Isolation of pure cultures MCQs With Answer

Isolation of pure cultures MCQs With Answer

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Introduction: Isolation of pure cultures MCQs With Answer is an essential resource for B. Pharm students learning microbiological techniques used in pharmaceutical analysis, sterility testing, and drug development. This introduction covers core concepts—aseptic technique, streak plate, spread and pour plate methods, serial dilution, selective and differential media, enrichment, colony morphology, contamination control, and microbial preservation. Mastery of isolation of pure cultures helps ensure accurate identification, quality control, and reproducibility in pharmaceutical microbiology labs. The following MCQs focus on practical and theoretical aspects to strengthen exam preparation and lab competence. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. What is the primary purpose of isolating a pure culture in pharmaceutical microbiology?

  • To increase the growth rate of organisms
  • To obtain a single species population for study
  • To produce mixed fermentation products
  • To reduce contamination in media

Correct Answer: To obtain a single species population for study

Q2. Which aseptic technique prevents contamination during transfer of cultures?

  • Using contaminated pipette tips
  • Flaming the inoculating loop
  • Working with open culture plates for long periods
  • Touching the culture tube rim with gloved hand

Correct Answer: Flaming the inoculating loop

Q3. The streak plate method is primarily used to:

  • Quantify viable cells in a sample
  • Isolate single colonies from a mixed culture
  • Measure optical density of cultures
  • Enrich fastidious organisms

Correct Answer: Isolate single colonies from a mixed culture

Q4. Which plate method is best for counting colony forming units (CFU) when sample concentration is low?

  • Direct microscopic count
  • Streak plate
  • Pour plate
  • Spread plate with undiluted sample

Correct Answer: Pour plate

Q5. In serial dilution, which factor reduces colony numbers to a countable range?

  • Handling errors
  • Using larger inoculum volumes
  • Dilution by a constant factor (e.g., 10-fold)
  • Prolonged incubation

Correct Answer: Dilution by a constant factor (e.g., 10-fold)

Q6. Which media type contains inhibitors to suppress unwanted microbes?

  • General-purpose media
  • Enrichment media
  • Selective media
  • Undefined media

Correct Answer: Selective media

Q7. Differential media are used to:

  • Kill all bacteria on a plate
  • Differentiate organisms based on biochemical reactions
  • Provide enrichment factors only
  • Measure antibiotic potency

Correct Answer: Differentiate organisms based on biochemical reactions

Q8. Which of the following is an example of a selective and differential medium?

  • Nutrient agar
  • MacConkey agar
  • Thioglycollate broth
  • Sabouraud dextrose agar

Correct Answer: MacConkey agar

Q9. The quadrant streak method aims to:

  • Increase colony density across the plate
  • Isolate individual colonies by progressive dilution on the plate
  • Culture anaerobic bacteria only
  • Determine growth rate constants

Correct Answer: Isolate individual colonies by progressive dilution on the plate

Q10. Which colony characteristic is NOT typically used for identification?

  • Color
  • Elevation
  • Odor
  • pH of ambient air

Correct Answer: pH of ambient air

Q11. Enrichment media are designed to:

  • Suppress target organisms
  • Increase numbers of a desired organism from a mixed sample
  • Prevent any growth
  • Measure extracellular enzyme activity only

Correct Answer: Increase numbers of a desired organism from a mixed sample

Q12. Which technique mixes the sample with molten agar before solidification?

  • Streak plate
  • Spread plate
  • Pour plate
  • Drop plate

Correct Answer: Pour plate

Q13. Spread plate method is best for:

  • Embedding cells within agar
  • Distributing a measured volume evenly on agar surface
  • Isolating anaerobes from soil
  • Counting spores only

Correct Answer: Distributing a measured volume evenly on agar surface

Q14. A countable plate for CFU typically contains how many colonies?

  • 1–10
  • 30–300
  • 500–1000
  • Over 2000

Correct Answer: 30–300

Q15. Why is aseptic technique critical in isolating pure cultures?

  • To ensure rapid growth of contaminants
  • To prevent introduction of foreign microbes and ensure pure isolates
  • To change colony morphology
  • To sterilize the incubator

Correct Answer: To prevent introduction of foreign microbes and ensure pure isolates

Q16. Which instrument is commonly used to transfer small volumes of liquid culture aseptically?

  • Incubator
  • Inoculating loop or sterile pipette
  • Autoclave
  • Spectrophotometer

Correct Answer: Inoculating loop or sterile pipette

Q17. Which preservative method is suitable for long-term maintenance of pure cultures?

  • Repeated subculturing on agar every day
  • Lyophilization (freeze-drying) or storage in cryovials at -80°C
  • Storing cultures at room temperature
  • Exposure to UV light

Correct Answer: Lyophilization (freeze-drying) or storage in cryovials at -80°C

Q18. Colony opacity (transparent vs. opaque) helps in:

  • Determining temperature requirements
  • Distinguishing microorganisms based on colony morphology
  • Measuring dilution factors
  • Calculating growth rate constants

Correct Answer: Distinguishing microorganisms based on colony morphology

Q19. Which condition is most important when isolating strict anaerobes?

  • High oxygen exposure during plating
  • Use of reducing agents and oxygen-free environment
  • Incubation at high light intensity
  • Frequent shaking of plates

Correct Answer: Use of reducing agents and oxygen-free environment

Q20. Selective media for Staphylococcus aureus often contain which compound to inhibit Gram negatives?

  • High salt (NaCl) and mannitol
  • Antibiotics like colistin and nalidixic acid
  • Bile salts and crystal violet
  • Low pH

Correct Answer: High salt (NaCl) and mannitol

Q21. Which parameter is most critical when performing serial dilution?

  • Using different diluents for each step
  • Accurate pipetting and mixing at each dilution step
  • Keeping tubes uncapped for long
  • Using non-sterile equipment

Correct Answer: Accurate pipetting and mixing at each dilution step

Q22. In a spread plate, what device is commonly used to distribute sample evenly?

  • Inoculating loop
  • Glass spreader (hockey stick) or sterile beads
  • Autoclave
  • Spectrophotometer

Correct Answer: Glass spreader (hockey stick) or sterile beads

Q23. Which growth medium is preferred for fungi isolation in pharmaceutical labs?

  • MacConkey agar
  • Blood agar
  • Sabouraud dextrose agar
  • TCBS agar

Correct Answer: Sabouraud dextrose agar

Q24. How does selective enrichment differ from simple enrichment?

  • Selective enrichment uses inhibitors to suppress competitors while promoting target organisms
  • Selective enrichment eliminates desired organisms
  • Simple enrichment always uses antibiotics
  • There is no difference

Correct Answer: Selective enrichment uses inhibitors to suppress competitors while promoting target organisms

Q25. What does CFU stand for in viable count methods?

  • Colony forming units
  • Culture forming units
  • Cell fluorescence units
  • Colony formation utility

Correct Answer: Colony forming units

Q26. A mixed culture yields colonies with different morphologies; the next step to obtain pure cultures is:

  • Discard the plate
  • Pick distinct colonies and subculture by streaking on fresh agar
  • Incubate longer without changes
  • Blend the colonies together

Correct Answer: Pick distinct colonies and subculture by streaking on fresh agar

Q27. Which indicator is used in MacConkey agar to detect lactose fermenters?

  • Phenol red
  • Methylene blue
  • Congo red
  • Neutral red

Correct Answer: Neutral red

Q28. Streaking for isolation works by:

  • Increasing inoculum density across the plate
  • Diluting cells across the agar surface to separate individuals
  • Embedding cells in agar
  • Staining cells before incubation

Correct Answer: Diluting cells across the agar surface to separate individuals

Q29. Which method is most appropriate to recover stressed or injured bacteria from clinical samples?

  • Direct plating on selective media only
  • Use of non-selective enrichment and gentle recovery conditions
  • Heating samples to increase recovery
  • Using high antibiotic concentrations

Correct Answer: Use of non-selective enrichment and gentle recovery conditions

Q30. Which of the following is a limitation of the streak plate method?

  • It provides quantitative counts as accurately as pour plate
  • It may not separate organisms with similar colony characteristics
  • It is the best method for counting anaerobes
  • It embeds cells in agar

Correct Answer: It may not separate organisms with similar colony characteristics

Q31. When calculating CFU/mL from a plate, which values are needed?

  • Dilution factor, plated volume, and colony count
  • Only incubation time
  • Only agar type
  • Only color of colonies

Correct Answer: Dilution factor, plated volume, and colony count

Q32. A fastidious organism requires which of the following for growth?

  • Minimal salts only
  • Special nutrients or growth factors in media
  • Large inoculum only
  • Exposure to ultraviolet light

Correct Answer: Special nutrients or growth factors in media

Q33. Aseptic transfer from broth to agar should include which step to minimize contamination?

  • Wipe the tube mouth with a sterile gauze
  • Flame the tube mouth and cap, and minimize exposure time
  • Leave the tube uncapped for several minutes
  • Touch the loop to the bench surface first

Correct Answer: Flame the tube mouth and cap, and minimize exposure time

Q34. Blood agar is primarily used to:

  • Differentiate organisms based on hemolysis
  • Select for Gram-negative bacteria
  • Grow fungi exclusively
  • Measure osmotic tolerance

Correct Answer: Differentiate organisms based on hemolysis

Q35. Which factor can lead to false high viable counts?

  • Using an accurately diluted sample
  • Plating multiple dilutions and choosing the correct plate
  • Clumping of cells leading to underestimation
  • Contamination during plating

Correct Answer: Contamination during plating

Q36. When isolating bacteria from a pharmaceutical product, the first step is usually:

  • Directly sequencing DNA
  • Sampling under sterile conditions and choosing appropriate media
  • Storing sample at room temperature for days
  • Adding antibiotics to the product

Correct Answer: Sampling under sterile conditions and choosing appropriate media

Q37. The term “pure culture” means:

  • Culture contains only one strain or species
  • Culture has no living cells
  • Culture contains multiple species with similar traits
  • Culture is always pathogenic

Correct Answer: Culture contains only one strain or species

Q38. Which plating technique minimizes heat damage to thermolabile cells?

  • Pour plate with very hot agar
  • Spread plate using cooled but molten agar
  • Streak plate where cells are not mixed with molten agar
  • Using an open flame directly on the cells

Correct Answer: Streak plate where cells are not mixed with molten agar

Q39. Which preservative technique is unsuitable for maintaining viability of most bacteria long-term?

  • Repeated subculturing at room temperature
  • Freezing at -80°C with cryoprotectant
  • Freeze-drying (lyophilization)
  • Storage in glycerol at low temperature

Correct Answer: Repeated subculturing at room temperature

Q40. Which indicator suggests successful isolation of single colonies?

  • Uniform lawn of growth across plate
  • Discrete, well-separated colonies each arising from a single CFU
  • Entire plate remains clear
  • Only filamentous growth observed

Correct Answer: Discrete, well-separated colonies each arising from a single CFU

Q41. Which media component often differentiates lactose fermenters by color change?

  • Carbohydrate and pH indicator
  • High salt concentration only
  • Antibiotics only
  • Protease enzymes

Correct Answer: Carbohydrate and pH indicator

Q42. Which statement about mixed cultures is correct?

  • They always simplify identification processes
  • They contain two or more different microbial species
  • They are ideal for producing pure pharmaceuticals
  • They do not require isolation steps for study

Correct Answer: They contain two or more different microbial species

Q43. When performing a streak plate, flame the loop between quadrants to:

  • Increase inoculum size
  • Sterilize and reduce cell transfer to achieve dilution
  • Cool the loop down
  • Add nutrients to the surface

Correct Answer: Sterilize and reduce cell transfer to achieve dilution

Q44. Which control is essential in culture isolation experiments to detect contamination?

  • Positive control only
  • Negative (sterility) control with sterile media incubated without inoculum
  • Using no controls
  • Only environmental monitoring without plates

Correct Answer: Negative (sterility) control with sterile media incubated without inoculum

Q45. Colony morphology alone is insufficient for definitive identification because:

  • All species have identical morphology
  • Different species can produce similar colony appearances
  • Morphology proves antibiotic sensitivity
  • It replaces biochemical testing

Correct Answer: Different species can produce similar colony appearances

Q46. In pharmaceutical quality control, isolation of pure cultures is important for:

  • Assessing sterility, contamination source tracking, and microbial identification
  • Only increasing product viscosity
  • Making products more stable to heat
  • Improving taste of oral formulations

Correct Answer: Assessing sterility, contamination source tracking, and microbial identification

Q47. Which practice reduces aerosol contamination when plating?

  • Vigorous shaking of plates
  • Working close to a flame or in a laminar flow hood and minimizing splashes
  • Leaving plates uncovered for long
  • Using high-speed vortexing over open agar

Correct Answer: Working close to a flame or in a laminar flow hood and minimizing splashes

Q48. Which technique allows rapid screening of multiple isolates for purity before biochemical tests?

  • Gram staining of isolated colonies
  • Mass spectrometry without culturing
  • Direct PCR from mixed sample only
  • Leaving cultures in mixed broth overnight

Correct Answer: Gram staining of isolated colonies

Q49. What is the effect of overcrowded plating on CFU counts?

  • Accurate counts within 30–300 range
  • Underestimation due to colony merging
  • No effect on counts
  • Always gives lower counts than actual

Correct Answer: Underestimation due to colony merging

Q50. Which best practice ensures reproducibility when isolating pure cultures?

  • Using documented standard operating procedures, sterile technique, and proper record keeping
  • Using random, undocumented methods
  • Ignoring incubation times
  • Reusing disposable loops

Correct Answer: Using documented standard operating procedures, sterile technique, and proper record keeping

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