What is Lincomycin used for? What is its mechanism of action?
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?
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)
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?
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