MCB 3020 FINAL EXAM (CH. 13-26) QUESTIONS AND
ANSWERS
Fermentation process for sucrose: - Answers - Breaks down into fructose and glucose.
Enzyme: sucrase
Fermentation process for lactose: - Answers - Breaks down into galactose and glucose.
Enzyme: β-galactosidase
Fermentation process for glucose: - Answers - Pyruvic acid is the intermediate and
acids/alcohols/gas are the end products.
Differential media: - Answers - Media that contains substances (indicators) that reveal
differences between organisms. Example: Phenol red broth (PRB).
PRB Tubes: - Answers - Phenol red broth, differential medium. Peptones are the source
of protein which supports growth. Contains pH indicator phenol red and peptones.
Durham tube - Answers - An inverted tube located inside the PRB tube which collects
produced gas. Should have avery large air bubble once fermentation is finished.
What is a PRB tube contained a small bubble? - Answers - This would be due to prep
error, should only see a large air bubble.
PRB result = broth turned yellow - Answers - positive for fermentation , pH is below 6.8
= acidic. Bacteria fermented glucose and produced acid which changed the broth color.
PRB result = broth turned pink - Answers - positive for alkaline but negative for
fermentation, pH is above 7.4 = basic. Bacteria degraded the peptones and produce
alkali which changes the broth color.
PRB result = broth stayed red - Answers - The pH is in between 6.8 and 7.4.
Why is it so important to compare the inoculated PRB-tubes to an uninoculated control
tube? - Answers - A control group is needed to see the difference between each tube to
make sure the color change is due to the inoculated bacteria instead of a contaminating
bacteria.
When can carbohydrate fermentation occur? - Answers - In the absence or presence of
oxygen.
,What happens when a carbohydrate is used up by the bacteria? - Answers - It can use
peptone as an additional energy source, which may mask the acid-producing reaction
by producing base end products.
Why is it crucial to incubate your culture no longer than 48 hours? - Answers - A culture
can use peptones as an energy source to produce a false alkaline positive result.
What happens if you get a pink tube and no air bubble? - Answers - This is no an
expected result, a pink tube would be positive for fermentation.
What happens if you get a pink tube and large air bubble? - Answers - This is due to a
prep error or the culture was left in the incubator for too long.
What happens if you get a positive result for lactose and glucose? - Answers - Can
draw the conclusion that both tubes should be positive for fermentation = yellow.
What happens if you get a positive result for lactose and a negative result for glucose? -
Answers - Can draw conclusion that both tubes have no glucose was both tubes should
be positive for fermentation = yellow.
What is pyruvic acid? - Answers - An intermediate of fermentation.
What are peptones? - Answers - An excellent natural sources of amino acids, peptides
and proteins in PRB (phenol red broth) tubes.
Aerobic respiration vs fermentation: - Answers - Aerobic respiration can only be carried
out in the presence of oxygen. Fermentation can occur with or without fermentation.
Aerobic respiration produces 6 carbon dioxide/6 water/and 30 ATP. Fermentation
produces 2 ATP.
Depending on the microorganism, pyruvic acid can be used in different ways. How so? -
Answers - Can be used for cell synthesis. Bacteria can produce lactate or ethanol from
the same pyruvate. The end product of pyruvate fermentation depends on the specific
organism.
Phenol red broth (PRB) indicator - Answers - phenol red
What does IMViC stand for? - Answers - Indole, Methyl Red, Voges-Proskauer, and
Citrate test.
What is the IMViC test used for? - Answers - Used to identify if gut bacteria to ensure
water doesn't have pathogens such as salmanella.
Indole test - Answers - Detects the presence of tryptophanase to break down tryptophan
into indole.
, Indole test indicator - Answers - Kovac's reagent
Positive indole test result - Answers - Produces red top layer
Negative indole test result - Answers - Produces colorless/pale orange top layer
Medium used for indole test - Answers - SIM (Sulfide Indole Motility) deep medium =
differential
Methyl red test - Answers - Detects the ability to use glucose for energy and produce
stable acids
Medium used for methyl red test AND Voges-Proskauer test - Answers - MRVP (Methyl
Red Voges Proskaur) Broth. Contains glucose, peptone, and phosphate buffer =
differential.
Positive methyl red test result - Answers - Red broth = acidic
Negative methyl red test result - Answers - Yellow broth = basic
Voges-Proskauer test - Answers - Detects the presence of acetoin.
Voges-Proskauer test indicator - Answers - Barrit's reagent: contains alcoholic α-
naphthol (VP-A) and 40% potassium hydroxide solution (VP-B).
Positive Voges-Proskauer test result - Answers - Produces dark red top layer
Negative Voges-Proskauer test result - Answers - Produces no layer, remains dark
yellow
Citrate Utilization Test - Answers - Detects the presence of citrate permease to use
citrate for energy = no glucose used!
Citrate Utilization Test indicator - Answers - Bromthymol blue
Positive Citrate Utilization Test - Answers - Broth turns blue = alkaline
Negative Citrate Utilization Test - Answers - Broth remains green = neutral
Medium for Citrate Utilization Test - Answers - Simmons citrate = selective and
differential since it shows growth (+ or -) and pH color change (blue/green)
Would you ever see a green tube with growth? - Answers - No, because growth should
only seen on a blue tube. Possible error due to glucose may have been present.
ANSWERS
Fermentation process for sucrose: - Answers - Breaks down into fructose and glucose.
Enzyme: sucrase
Fermentation process for lactose: - Answers - Breaks down into galactose and glucose.
Enzyme: β-galactosidase
Fermentation process for glucose: - Answers - Pyruvic acid is the intermediate and
acids/alcohols/gas are the end products.
Differential media: - Answers - Media that contains substances (indicators) that reveal
differences between organisms. Example: Phenol red broth (PRB).
PRB Tubes: - Answers - Phenol red broth, differential medium. Peptones are the source
of protein which supports growth. Contains pH indicator phenol red and peptones.
Durham tube - Answers - An inverted tube located inside the PRB tube which collects
produced gas. Should have avery large air bubble once fermentation is finished.
What is a PRB tube contained a small bubble? - Answers - This would be due to prep
error, should only see a large air bubble.
PRB result = broth turned yellow - Answers - positive for fermentation , pH is below 6.8
= acidic. Bacteria fermented glucose and produced acid which changed the broth color.
PRB result = broth turned pink - Answers - positive for alkaline but negative for
fermentation, pH is above 7.4 = basic. Bacteria degraded the peptones and produce
alkali which changes the broth color.
PRB result = broth stayed red - Answers - The pH is in between 6.8 and 7.4.
Why is it so important to compare the inoculated PRB-tubes to an uninoculated control
tube? - Answers - A control group is needed to see the difference between each tube to
make sure the color change is due to the inoculated bacteria instead of a contaminating
bacteria.
When can carbohydrate fermentation occur? - Answers - In the absence or presence of
oxygen.
,What happens when a carbohydrate is used up by the bacteria? - Answers - It can use
peptone as an additional energy source, which may mask the acid-producing reaction
by producing base end products.
Why is it crucial to incubate your culture no longer than 48 hours? - Answers - A culture
can use peptones as an energy source to produce a false alkaline positive result.
What happens if you get a pink tube and no air bubble? - Answers - This is no an
expected result, a pink tube would be positive for fermentation.
What happens if you get a pink tube and large air bubble? - Answers - This is due to a
prep error or the culture was left in the incubator for too long.
What happens if you get a positive result for lactose and glucose? - Answers - Can
draw the conclusion that both tubes should be positive for fermentation = yellow.
What happens if you get a positive result for lactose and a negative result for glucose? -
Answers - Can draw conclusion that both tubes have no glucose was both tubes should
be positive for fermentation = yellow.
What is pyruvic acid? - Answers - An intermediate of fermentation.
What are peptones? - Answers - An excellent natural sources of amino acids, peptides
and proteins in PRB (phenol red broth) tubes.
Aerobic respiration vs fermentation: - Answers - Aerobic respiration can only be carried
out in the presence of oxygen. Fermentation can occur with or without fermentation.
Aerobic respiration produces 6 carbon dioxide/6 water/and 30 ATP. Fermentation
produces 2 ATP.
Depending on the microorganism, pyruvic acid can be used in different ways. How so? -
Answers - Can be used for cell synthesis. Bacteria can produce lactate or ethanol from
the same pyruvate. The end product of pyruvate fermentation depends on the specific
organism.
Phenol red broth (PRB) indicator - Answers - phenol red
What does IMViC stand for? - Answers - Indole, Methyl Red, Voges-Proskauer, and
Citrate test.
What is the IMViC test used for? - Answers - Used to identify if gut bacteria to ensure
water doesn't have pathogens such as salmanella.
Indole test - Answers - Detects the presence of tryptophanase to break down tryptophan
into indole.
, Indole test indicator - Answers - Kovac's reagent
Positive indole test result - Answers - Produces red top layer
Negative indole test result - Answers - Produces colorless/pale orange top layer
Medium used for indole test - Answers - SIM (Sulfide Indole Motility) deep medium =
differential
Methyl red test - Answers - Detects the ability to use glucose for energy and produce
stable acids
Medium used for methyl red test AND Voges-Proskauer test - Answers - MRVP (Methyl
Red Voges Proskaur) Broth. Contains glucose, peptone, and phosphate buffer =
differential.
Positive methyl red test result - Answers - Red broth = acidic
Negative methyl red test result - Answers - Yellow broth = basic
Voges-Proskauer test - Answers - Detects the presence of acetoin.
Voges-Proskauer test indicator - Answers - Barrit's reagent: contains alcoholic α-
naphthol (VP-A) and 40% potassium hydroxide solution (VP-B).
Positive Voges-Proskauer test result - Answers - Produces dark red top layer
Negative Voges-Proskauer test result - Answers - Produces no layer, remains dark
yellow
Citrate Utilization Test - Answers - Detects the presence of citrate permease to use
citrate for energy = no glucose used!
Citrate Utilization Test indicator - Answers - Bromthymol blue
Positive Citrate Utilization Test - Answers - Broth turns blue = alkaline
Negative Citrate Utilization Test - Answers - Broth remains green = neutral
Medium for Citrate Utilization Test - Answers - Simmons citrate = selective and
differential since it shows growth (+ or -) and pH color change (blue/green)
Would you ever see a green tube with growth? - Answers - No, because growth should
only seen on a blue tube. Possible error due to glucose may have been present.
