BIOS 242 MICROBIOLOGY WEEK 2
QUESTIONS AND ANSWERS
1. Which component of the Gram stain acts as a mordant to form a large complex with the
crystal violet?
A. Safranin
B. Ethyl alcohol
C. Crystal violet
D. Gram’s iodine
Answer: D
Conceptual Explanation: Gram’s iodine serves as a mordant, binding with crystal violet to
form the CV-I complex which is trapped within the thick peptidoglycan layer of Gram-
positive bacteria.
2. Which type of microscopy uses a special condenser to block direct light, causing the
specimen to appear bright against a dark background?
A. Phase-contrast microscopy
B. Dark-field microscopy
C. Bright-field microscopy
,D. Fluorescence microscopy
Answer: B
Conceptual Explanation: Dark-field microscopy uses a dark-field condenser so only light
reflected off the specimen enters the objective lens, making it ideal for viewing living,
unstained cells.
3. In the Acid-fast staining procedure, which genus of bacteria is specifically targeted due to
the presence of mycolic acid?
A. Staphylococcus
B. Mycobacterium
C. Escherichia
D. Bacillus
Answer: B
Conceptual Explanation: Mycobacterium species, such as M. tuberculosis, contain high
concentrations of mycolic acid in their cell walls, making them ‘acid-fast’.
4. What is the total magnification of a specimen if the ocular lens is 10x and the objective lens
is 40x?
A. 50x
B. 40x
C. 4000x
, D. 400x
Answer: D
Conceptual Explanation: Total magnification is calculated by multiplying the ocular lens
power by the objective lens power (10 x 40 = 400).
5. Which type of culture medium contains inhibitors that suppress the growth of unwanted
microbes while encouraging the growth of desired ones?
A. Differential media
B. Enriched media
C. Selective media
D. General-purpose media
Answer: C
Conceptual Explanation: Selective media are designed to inhibit the growth of certain
microbes while allowing others to grow, such as using salt to select for halophiles.
6. During the Gram stain, what happens to Gram-negative bacteria when the decolorizer
(alcohol) is applied?
A. The cell wall becomes dehydrated and traps the dye.
B. The bacteria turn purple.
C. The outer membrane is dissolved, and the thin peptidoglycan layer loses the dye.
D. The cell wall thickens instantly.
QUESTIONS AND ANSWERS
1. Which component of the Gram stain acts as a mordant to form a large complex with the
crystal violet?
A. Safranin
B. Ethyl alcohol
C. Crystal violet
D. Gram’s iodine
Answer: D
Conceptual Explanation: Gram’s iodine serves as a mordant, binding with crystal violet to
form the CV-I complex which is trapped within the thick peptidoglycan layer of Gram-
positive bacteria.
2. Which type of microscopy uses a special condenser to block direct light, causing the
specimen to appear bright against a dark background?
A. Phase-contrast microscopy
B. Dark-field microscopy
C. Bright-field microscopy
,D. Fluorescence microscopy
Answer: B
Conceptual Explanation: Dark-field microscopy uses a dark-field condenser so only light
reflected off the specimen enters the objective lens, making it ideal for viewing living,
unstained cells.
3. In the Acid-fast staining procedure, which genus of bacteria is specifically targeted due to
the presence of mycolic acid?
A. Staphylococcus
B. Mycobacterium
C. Escherichia
D. Bacillus
Answer: B
Conceptual Explanation: Mycobacterium species, such as M. tuberculosis, contain high
concentrations of mycolic acid in their cell walls, making them ‘acid-fast’.
4. What is the total magnification of a specimen if the ocular lens is 10x and the objective lens
is 40x?
A. 50x
B. 40x
C. 4000x
, D. 400x
Answer: D
Conceptual Explanation: Total magnification is calculated by multiplying the ocular lens
power by the objective lens power (10 x 40 = 400).
5. Which type of culture medium contains inhibitors that suppress the growth of unwanted
microbes while encouraging the growth of desired ones?
A. Differential media
B. Enriched media
C. Selective media
D. General-purpose media
Answer: C
Conceptual Explanation: Selective media are designed to inhibit the growth of certain
microbes while allowing others to grow, such as using salt to select for halophiles.
6. During the Gram stain, what happens to Gram-negative bacteria when the decolorizer
(alcohol) is applied?
A. The cell wall becomes dehydrated and traps the dye.
B. The bacteria turn purple.
C. The outer membrane is dissolved, and the thin peptidoglycan layer loses the dye.
D. The cell wall thickens instantly.
