Chocolate agar (CHOC) or chocolate blood agar (CBA) is a nonselective, enriched growth medium used for isolation of

pathogenic In biology, a pathogen (, "suffering", "passion" and , "producer of"), in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a germ. The term ...

bacteria. It is a variant of the blood agar plate, containing

red blood cells Red blood cells (RBCs), referred to as erythrocytes (, with -''cyte'' translated as 'cell' in modern usage) in academia and medical publishing, also known as red cells, erythroid cells, and rarely haematids, are the most common type of blood cel ...

that have been lysed by slowly heating to 80°C. Chocolate agar is used for growing fastidious respiratory bacteria, such as ''

Haemophilus influenzae ''Haemophilus influenzae'' (formerly called Pfeiffer's bacillus or ''Bacillus influenzae'') is a Gram-negative, Motility, non-motile, Coccobacillus, coccobacillary, facultative anaerobic organism, facultatively anaerobic, Capnophile, capnophili ...

'' and ''

Neisseria meningitidis ''Neisseria meningitidis'', often referred to as the meningococcus, is a Gram-negative bacterium that can cause meningitis and other forms of meningococcal disease such as meningococcemia, a life-threatening sepsis. The bacterium is referred to a ...

''. In addition, some of these bacteria, most notably ''H. influenzae'', need growth factors such as

nicotinamide adenine dinucleotide Nicotinamide adenine dinucleotide (NAD) is a Cofactor (biochemistry), coenzyme central to metabolism. Found in all living cell (biology), cells, NAD is called a dinucleotide because it consists of two nucleotides joined through their phosphat ...

(factor V or NAD) and hemin (factor X), which are inside red blood cells; thus, a prerequisite to growth for these bacteria is the presence of red blood cell lysates. The heat also inactivates enzymes which could otherwise degrade NAD. The agar is named for its color and contains no

chocolate Chocolate is a food made from roasted and ground cocoa beans that can be a liquid, solid, or paste, either by itself or to flavoring, flavor other foods. Cocoa beans are the processed seeds of the cacao tree (''Theobroma cacao''); unprocesse ...

products.

Variants

Chocolate agar with the addition of

bacitracin Bacitracin is a polypeptide antibiotic. It is a mixture of related cyclic peptides produced by '' Bacillus licheniformis'' bacteria, that was first isolated from the variety "Tracy I" ( ATCC 10716) in 1945. These peptides disrupt Gram-positiv ...

becomes selective for the genus '' Haemophilus''. Another variant of chocolate agar called Thayer–Martin agar contains an assortment of antibiotics which select for ''

Neisseria ''Neisseria'' is a large genus of bacteria that colonize the mucous membranes of many animals. Of the 11 species that colonize humans, only two are pathogens: '' N. meningitidis'' and '' N. gonorrhoeae''. ''Neisseria'' species are Gram-negative ...

'' species.

Composition

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

History

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 The New York City Department of Health and Mental Hygiene (also known as NYC Health) is the department of the government of New York City responsible for public health along with issuing birth certificates, dog licenses, and conducting restaur ...

Bureau of Laboratories. The term "chocolate" comes from the brown color generated from the higher concentration of heated blood in the mixture and was first used by Warren Crowe in 1915.

References

{{DEFAULTSORT:Chocolate Agar Microbiological media