Microbiology Quiz
Questions: 16 · 10 minutes
1. In a PCR test, the no-template control produces an amplification signal. What is the most likely interpretation?
The tested sample definitely contains the target sequence
The control or reaction reagents were contaminated with amplifiable material
The PCR instrument failed to detect amplified material
The primers were unable to bind any DNA
2. A bacterium retains carbol fuchsin even after treatment with acid-alcohol. Which cell-envelope component best explains this result?
A waxy layer containing mycolic acids
A cellulose-rich cell wall
A membrane containing large amounts of cholesterol
A capsule composed mainly of simple sugars
3. A plate inoculated with 0.1 mL of a 10⁻⁵ dilution produces 80 colonies. What was the estimated concentration of the original sample?
Eight hundred thousand colony-forming units per mL
Eight million colony-forming units per mL
Eighty million colony-forming units per mL
Eighty thousand colony-forming units per mL
4. A culture containing Bacillus survives brief boiling and later produces actively growing cells under favorable conditions. Which structure most likely enabled survival?
A ribosome
A flagellum
An endospore
A conjugation pilus
5. A mixed sample must be separated so individual bacterial colonies can be examined. Which technique is designed for this purpose?
Directly observing an undiluted broth with a wet mount
Measuring the sample with a spectrophotometer
Incubating the entire sample in one sealed tube
Streaking an agar plate to progressively dilute cells across its surface
6. A laboratory needs to sterilize a heat-stable liquid growth medium before use. Which method is most appropriate?
Heating it under pressure in an autoclave
Exposing the medium to ultraviolet light
Wiping the outside of the container with disinfectant
Refrigerating the medium overnight
7. An antibiotic blocks bacterial ribosomes but does not interfere with viral replication inside human cells. What follows from this difference?
It should eliminate both bacterial and viral infections equally
It would not be expected to treat influenza, which is caused by a virus
It should work against viruses only when they have an envelope
It would convert influenza viruses into harmless bacterial cells
8. Food kept in a refrigerator usually spoils more slowly than food left at room temperature. What is the best microbiological explanation?
Cold temperatures remove all water required by microorganisms
Lower temperatures slow the metabolism and reproduction of many microbes
Cold air prevents microorganisms from reaching the food
Refrigeration permanently destroys microbial DNA
9. Why do typical Gram-positive bacteria remain purple after the Gram-staining procedure?
Their outer membrane prevents the counterstain from entering
Their thick peptidoglycan layer retains the crystal violet–iodine complex
Their capsules permanently bind crystal violet
Their DNA reacts with iodine to form a purple pigment
10. A student wants to watch living, unstained bacterial cells move in a liquid sample. Which method is most suitable?
Transmission electron microscopy
Bright-field microscopy after Gram staining
Phase-contrast microscopy
Scanning electron microscopy
11. Which feature most clearly distinguishes viruses from cellular microorganisms?
Viruses are always smaller than every bacterium
Viruses contain proteins but no genetic material
Viruses can reproduce independently on nutrient agar
Viruses require host-cell machinery to reproduce
12. A microscope uses a 10× ocular lens and a 40× objective lens. What is the total magnification?
400×
50×
100×
4,000×
13. During a transfer, a student sterilizes the inoculating loop before collecting a second bacterial culture. What is the main purpose of this step?
To make the second culture grow more quickly
To improve the visibility of cells under a microscope
To supply the second culture with additional oxygen
To reduce cross-contamination between cultures
14. Two bacterial cells look like one blurred object through a microscope, even after magnification is increased. Which microscope property is insufficient?
Magnification, the apparent enlargement of an object
Contrast, the visible difference between specimen and background
Working distance, the space between objective and specimen
Resolution, the ability to distinguish two nearby points
15. How would a facultative anaerobe typically grow in a tube containing an oxygen gradient?
Throughout the tube, with denser growth near the top
Only at the oxygen-free bottom
As a narrow band just below the surface
Only at the oxygen-rich surface
16. A bacterial culture has just been transferred into fresh medium. Cells are metabolically active but show little increase in number. Which growth phase is this?
Exponential phase
Stationary phase
Lag phase
Death phase