Antibiotics 2 Flashcards
Glycopeptides
glycosylated, cyclic non-ribosomally synthesized peptide antibiotics (bactericidal)
ex: Vancomycin
Mechanism of Action of Vancomycin
-Vancomycin binds to D-Ala-D-Ala at the end of peptide side chain in peptidoglycan precursors, blocking PBPs from catalyzing transglycosylation/transpeptidation steps of peptidoglycan synthesis
(blocking cross linking)
-Effective on many Gram-positives, not effective on Gram-negative bacteria
due to permeability barrier of Gram-negative outer membrane
What type of bacteris (gram positive or gram negative) is Vancomycin effective on
Gram positive bc it is too big to fit through the outer membrane permeability barrier in gram negative bacteria
When is vancomycin used
-gram positive infections
- Often used for β-lactam resistant infections
(e. g. MRSA) or in patients w/ β -lactam hypersensitivity
mechanism of resistance to Vancomycin
- Modification of antibiotic target
- bacteria acquire genes encoding machinery to produce altered peptidoglycan structure that lacks D-Ala-DAla groups (contain D-Ala-D-Lac in place of D-Ala-D-Ala);
-vancomycin is unable to bind efficiently to these modified precursors
-Genes encoding vancomycin resistance are usually found on plasmids or
transposons that can be easily transferred to other bacteria by horizontal exchange
-Vancomycin resistance often associated with enterococci in hospital settings (VRE
– vancomycin-resistant enterococci)
What is vancomycin resistance often associated with
enterococci in hospital settings ( VRE-vancomycin resistance enterococci)
How can a person with MRSA that is treated with Vancoymycin develop a vancoymycin resistant infection
- MRSA means resistant to all B-lactams, so you can’t use a B-lactam antibiotic, so you use Vancomycin which binds to/blocks the substrate fro PBP so if can’t from peptidoglycan
- BUT this MRSA infection transferred its resistance to the other infection making them both MRSA and vancymycin resistant
Cycloserine
- another peptidoglycan inhibitor
- structurally similar to D-alanine
- used as a second line anti-tuberculosis therapy
- competitive inhibitor of Alanine racemase adn D-alanyl-D-alanine synthetase with higher affinity for enzymes than natural substrate D-alanine
Cycloserine mechanism of action
- competitive inhibitor of D-alanine in two sequential reactions
- competitive for these two enzymes that are required for production of peptidoglycan precursors
- Alanine racemase
- D-alanyl-D-alanine synthetase
Bacitracin
-info and mechanism of action
- another peptidoglycan inhibitor
- peptide antibiotic-too toxic for systemic use so only topical use
- MOA: Bind to pyrophosphate on the lipid carrier for peptidoglycan precursors (bacteoprenol-PP) and blocks its recycling
- normally the reaction is bactoprenol-PP becomes Bactoprenol-P +Pi but bacitracin blocks this
- without available lipid carrier, peptidoglycan synthesis cannot continue
- group A streptococci are 10x more sensitive than other related bacteria
- Bacitracin “A disks” are a diagnostic test for Group A Streptococci (GAS)
which bug is 10x more sensitive to bacitracin than others
GAS
Group A Streptococci
-therefore Bacitracin “A disks” are a diagnostic test for GAS
MOA of bacitracin in my words
- there is a lipid tcarrier for peptidoglycan precursors (like bactoprenol) that is bound to pyrophosphate. Bactoprenol-PP
- This lipid carrier needs to be recycled, but bacitracin binds to to pyrophosphate which blocks recycling of the lipid carrier therefore stopping peptidoglycan synthesis
Daptomycin
- Lipopeptide (lipid modified peptide) antibiotic
- bactericidal, narrow spectrum (Gram positive-bacteria)
- MOA: thought to bind and disrupt the cytoplasmic membrane, possibly works via a loss of membrane potential, novel mechanism confers activity against antibiotic resistant bacteria
- WORKS ON GRAM POSITIVE BACTERIA
Polymyxins (polymyxin B, Colistin)
- lipopeptide antibiotics
- bactericidal, narrow spectrum (Gram Negative)
- adverse effects due to toxicity limit use to only serious infections caused by antibiotic resistant bacteria, or only topical use. This is a last resort drug
MOA: Bind to LPS in the outer membrane of Gram-Negative Bacteria, leading to disruption of the outer membrane and the cytoplasmic membrane
-novel mechanism confers activity against antibiotic resistant Gram negative bacteria
What are the two drugs that work against the cell envelope and are lipopeptide antibiotics (lipid-modiified peptide)
- Daptomycin- Gram positive-cellular membrane
2. Polymyxins- Gram negative, LPS
difference between prokaryotic and eukaryotic ribosomes
bacterial: 30s+50S=70s
eukaryotic: 402+60s=80s
- structural differences in ribosomes confer selectivity
quick reminder of how ribosomes work
- protein synthesis occurs in two phases, initiation and elongation
- 30s subunit forms an “initiation complex” with mRNA and initiator tRNA
- The 50s subunit joins resulting in a functional 70s ribosome that forms peptide bonds to produce peptides via translation
Drugs that target the 30s function
Tetracycline
Tigecycline
Aminoglycosides
tetracyclines
- spectrum
- static vs cidal
- MOA
-(e.g. tetracycline, doxycycline, minocycline)
-bacteriostatic, broad spectrum ( but now limited in use due to bacterial resistance)
-MOA: bind to 30S ribosomal subunit and interferes with the binding of aminoacyl tRNA to the ribosomal complex
-• Chemists have created derivatives that differ at chemical substituents at “R”
positions; these changes alter pharmacology but not mechanism of action
Tetracycline antibiotic resistance mechanisms
- tetracycline efflux pumps (reduction in concentration0 is the most common. It provides resistance to all tetracycline family Abx
- Less common is mutations on the ribosome (bc remember tetracycline targets the 30s ribosome) this is classified as modification of antibiotic target
- resistance is often encoded on a plasmid
Tigecycline
- “new” drug class, 2006
- bacteriostatic
- MOA: same as tetracyclines (bind to 30s ribosomal subunit and interfered with the binding of aminoacyl tRNA to the ribosome), but also binds additional unique sites on the ribosome
- does not exhibit cross-resistance with tetracyclines bc it also binds on a different unique part of the ribosome
What are the Aminoglycosides and are they static or cidal
- gentamicin, amikacin, tobramycin
- bactericidal (ONLY bactericidal ribosomal inhibitor)
Mechanism of action for aminoglycosides
bind irreversibly to 30S ribosomal subunit; stops initiation of protein synthesis, causes premature release of mRNA from ribosome, and causes misreading (incorporation of incorrect amino acid into growing protein) by translating ribosomes
What type of bacteria are aminoglycosides good for
useful for hard to kill gram negatives (eg. Pseudomonas aeruginosa)
-they do not penetrate many gram positives well and have poor activity against anaerobes
adverse effects of aminoglycosides
-ototoxic and nephrotoxic
Mechanism of resistance for aminoglycosides:
- Enzymatic modification of the antibiotic (transferases catalyze addition of adenyl, acetyl, or phosphorl group) to prevent aminoglycoside binding to ribosome
- genes encoding transferases are often located on mobile genetic elements (plasmids, transposons) that facilitate transfer to other bacteria
Macrolides
- general
- static/cidal
- effective against what
- MOA
-(erythromycin, azithromycin, clarithromycin)
- bacteriostatic, primarily active against Gram-positive bacteria
- often useful in patients allergic to B-lactams
- do not cross outer membrane of many gram negatives effectively
-MOA: binds 50s ribosomal subunit to block elongation of proteins
Mechanism of resistance for the macrolides (erythromycin, azithromycin, clarithromycin)
2 primary mechanisms:
- Enzymatic modification (methylation) of ribosomal RNA (erm methylase gene) so erythromycin can’t bind
- typically plasmid born - efflux pump: can expel macrolides from cells (reduction in antibiotic concentration) enabling protein synthesis to continues
Clindamycin
- static/cidal
- useful against
- used to treat
- MOA
- bacteriostatic
- generally effective against gram negative aerobes
- useful for treatment against community acquired MRSA ( but not hospital which is resistant to multiple antibiotics)
- treat infections of toxin producing S. aureus (bc the toxins are proteins) also treats C. difficile colitis
- MOA: Bind 50s ribosomal subunit to block elongation of proteins
Mechanism of resistance for Clindamycin
- Enzymatic modification (methylation) of rRNA (erm methylase gene) so clindamycin can’t bind (modification of antibiotic target)
- exhibits cross-resistance with macrolides (ie bacteria that are resistant to macrolides due to the presence of erm gene are also resistant to clindamycin
Chloramphenicol
- static vs cidal
- toxicity
- MOA
-bacteriostatic, broad spectrum
- Potential toxicity (aplastic anemia) limits use to very severe
infections (e.g. typhoid fever, Rocky Mountain Spotted fever)
-Toxicity probably derives from lack of selectivity – inhibits
ribosomes in mitochondria
- Mechanism of Action: Binds 50S ribosome subunit to inhibit peptidyl transferase activity (elongation)
Mechanism of Resistance for chloramphenicol:
Bacterial enzyme (chloramphenicol
acetyltransferase; CAT) catalyzes addition of acetyl group to the
drug, preventing ribosome binding by the drug (modification of antibiotic) – typically plasmid-borne
linezolid (oxazolidinone family)
-Narrow spectrum: Not effective against most Gram-negative bacteria or
anaerobes
-Oral availability
-Mechanism of action: binds unique site on 50S subunit (23S rRNA) to
prevent formation of 70S initiation complex (bacteriostatic)
-resistant strains developed within 1 year
Mechanism of antibiotic resisitance for Linezolid
•point mutations in ribosomal rRNA that prevent linezolid binding
(modification of antibiotic target) – no cross resistance with other
ribosome-targeting antibiotics observed
What Class of drugs inhibits DNA replication
Quinolones
-original=Nalidixic acid (synthetic quinolone)
-clinically useful= fluoroquinolones (norfloxacin, ciprofloxacin, moxifloxacin, levofloxacin)
What is the MOA for the synthetic quinolone Nalidixic Acid and what are problems with it
- Binds bacterial DNA gyrase and/or topoisomerase to
inhibit its catalytic function – disrupts DNA replication and repair, results in
DNA damage
-Problems: narrow anti microbial spectrum: rapid selection for resistant
mutants
What are the clinically useful quinolones
fluoroquinolones (norfloxacin, ciprofloxacin,
moxifloxacin, levofloxacin)
What is the MOA of the clinically useful quinolones
-bactericidal
-broad spectrum
-all quinolones exhibit same mechansim of action
- Binds bacterial DNA gyrase and/or topoisomerase to
inhibit its catalytic function – disrupts DNA replication and repair, results in
DNA damage
Mechanisms of resistance for the quinolones
- Point mutations in bacterial DNA gyrase prevent antibiotic binding and render the enzyme resistant to the action of qionolones (modification of antibiotic target)
- Efflux pump mediated and altered porin resistance also observed
metronidazole
- used to treat anaerobic bacterial infections;common treatment for C. difficle pseudomembranous colitis
- Bactericidal
- MOA: in an anaerobic environment, bacteria reduce the nitro group to produce a radical species, a toxic metabolite that damages DNA
-so basically the bacteria take the metronidazole from an inactive to an active form. they reduce the nitro group which turns the metronidazole into a radical species that will damage DNA
Mechanism of resistance for Metronidazole
alterations in metabolic pathways that cause bacteria to lose ability to reduce the nitro group
What are the drugs that target DNA
Quinolones
Metronidazole
Rifampin ( rifampicin)
-bactericidal
-MOA: Binds B subunit of bacterial RNA polymerase to
inhibit polymerase activity, thereby preventing RNA synthesis
-Problem: rapid selection for resistant mutants
Mechanism of resistance for rifampin (rifampicin)
- acquisition of mutations in B subunit of RNA
polymerase that prevent antibiotic from binding
Fidaxomicin
- bactericidal
- MOA: Noncompetive inhibitor of RNA synthesis by binding bacterial RNA polymerase
Mechanism of resistance to Fidaxomicin
-acquisition of mutations in B subunit of RNA polymerase that prevent antibiotic from binding
Anti-folates
- Metabolic analogs as antibiotics
- metabolic analogs are structurally similar to natural metabolic intermediates-the analogs act as competitive inhibitors to block the normal biosynthetic pathway
Describe the pathway or tetrahydrofolate biosynthesis in bacteria
- Pteridine and p-Aminobenzoic acid are combined to dihydropteroic acid
- Dihydropteroic acid is converted to Dihydrofolic Acid
- Dihydrofolic Acid is converted to Tetrahydrofolic acid
What is the purpose of tetrahydrofolic acid (FH4)
- it is required for bacterial growth
- it donates a single carbon group for production of key biosynthetic precursors (eg nucleotides)
Sulfonamides
- sulfamethoxazole, sulfadiazine
- metabolic analos of p-Aminobenzoic acid that competitively inhibit dihydropteroic acid synthesis
-this blocks the first step in the prodtuction of tetrahydrofolic acid which is needed for bacterial growth
Trimethoprim
-metabolic analog of dihydrofolate-compeitively inhibits dihydrofolate reductase
So you don’t convert dihydrofolic acid to tetrahydrofolic acid
Mechanism of resistance to trimethoprim
Acquisition of another gene
encoding dihydrofolate reductase (typically plasmidborne);
mutations in the gene encoding dihydrofolate
reductase
How do you block tetrahydrofolic acid biosynthesis as an antibiotic mechanism
you give sulfonamide to block the production of dihydropteroic acid.
and trimethoprim to block the conversion of dihydrofolic acid to tetrahydrofolic acid
these two drugs are given together for best results
List four scenarios where two antibiotics would likely be used
- Prompt treatment for undetermined pathogen causing life-threatening
infections: “coverage” for all likely pathogens until the actual cause
has been identified
(e.g., bacterial meningitis or bacteremia in an immunosuppressed
patient ) - Polymicrobial infection caused by bacteria with different susceptibility
profiles
3. To avoid or delay emergence of resistant mutants during long-term antibiotic therapy (e.g. anti-tuberculosis course of therapy typically lasts for 6-9 months)
- Synergy: the combined action of 2 drugs is greater than the sum of
individual drugs
Synergism
-combination of 2 agents (Abx) result in at least a 4-fold reduction in the MIC; or combination is more than additive
3 examples of synergy
- Two drugs act sequentially to block an essential metabolic pathway
Sulfonamides & trimethoprim: Block sequential steps in folic acid
biosynthesis to produce bactericidal synergistic effect - One drug enhances the uptake of another drug
Penicillins (vancomycin) enhance uptake of aminoglycosides - One drug may prevent the inactivation of another drug
Clavulanic acid inactivates
B-lactamase, preventing enzymatic
cleavage of
B-lactam antibiotics and preserving their effectiveness (giving a B-lactam and a B-lactamase inhibitor)
Antagonism
- the combination is less than the sum of the parts
- often occurs with the combination of a static and a cidal drug, especially is the static drug is given first
When is it not good to give a bacteriostatic and a bactericidal drug simulatneously
- it often has an antagonistic effect
- especially bad to give the static drug first
- most cidal drugs work by acting on growth and synthesis mechanisms. static drugs stop these mechanisms from occuring
- so if you are stoping the growth mechanisms from occurring, the cidal drug will have nothing to target, so it will be less effective
explain why combining some B-lactams can cause an antagonistic effect
– Some
B‐lactams(ampicillin,1stgeneration
cephalosporins,cefoxitin,imipenem)stronglyinduce
AmpC B-lactamasesthatcandegrademanypenicillins
and2nd&3rdgenerationcephalosporins
Indifference
2 agents together don’t work any better or worse than a single agent
Biofilm
- bacteria growing in a multicellular aggregate on surface
- likely responsible for many chronic infections ( endocarditis, otitis media)
-bacteria in the biofilm exhibit enhanced tolerance to antibiotics (NOT
genetically encoded resistance) – partly due to “non-growing” state
-Antibiotic therapy kills planktonic bacteria, symptoms abate
Antibiotics fail to eradicate the biofilm bacteria
After antibiotics are withdrawn, bacteria can disperse from the biofilm
and cause recurrent infection
-Standard antibiotic susceptibility
measurements (e.g. MIC) use planktonic
cells – do not account for biofilm tolerance
*** the bacteria in the biofilm aren’t really growing much so they aren’t a great target for antibiotics that target growth, and they also are a bit tucked away under layers so it is physically more challenging for the antibiotic to reach it
Antibiotics
inhibit bacterial growth in the host by interfering with critical
cellular processes in the bacteria and exploiting the differences
between bacterial physiology and host physiology to achieve selectivity
which types of antibiotics target cell wall biosynthesis at distinct steps
•B-lactams, glycopeptides, bacitracin, cycloserine, daptomycin and
polymyxin
Which antibiotics target early stages of
protein synthesis
•tetracyclines, aminoglycosides, and linezolid
Which antibiotics target elongation stage of protein synthesis
macrolides, chloramphenicol, and clindamycin
Which antibiotics inhibit DNA gyrase to produce DNA damage
fluoroquinolones
Which antibiotic targets DNA metabolism in anaerobic bacteria
metronidazole
Which antibiotics are antimetabolites that are competitive inhibitors of normal metabolic reactions
trimethoprim, sulfonamides
Resistance
has emerged to nearly all antibiotics in clinical use, often
facilitated by genetic transfer of resistance determinants between bacteria
