SM SM Antimicrobial Resistance & Stewardship 1 — Questions and Answers
Question 1: Which mechanism does Staphylococcus aureus use to confer resistance to beta-lactam antibiotics including methicillin?
- Production of beta-lactamase that hydrolyzes the antibiotic
- Expression of altered penicillin-binding protein PBP2a encoded by mecA (Correct answer)
- Active efflux of the antibiotic via MexAB-OprM pump
- Reduced outer membrane permeability preventing drug entry
Correct answer: Expression of altered penicillin-binding protein PBP2a encoded by mecA
MRSA expresses PBP2a, encoded by the mecA gene, which has low affinity for beta-lactam antibiotics, allowing cell wall synthesis to continue.
Question 2: Which phenotypic test is used in clinical microbiology labs to screen for extended-spectrum beta-lactamase (ESBL) production in Enterobacteriaceae?
- Double-disk synergy test using clavulanate and cephalosporins (Correct answer)
- Carbapenem inactivation method (CIM)
- Modified Hodge test
- Colistin broth disk elution
Correct answer: Double-disk synergy test using clavulanate and cephalosporins
The double-disk synergy test detects ESBL production by demonstrating enhanced inhibition when a cephalosporin disk is placed near an amoxicillin-clavulanate disk.
Question 3: A Klebsiella pneumoniae isolate tests resistant to all carbapenems and is positive on the carbapenem inactivation method (CIM). Which resistance mechanism is most likely?
- OXA-48 serine carbapenemase production
- KPC (Klebsiella pneumoniae carbapenemase) production (Correct answer)
- NDM (New Delhi metallo-beta-lactamase) production
- Loss of porin OmpK35/OmpK36 combined with AmpC overexpression
Correct answer: KPC (Klebsiella pneumoniae carbapenemase) production
KPC is the most common carbapenemase in Klebsiella pneumoniae in the US, and isolates are CIM-positive due to its serine-based hydrolytic activity.
Question 4: According to antimicrobial stewardship principles, which intervention most effectively reduces unnecessary broad-spectrum antibiotic use?
- Automatic stop orders after 72 hours for all antibiotics
- Prospective audit and feedback by infectious disease-trained pharmacists (Correct answer)
- Restricting vancomycin to ID physician approval only
- Posting antibiogram data in nursing units
Correct answer: Prospective audit and feedback by infectious disease-trained pharmacists
Prospective audit and feedback by ID pharmacists is considered a core element of stewardship, directly targeting individual prescriptions with evidence-based recommendations.
Question 5: Which of the following correctly describes horizontal gene transfer of resistance genes via conjugation?
- Transfer of DNA through bacteriophage between bacteria
- Direct cell-to-cell transfer of plasmids through a pilus (Correct answer)
- Uptake of naked DNA from the environment
- Transposon insertion within the bacterial chromosome
Correct answer: Direct cell-to-cell transfer of plasmids through a pilus
Conjugation involves direct cell-to-cell contact via a pilus, allowing plasmid DNA carrying resistance genes to transfer between bacteria of the same or different species.
Question 6: The minimum inhibitory concentration (MIC) of an antibiotic is best defined as:
- The concentration that kills 99.9% of bacteria in 24 hours
- The lowest concentration that visibly inhibits bacterial growth (Correct answer)
- The concentration at which bactericidal activity begins
- The serum drug level achievable at standard dosing
Correct answer: The lowest concentration that visibly inhibits bacterial growth
The MIC is the lowest concentration of an antibiotic that visibly inhibits growth of a microorganism after overnight incubation.
SM SM Antimicrobial Resistance & Stewardship 1
Which mechanism does Staphylococcus aureus use to confer resistance to beta-lactam antibiotics including methicillin?