Microbiology Lab Exam 3 – Flashcards
Unlock all answers in this set
Unlock answersquestion
5-4: which of the two tests would most likely produce more false negatives with a shorter incubation time? why?
answer
a shorter incubation time runs the risk of a false negative on the VP test, since not enough incubation time may have elapsed to produce an adequate amount of acetoin to give a positive result. The pH change associated with mixed acid fermentation (MR)occurs fairly quickly and is less likely to be affected by shorter incubation.
question
5-4 which test would likely benefit from a longer incubation time? why?
answer
the VP test would benefit most from a longer incubation time because it take a while for the 2,3-butanediol to be produced.
question
5-4 would a false negative result for 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.
question
5-4 why were you told to shake the VP test after the reagents were added?
answer
the indicator reaction requires oxygen to oxidize acetoin to diacetyl.
question
5-4 why is the methyl red test read immediately after addition of methyl red reagents and the VP read up to 60 minutes after addition of VP reagents A and B?
answer
the MR test is based on a pH change, which can be demonstrated immediately. the VP test requires a chemical reaction and time must be allowed for this to occur.
question
5-4 some microbiologists recommend 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 minimalizes the possibility of a false negative.
question
5-8 is it a positive or negative control?
answer
this is a negative control
question
5-8 what information is provided by the uninoculated control?
answer
it demonstrates that no ingredients in the medium produces a positive result at any stage of testing. it also show the original color of the broth, which allows detection of slight color changes in the experimental tubes that might not be apparent without comparison to the original.
question
5-8 why is gas production not recognized as nitrate reduction when the organism is a known fermenter?
answer
if the organism is a known fermenter, it could be that the gas is a fermentation product and not the result of nitrate reduction. if the organism is a known nonfermenter, then gas production would be considered positive for nitrate reduction, specifically denitrification.
question
5-8 suppose you remove your test cultures from the incubator and notive that one of them- a known fermenter- has a gas bubble in the durham tube. knowing that fermenters frequently produce gas, you ignore the bubble and proceed to the next step. adding reagents produces no change and neither does adding zinc. is this occurrence consistent with what you have learned about this test? why?
answer
an organism that reduces nitrate to N2 gas or something other than nitrate would show no color change after adding reagents A and B because it didn't reduce nitrate to nitrite. likewise, addition of the zinc indicator wouldn't produce a color change because the nitrate has been reduced and is no longer present in the medium. the question is what has the nitrate been reduced to? these results are unequivocally positive for nitrate reduction to something other than nitrite but the fact that the organism is a fermenter lends double as to whether the bubble is from fermentation or nitrate reduction.
question
5-8 would you change your answer to the question above if the control both did not change color after the addition of reagents? what if the control broth had changed color only after the addition of reagents and zinc?
answer
no, because the organism in the question before reduced nitrate to something other than nitrite. the control broth should not change color after the addition of reagents, so everything is fine so far however, the control should change (to red) after addition of zinc. both of these results are expected for the control, so no change in the interpretation of the situation in the question before is necessary.
question
5-8when testing microaerophiles, some microbiologists prefer to use a semisolid nitrate medium that contains a small amount of agar. why do you think that this is done?
answer
the agar slows the diffusion of oxygen into the medium and maintains an environment more suitable for microaerophilic growth.
question
5-20 is it a positive or negative control in each test?
answer
it is a negative control and minimized concern about false positive results.
question
5-20 what purpose does it serve in the sulfur reduction test?
answer
it demonstrates that nonenzymatic reduction of
