• Blood agar is used to grow a wide range of pathogenic organisms particularly those which do not grow in ordinary culture media. Microorganisms such as Haemophilus influenza, Streptococcus pneumonia and Neissaria species require Blood agar to grow. It is also used to detect haemolytic bacteria especially Streptococcus species.

Composition of blood agar:

1. Nutrient agar ………………………….                1000 ml
2. Sterile defibrinated blood ………. 50 ml

Choice of blood:

• For most pathogens, haemolysis free defibrinated horse, sheep, goat or rabbit blood can be used.
• Sheep blood cannot be used for Haemophilus influenzae because it contains inhibitors for H. influenza.
• Oxalated horse blood is used in blood agar for testing Satellitism.
• Human blood (expired citrated blood from blood bank) should not be used because it contains inhibitors for pathogenic organisms. Also human blood inhibits Streptococcus and interfere with haemolysis pattern.

Preparation of blood agar:

1. Prepare and sterilize nutrient agar in a conical flask by dissolving required amount in distilled water and then autoclaving
2. Transfer the nutrient agar in water bath at 50°C
3. Bring blood at room temperature before adding to NA
4. Add sterile blood (50 ml for 1000 ml NA) aseptically and mix well, gently avoiding bubble formation.
5. Dispense aseptically in 15 ml amount in sterile petri dishes
6. Allow to solidified at room temperature
7. Label the petri dishes with date and batch numbers
8. Store the petri dishes at 4 °C in sealed plastic bag to prevent loss of moisture. It can be used up to 4 weeks

Uses of blood agar:

1. Used to culture Streptococcus pyogens and Streptococcus pneumonia
2. Used for Satellitism test of Hemophiuls infuenzae
   • Inoculate the Blood agar with infulenzae and then streak Staphylococcus aureus on the plate.
   • Incubate the plate in CO2 incubator at 35-37 C for 18-24 hours
   • Observe characteristic satellitism of influenzae

3. Used to culture Neisseria species
4. Blood agar can be made selective medium for certain pathogens by adding antibiotics, dyes, chemicals or by heating
   • Crystal violet blood agar is selective for Streptococcus pyogens
   • Kanamycin or neomycin blood agar is selectives for anaerobes such as Neisseria species
   • Chocolate agar (heated blood agar) is selective for Haemophilus influenzae

Blood agar: composition, preparation and uses

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