4-4 In your own words, what is the application (purpose) of MSA? -Answer- Most clinical isolates are incapable of growing in 7.5% salt, but Staph species can, which makes this a selective medium used for isolating Staph species. It is also differential because most Staph species are not able to fe...
4-4 In your own words, what is the application (purpose) of MSA? -Answer- Most
clinical isolates are incapable of growing in 7.5% salt, but Staph species can, which
makes this a selective medium used for isolating Staph species. It is also differential
because most Staph species are not able to ferment mannitol except S. aureus
4-4 Which ingredients in MSA supplies carbon? Nitrogen? -Answer- Carbon: beef
extract, peptone, and mannitol
Nitrogen: Beef extract and peptone
4-4 Is MSA a defined or undefined medium? -Answer- MSA is an undefined medium
because the exact composition of beef extract and peptone are unknown
4-4 In your own words, what is the role of NaCl in MSA and how does it work? -Answer-
NaCl is the selective agent and its presence dehydrates most cells
4-4 In your own words, what are the roles of mannitol and phenol red in MSA? -Answer-
The ability to ferment mannitol to acid end products makes this a differential medium.
The acid end products are detected by the pH indicator in phenol red, which will turn
yellow
4-4 Growth on the MSA and NA plates was recorded as "good growth," "poor growth,"
or "no growth." These are qualitative and at least for the first two, subjective terms.
What did you use to establish what constituted "good growth?" -Answer- The NA
inoculated with the same organisms provided examples of what "good growth" for each
organism looks like on a nonselective medium. Without these for comparison, it wouldn't
be possible to tell is sparse growth on MSA was due to inhibition or was just normal
growth for that species
4-4 What wouldn't it be advisable to compare growth of the organisms on each plate to
each other? (2 answers) -Answer- On the NA plate, "good growth" for one organism
may be very dense, whereas it may be thin for another. In the context of this exercise
you want to compare the amount of growth of each organism on the selective medium
against its growth on NA to see if inhibition occurs. You wouldn't want to compare
growth of the organisms on the MSA plate to each other because you wouldn't know
which ones were inhibited or normal
4-4 Would removal of NaCl from MSA alter the medium's sensitivity or specificity? -
Answer- Specificity; organisms that shouldn't grow on it would, and some of those
might be able to ferment mannitol to acid end products
, 4-4 Suppose a mistake was made in preparing a batch of MSA and the starting pH is
7.4 instead of 7.0-7.2. Would this affect the medium's sensitivity or specificity? -Answer-
Sensitivity; it would take more acid production from mannitol fermentation to produce
the yellow color of a positive.
4-4 With the diversity of microorganisms in the world, how can a single test such as
MSA be used to "confidently" identify Staph aureus? -Answer- If the sample plated is
one taken from a random source, identification would not be as certain. However, in
clinical situations, the source is known. Since the salt is selective for staphs, and S.
aureus is the only staph to ferment mannitol, presumptive identification can be made
5-14 The disassembly of DNA was described as a "depolymerization." What other terms
apply to this process? -Answer- "Catabolic" and "hydrolysis"
5-14 A positive result fort the DNA hydrolysis test does not distinguish between Staph
DNase and the DNase produced by Serratia. If you had the expertise to correct this
weakness in the system, would you be improving the test sensitivity or specificity? -
Answer- Specificity because it would now be able to discriminate between the 2 types
of DNase
5-14 Suggest a reason why this test is read after 24 hours while other tests may take a
week. What would be the likely consequence of incubating DNase agar for a week? -
Answer- The suggested incubation time is related to the activity of the enzyme. Some
enzymes produce a result faster than others. In fact, DNase + organisms may clear the
entire plate if incubated too long
5-14 Why is the unincolulated control relatively unnecessary in this test? -Answer-
Usually there is plenty of medium that is not cleared to illustrate "no reaction" even
when a specific sector o the plate is not designated as the uninoculated control.
5-14 Why is it advisable to use a + control along with organisms that you are testing? -
Answer- As usual, a + control indicated that the medium was prepared correctly and
lends confidence in - results being true -, not the resuting of the medium not functioning
correctly
5-14 In what ways can a DNase + organism use the products of DNA hydrolysis? -
Answer- The easiest use of the fragments would be to convert the nucleotides into their
triphosphate form and incorporate them into the cell's DNA during its next round of
replication. Deoxyribose can be converted into ribose and used to make
ribonucleotides. Deoxyribose, purines, and pyrimidines can also be used as energy
sources in catabolic pathways
5-25 The streak-stab technique, used to promote strep activity, is preferred over
incubating the plates anaerobically. Why do you think this is so? -Answer- The primary
goal of the microbiologist at this point is to isolate the organisms obtained from the
aerobic environment- the phayrnx
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