Microbiology Lab Practical Exam 2
(2026/2027) | Latest Questions | 100%
Verified Answers | A+
• What is the purpose of the oxidase test? -✓✓-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? -✓✓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? -✓✓-
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 -✓✓*Positive Catalase*
-Visible Bubble production
*Negative Catalase Test*
-No Bubbles
*Reagent Used:*
-Hydrogen Peroxide
• Why is the Catalase Test Used? -✓✓-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? -✓✓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 -✓✓*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? -✓✓*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? -✓✓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? -✓✓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 -✓✓*Positive Malonate:*
-Color change in medium from green to blue (malonate is utilized)
*Negative Malonate:*
-No color change from uninoculated tube, remains green (malonate is not
utilized)
• Why is the malonate test used? -✓✓The malonate test is used to assess a
bacteria's ability to use malonate as its sole carbon source.
• What is the chemical reaction that takes place during the Malonate Test? -
✓✓In one portion of the Krebs cycle, which requires the enzyme succinate
dehydrogenase, the coenzyme FAD is reduced to FADH2. Malonate, when added
to the media, is similar enough to succinate to replace it as the substrate in the
reaction. When combined with subsequent build up of succinate in the cell, shuts
down the Krebs cycle and will kill the organism unless it can ferment malonate as
its sole remaining carbon source.
• What causes the color change in the malonate test? -✓✓-Bromthymol blue dye,
which is green when in uninoculated media, is added to indicate any shift in the
(2026/2027) | Latest Questions | 100%
Verified Answers | A+
• What is the purpose of the oxidase test? -✓✓-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? -✓✓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? -✓✓-
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 -✓✓*Positive Catalase*
-Visible Bubble production
*Negative Catalase Test*
-No Bubbles
*Reagent Used:*
-Hydrogen Peroxide
• Why is the Catalase Test Used? -✓✓-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? -✓✓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 -✓✓*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? -✓✓*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? -✓✓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? -✓✓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 -✓✓*Positive Malonate:*
-Color change in medium from green to blue (malonate is utilized)
*Negative Malonate:*
-No color change from uninoculated tube, remains green (malonate is not
utilized)
• Why is the malonate test used? -✓✓The malonate test is used to assess a
bacteria's ability to use malonate as its sole carbon source.
• What is the chemical reaction that takes place during the Malonate Test? -
✓✓In one portion of the Krebs cycle, which requires the enzyme succinate
dehydrogenase, the coenzyme FAD is reduced to FADH2. Malonate, when added
to the media, is similar enough to succinate to replace it as the substrate in the
reaction. When combined with subsequent build up of succinate in the cell, shuts
down the Krebs cycle and will kill the organism unless it can ferment malonate as
its sole remaining carbon source.
• What causes the color change in the malonate test? -✓✓-Bromthymol blue dye,
which is green when in uninoculated media, is added to indicate any shift in the
