reductase that doesn’t bind trimethoprim (bypass)
How does metronidazole work?
damages DNA, inhibits DNA
synthesis
How is metronidazole activated?
By bacterial
housekeeping proteins flavodoxin
and ferredoxin in microaerophilic
and obligate anaerobes
Why is metronidazole only activated in bacterial cells?
Lack flavodoxin and ferredoxin
Which bacteria is metronidazole used for?
Helicobacter pylori and Clostridium difficile, also effective against some eukaryotic pathogens that can activate metronidazole
Which bacteria are sulfonamides and trimethoprim used for?
Many species - broad spectrum
How does resistance develop against metronidazole?
Upregulation of DNA repair enzymes (RecA) (bypass/immunity)
How do quinolones and fluoroquinolones work?
Inhibit DNA replication, quinolones bind to and stabilize the gyrase:DNA complex in DNA gyrase and prevent strand resealing, resulting in double strand breaks in the DNA.
What do quinolones/fluoroquinolones target?
DNA gyrase (Topoisomerase II)
Difference between quinolones and fluoroquinolones?
Fluoroquinolones are just fluorinated, more active and effective quinolones
Which bacteria is ciproflaxin(quinolone) used for?
B anthracis (anthrax)
How is resistance developed against quinolones/fluoroquinolones?
Single mutation in DNA gyrase can make bacteria resistant - gyrase still active but no longer binds (immunity)
What does rifampicin target?
Exit channel on RNA polymerase beta subunit
How does rifampicin work?
Blocks emergence of new RNA from exit channel in bacterial RNA pol beta subunit
How is resistance developed against rifampicin?
Single mutation in
RNA polymerase that prevents rifampin binding
but not RNA polymerization.
What do aminoglycosides like streptomycin target?
Bind to the bacterial ribosomal 30S subunit, inhibit the initiation and
elongation steps in protein synthesis. Allows the wrong tRNA to bind inhibiting protein synthesis.
How does resistance develop against aminoglycosides?
Aminoglycosidal modifying enzymes - modification reduces their transport into cell (too big) and their ability to bind to the 30S subunit
What do tetracycline target?
Bind to the 30S subunit and inhibits tRNA binding at the A site
How does tetracycline work?
Distort A site and prevent alignment of
aminoacylated tRNA with codon on mRNA –
block peptide synthesis
How is resistance developed against tetracycline?
Efflux pumps, modification by tetracycline enzymes, cytoplasmic proteins that protect 30S subunit by preventing binding without disrupting protein synthesis
What do chloramphenicol target?
Binds to the peptidyl transferase on the 50S subunit, inhibits the peptidyl transfer step in protein synthesis
How is resistance to chloramphenicol developed?
Bacteria
have/acquire an enzyme, chloramphenicol
acetyltransferase, which acetylates chloramphenicol,
inactivating it.
What do macrolides target?
Bind to the 50S subunit and inhibit
translocation
What is azithromycin? What is it used for?
Macrolide. Very effective at treating
sexually transmitted infections such as
Chlamydia and N. gonorrhoeae – single
dose (cf. a full course) but very expensive
How is resistance against macrolides developed?
Efflux pump
What is the difference between lincosamides/streptogramins and macrolides?
Differ in structure from macrolides but have the same mechanism - bind to roughly the same site on the 50S ribosome and inhibit translocation step. Lincosamides kill intestinal microbiota, associated with C difficile infections. Streptogramins used in agriculture - many resistance strains.
Which antibiotics are used against 30S subunit of bacterial ribosome? 50S subunit?
30S - Tetracycline, aminoglycosides
50S - Chloramphenicol, macrolides, streptogramins, lincosamides
What does teixobactin target?
Binds to precursors of peptidoglycan – Lipid II and Lipid II precursors and prevents PBP/transpeptidase activity, blocking peptidoglycan synthesis
Which bacteria is teixobactin effective against?
Gram-positive bacteria – S. aureus, S. pneumoniae, M. tuberculosis,
vancomycin-resistant enterococci (VRE, have D-Ala-D-Lac but can still bind teixobactin). Gram-negatives are resistant
How are gram-negatives resistant to teixobactin?
Act on PG and WTA in gram-positive bacteria which lack OM - presence of the outer membrane prevents uptake of teixobactin
Why is resistance less likely to occur with teixobactin?
Acts on lipid II, a non-protein molecule - can’t change lipid II
What kinds of organisms can be found in the microbiota?
Bacteria, archaea, fungi, some protozoa and viruses/phages
Why are most microbes found in the gut?
Provides stable nutrients and temperature
Difference between microbiota and microbiome?
Microbiota - refers to the community microbes
Microbiome - refers to the combined genomes
How are the 60-90% non-culturable gut microbiota identified?
(1) targeted sequencing of phylogenetically
informative genes, usually 16s rRNA, a taxonomic marker for bacteria, or (2) by random sequencing of
all genes in the microbial community (shotgun metagenomic sequencing of stool samples). Successful identification of bacterial species in a microbiome requires a match with existing genome
sequences in databases
Which bacteria dominate in the gut?
In adults, Gram-positive Firmicutes (eg.
Clostridia, Lactobacilli, Entercocci) and Gramnegative
Bacteroidetes dominate.
Most gut bacteria are anaerobic. Some are
adherent, most are in the lumen of the colon
(large intestine).
What are the metabolic functions of gut microbiota?
Aid in digestion by fermenting/breakdown of complex carbohydrates, amino acids, bile salts, lipids,
fatty acids, salvage urea, synthesize essential amino acids, vitamins (eg. vitamin K, folic acid) – these nutrients are used by the bacteria but are also available to host, modulate appetite, etc
What are the trophic functions of gut microbiota?
Facilitate gut development by controlling epithelial cell proliferation and differentiation; influence organ development, development of lymphatic tissues, angiogenesis, fat storage
What are the protective functions of gut microbiota?
Protective: out-compete pathogenic bacteria for nutrients, prevent colonization by pathogens,
stimulate the immune system, facilitate its maturation to eliminate pathogens but tolerate
commensals
How do C difficile infections occur?
May be present in the gut in low levels prior
to antibiotic treatment, but controlled by competition with commensal bacteria, or may be acquired
by ingestion of spores in hospital. C. difficile spores are robust and difficult to kill – are prevalent in
hospital settings.
What is C difficile?
Anaerobic spore-forming Gram-positive bacillus. Causes severe diarrhea, pseudomembranous colitis, toxic
megacolon - can be fatal. Resistant to multiple antibiotics. Vegetative cells produce toxins TcdA, TcdB
(for Toxin C. difficile A, B) that cause epithelial damage. Levels of glucose and other nutrients affect expression of C. difficile genes. Intestinal components such as bile acids induce C. difficile spores to germinate.
