Chocolate agar

Chocolate agar

Chocolate agar, also known as heated blood agar, is a type of growth medium used in microbiology for the cultivation of a wide range of bacteria, particularly those that are more fastidious in their growth requirements. This agar gets its name from the chocolate-brown color it takes on, which is a result of heating the blood that is added to the agar, causing the red blood cells to lyse. The lysis of red blood cells releases nutrients, including hemoglobin, which supports the growth of many pathogenic bacteria that would not grow on other types of media.

Composition

Chocolate agar consists of a base of agar, a gel-like substance derived from seaweed that provides a solid surface for bacterial growth. To this base, sheep blood or sometimes horse blood is added, typically at a concentration of 5-10%. The mixture is then heated until the blood cells lyse, giving the medium its characteristic chocolate-brown color. Additional supplements may be added to the medium to support the growth of specific bacteria or to inhibit the growth of others. These can include antibiotics, vitamins, and amino acids.

Uses

Chocolate agar is particularly useful for the cultivation of Haemophilus influenzae, a cause of meningitis, pneumonia, and septicemia, and Neisseria gonorrhoeae, the bacterium responsible for gonorrhea. It is also used for the isolation of Neisseria meningitidis, the causative agent of meningococcal meningitis. The enriched environment provided by the lysed blood cells allows these fastidious organisms to grow, which would not be possible on standard blood agar plates.

Preparation

The preparation of chocolate agar involves adding the blood to the agar base and then heating the mixture to a temperature sufficient to lyse the blood cells. This process not only releases the nutrients contained within the red blood cells but also inactivates any potential inhibitors of bacterial growth. Once prepared, the agar is poured into sterile petri dishes to solidify and is then ready for use.

Limitations

While chocolate agar is invaluable for the cultivation of certain bacteria, it is not suitable for all types. Some bacteria may require more specialized media for growth, or they may be inhibited by the components of chocolate agar. Additionally, because the medium is enriched, it can support the growth of a wide range of bacteria, which can sometimes make isolating a specific organism challenging.

See Also

• Agar plate
• Blood agar
• Microbiological culture
• Selective medium

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