Antimicrobial Agents and Antimicrobial Susceptibility Testing

  1. How does Dilution Antimicrobial testing work?
    • Bacteria are exposed to different concentrations of antimicrobial agents
    • The smallest concentration that inhibits growth of bacteria is recorded - this value is the minimal inhibitory concentration (MIC)
  2. How are Broth antimicrobial dilutions performed?
    • Dilutions of the antimicrobial agents are prepared in broth
    • Assays are generally performed in microtiter plates
  3. How are Agar Antimicrobial dilutions performed?
    • Dilutions of the antimicrobial agents are prepared in agar
    • Bacteria are inoculated onto the agar plates
  4. Define the minimum bactericidal concentration (MBC) of an antimicrobial agent
    The lowest concentration of an antimicrobial agent that kills at least 99.9% of the bacteria in the original inoculum
  5. What is Disk diffusion antimicrobial testing also referred to as?
    Kirby-Bauer sensitivity test
  6. What agar is used for the Standardization of Disk diffusion?
    • Mueller-Hinton agar (MHA) - 4 mm thick in Petri dish at a pH 7.2-7.4
    • Fastidious organisms (like S. pneumo) will use MHA with 5% sheep red blood cells
    • For Haemophilus influenzae - Haemophilus test medium (HTM) is used
  7. What does HTM stand for and what nutrients are in it?
    • Haemophilus test medium
    • Mueller-Hinton base supplemented with hematin, NAD, and yeast extract
  8. What is equal to a McFarland #0.5 turbidity standard?
    Bacterial inoculum (108 colony forming units/mL)
  9. How are disk diffusion rated for sensitivity?
    • After incubation, the diameters of the zones of inhibition are measured
    • Zone sizes are compared to standard interpretation charts
    • Results are reported as sensitive (S), intermediate (I), or resistant (R)
  10. What is MRSA actually resistant to?
    • Oxacillin or Nafcillin
    • (Methicillin is no longer available in the US)
  11. When dealing with MRSA, what is a powerful inducer of oxacillin resistance?
  12. What are the zones and results for Oxacillin resistance for MRSA?
    • Disk diffusion:  <=19 mm Resistant and >=20 mm Sensitive
    • Broth dilution tests: <2 microgram/mL Sensitive and >4 microgram/mL Resistant
  13. What test is done for Gradient diffusion?
    Etest (AB Biodisk) provides quantitative antimicrobial susceptibility testing results
  14. What is the procedure for the Gradient diffusion Etest antimicrobial testing?
    • Bacterial suspension equal to a McFarland #0.5 turbidity standard is prepared
    • Bacteria are lawned onto Mueller-Hinton agar plate and the Etest strips are placed on top of the agar - each strip contains a different antimicrobial agent
    • After incubation, the bacteria produce and elliptical zone of inhibition around the strip where the zone of inhibition crosses the strip
  15. What does beta-lactamase do?
    An enzyme that confers resistance to penicillin and some of the semisynthetic penicillins (i.e. ampicillin)
  16. What organisms produce extended-spectrum beta-lactamase (ESBL)?
  17. What is ESBL and what does it do?
    • Extended-spectrum beta-lactamase
    • These enzymes inactivate the extended spectrum cephalosporins (i.e. ceftriaxone and cefotaxime)
  18. What are the organisms Enterobacteriaceae capable of producing?
    • Extended-spectrum beta-lactamase (ESBL)
    • These enzymes inactivate the extended spectrum cephalosporins (i.e. ceftriaxone and cefotaxime)
  19. What is the D-zone test used for?
    • Used to detect the presence of inducible clindamycin resistance by erythromycin
    • A plate is inoculated for disk diffusion assay with erythromycin and clindamycin disk
    • After incubation, the plate is examined for a flattening of the zone of inhibition around the clindamycin disk, resembling the letter D - indicating the presence of inducible resistance to clindamycin
  20. How do Beta-lactam antibiotics work?
    Inhibit cell wall synthesis
  21. What are some examples of Beta-lactams?
    • Penicillins
    • Cephalosporins
    • Monobactams
    • Carbapenems
  22. How are the class of cephalosporins categorized?
    • Narrow spectrum (1st generation)
    • Expanded spectrum (2nd gen)
    • Broad spectrum (3rd gen)
    • Extended spectrum (4th gen)
  23. How do Beta-lactamase inhibitors work?
    • Bacteria can exhibit resistance to the beta-lactam antibiotics by producing and enzyme (beta-lactamase) that cleaves the beta-lactam ring - inactivating the antibiotic
    • Beta-lactamase inhibitors can be given with a beta-lactam antibiotic to provide effective treatment
  24. What are examples of Beta-lactamase inhibitors?
    • Clavulanic acid
    • Sulbactam
    • Tazobactam
  25. How do Aminoglycosides work?
    • Inhibit protein synthesis at the 30S ribosomal subunit
    • Active against gram-negative and gram-positive bacteria
    • No activity against obligate anaerobes
    • Because of potential toxicity, dosage should be monitored using peak and trough values in peripheral blood
  26. What are some examples of Aminoglycosides?
    • Gentamicin
    • Tobramycin
    • Netilmicin
  27. How is Tobramycin classified, compared to the other Aminoglycosides?
    • Tobramycin is a bactericidal
    • Others are bacteristatic
  28. How do Tetracyclines work?
    • Inhibit protein synthesis at the 30S ribosomal subunit
    • Active against gram-positive, gram-negative, Mycoplasma, and Chlamydia
    • Increased resistance has limited their use
  29. What are some examples of Tetracyclines?
    • Doxycycline
    • Minocycline
  30. How do Chloramphenicols work?
    • Inhibits protein synthesis by binding to the 50S ribosomal subunit
    • Broad spectrum
    • Used to treat serious gram-negative infections, such as meningitis
    • Risk of bone marrow toxicity, aplastic anemia (bone marrow suppression)
  31. How do Macrolides work?
    Inhibit protein synthesis
  32. What are some examples of Macrolides?
    • Erythromycin
    • Clarithromycin
  33. How do Sulfonamides work?
    Inhibit folic acid synthesis by forming nonfunctional analogs of folic acid
  34. How do Glycopeptides work?
    Inhibit cell wall formation by inhibiting peptidoglycan synthesis
  35. What is the only approved Glycopeptide for the U.S.?
  36. What are some Vancomycin resistant species isolated?
    • Enterococcus
    • Staph aureus
  37. How do Quinolones work?
    • Inhibit DNA activity by inactivating DNA gyrase
    • Newer agents are known as fluoroquinolones
  38. What are some examples of Fluoroquinolones (Quinolones)?
    • Ciprofloxacin
    • Levofloxacin
  39. How do Polymyxins work?
    • Disrupt plasma membranes
    • Used to treat infections caused by gram-negative bacteria
  40. What are some examples of Polymyxins?
    • Polymixin B
    • Polymixin E
  41. How do Nitrofurantoins work?
    • Inhibits bacterial enzymes
    • Used to treat UTIs
  42. Define antibiotic
    • Molecule produced my microorganism that inhibits the growth of other microorganisms
    • Antibiotics can also be synthetic
  43. Define Cidal
    • Kills microorganisms
    • (i.e. bactericidal compound kills bacteria)
  44. Define Static
    • Inhibits the growth of microorganisms
    • (i.e. bacteristatic compound inhibits bacterial growth)
  45. Define Synergy
    When 2 or more antimicrobials are used and the combined effect is greater than what would be expected for the simple additive effect of the agents
  46. What does Narrow-spectrum antimicrobial agent mean?
    Limitied range of action
  47. What does Broad-spectrum antimicrobial agent mean?
    Active against a wide range of bacteria
Card Set
Antimicrobial Agents and Antimicrobial Susceptibility Testing
Antimicrobial Susceptibility Testing, Classes of Antimicrobials, Definitions