MCB 1000L Oxidase Test Virtual Lab | and
Answers | Microbiology Laboratory |
2026/2027 Updated | 100% Correct
Practice Question Bank & Answer Rationale
THIS PACK CONTAINS
96 exam-style multiple-choice questions. Each question includes the correct answer and a complete rationale
for focused revision.
COVERAGE
This exam assesses mastery of the cytochrome c oxidase (oxidase) test as performed in a virtual
microbiology laboratory, integrating the biochemical basis of the test, reagent chemistry, procedural technique,
interpretation, quality control, and clinical/ecological applications. Questions span the full laboratory workflow
from enzyme mechanism to organism identification and troubleshooting.
STUDY GUIDE
Recommended duration: 2 hours. Passing target: 70%.
HIGHP - Page 1 of 32
,1. A bacterium isolated from a wound is catalase-positive and oxidase-negative. Which
organism is most consistent with these results?
[ ] A. Pseudomonas aeruginosa
[ ] B. Neisseria gonorrhoeae
[ ] C. Staphylococcus aureus
[ ] D. Campylobacter jejuni
CORRECT: C. Staphylococcus aureus
S. aureus is catalase-positive (staphylococci) and oxidase-negative, matching both results. P. aeruginosa,
Neisseria, and Campylobacter are all oxidase-positive, so they would not fit an oxidase-negative profile.
2. In the oxidase test, the reagent N,N,N',N'-tetramethyl-p-phenylenediamine
dihydrochloride functions primarily as which of the following?
[ ] A. A terminal electron acceptor for cytochrome c oxidase
[ ] B. An artificial electron donor that is oxidized to a colored product
[ ] C. A chelator that removes iron from cytochrome c
[ ] D. A substrate that is hydrolyzed by the enzyme to release a chromophore
CORRECT: B. An artificial electron donor that is oxidized to a colored product
The reagent acts as an artificial electron donor; when cytochrome c oxidase is present, it is oxidized to a
colored (purple/blue) product. It is not a terminal acceptor, chelator, or hydrolyzed substrate, which
distinguishes it from chromogenic substrate tests like ONPG or esculin.
3. A student reads an oxidase test strip at 45 minutes and observes a faint purple color.
How should this result be interpreted?
[ ] A. Positive; the enzyme is present but slow
[ ] B. Negative; color development after 30 seconds is not diagnostic
[ ] C. Invalid; the reagent has degraded
[ ] D. Positive; any color change indicates oxidase activity
CORRECT: B. Negative; color development after 30 seconds is not diagnostic
Oxidase reactions must be read within 10-30 seconds; color appearing after 30 seconds is considered
negative because it reflects non-enzymatic auto-oxidation of the reagent. Interpreting late color as positive is
a common error that can misidentify organisms.
4. Which of the following best explains why the oxidase test is especially useful in the
presumptive identification of Neisseria species from clinical specimens?
[ ] A. Neisseria are the only oxidase-positive gram-negative diplococci
[ ] B. Oxidase positivity distinguishes Neisseria from most gram-positive cocci and many gram-negative
bacilli
HIGHP - Page 2 of 32
, [ ] C. Oxidase-negative Neisseria are always nonpathogenic
[ ] D. The test directly detects the presence of the Neisseria capsule
CORRECT: B. Oxidase positivity distinguishes Neisseria from most gram-positive cocci and many
gram-negative bacilli
Neisseria are oxidase-positive, which helps separate them from oxidase-negative gram-positive cocci and
many gram-negative bacilli. However, other organisms can also be oxidase-positive, so it is presumptive, not
definitive, and the test does not detect capsules.
5. When performing the oxidase test using a filter paper method, which of the following is
the most appropriate way to obtain the bacterial sample?
[ ] A. Use a nichrome loop to pick colonies from blood agar
[ ] B. Use a wooden applicator stick to pick colonies from tryptic soy agar
[ ] C. Use a platinum loop to pick colonies from MacConkey agar
[ ] D. Use a plastic loop to pick colonies from chocolate agar
CORRECT: D. Use a plastic loop to pick colonies from chocolate agar
Plastic or wooden applicators should be used because metal loops (especially nichrome or iron-containing)
can catalyze false-positive reactions. Chocolate agar is acceptable, but the key is avoiding metal; blood agar
can also interfere, so plastic is preferred.
6. In a virtual oxidase test, a student observes a deep blue color within 5 seconds using a
filter paper method. Which of the following organisms is most likely responsible?
[ ] A. Escherichia coli
[ ] B. Pseudomonas aeruginosa
[ ] C. Staphylococcus epidermidis
[ ] D. Enterococcus faecalis
CORRECT: B. Pseudomonas aeruginosa
P. aeruginosa is oxidase-positive, producing a rapid blue color. E. coli, S. epidermidis, and E. faecalis are all
oxidase-negative, so they would not yield a positive reaction.
7. A quality control program for the oxidase test should include which pair of organisms?
[ ] A. E. coli (positive) and P. aeruginosa (negative)
[ ] B. P. aeruginosa (positive) and E. coli (negative)
[ ] C. S. aureus (positive) and S. epidermidis (negative)
[ ] D. Bacillus subtilis (positive) and E. faecalis (negative)
CORRECT: B. P. aeruginosa (positive) and E. coli (negative)
P. aeruginosa is oxidase-positive and E. coli is oxidase-negative, making them ideal QC organisms to verify
HIGHP - Page 3 of 32
, both positive and negative reactions. The other pairs either reverse the expected results or use organisms
that are both oxidase-negative.
8. Which of the following statements about the oxidase test and bacterial respiration is
correct?
[ ] A. Oxidase-positive bacteria lack cytochrome c oxidase but possess alternative oxidases
[ ] B. The test detects cytochrome c oxidase, which is part of the electron transport chain in many
aerobic bacteria
[ ] C. Oxidase-negative bacteria cannot perform aerobic respiration
[ ] D. The test is specific for anaerobic bacteria that use nitrate as a terminal electron acceptor
CORRECT: B. The test detects cytochrome c oxidase, which is part of the electron transport chain in
many aerobic bacteria
The oxidase test detects cytochrome c oxidase, a component of the electron transport chain in many aerobic
organisms. Oxidase-negative bacteria may still respire aerobically using other terminal oxidases, and the test
is not specific for anaerobes.
9. A student performs the oxidase test on a colony from MacConkey agar and obtains a
positive result. What is the most likely explanation?
[ ] A. The organism is a lactose fermenter
[ ] B. The organism is oxidase-positive, such as Pseudomonas
[ ] C. The result is a false positive due to the medium's pH indicator
[ ] D. The organism is a non-fermenter, which is always oxidase-positive
CORRECT: B. The organism is oxidase-positive, such as Pseudomonas
MacConkey agar can support growth of oxidase-positive organisms like Pseudomonas, which would yield a
true positive. The medium's pH indicator does not cause false oxidase positives, and not all non-fermenters
are oxidase-positive (e.g., Acinetobacter is oxidase-negative).
10. Which of the following best describes the expected oxidase result for Campylobacter
jejuni and its clinical significance?
[ ] A. Oxidase-negative; helps distinguish it from Salmonella
[ ] B. Oxidase-positive; supports its identification as a microaerophilic, curved gram-negative rod
[ ] C. Oxidase-positive; confirms it is a facultative anaerobe
[ ] D. Oxidase-negative; indicates it is a non-motile organism
CORRECT: B. Oxidase-positive; supports its identification as a microaerophilic, curved
gram-negative rod
C. jejuni is oxidase-positive, which is consistent with its classification as a microaerophilic, curved
HIGHP - Page 4 of 32
Answers | Microbiology Laboratory |
2026/2027 Updated | 100% Correct
Practice Question Bank & Answer Rationale
THIS PACK CONTAINS
96 exam-style multiple-choice questions. Each question includes the correct answer and a complete rationale
for focused revision.
COVERAGE
This exam assesses mastery of the cytochrome c oxidase (oxidase) test as performed in a virtual
microbiology laboratory, integrating the biochemical basis of the test, reagent chemistry, procedural technique,
interpretation, quality control, and clinical/ecological applications. Questions span the full laboratory workflow
from enzyme mechanism to organism identification and troubleshooting.
STUDY GUIDE
Recommended duration: 2 hours. Passing target: 70%.
HIGHP - Page 1 of 32
,1. A bacterium isolated from a wound is catalase-positive and oxidase-negative. Which
organism is most consistent with these results?
[ ] A. Pseudomonas aeruginosa
[ ] B. Neisseria gonorrhoeae
[ ] C. Staphylococcus aureus
[ ] D. Campylobacter jejuni
CORRECT: C. Staphylococcus aureus
S. aureus is catalase-positive (staphylococci) and oxidase-negative, matching both results. P. aeruginosa,
Neisseria, and Campylobacter are all oxidase-positive, so they would not fit an oxidase-negative profile.
2. In the oxidase test, the reagent N,N,N',N'-tetramethyl-p-phenylenediamine
dihydrochloride functions primarily as which of the following?
[ ] A. A terminal electron acceptor for cytochrome c oxidase
[ ] B. An artificial electron donor that is oxidized to a colored product
[ ] C. A chelator that removes iron from cytochrome c
[ ] D. A substrate that is hydrolyzed by the enzyme to release a chromophore
CORRECT: B. An artificial electron donor that is oxidized to a colored product
The reagent acts as an artificial electron donor; when cytochrome c oxidase is present, it is oxidized to a
colored (purple/blue) product. It is not a terminal acceptor, chelator, or hydrolyzed substrate, which
distinguishes it from chromogenic substrate tests like ONPG or esculin.
3. A student reads an oxidase test strip at 45 minutes and observes a faint purple color.
How should this result be interpreted?
[ ] A. Positive; the enzyme is present but slow
[ ] B. Negative; color development after 30 seconds is not diagnostic
[ ] C. Invalid; the reagent has degraded
[ ] D. Positive; any color change indicates oxidase activity
CORRECT: B. Negative; color development after 30 seconds is not diagnostic
Oxidase reactions must be read within 10-30 seconds; color appearing after 30 seconds is considered
negative because it reflects non-enzymatic auto-oxidation of the reagent. Interpreting late color as positive is
a common error that can misidentify organisms.
4. Which of the following best explains why the oxidase test is especially useful in the
presumptive identification of Neisseria species from clinical specimens?
[ ] A. Neisseria are the only oxidase-positive gram-negative diplococci
[ ] B. Oxidase positivity distinguishes Neisseria from most gram-positive cocci and many gram-negative
bacilli
HIGHP - Page 2 of 32
, [ ] C. Oxidase-negative Neisseria are always nonpathogenic
[ ] D. The test directly detects the presence of the Neisseria capsule
CORRECT: B. Oxidase positivity distinguishes Neisseria from most gram-positive cocci and many
gram-negative bacilli
Neisseria are oxidase-positive, which helps separate them from oxidase-negative gram-positive cocci and
many gram-negative bacilli. However, other organisms can also be oxidase-positive, so it is presumptive, not
definitive, and the test does not detect capsules.
5. When performing the oxidase test using a filter paper method, which of the following is
the most appropriate way to obtain the bacterial sample?
[ ] A. Use a nichrome loop to pick colonies from blood agar
[ ] B. Use a wooden applicator stick to pick colonies from tryptic soy agar
[ ] C. Use a platinum loop to pick colonies from MacConkey agar
[ ] D. Use a plastic loop to pick colonies from chocolate agar
CORRECT: D. Use a plastic loop to pick colonies from chocolate agar
Plastic or wooden applicators should be used because metal loops (especially nichrome or iron-containing)
can catalyze false-positive reactions. Chocolate agar is acceptable, but the key is avoiding metal; blood agar
can also interfere, so plastic is preferred.
6. In a virtual oxidase test, a student observes a deep blue color within 5 seconds using a
filter paper method. Which of the following organisms is most likely responsible?
[ ] A. Escherichia coli
[ ] B. Pseudomonas aeruginosa
[ ] C. Staphylococcus epidermidis
[ ] D. Enterococcus faecalis
CORRECT: B. Pseudomonas aeruginosa
P. aeruginosa is oxidase-positive, producing a rapid blue color. E. coli, S. epidermidis, and E. faecalis are all
oxidase-negative, so they would not yield a positive reaction.
7. A quality control program for the oxidase test should include which pair of organisms?
[ ] A. E. coli (positive) and P. aeruginosa (negative)
[ ] B. P. aeruginosa (positive) and E. coli (negative)
[ ] C. S. aureus (positive) and S. epidermidis (negative)
[ ] D. Bacillus subtilis (positive) and E. faecalis (negative)
CORRECT: B. P. aeruginosa (positive) and E. coli (negative)
P. aeruginosa is oxidase-positive and E. coli is oxidase-negative, making them ideal QC organisms to verify
HIGHP - Page 3 of 32
, both positive and negative reactions. The other pairs either reverse the expected results or use organisms
that are both oxidase-negative.
8. Which of the following statements about the oxidase test and bacterial respiration is
correct?
[ ] A. Oxidase-positive bacteria lack cytochrome c oxidase but possess alternative oxidases
[ ] B. The test detects cytochrome c oxidase, which is part of the electron transport chain in many
aerobic bacteria
[ ] C. Oxidase-negative bacteria cannot perform aerobic respiration
[ ] D. The test is specific for anaerobic bacteria that use nitrate as a terminal electron acceptor
CORRECT: B. The test detects cytochrome c oxidase, which is part of the electron transport chain in
many aerobic bacteria
The oxidase test detects cytochrome c oxidase, a component of the electron transport chain in many aerobic
organisms. Oxidase-negative bacteria may still respire aerobically using other terminal oxidases, and the test
is not specific for anaerobes.
9. A student performs the oxidase test on a colony from MacConkey agar and obtains a
positive result. What is the most likely explanation?
[ ] A. The organism is a lactose fermenter
[ ] B. The organism is oxidase-positive, such as Pseudomonas
[ ] C. The result is a false positive due to the medium's pH indicator
[ ] D. The organism is a non-fermenter, which is always oxidase-positive
CORRECT: B. The organism is oxidase-positive, such as Pseudomonas
MacConkey agar can support growth of oxidase-positive organisms like Pseudomonas, which would yield a
true positive. The medium's pH indicator does not cause false oxidase positives, and not all non-fermenters
are oxidase-positive (e.g., Acinetobacter is oxidase-negative).
10. Which of the following best describes the expected oxidase result for Campylobacter
jejuni and its clinical significance?
[ ] A. Oxidase-negative; helps distinguish it from Salmonella
[ ] B. Oxidase-positive; supports its identification as a microaerophilic, curved gram-negative rod
[ ] C. Oxidase-positive; confirms it is a facultative anaerobe
[ ] D. Oxidase-negative; indicates it is a non-motile organism
CORRECT: B. Oxidase-positive; supports its identification as a microaerophilic, curved
gram-negative rod
C. jejuni is oxidase-positive, which is consistent with its classification as a microaerophilic, curved
HIGHP - Page 4 of 32
