Chocolate agar

Chocolate agar is used for growing fastidious respiratory bacteria, such as Haemophilus influenzae and Neisseria meningitidis.

Chocolate agar with the addition of bacitracin becomes selective for the genus Haemophilus.

The composition of chocolate agar includes the following components:[5] The exact concentrations of these ingredients may vary slightly depending on the specific formulation used in different laboratories or by different manufacturers.

Addition of heated blood to media was first documented for use by Cohen and Fitzgerald in 1910 and then by Dr. Olga Povitzky at the New York City Department of Health Bureau of Laboratories.

[6][7] The term "chocolate agar" comes from the brown color generated from the higher concentration of heated blood in the mixture and was given the distinctive description first by Warren Crowe in 1915.

Chocolate agar showing Francisella tularensis colonies
Comparison of two culture media types used to grow Neisseria gonorrhoeae bacteria.

Known as overgrowth, the nonselective chocolate agar medium on the left, due to its composition, allowed for the growth of organismal colonies other than those of N. gonorrhoeae , while the selective Thayer–Martin medium on the right, containing antimicrobials that inhibit the growth of organisms other than N. gonorrhoeae , shows no overgrowth, but is positive for N. gonorrhoeae bacteria. (Enlarge image to see N. gonorrhoeae colonies)