Microbiology Lab 9 Exam 2025
Questions and Answers
Describe the selective and differential properties of mannitol salt agar (MSA) for
the isolation and identification of staphylococci. - --CORRECT 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. - --CORRECT 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.
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,Why is the coagulase test considered to be the definitive test for S. aureus? - --
CORRECT 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? - --CORRECT
ANSWER--S. aureus produces coagulase during infection to surround 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? - --CORRECT
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? - --CORRECT ANSWER--Nosocomial
infections are infections acquired in a hospital or healthcare setting.
Why are the staphylococci among the leading causes of nosocomial infections? - --
CORRECT 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.
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, Why are staphylococcal infections becoming increasingly difficult to treat? - --
CORRECT 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? - --
CORRECT ANSWER--At-risk patients are tested for S. aureus nasal carriage
because of the risk for endogenous infection.
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. - --CORRECT
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.
Why is MRSA not confined to transmission only in hospitals? - --CORRECT
ANSWER--
When bacteria from a throat swab are streaked on blood agar, why is the agar
stabbed several times with the loop? - --CORRECT ANSWER--Stabbing of the
blood agar creates an anaerobic environment for the streptococci and improves the
development of hemolysis.
....COPYRIGHT ©️ 2025 ALL RIGHTS RESERVED...TRUSTED & VERIFIED 3
Questions and Answers
Describe the selective and differential properties of mannitol salt agar (MSA) for
the isolation and identification of staphylococci. - --CORRECT 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. - --CORRECT 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.
....COPYRIGHT ©️ 2025 ALL RIGHTS RESERVED...TRUSTED & VERIFIED 1
,Why is the coagulase test considered to be the definitive test for S. aureus? - --
CORRECT 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? - --CORRECT
ANSWER--S. aureus produces coagulase during infection to surround 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? - --CORRECT
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? - --CORRECT ANSWER--Nosocomial
infections are infections acquired in a hospital or healthcare setting.
Why are the staphylococci among the leading causes of nosocomial infections? - --
CORRECT 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.
....COPYRIGHT ©️ 2025 ALL RIGHTS RESERVED...TRUSTED & VERIFIED 2
, Why are staphylococcal infections becoming increasingly difficult to treat? - --
CORRECT 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? - --
CORRECT ANSWER--At-risk patients are tested for S. aureus nasal carriage
because of the risk for endogenous infection.
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. - --CORRECT
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.
Why is MRSA not confined to transmission only in hospitals? - --CORRECT
ANSWER--
When bacteria from a throat swab are streaked on blood agar, why is the agar
stabbed several times with the loop? - --CORRECT ANSWER--Stabbing of the
blood agar creates an anaerobic environment for the streptococci and improves the
development of hemolysis.
....COPYRIGHT ©️ 2025 ALL RIGHTS RESERVED...TRUSTED & VERIFIED 3
