BIOD 171 Essential Microbiology - Module 3 Exam 2026/2027 UPDATE
1. Which component of the Gram-negative cell wall contains Lipid A, also known
as endotoxin?
A. Peptidoglycan
B. Lipopolysaccharide (LPS)
C. Teichoic acid
D. Periplasmic space
Answer: B
Rationale: Lipopolysaccharide (LPS) is found in the outer membrane of Gram-negative
bacteria; the Lipid A portion acts as an endotoxin.
2. What is the primary purpose of using immersion oil with a 100x objective
lens?
A. To increase the magnification of the specimen
B. To lubricate the slide for easier movement
C. To reduce light refraction and increase resolution
D. To stain the bacteria for better contrast
Answer: C
Rationale: Immersion oil has the same refractive index as glass, preventing light from
bending away and thus improving the resolution.
,3. In the Gram stain procedure, what is the function of Iodine?
A. Primary stain
B. Mordant
C. Decolorizer
D. Counterstain
Answer: B
Rationale: Iodine acts as a mordant, forming a complex with crystal violet that gets
trapped in the thick peptidoglycan layer of Gram-positive cells.
4. Which type of microscopy is most effective for viewing living, unstained
specimens by creating contrast through light phase shifts?
A. Bright-field microscopy
B. Electron microscopy
C. Phase-contrast microscopy
D. Fluorescence microscopy
Answer: C
Rationale: Phase-contrast microscopy converts small differences in the refractive index
and cell density into easily detected variations in light intensity.
5. Gram-positive bacteria appear what color after a successful Gram stain?
A. Pink/Red
B. Purple/Blue
C. Green
D. Colorless
Answer: B
Rationale: Gram-positive bacteria retain the primary stain (Crystal Violet) and appear
purple.
, 6. What is the total magnification of a specimen if the ocular lens is 10x and the
objective lens is 40x?
A. 40x
B. 50x
C. 4000x
D. 400x
Answer: D
Rationale: Total magnification is calculated by multiplying the ocular lens (10x) by the
objective lens (40x), which equals 400x.
7. Acid-fast staining is specifically used to identify which genus of bacteria?
A. Mycobacterium
B. Escherichia
C. Staphylococcus
D. Bacillus
Answer: A
Rationale: Mycobacterium species have mycolic acids in their cell walls that require acid-
fast staining to be visualized.
8. Which microscope uses an electron beam to visualize the internal
ultrastructure of a specimen?
A. Transmission Electron Microscope (TEM)
B. Scanning Electron Microscope (SEM)
C. Confocal Microscope
D. Dark-field Microscope
Answer: A
Rationale: TEM passes electrons through a thin section of a specimen to image internal
structures.
1. Which component of the Gram-negative cell wall contains Lipid A, also known
as endotoxin?
A. Peptidoglycan
B. Lipopolysaccharide (LPS)
C. Teichoic acid
D. Periplasmic space
Answer: B
Rationale: Lipopolysaccharide (LPS) is found in the outer membrane of Gram-negative
bacteria; the Lipid A portion acts as an endotoxin.
2. What is the primary purpose of using immersion oil with a 100x objective
lens?
A. To increase the magnification of the specimen
B. To lubricate the slide for easier movement
C. To reduce light refraction and increase resolution
D. To stain the bacteria for better contrast
Answer: C
Rationale: Immersion oil has the same refractive index as glass, preventing light from
bending away and thus improving the resolution.
,3. In the Gram stain procedure, what is the function of Iodine?
A. Primary stain
B. Mordant
C. Decolorizer
D. Counterstain
Answer: B
Rationale: Iodine acts as a mordant, forming a complex with crystal violet that gets
trapped in the thick peptidoglycan layer of Gram-positive cells.
4. Which type of microscopy is most effective for viewing living, unstained
specimens by creating contrast through light phase shifts?
A. Bright-field microscopy
B. Electron microscopy
C. Phase-contrast microscopy
D. Fluorescence microscopy
Answer: C
Rationale: Phase-contrast microscopy converts small differences in the refractive index
and cell density into easily detected variations in light intensity.
5. Gram-positive bacteria appear what color after a successful Gram stain?
A. Pink/Red
B. Purple/Blue
C. Green
D. Colorless
Answer: B
Rationale: Gram-positive bacteria retain the primary stain (Crystal Violet) and appear
purple.
, 6. What is the total magnification of a specimen if the ocular lens is 10x and the
objective lens is 40x?
A. 40x
B. 50x
C. 4000x
D. 400x
Answer: D
Rationale: Total magnification is calculated by multiplying the ocular lens (10x) by the
objective lens (40x), which equals 400x.
7. Acid-fast staining is specifically used to identify which genus of bacteria?
A. Mycobacterium
B. Escherichia
C. Staphylococcus
D. Bacillus
Answer: A
Rationale: Mycobacterium species have mycolic acids in their cell walls that require acid-
fast staining to be visualized.
8. Which microscope uses an electron beam to visualize the internal
ultrastructure of a specimen?
A. Transmission Electron Microscope (TEM)
B. Scanning Electron Microscope (SEM)
C. Confocal Microscope
D. Dark-field Microscope
Answer: A
Rationale: TEM passes electrons through a thin section of a specimen to image internal
structures.
