Quantitative measurement of bacterial growth – total count MCQs With Answer

Quantitative measurement of bacterial growth – total count MCQs With Answer is an essential topic for B. Pharm students studying microbiology and pharmaceutical analysis. This concise introduction covers methods to enumerate microorganisms: viable plate counts, membrane filtration, most probable number (MPN), microscopic counts, turbidity (OD) measurements, and colony-forming unit (CFU) calculations. Key practical concepts include serial dilution, pour vs. spread plate techniques, counting chambers, spectrophotometer calibration, generation time, detection limits, and common sources of error in bioburden testing and quality control. These focused MCQs will deepen your lab skills, data interpretation, and exam readiness. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. Which method measures only viable bacteria capable of forming colonies?

• Direct microscopic count
• Viable plate count (CFU)
• Turbidity (optical density)
• Dry weight measurement

Correct Answer: Viable plate count (CFU)

Q2. Which technique is most appropriate for counting bacteria in very dilute water samples?

• Spread plate of undiluted sample
• Membrane filtration
• Direct microscopic count using hemocytometer
• Dry weight

Correct Answer: Membrane filtration

Q3. Colony Forming Unit (CFU) represents:

• Number of dead cells in a sample
• Total number of particles suspended
• Number of viable cells able to produce colonies
• Optical density at 600 nm

Correct Answer: Number of viable cells able to produce colonies

Q4. What is the primary limitation of direct microscopic counts for bacterial enumeration?

• It only detects viable cells
• It requires long incubation times
• It cannot distinguish live from dead cells without special stains
• It is unsuitable for concentrated samples

Correct Answer: It cannot distinguish live from dead cells without special stains

Q5. In serial dilution, the dilution factor when mixing 0.1 mL of culture into 9.9 mL diluent is:

• 1:10
• 1:100
• 1:1000
• 1:2

Correct Answer: 1:100

Q6. The most probable number (MPN) method is especially useful for:

• Counting cells on a membrane filter
• Estimating low concentrations using statistical inference from serial dilutions
• Measuring dry biomass
• Directly counting colonies on agar plates

Correct Answer: Estimating low concentrations using statistical inference from serial dilutions

Q7. Which plating method involves pouring molten agar with the sample into a Petri dish?

• Spread plate
• Pour plate
• Membrane filtration
• Direct streaking

Correct Answer: Pour plate

Q8. For accurate viable counts, plates with how many colonies are generally considered ideal?

• Less than 20 colonies
• 30–300 colonies
• More than 500 colonies
• Exactly 10 colonies

Correct Answer: 30–300 colonies

Q9. Optical density (OD) measurement correlates with bacterial concentration best when:

• Cells are in a highly turbid, saturated culture
• A calibration curve relating OD to CFU or biomass is available
• Only dead cells are present
• No standard curve is used

Correct Answer: A calibration curve relating OD to CFU or biomass is available

Q10. Which instrument measures scattered light to estimate turbidity of a bacterial culture?

• Spectrophotometer
• Hemocytometer
• Electronic balance
• Thermocycler

Correct Answer: Spectrophotometer

Q11. The Petroff-Hausser counting chamber is used for:

• Plate culturing of bacteria
• Direct microscopic counting of cells
• Measuring optical density
• Filtering bacteria onto membranes

Correct Answer: Direct microscopic counting of cells

Q12. When converting colony counts to CFU/mL after plating 0.1 mL of a 10^-5 dilution and counting 150 colonies, the CFU/mL is:

• 1.5 × 10^6 CFU/mL
• 1.5 × 10^7 CFU/mL
• 1.5 × 10^8 CFU/mL
• 1.5 × 10^5 CFU/mL

Correct Answer: 1.5 × 10^8 CFU/mL

Q13. Which staining technique allows differentiation of live and dead bacteria for microscopic counts?

• Gram staining
• Live/Dead fluorescent staining (e.g., SYTO9/PI)
• Endospore staining
• Simple crystal violet stain

Correct Answer: Live/Dead fluorescent staining (e.g., SYTO9/PI)

Q14. Which parameter describes the time required for a population to double during exponential growth?

• Lag time
• Generation time (doubling time)
• Stationary phase
• Plate count threshold

Correct Answer: Generation time (doubling time)

Q15. A cause of underestimation in plate counts is:

• Using a selective medium for a general count
• Cell clumping forming single colonies from multiple cells
• Plating an appropriate dilution range
• Counting plates with 50 colonies

Correct Answer: Cell clumping forming single colonies from multiple cells

Q16. What is plating efficiency?

• Ratio of dead cells to total cells
• Percentage of viable cells that form colonies under plating conditions
• Number of colonies per plate
• Growth rate during lag phase

Correct Answer: Percentage of viable cells that form colonies under plating conditions

Q17. Which method provides rapid enumeration and can discriminate viable from non-viable cells using fluorescence?

• Standard plate count
• Flow cytometry with viability dyes
• Dry weight measurement
• MPN by multiple tube fermentation

Correct Answer: Flow cytometry with viability dyes

Q18. In spread plate technique, the sample volume is typically:

• Greater than 5 mL
• Approximately 0.1 mL
• Mixed with molten agar
• Applied after incubation

Correct Answer: Approximately 0.1 mL

Q19. Which medium type inhibits growth of unwanted organisms while allowing target bacteria to grow?

• Non-selective medium
• Selective medium
• Enrichment broth
• Minimal salt agar

Correct Answer: Selective medium

Q20. Which factor least affects optical density measurements?

• Cell size and shape
• Presence of cell debris or precipitates
• Wavelength selection
• Plating temperature

Correct Answer: Plating temperature

Q21. Which unit is commonly used to report viable bacterial counts?

• Cells per liter
• CFU per mL
• mg dry weight
• Absorbance units

Correct Answer: CFU per mL

Q22. During which growth phase do bacterial cells exhibit maximum division rate?

• Lag phase
• Exponential (log) phase
• Stationary phase
• Death phase

Correct Answer: Exponential (log) phase

Q23. Which approach helps reduce counting errors due to overlapping colonies on a plate?

• Using too concentrated samples
• Plating a higher volume without dilution
• Performing appropriate serial dilutions to reach 30–300 colonies
• Incubating longer than required

Correct Answer: Performing appropriate serial dilutions to reach 30–300 colonies

Q24. In membrane filtration, bacteria are retained on the membrane by:

• Chemical binding to agar
• Physical entrapment on the membrane surface pore structure
• Evaporation of the sample
• Fluorescent staining

Correct Answer: Physical entrapment on the membrane surface pore structure

Q25. Dry weight measurement is most suitable for:

• Counting viable cells in a dilute sample
• Quantifying total biomass of filamentous microorganisms
• Rapid detection of pathogens in water
• Measuring turbidity by spectroscopy

Correct Answer: Quantifying total biomass of filamentous microorganisms

Q26. Which is true about pour plate vs spread plate techniques?

• Pour plate embeds colonies within the agar while spread plate grows colonies only on the surface
• Spread plate embeds colonies in agar and pour plate grows only on surface
• Both techniques always yield identical colony morphology
• Pour plate cannot be used with anaerobes

Correct Answer: Pour plate embeds colonies within the agar while spread plate grows colonies only on the surface

Q27. The detection limit of a plating method depends mainly on:

• Volume plated and dilution scheme
• Type of microscope used
• Color of agar
• Time of day the sample was taken

Correct Answer: Volume plated and dilution scheme

Q28. When using OD600 to estimate bacterial concentration, a single OD value corresponds to CFU only if:

• The culture is in death phase
• A species-specific calibration curve is available
• No calibration is needed
• All cells are dead

Correct Answer: A species-specific calibration curve is available

Q29. Which error will result from incubating plates too long before counting?

• Underestimation due to faded colonies
• Overgrowth and merging of colonies leading to underestimation
• No change in count
• Increased precision of counts

Correct Answer: Overgrowth and merging of colonies leading to underestimation

Q30. Which regulatory test in pharmaceuticals involves quantitative microbial enumeration?

• Bioburden testing of nonsterile products
• Viscosity testing
• Assay of active ingredient by titration
• pH measurement

Correct Answer: Bioburden testing of nonsterile products

Q31. Which of the following best describes the MPN endpoint?

• Counting colonies on plates
• Observing presence/absence of growth in multiple dilution series tubes and using statistical tables
• Measuring OD600 directly
• Filtering and counting colonies on membrane

Correct Answer: Observing presence/absence of growth in multiple dilution series tubes and using statistical tables

Q32. Which medium is used when both selection and differential visualization are needed?

• Enrichment broth
• Selective-differential medium (e.g., MacConkey agar)
• Plain nutrient agar only
• Minimal salts medium

Correct Answer: Selective-differential medium (e.g., MacConkey agar)

Q33. The primary advantage of automated colony counters is:

• They eliminate the need for incubation
• They increase speed and objectivity of colony counting
• They require no calibration
• They can measure OD600 directly

Correct Answer: They increase speed and objectivity of colony counting

Q34. Which approach helps distinguish live vs dead bacteria when using molecular methods?

• Using culture plating only
• Using viability PCR with DNA-intercalating dyes (e.g., PMA)
• Measuring OD without staining
• Dry weight estimation

Correct Answer: Using viability PCR with DNA-intercalating dyes (e.g., PMA)

Q35. When calculating CFU/mL from a spread plate, which formula is correct?

• CFU/mL = colony count × dilution factor × plated volume
• CFU/mL = colony count / (dilution factor × plated volume)
• CFU/mL = colony count × dilution factor / plated volume
• CFU/mL = colony count / dilution factor

Correct Answer: CFU/mL = colony count × dilution factor / plated volume

Q36. A common source of false positives in membrane filtration is:

• Using sterile forceps
• Contaminated filtration apparatus or membrane
• Filtering a sterile sample
• Using sterile media for incubation

Correct Answer: Contaminated filtration apparatus or membrane

Q37. Which growth phase often shows little or no increase in cell number while cells adapt to new environment?

• Exponential phase
• Lag phase
• Death phase
• Stationary phase

Correct Answer: Lag phase

Q38. Which of these methods provides the fastest approximate estimate of bacterial concentration in a lab culture?

• Standard plate count with incubation
• Optical density (OD) measurement
• Most probable number requiring multiple days
• Dry weight requiring drying and weighing

Correct Answer: Optical density (OD) measurement

Q39. In pharmaceutical microbiology, acceptable bioburden limits depend on:

• Product type, route of administration, and regulatory guidelines
• Only the color of the product
• Temperature at which product is stored exclusively
• Number of operators in the lab

Correct Answer: Product type, route of administration, and regulatory guidelines

Q40. What is the purpose of using a replicate plating approach?

• To decrease precision of results
• To increase statistical confidence and accuracy of counts
• To avoid serial dilutions
• To measure OD600 more accurately

Correct Answer: To increase statistical confidence and accuracy of counts

Q41. Which factor can cause overestimation of cell numbers in direct microscopic counting?

• Counting only viable cells
• Counting non-viable cells and debris as cells
• Using selective medium
• Plating diluted samples

Correct Answer: Counting non-viable cells and debris as cells

Q42. When preparing serial tenfold dilutions, transferring 1 mL into 9 mL gives a dilution of:

• 1:2
• 1:10
• 1:100
• 10:1

Correct Answer: 1:10

Q43. Which method is preferable for enumerating heat-injured bacteria that may not form colonies on standard media?

• Standard plate count on selective agar only
• Use of resuscitation/enrichment media before plating
• Direct OD600 measurement
• Dry weight measurement

Correct Answer: Use of resuscitation/enrichment media before plating

Q44. Which statement about CFU and cell number is correct?

• CFU always equals exact cell count
• CFU may underestimate actual cell number due to clumping or non-culturable cells
• CFU overestimates because one colony is always multiple cells
• CFU is unrelated to viability

Correct Answer: CFU may underestimate actual cell number due to clumping or non-culturable cells

Q45. Which analytical method can be used to quantify bacterial proteins as a surrogate for biomass?

• Spectrophotometric OD600 only
• Total protein assay (e.g., Bradford) after cell lysis
• Hemocytometer counting
• MPN directly

Correct Answer: Total protein assay (e.g., Bradford) after cell lysis

Q46. Which is a reason to include blank/control samples when measuring OD600?

• To calibrate incubation temperature
• To zero the spectrophotometer for media absorbance and turbidity
• To increase colony counts
• To sterilize the spectrophotometer

Correct Answer: To zero the spectrophotometer for media absorbance and turbidity

Q47. For anaerobic bacteria enumeration, which modification is necessary?

• Incubate plates aerobically
• Use anaerobic incubation conditions and appropriate reducing media
• Only measure OD600
• Use only membrane filtration

Correct Answer: Use anaerobic incubation conditions and appropriate reducing media

Q48. Which quality control practice improves reliability of plate counts?

• Using contaminated pipettes
• Including sterility controls and positive controls, and performing replicates
• Plating only a single sample without repeats
• Ignoring incubation times

Correct Answer: Including sterility controls and positive controls, and performing replicates

Q49. In calculating generation time from growth curve data, which phase data should be used?

• Lag phase data
• Exponential (log) phase data
• Stationary phase data
• Death phase data

Correct Answer: Exponential (log) phase data

Q50. When interpreting MCQ results about bacterial enumeration, which mindset is most important for B. Pharm students?

• Focus only on memorizing numbers without understanding methods
• Understanding principles, limitations, and practical implications of each enumeration method
• Avoid learning statistical considerations for counts
• Assume all methods give identical results in every situation

Correct Answer: Understanding principles, limitations, and practical implications of each enumeration method

G S Sachin

I am a Registered Pharmacist under the Pharmacy Act, 1948, and the founder of PharmacyFreak.com. I hold a Bachelor of Pharmacy degree from Rungta College of Pharmaceutical Science and Research. With a strong academic foundation and practical knowledge, I am committed to providing accurate, easy-to-understand content to support pharmacy students and professionals. My aim is to make complex pharmaceutical concepts accessible and useful for real-world application.

Mail- Sachin@pharmacyfreak.com