MCB 1000L Lab Quiz 2 - Weeks 1-6 Catalase
MR-VP - Requires Respondus LockDown
Browser | Questions and Answers |
2026/2027 Updated | 100% Correct
Practice Question Bank & Answer Rationale
THIS PACK CONTAINS
150 exam-style multiple-choice questions. Each question includes the correct answer and a complete
rationale for focused revision.
COVERAGE
This laboratory quiz assesses students' mastery of core biochemical and physiological tests used in microbial
identification, with emphasis on the catalase test, methyl red and Voges-Proskauer (MR-VP) tests, and
related week 1-6 lab techniques including aseptic technique, streak plating, and differential media
interpretation. Students must apply principles of enzyme activity, fermentation pathways, and reagent
chemistry to interpret experimental results and troubleshoot common procedural errors.
STUDY GUIDE
Recommended duration: 60 minutes. Passing target: 70%.
HIGHP - Page 1 of 48
,1. After adding 3% hydrogen peroxide to a bacterial smear, vigorous bubbling occurs.
Which enzyme is directly responsible for this observation?
[ ] A. Catalase
[ ] B. Oxidase
[ ] C. Peroxidase
[ ] D. Superoxide dismutase
CORRECT: A. Catalase
Catalase catalyzes the decomposition of hydrogen peroxide into water and oxygen gas, producing the
observed bubbling. Oxidase and peroxidase act on different substrates, and superoxide dismutase detoxifies
superoxide radicals, not peroxide.
2. In the MR-VP test, a red color after adding Barritt's reagents indicates a positive result
for which metabolic pathway?
[ ] A. Mixed acid fermentation
[ ] B. 2,3-butanediol fermentation
[ ] C. Butanediol fermentation
[ ] D. Lactic acid fermentation
CORRECT: B. 2,3-butanediol fermentation
Barritt's reagents detect acetoin, a precursor of 2,3-butanediol, which is produced in the butanediol
fermentation pathway. Mixed acid fermentation is detected by the methyl red test, and lactic acid fermentation
does not produce acetoin.
3. A student streaks a plate for isolation but after incubation observes confluent growth
with no isolated colonies. Which is the most likely explanation?
[ ] A. The loop was flamed between each quadrant.
[ ] B. The plate was incubated at the wrong temperature.
[ ] C. The loop was not flamed between quadrants.
[ ] D. The inoculum was too dilute.
CORRECT: C. The loop was not flamed between quadrants.
Failure to flame the loop between quadrants carries over excess inoculum, preventing thinning and isolation
of colonies. Flaming between quadrants and using a proper inoculum concentration promote isolated
colonies.
4. Which of the following best explains why the catalase test is performed on colonies
from a TSA plate rather than from a blood agar plate?
[ ] A. Blood agar contains catalase that can cause false-positive results.
[ ] B. TSA inhibits catalase production.
HIGHP - Page 2 of 48
, [ ] C. Blood agar enhances catalase activity.
[ ] D. TSA contains hydrogen peroxide.
CORRECT: A. Blood agar contains catalase that can cause false-positive results.
Blood agar contains catalase from red blood cells, which can generate bubbles and cause a false-positive
catalase test. TSA is a general-purpose medium free of catalase, making it suitable for this test.
5. In the Voges-Proskauer test, which reagent is added first to the broth culture?
[ ] A. Barritt's reagent A (alpha-naphthol)
[ ] B. Barritt's reagent B (40% KOH)
[ ] C. Methyl red
[ ] D. Kovac's reagent
CORRECT: A. Barritt's reagent A (alpha-naphthol)
Barritt's reagent A (alpha-naphthol) is added first, followed by reagent B (KOH). This order is critical because
alpha-naphthol enhances the color reaction, and reversing the order can lead to false results. Methyl red and
Kovac's reagent are used in different tests.
6. Which bacterial species is expected to give a positive methyl red test and a negative
Voges-Proskauer test?
[ ] A. Enterobacter aerogenes
[ ] B. Escherichia coli
[ ] C. Klebsiella pneumoniae
[ ] D. Serratia marcescens
CORRECT: B. Escherichia coli
Escherichia coli carries out mixed acid fermentation, producing stable acids that turn methyl red red, while
acetoin is not produced, yielding a negative VP test. Enterobacter, Klebsiella, and Serratia typically produce
acetoin and are VP-positive, MR-negative.
7. When performing a catalase test, a student observes bubbling after adding hydrogen
peroxide to a colony from a blood agar plate. What is the most appropriate interpretation?
[ ] A. The organism is catalase-positive.
[ ] B. The organism is catalase-negative.
[ ] C. The result is invalid due to possible false positive from blood agar.
[ ] D. The organism is oxidase-positive.
CORRECT: C. The result is invalid due to possible false positive from blood agar.
Blood agar contains catalase from red blood cells, which can cause bubbling independent of the organism's
catalase activity, making the result unreliable. A subculture on TSA should be used to confirm the result.
HIGHP - Page 3 of 48
, 8. Which of the following is the primary purpose of the methyl red test?
[ ] A. To detect acetoin production
[ ] B. To detect mixed acid fermentation
[ ] C. To detect indole production
[ ] D. To detect citrate utilization
CORRECT: B. To detect mixed acid fermentation
The methyl red test detects the production of stable acids from glucose fermentation, characteristic of mixed
acid fermentation. Acetoin production is detected by the VP test, indole by the indole test, and citrate
utilization by the citrate test.
9. A student performs a VP test and observes a red color after 15 minutes. What does this
indicate?
[ ] A. The organism produces acetoin.
[ ] B. The organism produces mixed acids.
[ ] C. The organism produces indole.
[ ] D. The organism produces hydrogen sulfide.
CORRECT: A. The organism produces acetoin.
A red color in the VP test indicates the presence of acetoin, which is converted to a red complex by Barritt's
reagents. Mixed acids are detected by the MR test, indole by the indole test, and H2S by SIM medium.
10. Which of the following is a key difference between the MR and VP tests?
[ ] A. MR test detects acetoin; VP test detects acids.
[ ] B. MR test uses Barritt's reagents; VP test uses methyl red.
[ ] C. MR test detects acids; VP test detects acetoin.
[ ] D. MR test requires anaerobic incubation; VP test requires aerobic incubation.
CORRECT: C. MR test detects acids; VP test detects acetoin.
The MR test detects stable acids produced during mixed acid fermentation, while the VP test detects acetoin,
a neutral end product. The reagents and pathways differ, but both tests use the same MR-VP broth and are
typically incubated aerobically.
11. A student performs the catalase test on an unknown organism and observes no
bubbles after adding 3% HO. Which organism is most consistent with this result?
[ ] A. Staphylococcus aureus
[ ] B. Streptococcus pyogenes
[ ] C. Bacillus subtilis
[ ] D. Micrococcus luteus
HIGHP - Page 4 of 48
MR-VP - Requires Respondus LockDown
Browser | Questions and Answers |
2026/2027 Updated | 100% Correct
Practice Question Bank & Answer Rationale
THIS PACK CONTAINS
150 exam-style multiple-choice questions. Each question includes the correct answer and a complete
rationale for focused revision.
COVERAGE
This laboratory quiz assesses students' mastery of core biochemical and physiological tests used in microbial
identification, with emphasis on the catalase test, methyl red and Voges-Proskauer (MR-VP) tests, and
related week 1-6 lab techniques including aseptic technique, streak plating, and differential media
interpretation. Students must apply principles of enzyme activity, fermentation pathways, and reagent
chemistry to interpret experimental results and troubleshoot common procedural errors.
STUDY GUIDE
Recommended duration: 60 minutes. Passing target: 70%.
HIGHP - Page 1 of 48
,1. After adding 3% hydrogen peroxide to a bacterial smear, vigorous bubbling occurs.
Which enzyme is directly responsible for this observation?
[ ] A. Catalase
[ ] B. Oxidase
[ ] C. Peroxidase
[ ] D. Superoxide dismutase
CORRECT: A. Catalase
Catalase catalyzes the decomposition of hydrogen peroxide into water and oxygen gas, producing the
observed bubbling. Oxidase and peroxidase act on different substrates, and superoxide dismutase detoxifies
superoxide radicals, not peroxide.
2. In the MR-VP test, a red color after adding Barritt's reagents indicates a positive result
for which metabolic pathway?
[ ] A. Mixed acid fermentation
[ ] B. 2,3-butanediol fermentation
[ ] C. Butanediol fermentation
[ ] D. Lactic acid fermentation
CORRECT: B. 2,3-butanediol fermentation
Barritt's reagents detect acetoin, a precursor of 2,3-butanediol, which is produced in the butanediol
fermentation pathway. Mixed acid fermentation is detected by the methyl red test, and lactic acid fermentation
does not produce acetoin.
3. A student streaks a plate for isolation but after incubation observes confluent growth
with no isolated colonies. Which is the most likely explanation?
[ ] A. The loop was flamed between each quadrant.
[ ] B. The plate was incubated at the wrong temperature.
[ ] C. The loop was not flamed between quadrants.
[ ] D. The inoculum was too dilute.
CORRECT: C. The loop was not flamed between quadrants.
Failure to flame the loop between quadrants carries over excess inoculum, preventing thinning and isolation
of colonies. Flaming between quadrants and using a proper inoculum concentration promote isolated
colonies.
4. Which of the following best explains why the catalase test is performed on colonies
from a TSA plate rather than from a blood agar plate?
[ ] A. Blood agar contains catalase that can cause false-positive results.
[ ] B. TSA inhibits catalase production.
HIGHP - Page 2 of 48
, [ ] C. Blood agar enhances catalase activity.
[ ] D. TSA contains hydrogen peroxide.
CORRECT: A. Blood agar contains catalase that can cause false-positive results.
Blood agar contains catalase from red blood cells, which can generate bubbles and cause a false-positive
catalase test. TSA is a general-purpose medium free of catalase, making it suitable for this test.
5. In the Voges-Proskauer test, which reagent is added first to the broth culture?
[ ] A. Barritt's reagent A (alpha-naphthol)
[ ] B. Barritt's reagent B (40% KOH)
[ ] C. Methyl red
[ ] D. Kovac's reagent
CORRECT: A. Barritt's reagent A (alpha-naphthol)
Barritt's reagent A (alpha-naphthol) is added first, followed by reagent B (KOH). This order is critical because
alpha-naphthol enhances the color reaction, and reversing the order can lead to false results. Methyl red and
Kovac's reagent are used in different tests.
6. Which bacterial species is expected to give a positive methyl red test and a negative
Voges-Proskauer test?
[ ] A. Enterobacter aerogenes
[ ] B. Escherichia coli
[ ] C. Klebsiella pneumoniae
[ ] D. Serratia marcescens
CORRECT: B. Escherichia coli
Escherichia coli carries out mixed acid fermentation, producing stable acids that turn methyl red red, while
acetoin is not produced, yielding a negative VP test. Enterobacter, Klebsiella, and Serratia typically produce
acetoin and are VP-positive, MR-negative.
7. When performing a catalase test, a student observes bubbling after adding hydrogen
peroxide to a colony from a blood agar plate. What is the most appropriate interpretation?
[ ] A. The organism is catalase-positive.
[ ] B. The organism is catalase-negative.
[ ] C. The result is invalid due to possible false positive from blood agar.
[ ] D. The organism is oxidase-positive.
CORRECT: C. The result is invalid due to possible false positive from blood agar.
Blood agar contains catalase from red blood cells, which can cause bubbling independent of the organism's
catalase activity, making the result unreliable. A subculture on TSA should be used to confirm the result.
HIGHP - Page 3 of 48
, 8. Which of the following is the primary purpose of the methyl red test?
[ ] A. To detect acetoin production
[ ] B. To detect mixed acid fermentation
[ ] C. To detect indole production
[ ] D. To detect citrate utilization
CORRECT: B. To detect mixed acid fermentation
The methyl red test detects the production of stable acids from glucose fermentation, characteristic of mixed
acid fermentation. Acetoin production is detected by the VP test, indole by the indole test, and citrate
utilization by the citrate test.
9. A student performs a VP test and observes a red color after 15 minutes. What does this
indicate?
[ ] A. The organism produces acetoin.
[ ] B. The organism produces mixed acids.
[ ] C. The organism produces indole.
[ ] D. The organism produces hydrogen sulfide.
CORRECT: A. The organism produces acetoin.
A red color in the VP test indicates the presence of acetoin, which is converted to a red complex by Barritt's
reagents. Mixed acids are detected by the MR test, indole by the indole test, and H2S by SIM medium.
10. Which of the following is a key difference between the MR and VP tests?
[ ] A. MR test detects acetoin; VP test detects acids.
[ ] B. MR test uses Barritt's reagents; VP test uses methyl red.
[ ] C. MR test detects acids; VP test detects acetoin.
[ ] D. MR test requires anaerobic incubation; VP test requires aerobic incubation.
CORRECT: C. MR test detects acids; VP test detects acetoin.
The MR test detects stable acids produced during mixed acid fermentation, while the VP test detects acetoin,
a neutral end product. The reagents and pathways differ, but both tests use the same MR-VP broth and are
typically incubated aerobically.
11. A student performs the catalase test on an unknown organism and observes no
bubbles after adding 3% HO. Which organism is most consistent with this result?
[ ] A. Staphylococcus aureus
[ ] B. Streptococcus pyogenes
[ ] C. Bacillus subtilis
[ ] D. Micrococcus luteus
HIGHP - Page 4 of 48
