Introduction: Methods of isolation, cultivation and maintenance of pure cultures is a core topic in Microbial and Cellular Biology for M.Pharm students. Mastery of these techniques is essential for drug development, quality control, and microbiological research. This blog presents targeted multiple-choice questions designed to test conceptual understanding and practical knowledge of isolation methods (streak, pour, spread, enrichment), selective and differential media, cultivation conditions for fastidious and anaerobic organisms, and long-term culture preservation strategies such as cryopreservation and lyophilization. Each question emphasizes decision-making relevant to laboratory practice and troubleshooting of pure culture maintenance.
Q1. Which isolation method is most suitable for obtaining well-separated colonies from a mixed bacterial sample while minimizing the risk of heat shock to organisms?
- Pour plate method
- Spread plate method
- Streak plate method
- Enrichment culture
Correct Answer: Streak plate method
Q2. For quantifying viable bacteria in a water sample, which technique gives the most accurate colony-forming unit (CFU) count when properly performed?
- Direct microscopic count
- Hemocytometer counting
- Serial dilution followed by spread plate
- Optical density measurement at 600 nm
Correct Answer: Serial dilution followed by spread plate
Q3. Which type of medium contains inhibitors that suppress unwanted organisms while permitting the growth of the target organism?
- Enrichment medium
- Selective medium
- Differential medium
- Minimal medium
Correct Answer: Selective medium
Q4. When cultivating obligate anaerobes, which approach creates an oxygen-free environment for growth?
- Using a candle jar
- Incubation in a CO2 incubator
- Using an anaerobic chamber or GasPak system
- Growing on selective aerobic media
Correct Answer: Using an anaerobic chamber or GasPak system
Q5. Enrichment culture is primarily used to:
- Reduce the number of viable cells
- Isolate fastidious organisms by lowering nutrient levels
- Favor growth of a desired organism from a mixed population
- Determine total microbial biomass
Correct Answer: Favor growth of a desired organism from a mixed population
Q6. Which preservation method is preferred for long-term maintenance of genetic stability and viability of bacterial strains in a culture collection?
- Frequent serial sub-culturing on agar slants
- Short-term refrigeration at 4°C
- Cryopreservation in liquid nitrogen with cryoprotectant
- Incubation at ambient room temperature
Correct Answer: Cryopreservation in liquid nitrogen with cryoprotectant
Q7. Lyophilization (freeze-drying) of bacterial cultures is advantageous because it:
- Requires constant cold-chain storage at −80°C
- Destroys plasmid DNA and reduces contaminating viruses
- Maintains long-term viability with minimal metabolic activity
- Is ideal for maintaining obligate anaerobes in an oxygenated state
Correct Answer: Maintains long-term viability with minimal metabolic activity
Q8. Which technique is most appropriate to separate colonies based on metabolic differences such as lactose fermentation?
- Selective medium alone
- Differential medium (e.g., MacConkey agar)
- Pour plate method
- Serial dilution in saline
Correct Answer: Differential medium (e.g., MacConkey agar)
Q9. A culture shows variable colony morphology after repeated sub-culturing. The most likely cause is:
- Strict maintenance of single-use aliquots
- Genetic drift or contamination during serial passage
- Cryopreservation at −196°C
- Use of selective media to maintain purity
Correct Answer: Genetic drift or contamination during serial passage
Q10. Which aseptic practice is critical when transferring microorganisms using an inoculating loop to prevent contamination?
- Cooling the loop by touching the agar surface before inoculation
- Passing the loop through a flame until red-hot and allowing it to cool briefly
- Wetting the loop with sterile saline before flaming
- Flicking the loop in air to cool quickly
Correct Answer: Passing the loop through a flame until red-hot and allowing it to cool briefly
Q11. Which method is best suited to isolate aerobic bacteria from a clinical specimen containing mixed flora?
- Incubation in a GasPak anaerobic jar
- Enrichment in selective broth followed by streak plate on selective agar
- Direct inoculation into thioglycollate broth only
- Lyophilization of the specimen prior to culture
Correct Answer: Enrichment in selective broth followed by streak plate on selective agar
Q12. When working with fastidious bacteria requiring specific growth factors, which cultivation strategy is most appropriate?
- Use of rich, supplemented media (e.g., blood or chocolate agar) and controlled atmospheric conditions
- Growing on minimal salts agar only
- Exposure to open air to increase oxygen tension
- Using only selective media with high salt concentration
Correct Answer: Use of rich, supplemented media (e.g., blood or chocolate agar) and controlled atmospheric conditions
Q13. The streak plate dilution method relies mainly on which principle to obtain isolated colonies?
- Serial dilution in liquid followed by pour plating
- Progressive spatial dilution of cells across an agar surface
- Heat inactivation of most cells before plating
- Use of antibiotics to select single colonies
Correct Answer: Progressive spatial dilution of cells across an agar surface
Q14. Which medium component is commonly used to solidify culture media and remain solid at incubation temperatures?
- Agar
- Gelatin
- Starch
- Cellulose
Correct Answer: Agar
Q15. For maintaining a culture of a slow-growing bacterium without frequent subculturing, which storage approach is least likely to promote genetic changes?
- Continuous culture at sub-optimal temperature
- Repeated streaking on fresh plates every week
- Storage as lyophilized ampoules
- Frequent serial passaging in rich broth
Correct Answer: Storage as lyophilized ampoules
Q16. Which plate method requires molten agar to be mixed with the sample before solidification and can detect microaerophiles embedded in agar?
- Spread plate
- Streak plate
- Pour plate
- Enrichment plate
Correct Answer: Pour plate
Q17. What is the primary purpose of including a cryoprotectant like glycerol when freezing bacterial stocks?
- To increase ice crystal formation that helps cells lyse
- To prevent osmotic shock and reduce ice crystal damage during freezing
- To serve as a nutrient for the bacteria during storage
- To lower the freezing point so samples remain liquid
Correct Answer: To prevent osmotic shock and reduce ice crystal damage during freezing
Q18. How does using a selective enrichment medium enhance recovery of a pathogen present in low numbers within a sample?
- By inhibiting growth of the target pathogen while promoting others
- By physically separating cells based on motility
- By providing conditions that preferentially favor growth of the target pathogen over competing flora
- By increasing oxygen tension only
Correct Answer: By providing conditions that preferentially favor growth of the target pathogen over competing flora
Q19. A contaminated frozen stock is discovered. The best immediate action to prevent spread and preserve evidence for investigation is to:
- Thaw and immediately subculture into fresh medium
- Discard the entire culture without documentation
- Quarantine the vial, document contamination, and retain an aliquot for further analysis before decontamination
- Label and return to the freezer for later handling
Correct Answer: Quarantine the vial, document contamination, and retain an aliquot for further analysis before decontamination
Q20. In maintaining pure cultures, what practice minimizes adaptive mutations and contamination over long-term laboratory use?
- Maintaining a single working stock that is repeatedly propagated
- Preparing multiple single-use working aliquots from a low-passage master stock and using aseptic technique
- Storing all cultures at room temperature to avoid freeze–thaw stress
- Using antibiotics in all routine maintenance media to prevent contamination
Correct Answer: Preparing multiple single-use working aliquots from a low-passage master stock and using aseptic technique
