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Blood Agar

Blood agar is a type of bacterial growth medium. It is mainly used to grow fastidious microorganisms such as streptococci. Such organisms do not grow well using an ordinary growth medium.

They only grow in blood agar because this medium has inhibitors for some family of bacteria. The medium needs to be heated to inactivate the inhibitor and release a specific growth factor. Once the blood agar is heated, it will become chocolate agar.

Blood Agar

Examples of bacteria that grow in blood agar medium.

  • Haemophilus influenza
  • Neisseria species
  • Strep Pneumonia

Agar blood as a differential medium.

Blood agar is a differential medium because it helps detect and differentiate hemolytic bacteria such as Streptococcus species. Detects hemolysis by cytolytic toxins that are secreted by a strain of bacteria.

Agar blood as a selective medium.

Blood agar can be used as a selective medium for some types of pathogens. To be selective, you must add a chemical, a dye or an antibiotic.

Blood agar Components

  • Agar
  • Pancreatic digest of casein
  • Sodium chloride
  • Papaic digest of soy meal
  • Beef extract
  • Peptone
  • Sheep blood, defibrinated
  • Distilled water
  • pH 7.3 ± 0.2 at 25°C

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For 1 Liter Preparation

Agar 15.0 g
Beef extract 10.0 g
Peptone 10.0 g
Sodium Chloride (NaCl) 5,0 g
Sheep blood, defibrinated 50 ml
pH 7.3 ± 0.2 at 25°C
Distilled/Deionized 950 ml

Blood agar Preparation

  • Add above components (40 gm), except sheep blood, to distilled/deionized water and bring volume to 950.0 mL.
  • Mix thoroughly.
  • Heat with frequent agitation and boil for 1 min to completely dissolve.
  • Autoclave for 15 min at 15 psi pressure at 121°C.
  • Cool to 45°- 50°C.
  • Aseptically add 50.0 mL of sterile, defibrinated sheep blood.
  • Mix thoroughly and pour into sterile Petri dishes.

What is blood agar used for?

  • It is used to isolate and identify antimicrobial susceptibility of Streptococci.
  • It is used to isolate and cultivate Streptococci, Neisseria, and fastidious microorganisms.
  • Blood agar helps differentiate bacteria based on their haemolytic properties.
  • It is used to prepare Salmonella typhi antigens.
  • It determines the salinity range of marine Flavobacteria.
  • Blood agar base can be used for testing food samples as per the recommendation of APHA.
  • It helps determine the type of hemolysis.

What Bacteria grow on blood agar?

  • Neisseria
  • Haemophilus genera
  • Streptococcus pyogenes
  • S. pneumonia
  • Satellitism of H. influenza
  • Streptococcus pneumonia
  • Streptococci-Streptococcus agalactiace
  • Clostridium perfringens

What is the difference between blood agar and chocolate agar?

Blood agar and chocolate agar are quite similar but they differ in preparation. Blood agar consists of many ingredients but the primary ingredient is the blood, which came from a rabbit or a sheep.

It is treated and water and a number of other ingredients are added to sterilize the solution. The blood is treated to remove fibrin, the clotting factor of the blood. On the other hand, chocolate agar contains lysed red blood cells, which turns brown giving the medium its chocolate color.

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