MCB 1000L Oxidase Test Virtual Lab | 12
Questions & Answers | Microbiology
Laboratory Study Guide | 2026/2027
Updated
The following questions continue the original practice set, with multiple-choice questions,
correct Resolution Result Verified Answerss, and rationales. They cover bacterial identification,
reagent chemistry, laboratory observations, controls, experimental errors, and interpretation of
results.
Important: Follow the reaction-time instructions for your specific test format. Some protocols
require reading results within 10–20 seconds, while others allow a different interval. Delayed
color changes can result from spontaneous oxidation.
Microbiology Laboratory Manual
+2
Section 2: Test principles and bacterial identification
Question 13
Which substance serves as the terminal electron acceptor during aerobic respiration?
A. Glucose B. Oxygen C. Nitrogen D. Carbon dioxide
Correct Resolution Result Verified Answers: B. Oxygen
Rationale: During aerobic respiration, oxygen accepts electrons at the end of the electron
transport chain. Cytochrome c oxidase facilitates this process in organisms that possess the
enzyme.
Question 14
What is the main purpose of using an oxidase reagent?
,A. To stain the bacterial cell wall B. To reveal cytochrome c oxidase activity through a color
reaction C. To measure the organism's size D. To determine the organism's shape
Correct Resolution Result Verified Answers: B. To reveal cytochrome c oxidase activity
through a color reaction
Rationale: The reagent acts as an artificial electron donor. Its oxidation produces the
characteristic blue-purple color associated with a positive result.
Question 15
Which organism is typically oxidase-negative?
A. Pseudomonas aeruginosa B. Vibrio cholerae C. Escherichia coli D. Neisseria species
Correct Resolution Result Verified Answers: C. Escherichia coli
Rationale: E. coli is typically oxidase-negative, whereas Pseudomonas aeruginosa, Vibrio
cholerae, and many Neisseria species are oxidase-positive. These patterns help differentiate
bacteria but do not independently establish species identity.
Question 16
What does the abbreviation ETS mean in microbiology?
A. Enzyme transfer solution B. Electron transport system C. External testing sample D. Energy
transfer stain
Correct Resolution Result Verified Answers: B. Electron transport system
Rationale: The electron transport system consists of components that transfer electrons and
contribute to energy conservation. Cytochrome c oxidase is one component of certain respiratory
electron transport chains.
Question 17
Which observation most strongly supports an oxidase-positive result?
A. Immediate bubbling B. A blue-purple color developing within the prescribed interval C. A
yellow colony after incubation D. A clear zone around the colony
Correct Resolution Result Verified Answers: B. A blue-purple color developing within the
prescribed interval
, Rationale: The defining observation is the appropriate color change within the test's specified
reading window. Bubbling is associated with a different test principle, while colony color and
clear zones are not the defining oxidase reaction.
Question 18
Why is the oxidase test considered a biochemical test?
A. It measures a biological activity involving an enzyme B. It measures only the physical size of
bacteria C. It identifies bacterial shape using magnification D. It counts colonies without
chemical reactions
Correct Resolution Result Verified Answers: A. It measures a biological activity involving an
enzyme
Rationale: Biochemical tests detect metabolic activities, enzymes, or chemical products of
organisms. The oxidase test detects the relevant enzyme activity through a chemical indicator
reaction.
Question 19
Which pair is correctly matched?
A. E. coli — typically oxidase-positive B. P. aeruginosa — typically oxidase-negative C. E. coli
— typically oxidase-negative D. V. cholerae — always oxidase-negative
Correct Resolution Result Verified Answers: C. E. coli — typically oxidase-negative
Rationale: E. coli is a common example of an oxidase-negative organism. P. aeruginosa and V.
cholerae are typically oxidase-positive.
Question 20
What does a positive oxidase result suggest about a bacterial isolate?
A. It has detectable cytochrome c oxidase activity under the test conditions B. It must be a Gram-
positive bacterium C. It necessarily causes disease D. It is resistant to all antibiotics
Correct Resolution Result Verified Answers: A. It has detectable cytochrome c oxidase activity
under the test conditions
Rationale: The result provides information about an enzyme, not the organism's Gram
classification, pathogenicity, or antibiotic susceptibility.
Question 21
Questions & Answers | Microbiology
Laboratory Study Guide | 2026/2027
Updated
The following questions continue the original practice set, with multiple-choice questions,
correct Resolution Result Verified Answerss, and rationales. They cover bacterial identification,
reagent chemistry, laboratory observations, controls, experimental errors, and interpretation of
results.
Important: Follow the reaction-time instructions for your specific test format. Some protocols
require reading results within 10–20 seconds, while others allow a different interval. Delayed
color changes can result from spontaneous oxidation.
Microbiology Laboratory Manual
+2
Section 2: Test principles and bacterial identification
Question 13
Which substance serves as the terminal electron acceptor during aerobic respiration?
A. Glucose B. Oxygen C. Nitrogen D. Carbon dioxide
Correct Resolution Result Verified Answers: B. Oxygen
Rationale: During aerobic respiration, oxygen accepts electrons at the end of the electron
transport chain. Cytochrome c oxidase facilitates this process in organisms that possess the
enzyme.
Question 14
What is the main purpose of using an oxidase reagent?
,A. To stain the bacterial cell wall B. To reveal cytochrome c oxidase activity through a color
reaction C. To measure the organism's size D. To determine the organism's shape
Correct Resolution Result Verified Answers: B. To reveal cytochrome c oxidase activity
through a color reaction
Rationale: The reagent acts as an artificial electron donor. Its oxidation produces the
characteristic blue-purple color associated with a positive result.
Question 15
Which organism is typically oxidase-negative?
A. Pseudomonas aeruginosa B. Vibrio cholerae C. Escherichia coli D. Neisseria species
Correct Resolution Result Verified Answers: C. Escherichia coli
Rationale: E. coli is typically oxidase-negative, whereas Pseudomonas aeruginosa, Vibrio
cholerae, and many Neisseria species are oxidase-positive. These patterns help differentiate
bacteria but do not independently establish species identity.
Question 16
What does the abbreviation ETS mean in microbiology?
A. Enzyme transfer solution B. Electron transport system C. External testing sample D. Energy
transfer stain
Correct Resolution Result Verified Answers: B. Electron transport system
Rationale: The electron transport system consists of components that transfer electrons and
contribute to energy conservation. Cytochrome c oxidase is one component of certain respiratory
electron transport chains.
Question 17
Which observation most strongly supports an oxidase-positive result?
A. Immediate bubbling B. A blue-purple color developing within the prescribed interval C. A
yellow colony after incubation D. A clear zone around the colony
Correct Resolution Result Verified Answers: B. A blue-purple color developing within the
prescribed interval
, Rationale: The defining observation is the appropriate color change within the test's specified
reading window. Bubbling is associated with a different test principle, while colony color and
clear zones are not the defining oxidase reaction.
Question 18
Why is the oxidase test considered a biochemical test?
A. It measures a biological activity involving an enzyme B. It measures only the physical size of
bacteria C. It identifies bacterial shape using magnification D. It counts colonies without
chemical reactions
Correct Resolution Result Verified Answers: A. It measures a biological activity involving an
enzyme
Rationale: Biochemical tests detect metabolic activities, enzymes, or chemical products of
organisms. The oxidase test detects the relevant enzyme activity through a chemical indicator
reaction.
Question 19
Which pair is correctly matched?
A. E. coli — typically oxidase-positive B. P. aeruginosa — typically oxidase-negative C. E. coli
— typically oxidase-negative D. V. cholerae — always oxidase-negative
Correct Resolution Result Verified Answers: C. E. coli — typically oxidase-negative
Rationale: E. coli is a common example of an oxidase-negative organism. P. aeruginosa and V.
cholerae are typically oxidase-positive.
Question 20
What does a positive oxidase result suggest about a bacterial isolate?
A. It has detectable cytochrome c oxidase activity under the test conditions B. It must be a Gram-
positive bacterium C. It necessarily causes disease D. It is resistant to all antibiotics
Correct Resolution Result Verified Answers: A. It has detectable cytochrome c oxidase activity
under the test conditions
Rationale: The result provides information about an enzyme, not the organism's Gram
classification, pathogenicity, or antibiotic susceptibility.
Question 21
