Blood Agar:
Blood agar is both differential and enriched medium. The blood that is incorporated into this medium is an enrichment ingredient for the cultivation of fastidious organisms such as the Streptococcus species.
A number of streptococcal species produce substances that destroy red blood cells; that is, they cause lysis of the red cell wall with subsequent release of hemoglobin. Such substances are referred to as hemolysins. The activity of streptococcal hemolysins also known as streptolysins can be readily observed when the organisms are growing on a blood agar plate.
Different streptococci produce different effects on the red blood cells in blood agar. Those that produce incomplete hemolysis and only partial destruction of the cells around colonies are called alpha-hemolytic Streptococci. Characteristically, this type of hemolysis is seen as a distinct greening of the agar in the hemolytic zone, and thus this group of streptococci has also been referred to as the viridans group.
Species whose hemolysins cause complete destruction of red cells in the agar zones surrounding their colonies are said to be beta-hemolytic. When growing on blood agar, beta-hemolytic streptococci are small opaque or semi translucent colonies surrounded by clear zones in a red opaque medium. Two types of beta lysins are produced: Streptolysin O and Streptolysin S. Streptolysin O, an antigenic, oxygen-labile enzyme, and streptolysin S, a nonantigenic, oxygen-stable lysin. The hemolytic reaction is enhanced when blood agar plates are streaked and simultaneously stabbed to show subsurface hemolysis by Streptolysin O in an environment with reduced oxygen tension. Some strains of Staphylococci, Escherichia coli, and other bacteria also may show beta-hemolysis.
Some species of Streptococci do not produce hemolysins. Therefore, when their colonies grow on blood agar, no change is seen in the red blood cells around them. These species are referred to as nonhemolytic or gamma hemolytic streptococci.
On blood agar, S. aureus usually displays a light to golden yellow pigment, whereas S.epidermidis has a white pigment and S.saprophyticus either a bright yellow or white pigment. However, pigmentation is not always a reliable characteristic. On blood agar, S.aureus is usually, but not always, beta-hemolytic; S. epidermidis and S. saprophyticus are almost always nonhemolytic.
Formula:
Ingredients per liter of deionized water
Note: Dissolve the above ingredients and autoclave. Cool the sterile blood agar base to 45° to 50°C and aseptically add 50 ml of sterile, defibrinated blood. Mix thoroughly and then dispense into plates while a liquid. Blood agar base for use in making blood agar also can be purchased. A combination of hemoglobin and a commercial nutrient supplement can be used in place of defibrinated blood.