bloedagar

Bloedagar, or blood agar in English, is an enriched, differential growth medium used in microbiology. It consists of a non-selective nutrient agar base (such as trypticase soy or Columbia agar) into which about 5% defibrinated blood, usually from sheep, is added to yield a solid medium that supports growth of a wide range of bacteria and enables observation of hemolysis.

Because the erythrocytes in the medium remain largely intact, colonies may be surrounded by zones of clear or altered red cells that indicate hemolysis. Depending on the organism, the hemolysis pattern is classified as alpha, beta, or gamma.

Alpha-hemolysis shows a partial, often greenish discoloration around colonies due to partial red blood cell lysis. Beta-hemolysis produces a clear or pale zone of complete clearing around colonies. Gamma-hemolysis indicates no hemolysis.

Blood agar is widely used in clinical microbiology for isolation and preliminary differentiation of many bacterial species, particularly streptococci and staphylococci. Beta-hemolytic streptococci (for example Streptococcus pyogenes) and certain staphylococci can be distinguished by their hemolysis patterns.

Variants include commercial blood agar plates and formulations with selective agents (for example for Gram-positive bacteria). Chocolate agar is a related medium in which the red blood cells are lysed before plating, releasing nutrients and enabling growth of fastidious organisms.

Preparation and storage: plates are prepared from a sterile base; blood is added to the cooled base (around 50°C) or pre-mixed; plates are poured and allowed to solidify. They are typically incubated at 35–37°C in air; plates should be stored refrigerated and used within a limited shelf life.