Original Westcott Microbiology Final Exam
Practice Questions 2026
1. Which of the following best describes the relationship between the resolution
and the wavelength of light in microscopy?
A) Resolution improves as wavelength increases
B) Resolution improves as wavelength decreases
C) Resolution is independent of wavelength
D) Resolution doubles when wavelength doubles
B) Resolution improves as wavelength decreases
Resolution is the ability to distinguish two closely spaced objects. Shorter wavelengths
provide better resolution. This is why electron microscopes (using electrons with much
shorter wavelengths than visible light) achieve far greater resolution than light
microscopes.
2. What is the function of the ocular lens in a compound light microscope?
A) To collect light from the specimen
B) To further magnify the image produced by the objective lens
C) To focus light onto the specimen
D) To adjust the amount of light
B) To further magnify the image produced by the objective lens
The ocular lens (eyepiece) further magnifies the image produced by the objective lens.
Total magnification is calculated by multiplying the ocular lens magnification by the
objective lens magnification.
,3. A microscope has a 15x ocular lens and a 100x oil immersion objective lens.
What is the total magnification?
A) 115x
B) 1,500x
C) 1,000x
D) 150x
B) 1,500x
Total magnification = ocular lens magnification × objective lens magnification = 15 × 100
= 1,500x.
4. Why is immersion oil used with high-power objective lenses?
A) To stain the specimen
B) To increase the numerical aperture and improve resolution
C) To fix the specimen to the slide
D) To prevent the specimen from drying out
B) To increase the numerical aperture and improve resolution
Immersion oil has the same refractive index as glass, preventing light from bending as it
passes from the specimen to the objective lens. This increases the numerical aperture and
improves resolution.
5. Which of the following is NOT a step in the Gram stain procedure?
A) Application of crystal violet
B) Application of iodine
C) Application of alcohol
D) Application of methylene blue
D) Application of methylene blue
,The Gram stain procedure involves crystal violet (primary stain), iodine (mordant), alcohol
(decolorizer), and safranin (counterstain). Methylene blue is not used in the Gram stain; it
is used in simple stains.
6. What color do Gram-negative bacteria appear after completing the Gram stain?
A) Purple
B) Pink/red
C) Green
D) Colorless
B) Pink/red
Gram-negative bacteria lose the crystal violet-iodine complex during decolorization due to
their thin peptidoglycan layer and outer membrane. They take up the safranin
counterstain and appear pink/red.
7. What is the purpose of heat fixation in smear preparation?
A) To kill the bacteria and attach them to the slide
B) To stain the bacteria
C) To increase magnification
D) To preserve the bacteria for future use
A) To kill the bacteria and attach them to the slide
Heat fixation kills the microorganisms, makes them adhere to the slide, and makes them
more permeable to stains. It is a critical step in smear preparation.
8. Which of the following is a differential stain?
A) Methylene blue
B) Gram stain
, C) India ink
D) Crystal violet
B) Gram stain
The Gram stain is a differential stain that distinguishes bacteria based on cell wall
structure. Methylene blue and crystal violet are simple stains; India ink is a negative stain.
9. What is the purpose of a negative stain?
A) To stain the microorganism
B) To stain the background, leaving the microorganism unstained
C) To stain both the microorganism and background
D) To stain neither
B) To stain the background, leaving the microorganism unstained
Negative stains (e.g., India ink, nigrosin) stain the background, leaving the microorganism
unstained. This is useful for observing capsule formation and cell shape.
10. Which of the following is a selective and differential medium?
A) Nutrient agar
B) MacConkey agar
C) Blood agar
D) Tryptic soy agar
B) MacConkey agar
MacConkey agar is both selective (inhibits Gram-positive bacteria) and differential
(distinguishes lactose fermenters from non-fermenters). Nutrient agar and tryptic soy agar
are general purpose; blood agar is differential but not selective.
Practice Questions 2026
1. Which of the following best describes the relationship between the resolution
and the wavelength of light in microscopy?
A) Resolution improves as wavelength increases
B) Resolution improves as wavelength decreases
C) Resolution is independent of wavelength
D) Resolution doubles when wavelength doubles
B) Resolution improves as wavelength decreases
Resolution is the ability to distinguish two closely spaced objects. Shorter wavelengths
provide better resolution. This is why electron microscopes (using electrons with much
shorter wavelengths than visible light) achieve far greater resolution than light
microscopes.
2. What is the function of the ocular lens in a compound light microscope?
A) To collect light from the specimen
B) To further magnify the image produced by the objective lens
C) To focus light onto the specimen
D) To adjust the amount of light
B) To further magnify the image produced by the objective lens
The ocular lens (eyepiece) further magnifies the image produced by the objective lens.
Total magnification is calculated by multiplying the ocular lens magnification by the
objective lens magnification.
,3. A microscope has a 15x ocular lens and a 100x oil immersion objective lens.
What is the total magnification?
A) 115x
B) 1,500x
C) 1,000x
D) 150x
B) 1,500x
Total magnification = ocular lens magnification × objective lens magnification = 15 × 100
= 1,500x.
4. Why is immersion oil used with high-power objective lenses?
A) To stain the specimen
B) To increase the numerical aperture and improve resolution
C) To fix the specimen to the slide
D) To prevent the specimen from drying out
B) To increase the numerical aperture and improve resolution
Immersion oil has the same refractive index as glass, preventing light from bending as it
passes from the specimen to the objective lens. This increases the numerical aperture and
improves resolution.
5. Which of the following is NOT a step in the Gram stain procedure?
A) Application of crystal violet
B) Application of iodine
C) Application of alcohol
D) Application of methylene blue
D) Application of methylene blue
,The Gram stain procedure involves crystal violet (primary stain), iodine (mordant), alcohol
(decolorizer), and safranin (counterstain). Methylene blue is not used in the Gram stain; it
is used in simple stains.
6. What color do Gram-negative bacteria appear after completing the Gram stain?
A) Purple
B) Pink/red
C) Green
D) Colorless
B) Pink/red
Gram-negative bacteria lose the crystal violet-iodine complex during decolorization due to
their thin peptidoglycan layer and outer membrane. They take up the safranin
counterstain and appear pink/red.
7. What is the purpose of heat fixation in smear preparation?
A) To kill the bacteria and attach them to the slide
B) To stain the bacteria
C) To increase magnification
D) To preserve the bacteria for future use
A) To kill the bacteria and attach them to the slide
Heat fixation kills the microorganisms, makes them adhere to the slide, and makes them
more permeable to stains. It is a critical step in smear preparation.
8. Which of the following is a differential stain?
A) Methylene blue
B) Gram stain
, C) India ink
D) Crystal violet
B) Gram stain
The Gram stain is a differential stain that distinguishes bacteria based on cell wall
structure. Methylene blue and crystal violet are simple stains; India ink is a negative stain.
9. What is the purpose of a negative stain?
A) To stain the microorganism
B) To stain the background, leaving the microorganism unstained
C) To stain both the microorganism and background
D) To stain neither
B) To stain the background, leaving the microorganism unstained
Negative stains (e.g., India ink, nigrosin) stain the background, leaving the microorganism
unstained. This is useful for observing capsule formation and cell shape.
10. Which of the following is a selective and differential medium?
A) Nutrient agar
B) MacConkey agar
C) Blood agar
D) Tryptic soy agar
B) MacConkey agar
MacConkey agar is both selective (inhibits Gram-positive bacteria) and differential
(distinguishes lactose fermenters from non-fermenters). Nutrient agar and tryptic soy agar
are general purpose; blood agar is differential but not selective.
