Microbiological culture and sensitivity interpretation MCQs With Answer

Introduction: Microbiological culture and sensitivity interpretation MCQs With Answer is a focused resource for M.Pharm students preparing for clinical pharmacy practice exams and residency placements. This collection emphasizes practical aspects of specimen handling, culture techniques, antimicrobial susceptibility testing (AST) methods, and interpretation principles that guide pharmacotherapy decisions. Questions probe standardized procedures (e.g., inoculum standards, incubation conditions), phenotypic and genotypic resistance detection (MRSA, ESBL, inducible resistance), MIC versus breakpoints (CLSI/EUCAST), and pharmacokinetic/pharmacodynamic indices relevant to AST. Designed for advanced learners, the quiz fosters critical thinking about laboratory reports, reporting pitfalls, and clinical application of AST data in antimicrobial stewardship.

Q1. Which skin antiseptic is preferred for preparing a venipuncture site for blood culture collection to minimize contamination?

• 70% isopropyl alcohol alone
• 2% chlorhexidine in 70% isopropyl alcohol
• Povidone-iodine alone
• 10% formalin

Correct Answer: 2% chlorhexidine in 70% isopropyl alcohol

Q2. Which transport medium is most appropriate for preserving obligate anaerobes during specimen transport?

• Cary-Blair transport medium
• Stuart transport medium
• Port-A-Cul anaerobic transport system
• Amies transport medium without charcoal

Correct Answer: Port-A-Cul anaerobic transport system

Q3. Which agar is both selective and differential for Enterobacteriaceae and commonly used to isolate Gram-negative enteric bacilli?

• Blood agar
• MacConkey agar
• Chocolate agar
• Mannitol salt agar

Correct Answer: MacConkey agar

Q4. What is the primary diagnostic value of the Gram stain performed on clinical specimens?

• To determine exact species identification
• To provide rapid information on Gram reaction and morphology
• To measure minimum inhibitory concentration (MIC)
• To detect specific resistance genes

Correct Answer: To provide rapid information on Gram reaction and morphology

Q5. What is the standard incubation temperature for routine culture of most human bacterial pathogens?

• 25°C
• 30°C
• 35–37°C
• 42°C

Correct Answer: 35–37°C

Q6. In the Kirby-Bauer disk diffusion method, what is the standardized turbidity of the bacterial inoculum before plating?

• 0.05 McFarland
• 0.5 McFarland
• 1.5 McFarland
• 3.0 McFarland

Correct Answer: 0.5 McFarland

Q7. How is the minimum inhibitory concentration (MIC) best defined?

• The highest concentration of antimicrobial that kills 99.9% of bacteria
• The lowest concentration of antimicrobial that inhibits visible growth
• The concentration that achieves maximum serum levels
• The dose required to prevent infection in 50% of subjects

Correct Answer: The lowest concentration of antimicrobial that inhibits visible growth

Q8. Which method uses a plastic strip impregnated with a gradient of antibiotic to determine MIC on an agar plate?

• Broth microdilution
• Etest (gradient diffusion)
• Kirby-Bauer disk diffusion
• Time-kill assay

Correct Answer: Etest (gradient diffusion)

Q9. Which organization’s breakpoints are most commonly used for antimicrobial susceptibility interpretation in the United States?

• EUCAST (European Committee on Antimicrobial Susceptibility Testing)
• CLSI (Clinical and Laboratory Standards Institute)
• WHO (World Health Organization)
• FDA (Food and Drug Administration)

Correct Answer: CLSI (Clinical and Laboratory Standards Institute)

Q10. Which phenotypic test is commonly used to screen for extended-spectrum β-lactamase (ESBL) production in Enterobacteriaceae?

• Oxacillin screening agar
• Combination disk test showing inhibition enhancement with clavulanic acid
• Nitrocefin chromogenic cephalosporin test only
• D-test for inducible clindamycin resistance

Correct Answer: Combination disk test showing inhibition enhancement with clavulanic acid

Q11. A positive D-test in a Staphylococcus aureus isolate indicates which of the following?

• Constitutive high-level macrolide resistance
• Inducible clindamycin resistance due to erm-mediated methylase
• Presence of mecA-mediated methicillin resistance
• Production of extended-spectrum β-lactamase

Correct Answer: Inducible clindamycin resistance due to erm-mediated methylase

Q12. Which phenotypic test is a reliable surrogate for detecting mecA-mediated methicillin resistance in Staphylococcus aureus?

• Oxacillin broth microdilution only
• Cefoxitin disk diffusion test
• Clindamycin D-test
• Penicillin zone edge test

Correct Answer: Cefoxitin disk diffusion test

Q13. In time-kill studies, bactericidal activity is conventionally defined as what reduction in colony count?

• ≥1 log10 reduction in CFU/mL
• ≥2 log10 reduction in CFU/mL
• ≥3 log10 reduction in CFU/mL
• No detectable growth regardless of baseline

Correct Answer: ≥3 log10 reduction in CFU/mL

Q14. Which pharmacokinetic/pharmacodynamic (PK/PD) index is most predictive of beta-lactam antibiotic efficacy?

• Peak concentration to MIC ratio (Cmax/MIC)
• Area under the concentration-time curve to MIC ratio (AUC/MIC)
• Time that free drug concentration remains above the MIC (fT>MIC)
• Post-antibiotic effect duration only

Correct Answer: Time that free drug concentration remains above the MIC (fT>MIC)

Q15. What does the term “inoculum effect” refer to in antimicrobial susceptibility testing?

• Decrease in MIC with increasing incubation time
• Increase in MIC when a higher bacterial inoculum is used
• Change in colony morphology at low inoculum
• Enhanced bactericidal activity at higher inoculum

Correct Answer: Increase in MIC when a higher bacterial inoculum is used

Q16. Heteroresistance is best described as which phenomenon?

• A uniformly resistant population detected by standard AST
• A population composed of susceptible majority and a resistant subpopulation
• An inducible resistance mechanism detected only by molecular testing
• Resistance that appears after prolonged antibiotic exposure only

Correct Answer: A population composed of susceptible majority and a resistant subpopulation

Q17. Which automated system commonly used in clinical microbiology laboratories provides both organism identification and susceptibility testing using disposable cards?

• Vitek 2
• MALDI-TOF MS
• PCR thermocycler
• Disk diffusion setup

Correct Answer: Vitek 2

Q18. Which American Type Culture Collection (ATCC) strain is commonly used as a quality control organism for disk diffusion susceptibility testing of Gram-negative bacilli?

• Staphylococcus aureus ATCC 25923
• Enterococcus faecalis ATCC 29212
• Escherichia coli ATCC 25922
• Pseudomonas aeruginosa ATCC 700603

Correct Answer: Escherichia coli ATCC 25922

Q19. When an isolate is reported as “intermediate” to an antibiotic, what is the most appropriate clinical interpretation?

• Drug will always be ineffective and should never be used
• Therapy may be effective if higher doses are used or if the drug concentrates at the infection site
• Isolate is resistant by all testing methods and requires combination therapy
• Laboratory error is likely and testing must be repeated

Correct Answer: Therapy may be effective if higher doses are used or if the drug concentrates at the infection site

Q20. Which PK/PD index best correlates with efficacy of aminoglycoside antibiotics?

• Time above MIC (fT>MIC)
• Peak concentration to MIC ratio (Cmax/MIC)
• Area under the curve to MIC ratio (AUC/MIC)
• Minimum bactericidal concentration (MBC) only

Correct Answer: Peak concentration to MIC ratio (Cmax/MIC)

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