AP-Micro Microbiology: Antimicrobial Susceptibility Testing 3 — Questions and Answers
Question 1: Which test is used to detect inducible clindamycin resistance caused by the erm gene in staphylococci?
- ESBL phenotypic confirmatory test
- D-zone test (double-disk diffusion) (Correct answer)
- Modified Hodge test
- Carbapenem inactivation method
Correct answer: D-zone test (double-disk diffusion)
The D-zone test places erythromycin and clindamycin disks adjacent to each other; flattening of the clindamycin zone toward the erythromycin disk indicates inducible resistance.
Question 2: What does the term 'bactericidal' mean in the context of antimicrobial testing?
- The drug inhibits bacterial growth without killing
- The drug kills at least 99.9% of the original bacterial inoculum (Correct answer)
- The drug is effective only against gram-positive bacteria
- The drug penetrates biofilms effectively
Correct answer: The drug kills at least 99.9% of the original bacterial inoculum
A bactericidal drug kills at least 99.9% (3 log reduction) of the original inoculum, distinguishing it from bacteriostatic agents that only inhibit growth.
Question 3: An organism that tests susceptible to a beta-lactam in vitro but carries a mecA gene should be reported as:
- Susceptible, since in vitro results override genotype
- Resistant, because mecA confers oxacillin/methicillin resistance regardless of phenotypic results (Correct answer)
- Intermediate, reflecting the genotypic-phenotypic discrepancy
- Susceptible-dose dependent
Correct answer: Resistant, because mecA confers oxacillin/methicillin resistance regardless of phenotypic results
mecA encodes an altered penicillin-binding protein (PBP2a) conferring MRSA resistance; organisms carrying mecA must be reported resistant to all beta-lactams regardless of in vitro appearance.
Question 4: What is the significance of the minimum bactericidal concentration (MBC) relative to the MIC?
- MBC is always lower than the MIC
- MBC is the lowest concentration that kills 99.9% of bacteria, typically equal to or greater than the MIC (Correct answer)
- MBC measures biofilm resistance only
- MBC replaces MIC in routine clinical testing
Correct answer: MBC is the lowest concentration that kills 99.9% of bacteria, typically equal to or greater than the MIC
The MBC is the lowest drug concentration that kills 99.9% of the original inoculum and is equal to or higher than the MIC because killing requires higher concentrations than inhibition.
Question 5: Which method of AST is best suited for testing slow-growing or fastidious organisms like Haemophilus influenzae?
- Standard Mueller-Hinton disk diffusion without supplements
- Disk diffusion on Haemophilus Test Medium (HTM) with appropriate supplements (Correct answer)
- Broth macrodilution only
- The direct colony suspension method on blood agar
Correct answer: Disk diffusion on Haemophilus Test Medium (HTM) with appropriate supplements
Haemophilus Test Medium (HTM) provides the growth factors (NAD and hemin) needed for H. influenzae and gives reliable, reproducible susceptibility results.
Question 6: What does a zone of inhibition of exactly zero (no zone) around an antibiotic disk indicate?
- The organism is susceptible and absorbed all the drug
- The organism is fully resistant to that antibiotic (Correct answer)
- The disk was defective and should be repeated
- The organism is intermediate in susceptibility
Correct answer: The organism is fully resistant to that antibiotic
No zone of inhibition means the organism grew up to the disk, indicating complete resistance to the antibiotic.
Question 7: In E-test (epsilometer test) strips, how is the MIC determined?
- At the point where the elliptical inhibition zone intersects the MIC scale on the strip (Correct answer)
- By reading the highest concentration mark on the strip
- By measuring the longest diameter of the zone in millimeters
- By counting colonies inside the inhibition ellipse
Correct answer: At the point where the elliptical inhibition zone intersects the MIC scale on the strip
The MIC is read at the point where the elliptical zone of inhibition intersects the printed concentration gradient scale on the E-test strip.
AP-Micro Microbiology: Antimicrobial Susceptibility Testing 3
Which test is used to detect inducible clindamycin resistance caused by the erm gene in staphylococci?