Microbiology ASCP MLT medialab
UPDATED ACTUAL Exam Questions and
CORRECT Answers
Sheep's blood agar is least suitable for the growth of which of the following organisms?
- Haemophilus influenzae
- Staphylococcus aureus
- Streptococcus species
- Neisseria meningitidis - CORRECT ANSWER - - Haemophilus influenzae
Conventional sheep blood agar is not suitable for recovery of Haemophilus species. Enzymes in
native sheep blood inactivate V factor, which is required for growth of Haemophilus species
such as Haemophilus influenzae. If the agar medium is supplemented with rabbit or horse blood,
Haemophilus species will grow as rabbit or horse blood does not contain these enzymes.
Selective Haemophilus isolation media is available: these contain beef heart infusion, peptones,
yeast extract (provides V Factor), and defibrinated horse blood (provides X Factor, which is also
required for growth of Haemophilus influenza).
Staphylococcus aureus grows rapidly on sheep blood agar. Sheep blood is used as the indicator
for hemolysis. Observation and interpretation of the hemolysis and subsequent tests are useful in
the identification of this organism.
Streptococcus species grow well on sheep blood agar. Sheep blood is used as the indicator for
hemolysis. Observation and interpretation of the hemolytic properties of streptococci aid in
determining subsequent tests for species identification.
Neisseria meningitidis grows best on enriched media such as chocolate agar but is also able to
grow on sheep blood agar. This organism, like other Neisseria species, grows best in a CO2
enriched environment. This organism colonizes the nasopharyngeal and oropharyngeal mucous
membranes but is also an opportunistic pathogen causing bacterial meningitis.
A parasitic egg was found in stool and measured 170 µm by 63 µm (see image to the right).
Identify the parasitic egg.
- Schistosoma japonicum egg
- Fasciola hepatica egg
- Schistosoma mansoni egg
,- Ascaris lumbricoides egg - CORRECT ANSWER - - Schistosoma mansoni egg
Schistosoma mansoni egg is the correct answer. Schistosoma mansoni eggs measure 115-180 µm
x 40-75 µm and are found in stool specimens. The parasitic egg of this species has a large lateral
spine (as seen in the image), is embryonated, and unoperculated.
Schistosoma japonicum egg is incorrect. Schistosoma japonicum eggs measure 70-100 µm x 50-
85 µm and are found in the small intestines. The eggs have a small lateral spine, are
unoperculated, and embryonated.
Fasciola hepatica egg is incorrect. Fasciola hepatica eggs measures 130-150 µm x 70-90 µm, are
operculated, unembryonated, and have a brownish-yellow color. In addition, this species is
known as a liver fluke and can cause bile duct obstructions.
Ascaris lumbricoides egg is incorrect. Ascaris lumbricoides eggs measure 45-95 µm x 35-40 µm,
have a very thick shell, and may have a mammilated outer albuminoid coat.
The image shows a variety of colony elevations. Which colony elevation description matches
with figure B in the image?
- High Convex or Dome
- Low Convex
- Flat
- Raised - CORRECT ANSWER - - Flat
Elevation is observed in colonies by tilting the culture plates and looking at the side of the
colony to view how the colony is raised off the agar plate. Figure B shows a flat colony as there
is not much elevation shown off the surface.
High Convex or Dome is shown in Figure A. The colony has a high elevation off the agar
surface and is dome-shaped.
Low Convex is shown in Figure C. The colony elevation is a bit lower but still rather convex, or
round in shape.
Raised is shown in Figure D. The colony is not rounded, so it is not convex, and is only slightly
elevated off the media surface.
Other colony elevations include umbilicate, which has a central depression in the colony, and
umbonate which as a central elevation in the colony.
,Which of the following best describes the organisms seen in this illustration:
- Gram-positive
- Gram-negative
- Gram-variable
- Acid-fast - CORRECT ANSWER - - Gram-variable
The organisms in this image are demonstrating a gram-variable phenomenon. For the most part,
the organisms are staining primarily gram-negative, however, on the tips of some of the rods,
there is a gram-positive staining morphology. This can be defined as gram-variable.
Gram-positive organisms stain dark purple due to the crystal violet. During the gram stain
procedure, crystal violet dye is added first to the slide for 30 sec. The crystal violet is washed off
with water and Gram's iodine is added to the slide. The crystal violet and iodine form a complex
together that does not wash out in organisms with thick cell walls containing teichoic acid.
Therefore, these organisms retain the purple color and do not counterstain with safranin.
Gram-negative organisms stain pink from the counterstain, safranin dye. These organisms do not
have a thick cell wall for the crystal violet and iodine complex to adhere to. So during the gram
stain procedure after the crystal violet and iodine complex is washed out, the slide is stained with
safranin dye. This is a pink dye and all organisms including epithelial cells stain pink with this
dye.
Acid-fast stain is used for Mycobacteria. The carbolfuchsin stain binds to mycolic acid in the cell
walls, which is red in color. After the de-colorization stage, the sample is stained with methylene
blue. So, any Mycobacterium in the sample will stain red and all other organisms and cells will
stain blue.
All of the following are true concerning clinical specimens that are gram stained before
culturing, EXCEPT:
- The gram stain allows you to judge the quality of the specimen.
- The gram stain provides the clinician with same day information regarding possible pathogenic
organisms.
- The gram stain eliminates the need for a culture.
- The gram stain provides internal quality control when direct smear results are compared to
culture results. - CORRECT ANSWER - - The gram stain eliminates the need for a
culture.
, The correct answer is the gram stain eliminates the need for a culture.
A Gram stain does not replace a culture; cultures still need to be processed even though a Gram
stain was performed.
Gram-stained direct smears can be used to:
- Judge the quality of the specimen.
- Provide the clinician with same-day information regarding possible pathogenic organisms,
pending results of culture and sensitivity.
- Provide internal quality control when direct smear results are compared to culture results.
Shigella serotyping is performed by using a polyvalent somatic (O) antisera. A Shigella species
grew from culture and serotyped as Group D. Which of the following Shigella species serotypes
as Group D?
- Shigella boydii
- Shigella dysenteriae
- Shigella sonnei
- Shigella flexneri - CORRECT ANSWER - - Shigella sonnei
Shigella sonnei is the correct answer because this species types as Group D. Shigella sonnei
infection is caused by the ingestion of contaminated food and water. The mode of pathogenicity
is due to invasion of the intestinal mucosa, which will be demonstrated by the presence of white
blood cells in stool specimens.
Shigella boydii is incorrect because this species types as Group C. Shigella boydii infection is
caused by the ingestion of contaminated food and water. The mode of pathogenicity is due to
invasion of the intestinal mucosa, which will be demonstrated by the presence of white blood
cells in stool specimens.
Shigella dysenteriae is the incorrect answer because this species types as Group A. Shigella
dysenteriae produces the most severe form of illness and can lead to hemolytic uremic syndrome
(HUS). However, all Shigella species can cause dysentery that includes watery diarrhea, fever,
and abdominal cramps. As the infection progresses, blood and mucus can be seen in stool
specimens.
UPDATED ACTUAL Exam Questions and
CORRECT Answers
Sheep's blood agar is least suitable for the growth of which of the following organisms?
- Haemophilus influenzae
- Staphylococcus aureus
- Streptococcus species
- Neisseria meningitidis - CORRECT ANSWER - - Haemophilus influenzae
Conventional sheep blood agar is not suitable for recovery of Haemophilus species. Enzymes in
native sheep blood inactivate V factor, which is required for growth of Haemophilus species
such as Haemophilus influenzae. If the agar medium is supplemented with rabbit or horse blood,
Haemophilus species will grow as rabbit or horse blood does not contain these enzymes.
Selective Haemophilus isolation media is available: these contain beef heart infusion, peptones,
yeast extract (provides V Factor), and defibrinated horse blood (provides X Factor, which is also
required for growth of Haemophilus influenza).
Staphylococcus aureus grows rapidly on sheep blood agar. Sheep blood is used as the indicator
for hemolysis. Observation and interpretation of the hemolysis and subsequent tests are useful in
the identification of this organism.
Streptococcus species grow well on sheep blood agar. Sheep blood is used as the indicator for
hemolysis. Observation and interpretation of the hemolytic properties of streptococci aid in
determining subsequent tests for species identification.
Neisseria meningitidis grows best on enriched media such as chocolate agar but is also able to
grow on sheep blood agar. This organism, like other Neisseria species, grows best in a CO2
enriched environment. This organism colonizes the nasopharyngeal and oropharyngeal mucous
membranes but is also an opportunistic pathogen causing bacterial meningitis.
A parasitic egg was found in stool and measured 170 µm by 63 µm (see image to the right).
Identify the parasitic egg.
- Schistosoma japonicum egg
- Fasciola hepatica egg
- Schistosoma mansoni egg
,- Ascaris lumbricoides egg - CORRECT ANSWER - - Schistosoma mansoni egg
Schistosoma mansoni egg is the correct answer. Schistosoma mansoni eggs measure 115-180 µm
x 40-75 µm and are found in stool specimens. The parasitic egg of this species has a large lateral
spine (as seen in the image), is embryonated, and unoperculated.
Schistosoma japonicum egg is incorrect. Schistosoma japonicum eggs measure 70-100 µm x 50-
85 µm and are found in the small intestines. The eggs have a small lateral spine, are
unoperculated, and embryonated.
Fasciola hepatica egg is incorrect. Fasciola hepatica eggs measures 130-150 µm x 70-90 µm, are
operculated, unembryonated, and have a brownish-yellow color. In addition, this species is
known as a liver fluke and can cause bile duct obstructions.
Ascaris lumbricoides egg is incorrect. Ascaris lumbricoides eggs measure 45-95 µm x 35-40 µm,
have a very thick shell, and may have a mammilated outer albuminoid coat.
The image shows a variety of colony elevations. Which colony elevation description matches
with figure B in the image?
- High Convex or Dome
- Low Convex
- Flat
- Raised - CORRECT ANSWER - - Flat
Elevation is observed in colonies by tilting the culture plates and looking at the side of the
colony to view how the colony is raised off the agar plate. Figure B shows a flat colony as there
is not much elevation shown off the surface.
High Convex or Dome is shown in Figure A. The colony has a high elevation off the agar
surface and is dome-shaped.
Low Convex is shown in Figure C. The colony elevation is a bit lower but still rather convex, or
round in shape.
Raised is shown in Figure D. The colony is not rounded, so it is not convex, and is only slightly
elevated off the media surface.
Other colony elevations include umbilicate, which has a central depression in the colony, and
umbonate which as a central elevation in the colony.
,Which of the following best describes the organisms seen in this illustration:
- Gram-positive
- Gram-negative
- Gram-variable
- Acid-fast - CORRECT ANSWER - - Gram-variable
The organisms in this image are demonstrating a gram-variable phenomenon. For the most part,
the organisms are staining primarily gram-negative, however, on the tips of some of the rods,
there is a gram-positive staining morphology. This can be defined as gram-variable.
Gram-positive organisms stain dark purple due to the crystal violet. During the gram stain
procedure, crystal violet dye is added first to the slide for 30 sec. The crystal violet is washed off
with water and Gram's iodine is added to the slide. The crystal violet and iodine form a complex
together that does not wash out in organisms with thick cell walls containing teichoic acid.
Therefore, these organisms retain the purple color and do not counterstain with safranin.
Gram-negative organisms stain pink from the counterstain, safranin dye. These organisms do not
have a thick cell wall for the crystal violet and iodine complex to adhere to. So during the gram
stain procedure after the crystal violet and iodine complex is washed out, the slide is stained with
safranin dye. This is a pink dye and all organisms including epithelial cells stain pink with this
dye.
Acid-fast stain is used for Mycobacteria. The carbolfuchsin stain binds to mycolic acid in the cell
walls, which is red in color. After the de-colorization stage, the sample is stained with methylene
blue. So, any Mycobacterium in the sample will stain red and all other organisms and cells will
stain blue.
All of the following are true concerning clinical specimens that are gram stained before
culturing, EXCEPT:
- The gram stain allows you to judge the quality of the specimen.
- The gram stain provides the clinician with same day information regarding possible pathogenic
organisms.
- The gram stain eliminates the need for a culture.
- The gram stain provides internal quality control when direct smear results are compared to
culture results. - CORRECT ANSWER - - The gram stain eliminates the need for a
culture.
, The correct answer is the gram stain eliminates the need for a culture.
A Gram stain does not replace a culture; cultures still need to be processed even though a Gram
stain was performed.
Gram-stained direct smears can be used to:
- Judge the quality of the specimen.
- Provide the clinician with same-day information regarding possible pathogenic organisms,
pending results of culture and sensitivity.
- Provide internal quality control when direct smear results are compared to culture results.
Shigella serotyping is performed by using a polyvalent somatic (O) antisera. A Shigella species
grew from culture and serotyped as Group D. Which of the following Shigella species serotypes
as Group D?
- Shigella boydii
- Shigella dysenteriae
- Shigella sonnei
- Shigella flexneri - CORRECT ANSWER - - Shigella sonnei
Shigella sonnei is the correct answer because this species types as Group D. Shigella sonnei
infection is caused by the ingestion of contaminated food and water. The mode of pathogenicity
is due to invasion of the intestinal mucosa, which will be demonstrated by the presence of white
blood cells in stool specimens.
Shigella boydii is incorrect because this species types as Group C. Shigella boydii infection is
caused by the ingestion of contaminated food and water. The mode of pathogenicity is due to
invasion of the intestinal mucosa, which will be demonstrated by the presence of white blood
cells in stool specimens.
Shigella dysenteriae is the incorrect answer because this species types as Group A. Shigella
dysenteriae produces the most severe form of illness and can lead to hemolytic uremic syndrome
(HUS). However, all Shigella species can cause dysentery that includes watery diarrhea, fever,
and abdominal cramps. As the infection progresses, blood and mucus can be seen in stool
specimens.
