Bactericidal action – evaluation MCQs With Answer

Bactericidal action – evaluation MCQs With Answer

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Bactericidal action – evaluation MCQs With Answer

Understanding bactericidal action is essential for B. Pharm students preparing for pharmacology, microbiology, and antimicrobial stewardship. This focused introduction covers mechanisms of bacterial killing, evaluation methods like MIC, MBC, and time‑kill assays, and pharmacodynamic indices (Cmax/MIC, AUC/MIC, T>MIC). You will also encounter factors affecting bactericidal activity such as inoculum size, biofilms, and resistance mechanisms. These MCQs emphasize both theory and practical laboratory interpretation to strengthen clinical and research decision‑making. The questions promote deeper reasoning about assay design, interpretation, and therapeutic implications. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. Which parameter directly distinguishes bactericidal from bacteriostatic activity in vitro?

  • Reduction in colony-forming units (CFU) by ≥3 log10 from initial inoculum
  • Inhibition of visible growth after 24 hours
  • Delay in regrowth after antibiotic removal
  • Decrease in optical density without CFU count

Correct Answer: Reduction in colony-forming units (CFU) by ≥3 log10 from initial inoculum

Q2. The minimum bactericidal concentration (MBC) is defined as:

  • The lowest concentration preventing visible growth in broth
  • The lowest concentration killing ≥99.9% of the initial inoculum
  • The concentration producing maximal inhibition zone on agar
  • The antibiotic concentration at which no resistant mutants arise

Correct Answer: The lowest concentration killing ≥99.9% of the initial inoculum

Q3. Which assay measures bactericidal activity over time and can distinguish between time-dependent and concentration-dependent killing?

  • Broth microdilution MIC test
  • Time-kill curve assay
  • Agar diffusion (disk) test
  • Gram staining

Correct Answer: Time-kill curve assay

Q4. A drug showing greater killing with higher concentrations and a steep concentration–response curve is called:

  • Time-dependent bactericide
  • Concentration-dependent bactericide
  • Bacteriostatic at all concentrations
  • Post-antibiotic effect dependent

Correct Answer: Concentration-dependent bactericide

Q5. Which pharmacodynamic index best correlates with efficacy of concentration-dependent bactericidal agents like aminoglycosides?

  • Time above MIC (T>MIC)
  • Cmax/MIC ratio
  • AUC/MIC ratio
  • Minimal inhibitory concentration (MIC) alone

Correct Answer: Cmax/MIC ratio

Q6. A bactericidal antimicrobial that disrupts cell wall synthesis primarily causes cell death by:

  • Inhibiting protein synthesis at 50S ribosome
  • Interfering with DNA gyrase function
  • Weakening cell wall leading to osmotic lysis
  • Blocking folate synthesis

Correct Answer: Weakening cell wall leading to osmotic lysis

Q7. Which factor can convert a drug that is bactericidal in vitro into bacteriostatic in vivo?

  • High protein binding reducing free drug concentration
  • Using an initial low inoculum in testing
  • Dehydration of the patient
  • Presence of oxygen in tissues

Correct Answer: High protein binding reducing free drug concentration

Q8. The inoculum effect describes:

  • Higher bacterial inocula leading to decreased observed antibiotic efficacy
  • Faster growth rates at low inocula
  • Increased MBC values with lower bacterial counts
  • Change in MIC when using anaerobic conditions

Correct Answer: Higher bacterial inocula leading to decreased observed antibiotic efficacy

Q9. Post-antibiotic effect (PAE) refers to:

  • Immediate regrowth when antibiotic is removed
  • Suppression of bacterial growth after short exposure to antibiotic
  • Increase in MIC after repeated exposure
  • Antagonistic interactions between drugs

Correct Answer: Suppression of bacterial growth after short exposure to antibiotic

Q10. For beta-lactam antibiotics, the most predictive pharmacodynamic parameter of efficacy is:

  • AUC/MIC
  • Cmax/MIC
  • Time above MIC (T>MIC)
  • Post-antibiotic effect duration

Correct Answer: Time above MIC (T>MIC)

Q11. Which test is primarily used to determine MIC values using gradient diffusion?

  • Checkerboard assay
  • Time-kill assay
  • Etest
  • Disk diffusion

Correct Answer: Etest

Q12. In MBC determination after broth dilution, how is MBC typically confirmed?

  • Measuring optical density of the broth
  • Plating aliquots from wells with no visible growth onto agar to check for survivors
  • Observing color change indicators only
  • Performing Gram stain

Correct Answer: Plating aliquots from wells with no visible growth onto agar to check for survivors

Q13. Which mechanism most commonly produces bactericidal activity among aminoglycosides?

  • Inhibition of DNA-dependent RNA polymerase
  • Irreversible inhibition of 30S ribosomal subunit causing mistranslation and membrane damage
  • Blocking peptidoglycan cross-linking
  • Disruption of folate metabolism

Correct Answer: Irreversible inhibition of 30S ribosomal subunit causing mistranslation and membrane damage

Q14. When evaluating bactericidal synergy between two drugs, which in vitro method is commonly used?

  • Agar dilution MIC
  • Checkerboard assay and time-kill studies
  • Single-concentration disk diffusion only
  • Gram staining synergy test

Correct Answer: Checkerboard assay and time-kill studies

Q15. A drug killing bacteria by creating pores in the cytoplasmic membrane is characteristic of which class?

  • Beta-lactams
  • Polymyxins
  • Tetracyclines
  • Sulfonamides

Correct Answer: Polymyxins

Q16. Which statement about MIC and MBC is correct?

  • MIC is always equal to MBC for bactericidal drugs
  • MBC is the lowest concentration preventing visible turbidity
  • A large MBC/MIC ratio suggests primarily bacteriostatic action
  • MBC testing uses disk diffusion

Correct Answer: A large MBC/MIC ratio suggests primarily bacteriostatic action

Q17. Biofilms reduce bactericidal efficacy primarily because:

  • Cells in biofilms are more metabolically active and thus more susceptible
  • Matrix limits antibiotic penetration and cells show slow growth or persistence
  • Biofilms increase antibiotic diffusion rates
  • They convert antibiotics into inactive metabolites

Correct Answer: Matrix limits antibiotic penetration and cells show slow growth or persistence

Q18. Which factor does NOT typically affect in vitro bactericidal testing results?

  • Composition of the growth medium
  • Incubation temperature and time
  • Manufacturer of the plastic plates used
  • Inoculum size

Correct Answer: Manufacturer of the plastic plates used

Q19. Time-dependent killers require which dosing strategy for maximal efficacy?

  • Large single doses to maximize Cmax
  • Frequent dosing or prolonged infusion to maximize T>MIC
  • Administration only once daily
  • Intermittent dosing to exploit PAE

Correct Answer: Frequent dosing or prolonged infusion to maximize T>MIC

Q20. The emergence of tolerant bacteria is characterized by:

  • A higher MIC value than before
  • Reduced killing rate despite unchanged MIC
  • Complete resistance due to genetic mutation
  • Increased antibiotic uptake

Correct Answer: Reduced killing rate despite unchanged MIC

Q21. Which laboratory observation suggests bactericidal activity on an agar plate?

  • Small zone of inhibition around a disk
  • No distinct relationship between zone size and killing
  • Clear zone of inhibition where bacteria are absent
  • Swarming growth up to the disk edge

Correct Answer: Clear zone of inhibition where bacteria are absent

Q22. Endotoxin release during bactericidal therapy is a clinical concern mainly with:

  • Gram-positive cocci
  • Mycobacteria
  • Gram-negative rods due to lipopolysaccharide (LPS) release
  • Fungi

Correct Answer: Gram-negative rods due to lipopolysaccharide (LPS) release

Q23. Which antibiotic pair is classically considered synergistic for certain infections (e.g., enterococcal endocarditis)?

  • Aminoglycoside plus cell wall-active agent (e.g., gentamicin + ampicillin)
  • Tetracycline plus sulfonamide
  • Two beta-lactams with same target
  • Polymyxin plus chloramphenicol

Correct Answer: Aminoglycoside plus cell wall-active agent (e.g., gentamicin + ampicillin)

Q24. A bactericidal agent that inhibits DNA replication by targeting DNA gyrase is:

  • Vancomycin
  • Fluoroquinolone
  • Linezolid
  • Sulfonamide

Correct Answer: Fluoroquinolone

Q25. In a time-kill assay, bactericidal activity is commonly defined as:

  • 0.5 log10 decrease in CFU/ml over 24 hours
  • ≥3 log10 decrease in CFU/ml from starting inoculum at a specified time
  • No change in CFU/ml
  • Increase in turbidity only

Correct Answer: ≥3 log10 decrease in CFU/ml from starting inoculum at a specified time

Q26. Which antimicrobial class is primarily bacteriostatic rather than bactericidal?

  • Glycopeptides (e.g., vancomycin)
  • Aminoglycosides
  • Macrolides (e.g., erythromycin)
  • Fluoroquinolones

Correct Answer: Macrolides (e.g., erythromycin)

Q27. Determining MBC after MIC requires incubation on nonselective agar for how long typically?

  • 1 hour
  • 24 hours
  • 7 days
  • Immediate observation is sufficient

Correct Answer: 24 hours

Q28. Antagonism between two antibiotics may be observed when:

  • One drug inhibits the bactericidal action of the other
  • Both drugs increase each other’s killing rate
  • They have additive effects
  • They target unrelated pathways

Correct Answer: One drug inhibits the bactericidal action of the other

Q29. Which statement about bactericidal disinfectants (e.g., bleach) is true?

  • They are used therapeutically inside the body
  • They chemically destroy microbial structures leading to irreversible loss of viability
  • They only inhibit growth but do not kill microorganisms
  • They selectively target bacterial ribosomes

Correct Answer: They chemically destroy microbial structures leading to irreversible loss of viability

Q30. When assessing bactericidal action against intracellular pathogens, which factor is most important?

  • Ability of antibiotic to penetrate and accumulate within host cells
  • Only the drug’s effect in acellular medium
  • Rate of bacterial spore formation
  • Color change in medium

Correct Answer: Ability of antibiotic to penetrate and accumulate within host cells

Q31. Which enzyme modification confers bactericidal resistance by drug inactivation (e.g., aminoglycosides)?

  • Beta-lactamase hydrolysis
  • Aminoglycoside-modifying enzymes (acetyltransferases, nucleotidyltransferases)
  • Altered peptidoglycan cross-linking
  • PBP overexpression only

Correct Answer: Aminoglycoside-modifying enzymes (acetyltransferases, nucleotidyltransferases)

Q32. Which clinical scenario most clearly requires bactericidal rather than bacteriostatic therapy?

  • Uncomplicated skin infection in an outpatient
  • Bacterial endocarditis with vegetations
  • Minor throat infection
  • Asymptomatic bacteriuria in non-pregnant adult

Correct Answer: Bacterial endocarditis with vegetations

Q33. The checkerboard assay primarily determines which interaction between two antimicrobials?

  • PAE duration for a single agent
  • Synergy, additivity, indifference, or antagonism via fractional inhibitory concentration indices
  • Time-kill kinetics only
  • Protein binding characteristics

Correct Answer: Synergy, additivity, indifference, or antagonism via fractional inhibitory concentration indices

Q34. A high AUC/MIC ratio is most predictive of clinical success for which antibiotic class?

  • Beta-lactams
  • Fluoroquinolones
  • Polymyxins
  • Sulfonamides only

Correct Answer: Fluoroquinolones

Q35. Persister cells affect bactericidal evaluation because they:

  • Are genetically resistant mutants
  • Are dormant phenotypic variants tolerant to killing without increased MIC
  • Grow faster under antibiotic pressure
  • Enhance antibiotic uptake

Correct Answer: Are dormant phenotypic variants tolerant to killing without increased MIC

Q36. Which lab practice improves reproducibility of MBC determinations?

  • Using undefined inoculum quantities
  • Standardizing inoculum size (e.g., 5 x 10^5 CFU/ml) and incubation conditions
  • Varying incubation temperature each run
  • Using selective agar to detect survivors

Correct Answer: Standardizing inoculum size (e.g., 5 x 10^5 CFU/ml) and incubation conditions

Q37. Which outcome on a time-kill curve indicates bacteriostatic activity?

  • ≥3 log10 decrease in CFU at 24 hours
  • No net decrease in CFU but suppressed growth compared to control
  • Complete eradication within 2 hours
  • Immediate lysis within minutes

Correct Answer: No net decrease in CFU but suppressed growth compared to control

Q38. Which of the following alters interpretation of MBC when testing stationary-phase bacteria?

  • Stationary-phase cells are often more tolerant, requiring higher concentrations to kill
  • They always show lower MBC than log-phase cells
  • Phase of growth has no impact
  • Stationary-phase cells are more susceptible due to active division

Correct Answer: Stationary-phase cells are often more tolerant, requiring higher concentrations to kill

Q39. Which statement about bactericidal action and clinical outcomes is correct?

  • Bactericidal action guarantees clinical cure in all infections
  • Bactericidal action is one factor; host immunity and drug penetration also determine outcome
  • Bacteriostatic drugs are never useful clinically
  • Only bactericidal drugs can treat systemic infections

Correct Answer: Bactericidal action is one factor; host immunity and drug penetration also determine outcome

Q40. Which modification in a laboratory protocol would best detect small numbers of surviving bacteria after exposure?

  • Use a higher initial inoculum without plating
  • Plate large-volume aliquots or perform serial dilutions and plate to detect low CFU
  • Rely solely on turbidity measurements
  • Use shorter incubation times before plating

Correct Answer: Plate large-volume aliquots or perform serial dilutions and plate to detect low CFU

Q41. Which of the following is a limitation of disk diffusion for evaluating bactericidal capacity?

  • It provides quantitative MBC values directly
  • It cannot distinguish bacteriostatic from bactericidal effects reliably
  • It measures time-kill kinetics precisely
  • It is useful for intracellular pathogens

Correct Answer: It cannot distinguish bacteriostatic from bactericidal effects reliably

Q42. A bactericidal antibiotic showing prolonged PAE would allow which dosing advantage?

  • More frequent dosing is required
  • Less frequent dosing intervals may be effective
  • Higher doses every hour are needed
  • PAE has no impact on dosing

Correct Answer: Less frequent dosing intervals may be effective

Q43. Which in vitro observation suggests the presence of tolerant bacterial populations?

  • Unchanged MIC but much higher MBC than expected
  • Decreased MIC over time
  • Immediate bactericidal effect at MIC
  • Uniform rapid killing for all isolates

Correct Answer: Unchanged MIC but much higher MBC than expected

Q44. When interpreting time-kill studies, what is the significance of regrowth after initial killing?

  • It indicates permanent eradication
  • It may indicate selection of resistant mutants or surviving persisters
  • It means the assay was contaminated only
  • Regrowth is irrelevant to clinical therapy

Correct Answer: It may indicate selection of resistant mutants or surviving persisters

Q45. Which of the following enhances bactericidal action of certain antibiotics in combination with host defenses?

  • Neutropenia
  • Impaired phagocytosis
  • Synergy with immune-mediated opsonization and phagocytosis
  • Biofilm formation

Correct Answer: Synergy with immune-mediated opsonization and phagocytosis

Q46. Which laboratory guideline organization provides standards for MIC and MBC testing?

  • FDA only
  • CLSI (Clinical and Laboratory Standards Institute)
  • WHO malaria group
  • ISO for food packaging exclusively

Correct Answer: CLSI (Clinical and Laboratory Standards Institute)

Q47. Which characteristic is typical of bactericidal beta-lactams at clinically achievable concentrations?

  • They inhibit folate synthesis
  • They interfere with protein translation
  • They bind penicillin-binding proteins causing cell lysis during active growth
  • They create free radical-mediated DNA damage only

Correct Answer: They bind penicillin-binding proteins causing cell lysis during active growth

Q48. In designing a time-kill experiment, sampling at multiple time points is important because:

  • Killing is instantaneous and one time point is sufficient
  • It reveals the rate and extent of killing and potential regrowth dynamics
  • Only the 24-hour time point is informative
  • It eliminates the need to plate samples

Correct Answer: It reveals the rate and extent of killing and potential regrowth dynamics

Q49. Which statement best describes concentration-dependent killing with prolonged PAE?

  • Higher drug concentrations increase killing rate and sustained suppression after exposure
  • Killing is independent of concentration and depends only on time
  • PAE is absent for concentration-dependent drugs
  • They require continuous infusion for efficacy

Correct Answer: Higher drug concentrations increase killing rate and sustained suppression after exposure

Q50. For an antibiotic showing bactericidal activity only at high multiples of MIC, clinical implication is:

  • Standard dosing will always be effective
  • Achieving adequate serum/tissue concentrations may be challenging and dosing strategies must ensure sufficient levels
  • The drug is guaranteed to eradicate biofilms
  • No need to consider pharmacokinetics

Correct Answer: Achieving adequate serum/tissue concentrations may be challenging and dosing strategies must ensure sufficient levels

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