Chocolate agar

Chocolate agar (CHOC) or chocolate blood agar (CBA) – is a nonselective, enriched growth medium used for isolation of pathogenic bacteria. It is a variant of the blood agar plate, containing red blood cells that have been lysed by slowly heating to 80 °C. Chocolate agar is used for growing fastidious respiratory bacteria, such as Haemophilus influenzae and Neisseria meningitidis. In addition, some of these bacteria, most notably H. influenzae, need growth factors such as NAD (factor V) and hemin (factor X), which are inside red blood cells; thus, a prerequisite to growth for these bacteria is lysis of the red blood cells. The heat also inactivates enzymes which could otherwise degrade NAD. The agar is named for the color and contains no actual chocolate.

Chocolate agar with the addition of bacitracin becomes selective, most critically, for the genus Haemophilus. Another variant of chocolate agar called Thayer-Martin agar contains an assortment of antibiotics which select for Neisseria species.

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