Biology amp Biotechnology Department PURPOSE Blood agar is used for isolation and cultivation of many types of fastidious bacteria It is also used to differentiate bacteria based on their hemolytic characteristics especially within the genera ID: 542818
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Slide1
Blood Agar
Biology & Biotechnology DepartmentSlide2
PURPOSE :
Blood agar
is
used for
:
isolation and cultivation of many types of fastidious bacteria.
It is also used to differentiate bacteria based on their hemolytic characteristics, especially within the genera
Streptococcus, Enterococcus,
and
Aerococcus
. Slide3
PRINCIPLE
:
Several species of
Gram-positive
cocci
produce
exotoxins
called
hemolysins
able to destroy
red blood cells (RBCs) and hemoglobin.
Blood agar
, which is a mixture of
nutrient agar
and sheep blood, allows differentiation of bacteria based on their ability to
hemolyze
RBCs.
The
three major types of hemolysis
are
ß hemolysis, a hemolysis
, and
'Y hemolysis
. Slide4
ß
hemolysis
is the
complete
destruction of RBCs and hemoglobin, and
results in
a
clearing of the medium
around the colonies
a
hemolysis
is the
partial
destruction of RBCs and
produces
a
greenish discoloration of the agar around the
colonies
.
'Y hemolysis
is
actually non-hemolysis
and
appears as simple growth with no change to the
medium
. Slide5Slide6
Hemolysis
:
is the
breakdown of red blood cells.
The ability of bacterial colonies to
induce hemolysis
when
grown on blood agar
Is used to
classify certain microorganisms
.
This is particularly useful in classifying
streptococcal species
.
A substance that causes hemolysis is a
hemolysinSlide7
Preparation of Blood Agar:
Nutrient agar which
has been melted and
cooled
to( 40
to
45°C)
add 5% (
vol
/
vol
)
sterile
defibrinated
blood
that has been
warmed
to room temperature.
Swirl
the flask to mix thoroughly,
avoiding the formation of bubbles
, and dispense into
sterile plates
, continuing to
avoid bubbles and froth on the surface
.
NOTE
:
Cooling
the agar and warming the blood are
essential steps
in this procedure
.
Hot
agar
can
damage
red blood cells, and
cold blood
can
cause
the agar to
gel before pouring
.Slide8
Bacteria can produce different types of
hemolysins
:
Beta-
hemolysin
breaks down the red blood cells and hemoglobin completely.
This leaves
a clear zone
around the bacterial growth. Such results are referred to as β-hemolysis (beta hemolysis).
Alpha-
hemolysin
partially breaks down the red blood cells and leaves a
greenish color
behind.
This is referred to as α-hemolysis (alpha hemolysis).
The
greenish color is caused by
the presence of
biliverdin
, which is a by-product of the breakdown of hemoglobin. Slide9
If the organism
does not produce
hemolysins
and does not break down the blood cells,
no clearing will occur
.
This
is called γ-hemolysis (gamma hemolysis).Slide10Slide11Slide12Slide13Slide14
Hemolyses
of
Streptococcus spp
.
(left)
α-
hemolysis (
S.
mitis
);
(middle)
β-
hemolysis (
S.
pyogenes
);
(right)
γ-
hemolysis (non-hemolytic,
S.
salivarius
)