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How does a bacteria achieve resistance to Aminoglycosides?
- They make enzymes that add a functional group to the aminoglycoside. The possible changes are
What does the gentamycin assay show you?
Intracellular bacteria concentration (all extracellular are killed by the gentamycin, but the intracellular survive)
Which antibiotics inhibit tRNA Binding?
What are four important factors regarding Aminoglycosides?
- Require Oxygen for uptake (not effective on anaerobes)
- Do not enter mammalian cells
- Toxic concentration can easily be reached
- Synergistic with beta lactams
What classification of drug are Streptomycin, Neomycin, Gentamicin, Tobramycin, and Amikacin? What are they used for?
- They are all aminoglycosides
- Streptomycin – TB and plague
- Neomycin – topical component of 3 in one ointment
- Gentamycin – Broad spectrum IV
- Tobramycin – Broad spectrum IV
- Amikacin – reserved for resistant strains
Are aminoglycosides bacteriocidal or bacteriostatic?
- They stop 85% of protein synthesis, and the remaining 15% of proteins are incorrectly made
- These faulty proteins kill the cells
What class drug works synergistically with Aminoglycosides?
Beta lactams because they weaken the cell wall allowing aminoglycoside entry
Is spectinomycin bacteriocidal or bacteriostatic?
How do cells become resistant to Tetracycline? (3 mechanisms)
- “tet” protein on plasmid: mediates pumping tetracycline out of cells
- Ribosome protection: gene on plasmids produce RPP that binds to 30S preventing tetracycline binding
- Uncommon method: plasma coded enzyme that oxidizes tetracycline
What drug was designed to combat the resistance to tetracycline?
Tigecycline overcomes ribosome protection and has efflux resistance
What drugs bind to the 30s subunit of RNA? (prevent tRNA binding)
What drugs bind to the 50s RNA subunit? (prevent peptide bond formation)
What is Chloramphenicol used for? What drug is sometimes paired with it?
- Broad spectrum, static on most bacteria, but cidal for agents of meningitis
- Paired sometimes with Rifampin to prevent resistant population
What side effect was observed with chloramphenicol use?
Enters cells and disrupts mitochondria resulting in lethal anemia
How do cells become resistant to Chloramphenicol?
Chloramphenicol Acetyl Transferase (CAT) acetylates Chloramphenicol
What is Lincomycin used for? What is its mechanism of action?
- Gram +
- Good for penicillin allergic patients
- It binds to the 50s ribosomal subunit
What is Clindamycin used for? What is its mechanism of action?
- Gram + as well as anaerobes like Bacteroides
- It binds the 50s Ribosomal subunit
- Very similar to Lincomycin
What is a side effect of long courses of Clindamycin?
Overgrowth of C. difficile – antibiotic associated enterocolitis
What drugs make up Synercid and what are their mechanisms?
- Dalfopristin – blocks peptide bond formation
- Quinupristin – releases incomplete peptide
- Both drugs are binding to the 50s subunit
What is Synercid used for and what is it not good for?
- Cidal for VR MRSA
- Static for Enterococcus faecium
- Not effective for Enterococcus fecalis
- Care must be taken because it is toxic to blood vessel it is injected in – use a large vein
What are the three mechanisms for Synercid resistance?
- Drug efflux reaction
- Enzymes, either acetyl transferase or lyase
- Methylation of ribosome (“D” reaction)
What are the rare side effects of Synercid?
Hypotension or GI hemorrhage
What is Retapamulin used for? What is the mechanism?
- Used topically for impetigo or to eliminate staph carrier state
- Bacteriostatic – binds unique site on 50s
What class are the drugs erythromycin, clarithromycin, and azithromycin in? What step do they block?
- Macrolide antibiotics
- They block translocation
What class of drug are minocycline, doxycycline, and tigecycline?
- They bind the 30s subunit
What is the drug of choice for Legionella, Mycoplasma pneumoniae, and Campylobacter?
What is a “D” reaction? What is the example we have for it?
- Erythromycin induces the ribosome to methylation for resistance. The methylated ribosome is then resistant to Clindamycin or Lincomycin
- On a plate it makes the clindamycin or lincomycin disk look like a “D” instead of a circle of killed cells
How does Linezolid function? How is resistance achieved?
- Binds to the 50s subunit (specifically the 23s portion) and stops it from assembling with the 30s to form the 70s
- Resistance can be from a point mutation in the 23s subunit
- Works on gram +
What is the rare side effect of Linezolid?
It reacts with mitochrondria, especially in nerve cells causing “LION” – linesolid induced optic neuropathy
What is Nalidixic Acid?
- Quinolone (not fluoroquinolone) used for E. Coli UTI
- Rapid resistance so not very good
What is the method of action of the Fluoroquinolones?
- Inhibit DNA gyrase (esp. in gram -)
- Inhibit DNA topoisomerase IV (esp. in gram +)
What should you not use Fluoroquinolones for?
What generation Fluoroquinolone is Levofloxacin?
Third, also known as extended spectrum
What generation Fluoroquinolone are Moxifloxacin and Gatifloxacin?
- Good for most everything
Which antibiotic class can induce tendon damage?
- Other side effects of Fluoroquinolones –
- Rare CNS disturbances
- Photosensitization (sun damage)
- Not for most children
What is a drug targeted at Clostridium difficile but not normal gut flora?
- Binds to RNA polymerase
Why is Sulfa-Trimethoprin static in tissues but cidal in urine?
Urine lacks exogenous folic acid that can “rescue” bacteria found in tissues
What Is the mechanism of action of Polymyxin?
- Polymyxin has a hydrophilic head and a hydrophobic tail
- It inserts into bacterial membranes causing cidal leakage
- It binds to LPS so effective for gram negative
- Mainly topical use because it is toxic
What is the mechanism of Daptomycin?
- Binds Ca++ in the membrane of gram + bacteria
- Produces holes that kill bacteria
- ***not effective for pneumonia b/c binds to surfactant in lungs
What is the mechanism of Metronidazole?
Converted to a poison under strict anaerobic conditions
What is Nitrofurantoin used for?
- Broad spectrum bacteriostatic, mechanism unclear
- E coli, KES, staph, and enterococcus susceptible (not pseudomonas or proteus)
- Resistance is very rare (good for long term usage)
What is the mechanism of Mupirocin?
- Blocks binding of Isoleucine to tRNA
- Bacteriostatic for gram + strep or staph (impetigo)
- Topical (Bactroban)
A sample of drug susceptible Pseudomonas organisms were
placed on top of a monolayer of tissue culture cells and
incubated for 30 minutes at 37°C before gentamicin was added
(50 µ/ml). After incubating for an hour, the cells were removed
from the monolayer, washed, and then diluted. Then 500 cells
were inoculated onto a blood agar plate and incubated. The
next day, 40 Pseudomonas colonies were found on the plate.
Which of the following best explains these findings?
A. The colonies are gentamicin resistant mutants
B. These bacteria are contaminants
C. Gentamicin was mainly bacteriostatic
D. These bacteria are intracellular
(this multiple choice question has been scrambled)
An E. coli isolated from a urinary track infection was found to
be resistant to multiple antibiotics, but susceptible to trimethoprim.
Another lab found that the organism was resistant to trimethoprim.
The labs used the same methods except the one that found it
resistant to trimethoprim used a medium enriched with yeast
extract. Which of the following explains the findings?
a. Medium without yeast extract was nutritionally weak and
increased the susceptibility of the organism
b. Medium with yeast extract provided the end product of the
pathway blocked by trimethoprim
c. Yeast extract inhibited DNA gyrase so the organism was
more susceptible to trimethoprim
d. Yeast extract reacted with trimethoprim thus protecting the
What is the mechanism of Isoniazid?
- First it binds to Catalase and then to NAD-Mycolic Acid Synthetase
- The inactive Mycolic acid synthetase prevents building of mycolic acids
- Catalase-deficient mutants are INH resistant*****
What is the mechanism of Pryazinamide?
- Converted by bacterial enzyme pyrazinamidase at low pH into pyrazinoic acid which kills the cell
- Mutants that lack pyrazinamidase are resistant
- Side effect of liver damage
What is the mechanism of Ethambutol?
- Inhibits arabinosyl transferase, thus blocking synthesis of arabinoglycan
- Resistance occurs by mutation in arabinosyl transferase enzyme
- Side effect of neurotoxicity – especially optic nerve
What are Macrolides good for?