Blood Agar - PowerPoint Presentation

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

. Slide5
Slide6

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).Slide10
Slide11
Slide12
Slide13
Slide14

Hemolyses

of

Streptococcus spp

.

(left)

α-

hemolysis (

S.

mitis

);

(middle)

β-

hemolysis (

S.

pyogenes

);

(right)

γ-

hemolysis (non-hemolytic,

S.

salivarius

)