Micro1- Susceptibility

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  1. Resistance mechanisms are either _________ or _________, which are...
    • intrinsic: resistance d/t inherent metabolic attributes of the organism
    • acquired: resistance d/t acquisition of exogenous genes via mobile DNA elements or mutations
  2. Antimicrobial susceptibility testing is a(n) _________ assessment that attempts to predict __________.
    in vitro; in vivo activity
  3. What are the common AST methods?
    • broth microdilution
    • Kirby-Bauer disk diffusion
  4. Describe broth microdilution testing.
    • two fold dilution series of the drugĀ in Muller-Hinton broth
    • add a standardized amount of baceria to each tube and incubate
    • minimum inhibitory concentration (MIC) for this bacterial isolate is determined
  5. How do you interpret the MIC from broth microdilution testing? (3)
    • must know pharmacokinetics of the drug
    • the pharmcodynamics of the drug versus different type of bacteria
    • and the population distributions of the drug MICs for different bacteria
  6. Describe the Kirby-bauer disc testing method of susceptibility testing.
    • disc of antimicrobial on innoculated plate
    • measure zone of inhibition in mm, breakpoints outlined
    • disc testing is a correlate of MIC
  7. Is one method of susceptibility testing more favorable than the other?
    • sometimes MIC testing can inform drug dosage
    • slow-growing organisms MUST use MIC testing (broth microdilution) [b/c drug diffuses slowly from disc before organism can grow on agar]
  8. What are the population MIC parameters?
    • MIC50 and MIC90
    • calculated as percentiles
    • MIC90 is a reasonable bar to deciding on empirical therapy
    • MIC50 may or may not be normally distributed (bimodal distributions are common)
  9. Describe the general characteristics of the cephalosporin family. (4)
    • beta-lactams
    • eliminated via the kidney
    • low Vd (distribution is mostly ECF)
    • most drugs in this class do not cross the BBB
  10. Cephalosporins work by...
    interrupting cell wall function
  11. Describe the spectrum of activity of cephalosporins.
    • variable, esp among Enterobacteriacea (some have intrinsic resistance,some have acquired resistance)
    • dissemination of transferable resistance genes (beta-lactamase genes) on plasmids
  12. Describe the classification of cephalosporins by generation and the spectrum of each.
    • First gen: gram + aerobes and anaerobes, some strains of E. coli, Proteus, Klebsiella
    • Second gen: gram + aerobes and anaerobes, gram - aerobes and anaerobes (esp B. fragilis)
    • Third gen: Primarily gram - (good against most Enterobactericea), some gram +, [*exception: some of these cross the BBB]
    • Fourth gen: similar to third gen, just aerobes, not widely used in vetmed
  13. Name some first gen cephalosporins. (5)
    • Oral: cephalexin, cephalothin, cefadroxil
    • Parenteral: cefazolin
    • intramammary: cefapirin
  14. Name some second gen cephalosporins. (2)
    Parenteral: cefoxitin, cefotetan
  15. Name some third gen cephalosporins. (4)
    • Parenteral: Ceftazidime, Ceftriaxone (crosses the BBB), ceftiofur
    • Oral: cefpodoxime
  16. What are the major important exceptions to the activity of cephalosporins? (3)
    • Enterococcus: cephalosporins have no activity against these organisms because intrinsically lack PBP affinity
    • Bordatella: oral cephalosporins have no activity against this organism b/c it's impermeable (occasionally parenteral is useful)
    • Pseudomonas aeruginosa: intrinsic cephalosporinase production (SOME third gen may work, like ceftazidime)
Card Set:
Micro1- Susceptibility
2017-01-26 15:42:57
vetmed micro1

vetmed micro1
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