Practice Questions
Visualizing Microbes

(Write down your answers and then click at bottom of page for the correct answers.)

  1. During staining, the smear is heat-fixed in order to ________________________.
    1. kill the organism so that dyes will penetrate.
    2. attach the organism firmly to the slide
    3. kill the organism and attach the organism firmly to the slide
    4. neither kill the organism nor attach the organism firmly to the slide


  2. The colorization of bacteria with more than one dye solution to distinguish between microbial cell types is called a _________.
    1. wet mount
    2. simple stain
    3. differential stain
    4. structural stain
    5. hanging drop


  3. What do negative staining and positive staining have in common?
    1. They stain the specimen.
    2. They stain the background only.
    3. They are used only for light microscopy.
    4. They increase the contrast between the specimen and the background.
    5. They have colorless chromophores.


  4. A procedure that divides organisms into two or more groups depending on their individual reactions to the same staining procedure is referred to as __________.
    1. wet mount
    2. simple stain
    3. differential stain
    4. structural stain
    5. hanging drop


  5. If the iodine step were omitted in the Gram-staining procedure, what color would you expect Gram-positive bacteria to stain and what color would you expect Gram-negative bacteria to stain?
    1. Gram-positive = purple; Gram-negative = red
    2. Gram-positive = purple; Gram-negative = purple
    3. Gram-positive = purple; Gram-negative = colorless
    4. Gram-positive = red; Gram-negative = purple
    5. Gram-positive = red; Gram-negative = red


  6. The substances listed below are used in various differential staining techniques. Which of the following lists the correct order for the solutions used in the Gram stain?
    1. Alcohol
    2. Gram's iodine
    3. Carbol fuchsin
    4. Crystal violet
    5. Methyl red
    6. Methylene blue
    7. Safranin (or basic fuchsin)
    1. 1 - 3 - 4 - 2
    2. 1 - 7 - 2 - 4
    3. 4 - 2 - 1 - 7
    4. 1 - 4 - 3 - 6
    5. 4 - 7 - 1 - 5


  7. Let's assume that you were teaching the Gram stain to 5 high school students, and each student did one thing wrong during the Gram-staining procedure, as shown in the following list. All five students were staining slides containing a mixed culture of a Gram-positive rod and a Gram-negative coccus. Of the 5 students listed below which one will probably get a CORRECT Gram reactions (assuming everything else was done correctly)?
    1. One put the alcohol on first instead of when it should have been applied.
    2. One used safranin (a red dye) instead of basic fuchsin as a counter stain.
    3. One forgot the alcohol step completely.
    4. One forgot to use a counter stain.
    5. One left the alcohol on for 20 minutes instead of 20 seconds.


  8. The ACID-FAST STAIN is important, because it is a __________.
    1. differential stain that is helpful in diagnosing tuberculosis.
    2. simple stain that differentiates Mycobacterium species from other bacteria.
    3. simple stain that helps identify the causative agent of tuberculosis.
    4. acid (negatively charged) dye that stains cell walls of most eubacteria.
    5. basic dye that converts to an acid when motile (fast) bacteria are stained.


  9. Using the acid-fast stain's name as a guide, what would be a logical alternative name for the Gram-stain?
    1. the crystal violet safranin stain
    2. the counterstain
    3. the differential stain
    4. the carbol fuschin stain
    5. the alcohol-fast stain


  10. If you know you have a pure culture of spore-forming bacilli but the bacilli but the endospore stain on samples shows only pink rods, which choice below is reasonable explanation for what you see?
    1. Since the cells transmit pink light, any spores transmitting green light are not visible.
    2. You have accidentally used the India ink negative stain, so the spores are not stained.
    3. None of the cells happened to be producing spores at the time they were stained.
    4. Spores cannot be stained while they are in the cell; therefore you won't see any cells with spores.
    5. Bacilli do not make spores.
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