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Microbiology Lab 9 Questions with Correct
Answers | Updated (100% Correct Answers)
Describe the selective and differential properties of mannitol salt
agar (MSA) for the isolation and identification of staphylococci.
Answer: As a selective medium, MSA has a high concentration of
salt (7.5% NaCl), which is inhibitory to most bacteria other than the
staphylococci. As a differential medium, MSA contains the sugar
mannitol as a substrate for fermentation and phenol red as a pH
indicator to detect the production of acid. S. aureus ferments
mannitol and changes the color of the medium from red to yellow.
Most other staphylococci do not ferment mannitol.
Describe the differential property of blood agar for the isolation and
identification of staphylococci. Answer: Staphylococcus aureus
produces a toxin that lyses red blood cells whereas Staphylococcus
epidermidis and S. saprophyticus do not produce the toxin. Colonies
of S. aureus will produce β-hemolysis on blood agar.
Why is the coagulase test considered to be the definitive test for S.
aureus? Answer: Virtually all strains of S. aureus are coagulase
positive whereas all other species are negative.
What is the role of coagulase in the pathogenesis of S.aureus?
Answer: S. aureus produces coagulase during infection to surround
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,2
itself in a coat of clotted blood protein, which protects it against
host defenses as the bacterium multiplies in the host.
What is the role of alpha toxin in the pathogenesis of S. aureus?
Answer: Alpha toxin is an exotoxin that destroys red blood cells to
release nutrients and growth factors that increase S. aureus
multiplication in the host bloodstream
What are health care acquired infections? Answer: Nosocomial
infections are infections acquired in a hospital or healthcare setting.
Why are the staphylococci among the leading causes of nosocomial
infections? Answer: S. aureus is an opportunistic pathogen that
causes infections of compromised hosts (e.g. surgical patients, burn
and trauma patients) in the hospital. Thirty percent of the normal
population carries S. aureus so endogenous infections in patients
and transmission from healthcare worker to patient are common.
Why are staphylococcal infections becoming increasingly difficult to
treat? Answer: Numerous strains of S. aureus have developed
multiple resistances to common antibiotics including penicillins and
aminoglycosides and the resulting infections have become difficult
to treat effectively.
Why might hospital patients be tested for nasal carriage of S.
aureus? Answer: At-risk patients are tested for S. aureus nasal
carriage because of the risk for endogenous infection.
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, 3
Describe results from a coagulase, DNase, and novobiocin test that
would suggest a mixed culture was used for the tests, as opposed
to a pure culture. Answer: Mixed cultures of S. aureus and S.
saprophyticus would produce positive results for coagulase and
DNase and be resistant to novobiocin. A pure cultures of S. aureus
would be positive for coagulase and DNase but sensitive to
novobiocin. A pure culture of S. saprophyticus would be negative
for coagulase and DNase but resistant to novobiocin.
When bacteria from a throat swab are streaked on blood agar, why
is the agar stabbed several times with the loop? Answer: Stabbing
of the blood agar creates an anaerobic environment for the
streptococci and improves the development of hemolysis.
Differentiate between alpha and beta hemolysis Answer: Alpha
hemolysis is partial lysis of red blood cells and will create a halo of
greenish discoloration around colonies on the blood agar. Beta
hemolysis is complete lysis of red blood cells and will create a clear
halo around colonies on the blood agar.
What was Rebecca Lancefield's contribution to the study of
streptococci? Answer: Lancefield classified streptococci into several
major groups (e.g. group A, B, and C) based upon surface
carbohydrate antigens (Lancefield antigens).
© 2025 All rights reserved
Microbiology Lab 9 Questions with Correct
Answers | Updated (100% Correct Answers)
Describe the selective and differential properties of mannitol salt
agar (MSA) for the isolation and identification of staphylococci.
Answer: As a selective medium, MSA has a high concentration of
salt (7.5% NaCl), which is inhibitory to most bacteria other than the
staphylococci. As a differential medium, MSA contains the sugar
mannitol as a substrate for fermentation and phenol red as a pH
indicator to detect the production of acid. S. aureus ferments
mannitol and changes the color of the medium from red to yellow.
Most other staphylococci do not ferment mannitol.
Describe the differential property of blood agar for the isolation and
identification of staphylococci. Answer: Staphylococcus aureus
produces a toxin that lyses red blood cells whereas Staphylococcus
epidermidis and S. saprophyticus do not produce the toxin. Colonies
of S. aureus will produce β-hemolysis on blood agar.
Why is the coagulase test considered to be the definitive test for S.
aureus? Answer: Virtually all strains of S. aureus are coagulase
positive whereas all other species are negative.
What is the role of coagulase in the pathogenesis of S.aureus?
Answer: S. aureus produces coagulase during infection to surround
© 2025 All rights reserved
,2
itself in a coat of clotted blood protein, which protects it against
host defenses as the bacterium multiplies in the host.
What is the role of alpha toxin in the pathogenesis of S. aureus?
Answer: Alpha toxin is an exotoxin that destroys red blood cells to
release nutrients and growth factors that increase S. aureus
multiplication in the host bloodstream
What are health care acquired infections? Answer: Nosocomial
infections are infections acquired in a hospital or healthcare setting.
Why are the staphylococci among the leading causes of nosocomial
infections? Answer: S. aureus is an opportunistic pathogen that
causes infections of compromised hosts (e.g. surgical patients, burn
and trauma patients) in the hospital. Thirty percent of the normal
population carries S. aureus so endogenous infections in patients
and transmission from healthcare worker to patient are common.
Why are staphylococcal infections becoming increasingly difficult to
treat? Answer: Numerous strains of S. aureus have developed
multiple resistances to common antibiotics including penicillins and
aminoglycosides and the resulting infections have become difficult
to treat effectively.
Why might hospital patients be tested for nasal carriage of S.
aureus? Answer: At-risk patients are tested for S. aureus nasal
carriage because of the risk for endogenous infection.
© 2025 All rights reserved
, 3
Describe results from a coagulase, DNase, and novobiocin test that
would suggest a mixed culture was used for the tests, as opposed
to a pure culture. Answer: Mixed cultures of S. aureus and S.
saprophyticus would produce positive results for coagulase and
DNase and be resistant to novobiocin. A pure cultures of S. aureus
would be positive for coagulase and DNase but sensitive to
novobiocin. A pure culture of S. saprophyticus would be negative
for coagulase and DNase but resistant to novobiocin.
When bacteria from a throat swab are streaked on blood agar, why
is the agar stabbed several times with the loop? Answer: Stabbing
of the blood agar creates an anaerobic environment for the
streptococci and improves the development of hemolysis.
Differentiate between alpha and beta hemolysis Answer: Alpha
hemolysis is partial lysis of red blood cells and will create a halo of
greenish discoloration around colonies on the blood agar. Beta
hemolysis is complete lysis of red blood cells and will create a clear
halo around colonies on the blood agar.
What was Rebecca Lancefield's contribution to the study of
streptococci? Answer: Lancefield classified streptococci into several
major groups (e.g. group A, B, and C) based upon surface
carbohydrate antigens (Lancefield antigens).
© 2025 All rights reserved
