© EVERLY 2025 ALL RIGHTS RESERVED.
Microbiology Lab Practical Exam 2
Questions And Answers 100% Pass.
Oxidase Test - Answer✔*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? - Answer✔-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? - Answer✔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? - Answer✔-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 - Answer✔*Positive*
-Visible Bubble production
*Negative*
-No Bubbles
*Reagent Used:*
-Hydrogen Peroxide
, © EVERLY 2025 ALL RIGHTS RESERVED.
Why is the Catalase Test Used? - Answer✔-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? - Answer✔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 - Answer✔*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
What does the bacteria look like on a MacConkey Media? - Answer✔*Negative Growth:*
-Streak lines visible, but no colonies (Gram-positive)
*Positive For Growth, but colorless:*
-Gram-negative, but no lactose fermentation (noncoliform)
*Positive for growth and Lactose fermentation:*
-Pink growth and good growth indicates a Gram-negative bacteria and Lactose fermentation
(coliform)
What does the MacConkey Test Do? - Answer✔The test isolates and differentiates members of
the *Enterobacteriaceae* based on the ability to ferment lactose and their ability to grow in the
presence of bile salts and crystal violet.
Is the MacConkey test Selective or Differential? - Answer✔The MacConkey Agar is both
selective and differential for it is selective for only Gram-negative bacterial growth, and
differential for it differentiates between lactose fermenters and lactose nonfermenters
Malonate Test - Answer✔*Positive:*
-Color change in medium from green to blue (malonate is utilized)
*Negative*
-No color change from uninoculated tube, remains green (malonate is not utilized)
Microbiology Lab Practical Exam 2
Questions And Answers 100% Pass.
Oxidase Test - Answer✔*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? - Answer✔-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? - Answer✔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? - Answer✔-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 - Answer✔*Positive*
-Visible Bubble production
*Negative*
-No Bubbles
*Reagent Used:*
-Hydrogen Peroxide
, © EVERLY 2025 ALL RIGHTS RESERVED.
Why is the Catalase Test Used? - Answer✔-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? - Answer✔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 - Answer✔*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
What does the bacteria look like on a MacConkey Media? - Answer✔*Negative Growth:*
-Streak lines visible, but no colonies (Gram-positive)
*Positive For Growth, but colorless:*
-Gram-negative, but no lactose fermentation (noncoliform)
*Positive for growth and Lactose fermentation:*
-Pink growth and good growth indicates a Gram-negative bacteria and Lactose fermentation
(coliform)
What does the MacConkey Test Do? - Answer✔The test isolates and differentiates members of
the *Enterobacteriaceae* based on the ability to ferment lactose and their ability to grow in the
presence of bile salts and crystal violet.
Is the MacConkey test Selective or Differential? - Answer✔The MacConkey Agar is both
selective and differential for it is selective for only Gram-negative bacterial growth, and
differential for it differentiates between lactose fermenters and lactose nonfermenters
Malonate Test - Answer✔*Positive:*
-Color change in medium from green to blue (malonate is utilized)
*Negative*
-No color change from uninoculated tube, remains green (malonate is not utilized)
