MICRO TEST 3 DR AL LSUE FINAL PAPER
SOLVED QUESTIONS AND ANSWERS 100
PERCENT CORRECT
⩥ Ex. 5-4 (Methyl Red/VP)
Would a false negative result for the VP test more likely be attributable
to poor sensitivity or to poor specificity of the test system?
Answer: A false negative on the VP test is likely due to a lack of
sensitivity- there is not enough acetoin to react with the test reagents.
⩥ Ex. 5-4 (MR/VP)
Why were you told to shake the VP tubes after the reagents were added?
Answer: The indicator reaction requires oxygen to oxidize acetoin to
diacetyl.
⩥ Ex. 5-4 (MR/VP)
Why is the methyl red test read immediately after addition of methyl red
reagent and the Voges-Prokauer read up to 60 minutes after addition of
VP reagents A and B?
Answer: The methyl red test is based on a pH changed, which can be
demonstrated instantaneously. Reading the VP test requires a chemical
reaction and time must be allowed for this reaction to occur
,⩥ Ex. 5-4 (MR/VP)
Some microbiologists recommend re incubating organisms producing
methyl red negative results for an additional 2 to 3 days. Why do you
think this is done?
Answer: It is to give the organisms more time to produce enough acid to
lower the pH to the point of making the methyl red indicator turn red.
Longer incubation minimizes the possibility of a false negative
⩥ Ex. 5-9 Citrate Test
Consider the uninoculated tube.
a) Is it a positive or a negative control?
b) What information is provided by the uninoculated control?
Answer: a) It is a negative control
b) It provides a baseline color (green) for "no change' in the medium.
This allows detection of slight color changes in the experimental tubes
that might not be apparent without comparison to the original color. It
also verifies the sterility of the medium.
⩥ Ex. 5-9 Citrate Test
Many bacteria that are able to metabolize citrate (as seen in the citric
acid cycle) produce negative results in this test. Why? Be specific.
Answer: Citrate (citric acid) is the first intermediate of the citric acid
(Krebs) cycle where it is ultimately catabolized to CO2 and oxaloacetic
acid (which reenters the cycle). However, the citrate test does not detect
the ability of an organism to perform the citric acid cycle. It detects the
, ability of the organism to obtain citrate from the environment and use it.
Thus, an organism could synthesize its own citrate in the citric acid
cycle, but not be able to use citrate from the environment because it
lacks the ability to transport it into the cell; it is citrate permease (-).
⩥ Ex. 5-9 Citrate Test
explain how an organism that possesses the citrate lyase enzyme might
not test positively on Simmons citrate agar. Is this a false negative
result? Why or why not?
Answer: The citrate test determines if an organism has the ability to
transport citrate into the cell with the enzyme citrate permease and
survive with citrate as the sole carbon source. Citrate lyase, which
catalyzes the conversion of citrate into oxaloacetate and acetate, is
associated with how citrate is used, not how it gets into the cell. So, an
organism could be citrate-negative (it doesn't make citrate permease),
but still be citrate lyase-positive.
⩥ Ex. 5-20 SIM Medium
Consider the uninoculated SIM tube.
a) Is it a positive or a negative control in each test?
b) What purpose does it serve in the sulfur reduction test?
c) What purpose does it serve in the indole test?
d) What purpose doe sit serve in the motility test?
Answer: a) In each test it is a negative control and minimizes concern
about false positive results
SOLVED QUESTIONS AND ANSWERS 100
PERCENT CORRECT
⩥ Ex. 5-4 (Methyl Red/VP)
Would a false negative result for the VP test more likely be attributable
to poor sensitivity or to poor specificity of the test system?
Answer: A false negative on the VP test is likely due to a lack of
sensitivity- there is not enough acetoin to react with the test reagents.
⩥ Ex. 5-4 (MR/VP)
Why were you told to shake the VP tubes after the reagents were added?
Answer: The indicator reaction requires oxygen to oxidize acetoin to
diacetyl.
⩥ Ex. 5-4 (MR/VP)
Why is the methyl red test read immediately after addition of methyl red
reagent and the Voges-Prokauer read up to 60 minutes after addition of
VP reagents A and B?
Answer: The methyl red test is based on a pH changed, which can be
demonstrated instantaneously. Reading the VP test requires a chemical
reaction and time must be allowed for this reaction to occur
,⩥ Ex. 5-4 (MR/VP)
Some microbiologists recommend re incubating organisms producing
methyl red negative results for an additional 2 to 3 days. Why do you
think this is done?
Answer: It is to give the organisms more time to produce enough acid to
lower the pH to the point of making the methyl red indicator turn red.
Longer incubation minimizes the possibility of a false negative
⩥ Ex. 5-9 Citrate Test
Consider the uninoculated tube.
a) Is it a positive or a negative control?
b) What information is provided by the uninoculated control?
Answer: a) It is a negative control
b) It provides a baseline color (green) for "no change' in the medium.
This allows detection of slight color changes in the experimental tubes
that might not be apparent without comparison to the original color. It
also verifies the sterility of the medium.
⩥ Ex. 5-9 Citrate Test
Many bacteria that are able to metabolize citrate (as seen in the citric
acid cycle) produce negative results in this test. Why? Be specific.
Answer: Citrate (citric acid) is the first intermediate of the citric acid
(Krebs) cycle where it is ultimately catabolized to CO2 and oxaloacetic
acid (which reenters the cycle). However, the citrate test does not detect
the ability of an organism to perform the citric acid cycle. It detects the
, ability of the organism to obtain citrate from the environment and use it.
Thus, an organism could synthesize its own citrate in the citric acid
cycle, but not be able to use citrate from the environment because it
lacks the ability to transport it into the cell; it is citrate permease (-).
⩥ Ex. 5-9 Citrate Test
explain how an organism that possesses the citrate lyase enzyme might
not test positively on Simmons citrate agar. Is this a false negative
result? Why or why not?
Answer: The citrate test determines if an organism has the ability to
transport citrate into the cell with the enzyme citrate permease and
survive with citrate as the sole carbon source. Citrate lyase, which
catalyzes the conversion of citrate into oxaloacetate and acetate, is
associated with how citrate is used, not how it gets into the cell. So, an
organism could be citrate-negative (it doesn't make citrate permease),
but still be citrate lyase-positive.
⩥ Ex. 5-20 SIM Medium
Consider the uninoculated SIM tube.
a) Is it a positive or a negative control in each test?
b) What purpose does it serve in the sulfur reduction test?
c) What purpose does it serve in the indole test?
d) What purpose doe sit serve in the motility test?
Answer: a) In each test it is a negative control and minimizes concern
about false positive results
