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Veterinary bacteriology: information about important bacteria
Veterinary bacteriology

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Blood agar

Blood agar plate


Blood agar can be used for isolation and cultivation of fastidious and non-fastidious bacteria. The blood agar plates are incubated at different temperatures and in different atmospheres depending on the actual bacterium.

The medium contains:

  • beef extract
  • tryptone (contains amino acids)
  • NaCl (contributes to the osmotisc balance)
  • 5-10% blood (may originate from e.g. cattle, sheep or horses)
  • agar (gelifying agent)

Bacteria can also be differentiated, based on their capacity to secrete hemolysins, by cultivation on blood agar. Hemolysins are substances (usually enzymes or porins), which have the capacity to lyse red blood cells. The hemolysis will cause a clearing zone of the blood agar around the colonies. Bacteria can cause different types of hemolysis:

  • α-hemolysis, which means an incomplete clearing (green hemolysis).
  • β-hemolysis, which means a complete clearing.
  • Doubble hemolysis of some staphylococci consisting of an inner α-hemolysis zone and an outer β-hemolysis zone (see also below).
  • No hemolysis, which is sometimes referred to as γ-hemolysis, which may seem illogical.

Other comments:

Note that the α-hemolysin of staphylococci causes complete hemolysis, whereas their β-hemolysin causes incomplete hemolysis.
The capacity to produce hemolysins may vary between different strains of a particular bacterial species.

Updated: 2021-05-20.

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