MICRO LAB TEST 3 EXAM QUESTIONS AND ANSWERS
4-4
In your own words, what is the application (purpose) of MSA? - CORRECT ANSWER✅✅Most clinical
isolates are incapable of growing in 7.5% salt, but Staphylococcus species can, which makes this a
selective medium used for isolating Staphylococcus species. It is also differential, because most
Staphylococcus species are not able to ferment mannitol, but S.aureus.
4-4
Which Ingredients in MSA supplies
a) Carbon?
b) Nitrogen - CORRECT ANSWER✅✅a) beef extract, peptone, and mannitol all contain usable carbon
b) beef extract and peptone contain organic nitrogen
4-4
Is MSA a defined or undefined medium? Provide reasoning behind the your choice. - CORRECT
ANSWER✅✅MSA is an undefined medium because the exact composition of beef extract and peptone
are unknown. The goal of this medium is to isolate desired organisms, and once the undesired
organisms have been inhibited, you want the desired ones to grow well. The variety of biochemicals in
beef extract supplemented by the peptone make the medium more likely to satisfy the varied
nutritional needs of the organisms being selected for.
4-4
In your own words, What is the role of sodium chloride in MSA and how does it work? - CORRECT
ANSWER✅✅Sodium chloride is the selective agent in this medium. Its presence dehydrates most cells.
4-4
In your own words, what are the roles of mannitol and phenol red in MSA? - CORRECT ANSWER✅✅The
ability to ferment mannitol to acid end products make this a differential medium. The acid end products
are detected by the pH indicatory phenol red, which turns 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"? - CORRECT ANSWER✅✅The nutrient agar plate 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 if sparse growth on MSA was due
to inhibition or was just normal growth for that species.
4-4
Why wouldn't it be advisable to compare growth of the organisms on each plate to each other? There
are at least two answers to this question. - CORRECT ANSWER✅✅On the NA plate, "good growth" for
one organism may be very dense, whereas "good growth" for another might be fairly thin. 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 occurred. You wouldn't want to compare growth of the
organisms on the MSA plate (in the absence of information provided by the NA plate) to each other
because you wouldn't know which ones were inhibited and which were normal.
4-4
Would removal of sodium chloride from MSA alter the mediums sensitivity or specificity? Explain your
answer. - CORRECT ANSWER✅✅It would alter specificity because organisms that "shouldn't" grow on it
would, and some of those might be able to ferment mannitol to acid end products. It wouldn't likely
alter sensitivity because you would still probably be able to detect growth of organisms that "should"
grow on it.
4-4
Suppose a mistake is made in preparing a batch of MSA and the starting pH is 7.4 instead of 7.0-7.2.
Would that affect the medium's sensitivity or specificity? Explain your answer. - CORRECT
ANSWER✅✅If a batch of MSA were made at a higher than normal pH, it would take more acid
production from mannitol fermentation to produce the yellow color of a positive. ITs possible, then, that
a true positive for mannitol fermentation might not change the medium's color in the allotted
incubation time and would be incorrectly recorded as a negative. This would affect the medium's
sensitivity.
4-4
With the diversity of microorganisms in the world, how can a single test such as MSA be used to
"confidently" identify Staphylococcus aureus? - CORRECT ANSWER✅✅if the sample plated is one taken
from a random source, identification would not be as certain. However, in the clinical situations, the
4-4
In your own words, what is the application (purpose) of MSA? - CORRECT ANSWER✅✅Most clinical
isolates are incapable of growing in 7.5% salt, but Staphylococcus species can, which makes this a
selective medium used for isolating Staphylococcus species. It is also differential, because most
Staphylococcus species are not able to ferment mannitol, but S.aureus.
4-4
Which Ingredients in MSA supplies
a) Carbon?
b) Nitrogen - CORRECT ANSWER✅✅a) beef extract, peptone, and mannitol all contain usable carbon
b) beef extract and peptone contain organic nitrogen
4-4
Is MSA a defined or undefined medium? Provide reasoning behind the your choice. - CORRECT
ANSWER✅✅MSA is an undefined medium because the exact composition of beef extract and peptone
are unknown. The goal of this medium is to isolate desired organisms, and once the undesired
organisms have been inhibited, you want the desired ones to grow well. The variety of biochemicals in
beef extract supplemented by the peptone make the medium more likely to satisfy the varied
nutritional needs of the organisms being selected for.
4-4
In your own words, What is the role of sodium chloride in MSA and how does it work? - CORRECT
ANSWER✅✅Sodium chloride is the selective agent in this medium. Its presence dehydrates most cells.
4-4
In your own words, what are the roles of mannitol and phenol red in MSA? - CORRECT ANSWER✅✅The
ability to ferment mannitol to acid end products make this a differential medium. The acid end products
are detected by the pH indicatory phenol red, which turns 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"? - CORRECT ANSWER✅✅The nutrient agar plate 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 if sparse growth on MSA was due
to inhibition or was just normal growth for that species.
4-4
Why wouldn't it be advisable to compare growth of the organisms on each plate to each other? There
are at least two answers to this question. - CORRECT ANSWER✅✅On the NA plate, "good growth" for
one organism may be very dense, whereas "good growth" for another might be fairly thin. 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 occurred. You wouldn't want to compare growth of the
organisms on the MSA plate (in the absence of information provided by the NA plate) to each other
because you wouldn't know which ones were inhibited and which were normal.
4-4
Would removal of sodium chloride from MSA alter the mediums sensitivity or specificity? Explain your
answer. - CORRECT ANSWER✅✅It would alter specificity because organisms that "shouldn't" grow on it
would, and some of those might be able to ferment mannitol to acid end products. It wouldn't likely
alter sensitivity because you would still probably be able to detect growth of organisms that "should"
grow on it.
4-4
Suppose a mistake is made in preparing a batch of MSA and the starting pH is 7.4 instead of 7.0-7.2.
Would that affect the medium's sensitivity or specificity? Explain your answer. - CORRECT
ANSWER✅✅If a batch of MSA were made at a higher than normal pH, it would take more acid
production from mannitol fermentation to produce the yellow color of a positive. ITs possible, then, that
a true positive for mannitol fermentation might not change the medium's color in the allotted
incubation time and would be incorrectly recorded as a negative. This would affect the medium's
sensitivity.
4-4
With the diversity of microorganisms in the world, how can a single test such as MSA be used to
"confidently" identify Staphylococcus aureus? - CORRECT ANSWER✅✅if the sample plated is one taken
from a random source, identification would not be as certain. However, in the clinical situations, the
