Chocolate agar

Chocolate agar showing Francisella tularensis colonies

Comparison of two culture media types used to grow Neisseria gonorrhoeae bacteria.

Known as overgrowth, note that the non-selective chocolate agar medium on the left, due to its composition, allowed for the growth of organismal colonies other than those of Neisseria 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. (To see N. gonorrhoeae colonies, click to enlarge) .

Chocolate agar (CHOC) - is a non-selective, enriched growth medium. It is a variant of the blood agar plate. It contains red blood cells, which have been lysed by heating very slowly to 56 °C. Chocolate agar is used for growing fastidious (fussy) respiratory bacteria, such as Haemophilus influenzae. These bacteria need growth factors, like NAD and hemin, which are inside red blood cells; thus, a prerequisite to growth is lysis of the red blood cells. The agar is named for the color and contains no actual chocolate.

Chocolate agar alone is non-selective, however with the addition of Bacitracin the media becomes selective, most critically, for the genus Haemophilus. A further variant containing an assortment of antibiotics selects for Neisseria.