Question 1

Which enzyme is responsible for the coagulation of plasma, and what is its significance in distinguishing between Staphylococcus aureus and Staphylococcus epidermidis?

Accepted Answer

Coagulase; it distinguishes pathogenic S. aureus (coagulase positive) from non-pathogenic S. epidermidis (coagulase negative).

Question 2

A student performs a urease test and observes a bright pink color in the medium. Which chemical reaction is responsible for this color change, and what does it indicate about the organism?

Accepted Answer

The breakdown of urea into carbon dioxide and ammonia, indicating the organism produces urease.

Question 3

Which of the following equations correctly represents the reaction catalyzed by catalase?

Accepted Answer

$$2H_2O_2 \rightarrow 2H_2O + O_2$

Question 4

In the oxidase test, a rapid color change to purple on the filter disk indicates the presence of which enzyme, and what is its role in bacterial respiration?

Accepted Answer

Cytochrome oxidase; it transfers electrons to oxygen, reducing it to water.

Question 5

A bacterium is catalase negative but can still survive in the presence of oxygen. Which alternative enzyme might it use to detoxify hydrogen peroxide?

Accepted Answer

Peroxidase

Question 6

Which test would you use to distinguish between Streptococcaceae and Micrococcaceae, and what result would you expect for each?

Accepted Answer

Catalase test; Streptococcaceae are catalase negative, Micrococcaceae are catalase positive.

Question 7

Why might a negative coagulase test not definitively rule out Staphylococcus aureus?

Accepted Answer

Some strains of S. aureus may not express coagulase under certain conditions.

Question 8

Helicobacter pylori is able to survive in the acidic environment of the stomach due to the production of which enzyme, and what is the biochemical reaction involved?

Accepted Answer

Urease; $$CO(NH_2$)_2 + H_2O \rightarrow CO_2$ + 2NH_3$$

Question 9

A microbiologist observes bubbling when hydrogen peroxide is added to a bacterial colony. What does this indicate about the organism?

Accepted Answer

The organism produces catalase and is likely aerobic or facultative anaerobic.

Question 10

Which of the following best explains why hydrogen peroxide is an effective disinfectant against catalase-negative bacteria?

Accepted Answer

Catalase-negative bacteria cannot detoxify hydrogen peroxide, leading to accumulation of toxic reactive oxygen species.

Question 11

Which of the following is a correct interpretation of the urease test results shown in the second image?

Accepted Answer

A bright pink color indicates a positive urease test, meaning the organism produces urease.

Question 12

Which of the following statements about the oxidase test is correct?

Accepted Answer

A rapid purple color change indicates the presence of cytochrome oxidase, which is involved in aerobic respiration.

Question 13

Which of the following best describes the role of phenol red in the urease test?

Accepted Answer

It acts as a pH indicator, turning pink in alkaline conditions due to ammonia production.

Question 14

Which of the following organisms would you expect to be oxidase positive based on the oxidase test?

Accepted Answer

Pseudomonas aeruginosa

Question 15

Which equation represents the reaction catalyzed by urease?

Accepted Answer

$$CO(NH_2$)_2 + H_2O \rightarrow CO_2$ + 2NH_3$$

Question 16

Which test would be most useful for distinguishing between Escherichia coli and Proteus vulgaris?

Accepted Answer

Urease test; Proteus vulgaris is urease positive, E. coli is urease negative.

Question 17

Which of the following best describes the role of coagulase in bacterial pathogenesis?

Accepted Answer

It protects bacteria from phagocytosis by causing clot formation around the cells.

Question 18

Which of the following is the correct interpretation of a catalase test where bubbles are observed after adding hydrogen peroxide?

Accepted Answer

The organism is catalase positive.

Question 19

Which of the following best explains why Enterobacteriaceae are oxidase negative?

Accepted Answer

They lack cytochrome oxidase and use alternative terminal oxidases for oxygen reduction.

lab 14 - Microbiology Lab Study Guide: Biochemical Reactions (Coagulase, Urease, Catalase, Oxidase)

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