2025 Microbiology Laboratory Exam
Review and Practice Questions – Bachoon
Coursework
Why are pure cultures necessary for biochemical testing?
a) They save time and resources
b) Mixed cultures give more accurate results
c) They prevent skewed results caused by unwanted organisms
d) They allow faster bacterial growth
Rationale: Pure cultures ensure the test results are only from the organism being studied,
avoiding contamination. Mixed cultures can produce inaccurate or misleading results.
Why do you need to know the Gram reaction and morphology of an organism before
biochemical testing?
a) To determine the incubation temperature
b) To select the correct medium to expedite diagnosis
c) To measure enzyme activity
d) To count bacterial colonies
Rationale: Gram reaction and morphology help select the appropriate biochemical tests and
media. They do not determine temperature or colony numbers.
Which enzyme is linked to diaper rash in infants due to ammonia production?
a) Catalase
b) Amylase
c) Urease
d) Lipase
Rationale: Urease breaks down urea into ammonia, increasing skin irritation in wet diapers,
causing diaper rash. The other enzymes are not involved in urea breakdown.
,Which dye is used first in Gram staining?
a) Crystal Violet
b) Safranin
c) Methylene blue
d) Carbol fuchsin
Rationale: Crystal violet is the primary stain in Gram staining. Safranin is the counterstain used
last.
Which dye is used last in the Gram stain procedure?
a) Crystal violet
b) Gram’s iodine
c) Decolorizer
d) Safranin
Rationale: Safranin stains Gram-negative bacteria pink at the end of the process. It is applied
after decolorization.
What is the correct order of reagents in Gram staining?
a) Safranin → iodine → alcohol → crystal violet
b) Iodine → crystal violet → safranin → alcohol
c) Crystal violet → iodine → decolorizer → safranin
d) Alcohol → crystal violet → iodine → safranin
Rationale: The correct sequence ensures proper staining and differentiation of Gram-positive
and Gram-negative bacteria.
What type of media has a precise chemical composition?
a) Enriched media
b) Defined media
c) Selective media
d) Differential media
Rationale: Defined media contain known chemical quantities. Enriched, selective, and
differential media are more complex and often include unknown extracts.
What color does E. coli appear after Gram staining?
a) Purple
,b) Pink
c) Blue
d) Green
Rationale: E. coli is Gram-negative and appears pink after staining due to safranin retention.
What color does Bacillus subtilis appear under Gram stain?
a) Purple
b) Pink
c) Orange
d) Colorless
Rationale: Bacillus subtilis is Gram-positive and retains crystal violet, appearing purple.
Capsule-stained cells appear as:
a) Blue cells with dark centers
b) Black background with clear halos
c) Green rods with red spores
d) Red cells on blue background
Rationale: Capsule stains show a dark background with clear halos around the cells where
capsules repel stain.
Endospores appear what color in endospore staining?
a) Red
b) Green
c) Purple
d) Pink
Rationale: Malachite green stains endospores green, while vegetative cells appear red/pink
from safranin.
Acid-fast organisms appear what color in the acid-fast stain?
a) Red
b) Blue
c) Green
d) Purple
, Rationale: Acid-fast cells retain carbol fuchsin and appear red; non–acid-fast cells appear blue
with methylene blue counterstain.
What is the purpose of using the streak plate method?
a) To measure bacterial motility
b) To isolate individual bacterial colonies from a mixed culture
c) To determine bacterial oxygen requirements
d) To measure bacterial growth curves
Rationale: This method separates bacterial cells to obtain pure colonies. The other options do
not achieve isolation.
What does a positive urease test indicate?
a) Production of hydrogen sulfide
b) Breakdown of urea into ammonia and carbon dioxide
c) Fermentation of lactose
d) Reduction of nitrates to nitrites
Rationale: Urease breaks down urea into ammonia and CO₂, increasing pH. The other options
are unrelated to urease activity.
Which result indicates a positive oxidase test?
a) Yellow color change
b) Black precipitate
c) Purple or dark blue color within 10–30 seconds
d) Bubble formation
Rationale: A purple/blue color indicates cytochrome oxidase activity. Bubble formation relates
to the catalase test, not oxidase.
What is the main component of Gram-positive bacterial cell walls?
a) Lipopolysaccharide
b) Thick peptidoglycan layer
c) Mycolic acid
d) Teichoic acid only
Rationale: Gram-positive bacteria have thick peptidoglycan. LPS is in Gram-negative bacteria;
mycolic acid is in acid-fast bacteria.
Review and Practice Questions – Bachoon
Coursework
Why are pure cultures necessary for biochemical testing?
a) They save time and resources
b) Mixed cultures give more accurate results
c) They prevent skewed results caused by unwanted organisms
d) They allow faster bacterial growth
Rationale: Pure cultures ensure the test results are only from the organism being studied,
avoiding contamination. Mixed cultures can produce inaccurate or misleading results.
Why do you need to know the Gram reaction and morphology of an organism before
biochemical testing?
a) To determine the incubation temperature
b) To select the correct medium to expedite diagnosis
c) To measure enzyme activity
d) To count bacterial colonies
Rationale: Gram reaction and morphology help select the appropriate biochemical tests and
media. They do not determine temperature or colony numbers.
Which enzyme is linked to diaper rash in infants due to ammonia production?
a) Catalase
b) Amylase
c) Urease
d) Lipase
Rationale: Urease breaks down urea into ammonia, increasing skin irritation in wet diapers,
causing diaper rash. The other enzymes are not involved in urea breakdown.
,Which dye is used first in Gram staining?
a) Crystal Violet
b) Safranin
c) Methylene blue
d) Carbol fuchsin
Rationale: Crystal violet is the primary stain in Gram staining. Safranin is the counterstain used
last.
Which dye is used last in the Gram stain procedure?
a) Crystal violet
b) Gram’s iodine
c) Decolorizer
d) Safranin
Rationale: Safranin stains Gram-negative bacteria pink at the end of the process. It is applied
after decolorization.
What is the correct order of reagents in Gram staining?
a) Safranin → iodine → alcohol → crystal violet
b) Iodine → crystal violet → safranin → alcohol
c) Crystal violet → iodine → decolorizer → safranin
d) Alcohol → crystal violet → iodine → safranin
Rationale: The correct sequence ensures proper staining and differentiation of Gram-positive
and Gram-negative bacteria.
What type of media has a precise chemical composition?
a) Enriched media
b) Defined media
c) Selective media
d) Differential media
Rationale: Defined media contain known chemical quantities. Enriched, selective, and
differential media are more complex and often include unknown extracts.
What color does E. coli appear after Gram staining?
a) Purple
,b) Pink
c) Blue
d) Green
Rationale: E. coli is Gram-negative and appears pink after staining due to safranin retention.
What color does Bacillus subtilis appear under Gram stain?
a) Purple
b) Pink
c) Orange
d) Colorless
Rationale: Bacillus subtilis is Gram-positive and retains crystal violet, appearing purple.
Capsule-stained cells appear as:
a) Blue cells with dark centers
b) Black background with clear halos
c) Green rods with red spores
d) Red cells on blue background
Rationale: Capsule stains show a dark background with clear halos around the cells where
capsules repel stain.
Endospores appear what color in endospore staining?
a) Red
b) Green
c) Purple
d) Pink
Rationale: Malachite green stains endospores green, while vegetative cells appear red/pink
from safranin.
Acid-fast organisms appear what color in the acid-fast stain?
a) Red
b) Blue
c) Green
d) Purple
, Rationale: Acid-fast cells retain carbol fuchsin and appear red; non–acid-fast cells appear blue
with methylene blue counterstain.
What is the purpose of using the streak plate method?
a) To measure bacterial motility
b) To isolate individual bacterial colonies from a mixed culture
c) To determine bacterial oxygen requirements
d) To measure bacterial growth curves
Rationale: This method separates bacterial cells to obtain pure colonies. The other options do
not achieve isolation.
What does a positive urease test indicate?
a) Production of hydrogen sulfide
b) Breakdown of urea into ammonia and carbon dioxide
c) Fermentation of lactose
d) Reduction of nitrates to nitrites
Rationale: Urease breaks down urea into ammonia and CO₂, increasing pH. The other options
are unrelated to urease activity.
Which result indicates a positive oxidase test?
a) Yellow color change
b) Black precipitate
c) Purple or dark blue color within 10–30 seconds
d) Bubble formation
Rationale: A purple/blue color indicates cytochrome oxidase activity. Bubble formation relates
to the catalase test, not oxidase.
What is the main component of Gram-positive bacterial cell walls?
a) Lipopolysaccharide
b) Thick peptidoglycan layer
c) Mycolic acid
d) Teichoic acid only
Rationale: Gram-positive bacteria have thick peptidoglycan. LPS is in Gram-negative bacteria;
mycolic acid is in acid-fast bacteria.
