Microbiology Lab Quiz
Questions: 16 · 10 minutes
1. What is the main purpose of the streak-plate method?
To progressively separate cells so isolated colonies can develop
To identify bacteria solely by the color of the inoculating loop
To sterilize the agar surface during inoculation
To ensure every organism grows equally well on the medium
2. An uninoculated broth control becomes turbid during the same incubation in which test broths are growing. What is the best interpretation?
Contamination or a procedural problem occurred, so the test results may not be reliable.
Turbidity confirms that every test broth contains a pure culture.
The control proves the incubator temperature was optimal.
The test organism must be able to grow without nutrients.
3. Which sequence gives the standard order of reagents in a Gram stain?
Safranin, iodine, decolorizer, crystal violet
Crystal violet, decolorizer, iodine, safranin
Iodine, crystal violet, safranin, decolorizer
Crystal violet, iodine, decolorizer, safranin
4. After flaming an inoculating loop, why should you let it cool before touching a bacterial culture?
Cooling makes the loop hold a larger volume of culture.
A hot loop could kill cells in the sample being transferred.
A cool loop prevents oxygen from entering the culture tube.
Cooling converts the loop surface from selective to differential.
5. You have focused a stained bacterial smear under lower-power objectives and now need maximum useful magnification with a standard bright-field microscope. What should you do next?
Place immersion oil on the slide and use the 40× dry objective.
Place immersion oil over the specimen and rotate the 100× oil-immersion objective into position.
Use the 100× objective without oil and close the iris diaphragm completely.
Add water over the specimen and return to the 10× objective.
6. A streak plate grows two clearly different colony morphologies. You need pure cultures of both suspected organisms. What is the best next step?
Mix both colony types into one broth and treat the mixture as a pure culture.
Pick one well-isolated colony of each morphology and streak each onto a separate fresh plate.
Count all colonies together and identify the species from the total alone.
Incubate the original plate longer until the colony types become identical.
7. MacConkey agar inhibits many Gram-positive bacteria and distinguishes lactose fermenters by colony color. How is this medium classified?
Enriched but neither selective nor differential
Reducing because it removes oxygen from the medium
Selective only, because colony appearance provides no metabolic information
Both selective and differential
8. A broth that was clear before inoculation becomes visibly turbid after incubation. What can you reasonably conclude from this observation alone?
The broth contains exactly one bacterial species.
Every cell in the broth is alive and actively dividing.
The organism is necessarily pathogenic.
Microbial growth likely occurred, but turbidity alone does not establish culture purity.
9. A plate inoculated with 0.1 mL of a 10^-4 dilution develops 65 colonies. Assuming each colony represents one viable unit, what was the estimated concentration of the original sample?
6.5 × 10^3 CFU/mL
6.5 × 10^4 CFU/mL
6.5 × 10^8 CFU/mL
6.5 × 10^6 CFU/mL
10. Why do acid-fast bacteria resist decolorization during acid-fast staining?
They lack a cell envelope and retain every dye applied.
Their waxy, mycolic-acid-rich cell walls strongly retain the primary stain.
Their capsules chemically convert the decolorizer into a primary stain.
Their endospores release a counterstain during heating.
11. A sample is diluted 1:10, and then that dilution is diluted another 1:100. What is the overall dilution relative to the original sample?
1:1,000
1:100
1:10
1:10,000
12. A procedure involving a Biosafety Level 2 culture may generate infectious aerosols. Which engineering control is designed to provide appropriate containment during the procedure?
A properly operating Class II biological safety cabinet
A chemical fume hood used as a substitute for biological containment
A standard incubator with its door partly open
A bench shield with the work performed in open room air
13. Which microscopy method is especially useful for viewing living, unstained microorganisms with enhanced contrast?
Scanning electron microscopy
Transmission electron microscopy
Phase-contrast microscopy
Bright-field microscopy with a heat-fixed smear
14. How does a standard steam autoclave achieve sterilization?
By exposing materials to dry heat at room pressure
By filtering microorganisms out of sealed solid materials
By using pressurized saturated steam to deliver lethal moist heat
By freezing cells and then rapidly returning them to room temperature
15. Why are inoculated agar plates commonly incubated in an inverted position?
To expose the agar directly to more oxygen through the lid
To make the agar chemically selective during incubation
To reduce the chance that condensation will drip onto the agar and spread growth
To prevent all moisture from forming inside the plate
16. Which statement best distinguishes sterilization from routine disinfection?
Disinfection always destroys endospores, whereas sterilization does not.
Sterilization applies only to skin, while disinfection applies only to glassware.
A validated sterilization process is intended to eliminate all forms of microbial life, including bacterial endospores; routine disinfection may not.
The two terms describe identical levels of microbial control.