SM SM Antimicrobial Resistance & Stewardship 2 — Questions and Answers
Question 1: Which resistance phenotype is detected by a positive modified carbapenem inactivation method (mCIM) and a negative EDTA-CIM (eCIM)?
- Metallo-beta-lactamase (MBL) production such as NDM
- Serine carbapenemase production such as KPC or OXA-48 (Correct answer)
- Porin loss combined with AmpC overproduction
- Efflux pump-mediated carbapenem resistance
Correct answer: Serine carbapenemase production such as KPC or OXA-48
Serine carbapenemases (KPC, OXA-48) are mCIM-positive and eCIM-negative because EDTA chelates zinc and inhibits only metallo-beta-lactamases.
Question 2: Vancomycin-resistant Enterococcus (VRE) expresses the vanA operon. Which cellular change confers resistance?
- Thickening of the peptidoglycan cell wall to reduce drug penetration
- Substitution of D-Ala-D-Ala terminus with D-Ala-D-Lac, reducing vancomycin binding affinity (Correct answer)
- Overproduction of D-Ala-D-Ala ligase saturating the antibiotic
- Export of vancomycin via ABC transporters
Correct answer: Substitution of D-Ala-D-Ala terminus with D-Ala-D-Lac, reducing vancomycin binding affinity
The vanA operon encodes enzymes that substitute D-Lac for D-Ala at the peptidoglycan terminus, reducing vancomycin binding affinity 1,000-fold.
Question 3: In PK/PD-guided antibiotic dosing, aminoglycosides are optimally dosed using which pharmacodynamic principle?
- Time-dependent killing: maximize time above MIC (T>MIC)
- Concentration-dependent killing: maximize Cmax/MIC ratio (Correct answer)
- AUC-dependent killing: maximize AUC24/MIC ratio
- Post-antibiotic effect: extend intervals to exploit PAE
Correct answer: Concentration-dependent killing: maximize Cmax/MIC ratio
Aminoglycosides exhibit concentration-dependent killing, meaning efficacy correlates with the peak concentration (Cmax) relative to the MIC, supporting once-daily high-dose regimens.
Question 4: Which method is used to detect inducible clindamycin resistance (iMLSB) in Staphylococcus aureus?
- Oxacillin salt agar screen
- D-zone test (D-test) using erythromycin and clindamycin disks (Correct answer)
- Chromogenic agar plating
- Cefoxitin disk diffusion test
Correct answer: D-zone test (D-test) using erythromycin and clindamycin disks
The D-test places erythromycin and clindamycin disks 15-26mm apart; a flattened D-shaped inhibition zone around clindamycin indicates inducible methylase (erm gene) expression.
Question 5: Which CDC Core Elements of Hospital Antibiotic Stewardship Programs includes accountability?
- A pharmacy champion who reviews all antibiotic orders daily
- A physician leader responsible for outcomes and stewardship activities (Correct answer)
- An infection preventionist who tracks antibiotic-associated adverse events
- A microbiology laboratory director who approves susceptibility reporting
Correct answer: A physician leader responsible for outcomes and stewardship activities
The CDC specifies a physician leader (often an infectious disease physician) as the accountable leader responsible for stewardship program outcomes.
Question 6: A hospital antibiogram shows 40% susceptibility of E. coli to fluoroquinolones. According to stewardship guidelines, what is the recommended action for empiric UTI therapy?
- Continue fluoroquinolone empiric therapy until culture results are available
- Avoid fluoroquinolones for empiric therapy and choose an alternative agent (Correct answer)
- Use higher-dose fluoroquinolone to overcome resistance
- Add a second antibiotic to the fluoroquinolone regimen
Correct answer: Avoid fluoroquinolones for empiric therapy and choose an alternative agent
IDSA guidelines recommend avoiding antibiotics with <80% susceptibility on the local antibiogram for empiric UTI therapy to ensure adequate treatment.
SM SM Antimicrobial Resistance & Stewardship 2
Which resistance phenotype is detected by a positive modified carbapenem inactivation method (mCIM) and a negative EDTA-CIM (eCIM)?