MCB 1000L Oxidase Test Virtual Lab
Microbiology Laboratory Study Guide | 12
Original Practice Questions & Answers |
2026–2027
This study guide reviews the oxidase test commonly taught in introductory microbiology
laboratory courses. It includes multiple-choice questions, correct answers, and explanations to
help you prepare for MCB 1000L virtual labs, quizzes, and practical assessments.
Learning objectives
By the end of this guide, you should be able to:
• Explain the purpose of the oxidase test.
• Identify the enzyme detected by the test.
• Interpret positive and negative color reactions.
• Understand the test's role in bacterial identification.
• Recognize common testing errors and limitations.
12 practice questions with answers and rationales
Question 1: What is the primary purpose of the oxidase test?
A. To determine whether bacteria produce catalase B. To detect cytochrome c oxidase activity C.
To determine whether bacteria ferment lactose D. To measure bacterial motility
Correct answer: B. To detect cytochrome c oxidase activity
Rationale: The oxidase test detects cytochrome c oxidase, an enzyme involved in the bacterial
electron transport chain. The result helps differentiate bacterial groups during identification.
ATCC
+1
Question 2: Which chemical is commonly used as the oxidase reagent?
, A. Hydrogen peroxide B. Crystal violet C. Tetramethyl-p-phenylenediamine D. Methyl red
Correct answer: C. Tetramethyl-p-phenylenediamine
Rationale: This reagent acts as an artificial electron donor and develops a dark blue or purple
color when oxidized during a positive reaction. Hydrogen peroxide is used in the catalase test,
while crystal violet is a Gram-stain reagent.
ATCC
+1
Question 3: What color indicates a positive oxidase test?
A. Yellow B. Green C. Dark purple or blue D. Bright red
Correct answer: C. Dark purple or blue
Rationale: A rapid dark purple or blue color change indicates an oxidase-positive reaction. The
test must be interpreted within the time specified by the laboratory protocol, commonly about
10–30 seconds.
Microbiology Laboratory Manual
+1
Question 4: What does a negative oxidase test usually look like?
A. No qualifying color change within the specified time B. Immediate dark purple coloration C.
Vigorous bubbling D. Formation of a yellow precipitate
Correct answer: A. No qualifying color change within the specified time
Rationale: A negative result means the organism did not produce the expected rapid color
reaction under the test conditions. It does not change the reagent to the characteristic dark purple
or blue color within the specified reading window.
cwoer.ccbcmd.edu
+1
Question 5: Which bacterium is commonly oxidase-positive?
A. Escherichia coli B. Pseudomonas aeruginosa C. Proteus mirabilis D. Salmonella enterica
Microbiology Laboratory Study Guide | 12
Original Practice Questions & Answers |
2026–2027
This study guide reviews the oxidase test commonly taught in introductory microbiology
laboratory courses. It includes multiple-choice questions, correct answers, and explanations to
help you prepare for MCB 1000L virtual labs, quizzes, and practical assessments.
Learning objectives
By the end of this guide, you should be able to:
• Explain the purpose of the oxidase test.
• Identify the enzyme detected by the test.
• Interpret positive and negative color reactions.
• Understand the test's role in bacterial identification.
• Recognize common testing errors and limitations.
12 practice questions with answers and rationales
Question 1: What is the primary purpose of the oxidase test?
A. To determine whether bacteria produce catalase B. To detect cytochrome c oxidase activity C.
To determine whether bacteria ferment lactose D. To measure bacterial motility
Correct answer: B. To detect cytochrome c oxidase activity
Rationale: The oxidase test detects cytochrome c oxidase, an enzyme involved in the bacterial
electron transport chain. The result helps differentiate bacterial groups during identification.
ATCC
+1
Question 2: Which chemical is commonly used as the oxidase reagent?
, A. Hydrogen peroxide B. Crystal violet C. Tetramethyl-p-phenylenediamine D. Methyl red
Correct answer: C. Tetramethyl-p-phenylenediamine
Rationale: This reagent acts as an artificial electron donor and develops a dark blue or purple
color when oxidized during a positive reaction. Hydrogen peroxide is used in the catalase test,
while crystal violet is a Gram-stain reagent.
ATCC
+1
Question 3: What color indicates a positive oxidase test?
A. Yellow B. Green C. Dark purple or blue D. Bright red
Correct answer: C. Dark purple or blue
Rationale: A rapid dark purple or blue color change indicates an oxidase-positive reaction. The
test must be interpreted within the time specified by the laboratory protocol, commonly about
10–30 seconds.
Microbiology Laboratory Manual
+1
Question 4: What does a negative oxidase test usually look like?
A. No qualifying color change within the specified time B. Immediate dark purple coloration C.
Vigorous bubbling D. Formation of a yellow precipitate
Correct answer: A. No qualifying color change within the specified time
Rationale: A negative result means the organism did not produce the expected rapid color
reaction under the test conditions. It does not change the reagent to the characteristic dark purple
or blue color within the specified reading window.
cwoer.ccbcmd.edu
+1
Question 5: Which bacterium is commonly oxidase-positive?
A. Escherichia coli B. Pseudomonas aeruginosa C. Proteus mirabilis D. Salmonella enterica
