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CLINICAL BACTERIOLOGY
LABORATORY
STAPHYLOCOCCI
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Cards (45)
Procedure for investigating an unknown organism in microbiology laboratory
1. Undergo
screening
test
2. Perform
presumptive
test
3. Perform
confirmatory
test
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Screening test
Screens for the
presence
or
absence
of the organism in the specimen
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Presumptive test
Presumes the
identification
of the
unknown
organism
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Confirmatory
test
Confirms the presence of the
organism
of
interest
in the sample
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Microscopic
evaluation of the
morphology
of the organism is the first objective in the screening test
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Screening test is always done first before
presumptive
and
confirmatory
tests
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If the screening test is
negative
, it is impractical to perform the
confirmatory
test
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Presumptive tests
1.
Pigment
production on potato agar
2.
Catalase
production on trypticase soy agar (TSA)
3.
Hemolysis
and colonial
morphology
on blood agar (BA)
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Confirmatory tests
1.
Mannitol
fermentation test on
mannitol salt agar
(MSA)
2.
Coagulase
test
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Staphylococci
Gram
positive
cocci arranged in
clusters
Appear
purple
under the microscope
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Morphology alone cannot differentiate
Staphylococcus aureus
from
Staphylococcus epidermidis
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Catalase
test
Detects the ability of the organism to produce the enzyme
catalase
using
hydrogen peroxide
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Usefulness of catalase test
Differentiates
Staphylococci
from
Streptococci
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Performing catalase test
1. Grow colonies on
trypticase
soy agar (
TSA
)
2. Treat grown colonies with
hydrogen peroxide
(H2O2)
3. Appearance of
gas bubbles
indicates a
positive
test
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Positive catalase
test indicates the organism is from the
Staphylococci
family
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Blood agar
Enrichment
and
differential
medium
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Staphylococcus aureus
Produces small,
circular
,
golden-yellow
colonies
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Staphylococcus epidermidis
Produces
white
colonies
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Hemolysis
/
hemolytic
pattern
Indicates the ability of the organism to cause lysis of
red blood cells
in the
blood agar medium
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Agar
Medium that particular organism has to utilize
substances
found inside the
red blood cell
(a content of blood agar)
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Organism utilizing substance inside RBC
1. Destroy
RBC
to get
substance
2. Cause destruction or
lysis
of RBCs in
blood agar
medium
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Hemolytic
patterns
Different patterns observed in blood agar medium depending on organism's ability to cause
lysis
of
RBCs
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Hemolytic patterns
Beta-hemolysis
Alpha-hemolysis
Gamma-hemolysis
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Beta-hemolysis
Complete hemolysis/destruction/lysis of
RBCs
in
blood agar
medium
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Alpha-hemolysis
Incomplete destruction of
RBCs
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Gamma-hemolysis
No
hemolysis
of
red
blood cells
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Staphylococcus aureus produces
beta-hemolysis
, Staphylococcus epidermidis produces
gamma-hemolysis
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Mannitol
salt agar
Selective and differential medium containing
mannitol
and 7.5%
sodium chloride
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Selective
characteristic of mannitol salt agar
Only
salt-loving
organisms can
survive
and grow
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Differential characteristic of mannitol salt agar
Organism fermenting mannitol produces
acid
, changing agar color from light pink to
yellow
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Staphylococcus
aureus
is mannitol fermentation positive, Staphylococcus epidermidis is mannitol fermentation
negative
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Coagulase
Enzyme produced by
Staphylococcus aureus
that can convert
fibrinogen
to fibrin, causing clot formation
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Staphylococcus aureus is coagulase
positive
, Staphylococcus epidermidis is coagulase
negative
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Coagulase tests
Slide
coagulase
Tube
coagulase
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Staphylococcus
aureus
produces
yellow
pigment (lipochrome), Staphylococcus epidermidis produces white pigment
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Criteria for identifying Staphylococci
Colonial appearance on
blood
agar
Colonial appearance on
MSA
Gram
staining reaction
Microscopic
morphology
Catalase
&
coagulase
production
Mannitol
fermentation
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Catalase
test
Detects ability of bacteria to convert
hydrogen peroxide
to
oxygen
and water
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Staphylococci are catalase
positive
, Streptococci are catalase
negative
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Coagulase
test
Detects ability of
Staphylococcus aureus
to produce coagulase enzyme
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Coagulase test types
Slide
coagulase
Tube
coagulase
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