MCB 1000L
Oxidase Test
Virtual Lab Microbiology
Laboratory Study Guide | 2026–2027
This original 12-question practice set reviews the oxidase test, its purpose in bacterial
identification, reagent interpretation, common organisms, laboratory quality control, and virtual-
lab troubleshooting. Each question includes multiple-choice options, the correct Resolution
Result Verified Answers, and a rationale.
These are study questions for practice, not verified questions from a secured course examination.
Part 1: Practice questions and Resolution Result Verified
Answerss
Question 1. What is the primary purpose of the oxidase test?
A. To detect the production of catalase B. To detect cytochrome c oxidase activity C. To
determine whether bacteria ferment lactose D. To detect bacterial capsule formation
Correct Resolution Result Verified Answers: B. To detect cytochrome c oxidase activity
Rationale: The oxidase test detects the presence of cytochrome c oxidase, an enzyme involved in
the bacterial electron transport chain. The result helps distinguish groups of bacteria during
identification.
Question 2. Which reagent is commonly used in the oxidase test?
A. Gram's iodine B. Safranin C. Tetramethyl-p-phenylenediamine D. Methylene blue
, Correct Resolution Result Verified Answers: C. Tetramethyl-p-phenylenediamine
Rationale: This reagent acts as an artificial electron donor and changes color when oxidized. A
rapid dark-purple or deep-blue color change in the appropriate testing conditions indicates a
positive reaction.
Question 3. What indicates a positive oxidase test?
A. Formation of bubbles B. Immediate development of a dark purple or blue color C. A clear
zone around the colony D. A yellow color change
Correct Resolution Result Verified Answers: B. Immediate development of a dark purple or
blue color
Rationale: A rapid purple or blue color change indicates oxidation of the reagent by organisms
with detectable cytochrome c oxidase activity. Follow the timing specified by the laboratory
protocol because delayed color development may produce misleading results.
Question 4. Which bacterium is commonly oxidase-positive?
A. Escherichia coli B. Klebsiella pneumoniae C. Pseudomonas aeruginosa D. Shigella sonnei
Correct Resolution Result Verified Answers: C. Pseudomonas aeruginosa
Rationale: Pseudomonas aeruginosa typically produces cytochrome c oxidase and gives a
positive oxidase reaction. The other listed organisms belong to Enterobacterales, which are
generally oxidase-negative.
Microbiology Laboratory Manual
+1
Question 5. What does a negative oxidase result look like?
A. No characteristic purple-blue color develops within the specified reading time. B. The sample
immediately turns dark purple. C. Bubbles form throughout the sample. D. A red ring forms on
the surface.
Correct Resolution Result Verified Answers: A. No characteristic purple-blue color develops
within the specified reading time.
Rationale: A negative result indicates that detectable cytochrome c oxidase activity was not
demonstrated under the test conditions. It does not, by itself, identify the bacterium.
Question 6. Why must the oxidase test be read promptly?
Oxidase Test
Virtual Lab Microbiology
Laboratory Study Guide | 2026–2027
This original 12-question practice set reviews the oxidase test, its purpose in bacterial
identification, reagent interpretation, common organisms, laboratory quality control, and virtual-
lab troubleshooting. Each question includes multiple-choice options, the correct Resolution
Result Verified Answers, and a rationale.
These are study questions for practice, not verified questions from a secured course examination.
Part 1: Practice questions and Resolution Result Verified
Answerss
Question 1. What is the primary purpose of the oxidase test?
A. To detect the production of catalase B. To detect cytochrome c oxidase activity C. To
determine whether bacteria ferment lactose D. To detect bacterial capsule formation
Correct Resolution Result Verified Answers: B. To detect cytochrome c oxidase activity
Rationale: The oxidase test detects the presence of cytochrome c oxidase, an enzyme involved in
the bacterial electron transport chain. The result helps distinguish groups of bacteria during
identification.
Question 2. Which reagent is commonly used in the oxidase test?
A. Gram's iodine B. Safranin C. Tetramethyl-p-phenylenediamine D. Methylene blue
, Correct Resolution Result Verified Answers: C. Tetramethyl-p-phenylenediamine
Rationale: This reagent acts as an artificial electron donor and changes color when oxidized. A
rapid dark-purple or deep-blue color change in the appropriate testing conditions indicates a
positive reaction.
Question 3. What indicates a positive oxidase test?
A. Formation of bubbles B. Immediate development of a dark purple or blue color C. A clear
zone around the colony D. A yellow color change
Correct Resolution Result Verified Answers: B. Immediate development of a dark purple or
blue color
Rationale: A rapid purple or blue color change indicates oxidation of the reagent by organisms
with detectable cytochrome c oxidase activity. Follow the timing specified by the laboratory
protocol because delayed color development may produce misleading results.
Question 4. Which bacterium is commonly oxidase-positive?
A. Escherichia coli B. Klebsiella pneumoniae C. Pseudomonas aeruginosa D. Shigella sonnei
Correct Resolution Result Verified Answers: C. Pseudomonas aeruginosa
Rationale: Pseudomonas aeruginosa typically produces cytochrome c oxidase and gives a
positive oxidase reaction. The other listed organisms belong to Enterobacterales, which are
generally oxidase-negative.
Microbiology Laboratory Manual
+1
Question 5. What does a negative oxidase result look like?
A. No characteristic purple-blue color develops within the specified reading time. B. The sample
immediately turns dark purple. C. Bubbles form throughout the sample. D. A red ring forms on
the surface.
Correct Resolution Result Verified Answers: A. No characteristic purple-blue color develops
within the specified reading time.
Rationale: A negative result indicates that detectable cytochrome c oxidase activity was not
demonstrated under the test conditions. It does not, by itself, identify the bacterium.
Question 6. Why must the oxidase test be read promptly?
