Microbiology Lab Practical Exam 2 with
Verified Solutions
Oxidase Test Correct Ans-*Positive Test:*
-Turns blue within 20-30 seconds
*Negative Test*
-No color change within thirty seconds
*Reagent used:*
-Oxidase
What is the purpose of the oxidase test? Correct Ans--The tests is used to identify bacteria
containing the respiratory enzyme cytochrome c oxidase as an electron acceptor in the ETC.
How is the oxidase test performed? Correct Ans-1. Transfer bacterium to the paper
2. Add drops of complex A & B to bacterium
3. Observe for color change within 30 seconds
Why should you read the results of the oxidase test within 30 seconds? Correct Ans--The
reagents of this test are unstable, therefore they may oxidize independently shortly after
becoming moist. Therefore it is important to read within 30 seconds.
Catalase Test Correct Ans-*Positive*
-Visible Bubble production
, *Negative*
-No Bubbles
*Reagent Used:*
-Hydrogen Peroxide
Why is the Catalase Test Used? Correct Ans--This test is used to identify organisms that
produce the enzyme catalase, and its ability to break down hydrogen peroxide.
How is the catalase test performed? Correct Ans-1. Transfer a large amount of growth to slide
2. Place one to two drops of hydrogen peroxide on bacteria
3. Observe for bubbles
MacConkey Test Correct Ans-*Poor Growth or No growth:*
-Organism is inhibited by CV and/or bile, therefore is Gram-positive
*Good Growth:*
-Organism is not inhibited by CV and/or bile, therefore Gram-Negative
*Pink to red growth with or without bile precipitate:*
-Organism produces acid from lactose fermentation therefore is a probable coliform
*Growth is "colorless" (not red or pink):*
-Organism does not ferment lactose, therefore is a noncoliform
Verified Solutions
Oxidase Test Correct Ans-*Positive Test:*
-Turns blue within 20-30 seconds
*Negative Test*
-No color change within thirty seconds
*Reagent used:*
-Oxidase
What is the purpose of the oxidase test? Correct Ans--The tests is used to identify bacteria
containing the respiratory enzyme cytochrome c oxidase as an electron acceptor in the ETC.
How is the oxidase test performed? Correct Ans-1. Transfer bacterium to the paper
2. Add drops of complex A & B to bacterium
3. Observe for color change within 30 seconds
Why should you read the results of the oxidase test within 30 seconds? Correct Ans--The
reagents of this test are unstable, therefore they may oxidize independently shortly after
becoming moist. Therefore it is important to read within 30 seconds.
Catalase Test Correct Ans-*Positive*
-Visible Bubble production
, *Negative*
-No Bubbles
*Reagent Used:*
-Hydrogen Peroxide
Why is the Catalase Test Used? Correct Ans--This test is used to identify organisms that
produce the enzyme catalase, and its ability to break down hydrogen peroxide.
How is the catalase test performed? Correct Ans-1. Transfer a large amount of growth to slide
2. Place one to two drops of hydrogen peroxide on bacteria
3. Observe for bubbles
MacConkey Test Correct Ans-*Poor Growth or No growth:*
-Organism is inhibited by CV and/or bile, therefore is Gram-positive
*Good Growth:*
-Organism is not inhibited by CV and/or bile, therefore Gram-Negative
*Pink to red growth with or without bile precipitate:*
-Organism produces acid from lactose fermentation therefore is a probable coliform
*Growth is "colorless" (not red or pink):*
-Organism does not ferment lactose, therefore is a noncoliform
