MCB 1000L Oxidase Test Virtual Lab —
Questions Original Practice Questions &
Answers | 2026–2027
The following questions continue the original practice set, with multiple-choice questions, correct
answers, 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 answer: 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 answer: 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 answer: 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 answer: 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 answer: 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 answer: 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 answer: 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 answer: 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
Which statement about oxidase-negative bacteria is correct?
A. They cannot use any form of cellular respiration. B. They do not produce detectable cytochrome c
oxidase activity in the test. C. They are all anaerobic. D. They cannot grow on laboratory media.
Correct answer: B. They do not produce detectable cytochrome c oxidase activity in the test.
Rationale: A negative oxidase result does not mean an organism lacks all respiratory pathways. Bacteria
can use other electron transport components and metabolic strategies.
Question 22
Why should an oxidase test be combined with other identification methods?
A. The test identifies only the shape of bacteria. B. Many different organisms can share the same oxidase
result. C. The test cannot produce any interpretable results. D. All bacteria have identical biochemical
characteristics.
Correct answer: B. Many different organisms can share the same oxidase result.
Rationale: A positive or negative result narrows the possibilities but does not uniquely identify most
organisms. Additional biochemical tests, microscopy, or molecular methods may be needed.
Section 3: Reagents, equipment, and laboratory technique
Question 23
Which chemical is commonly used in the Kovács oxidase reagent?
A. Tetramethyl-p-phenylenediamine B. Sodium chloride alone C. Hydrogen peroxide D. Iodine solution
Correct answer: A. Tetramethyl-p-phenylenediamine
Rationale: This chemical functions as a chromogenic reducing agent that changes color when oxidized.
Hydrogen peroxide is commonly associated with the catalase test.
Question 24
Questions Original Practice Questions &
Answers | 2026–2027
The following questions continue the original practice set, with multiple-choice questions, correct
answers, 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 answer: 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 answer: 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 answer: 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 answer: 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 answer: 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 answer: 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 answer: 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 answer: 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
Which statement about oxidase-negative bacteria is correct?
A. They cannot use any form of cellular respiration. B. They do not produce detectable cytochrome c
oxidase activity in the test. C. They are all anaerobic. D. They cannot grow on laboratory media.
Correct answer: B. They do not produce detectable cytochrome c oxidase activity in the test.
Rationale: A negative oxidase result does not mean an organism lacks all respiratory pathways. Bacteria
can use other electron transport components and metabolic strategies.
Question 22
Why should an oxidase test be combined with other identification methods?
A. The test identifies only the shape of bacteria. B. Many different organisms can share the same oxidase
result. C. The test cannot produce any interpretable results. D. All bacteria have identical biochemical
characteristics.
Correct answer: B. Many different organisms can share the same oxidase result.
Rationale: A positive or negative result narrows the possibilities but does not uniquely identify most
organisms. Additional biochemical tests, microscopy, or molecular methods may be needed.
Section 3: Reagents, equipment, and laboratory technique
Question 23
Which chemical is commonly used in the Kovács oxidase reagent?
A. Tetramethyl-p-phenylenediamine B. Sodium chloride alone C. Hydrogen peroxide D. Iodine solution
Correct answer: A. Tetramethyl-p-phenylenediamine
Rationale: This chemical functions as a chromogenic reducing agent that changes color when oxidized.
Hydrogen peroxide is commonly associated with the catalase test.
Question 24
