Blood agar

Complex medium used routinely in clinical labs. Not selective. Differential because colonies of hemolytic organisms are surrounded by a zone of clearing of the red blood cells.

Chocolate agar

Complex medium used to culture fastidious bacteria, particularly those found in clinical specimens. Not selective or differential.


Chemically defined medium. Used in laboratory experiments to study nutritional requirements of bacteria. Not selective or differential.

MacConkey agar

Complex medium used to isolate Gram-negative rods that typically reside in the intestine. Selective because bile salts and dyes inhibit Gram-positive organisms and Gram-negative cocci. Differential because the pH indicator turns red when the sugar in the medium, lactose, is fermented.

Nutrient agar

Complex medium used for routine laboratory work. Supports the growth of a variety of nonfastidious bacteria.


Complex medium used to isolate Neisseria species, which are fastidious. Selective—contains antibiotics that inhibit most organisms except Neisseria species.

tine laboratory work, but they are invaluable when studying nutritional requirements of bacteria. Glucose-salts, which supports the growth of E. coli, contains only those chemicals listed in table 4.6. More elaborate recipes containing as many as 46 different ingredients can be used to make chemically defined media that support the growth of fastidious bacteria such as Neisseria gonorrhoeae, the bacterium that causes gonorrhea. ■ gonorrhea, p. 644

To maintain the pH near neutrality, buffers are often added to the medium. They are usually important in a defined medium because some bacteria can produce enough acid as a by-product of their metabolism to inhibit their own growth. This typically is not as much of a problem in complex media because the amino acids and other natural components provide at least some buffering function. ■ buffer, p. 24

Special Types of Culture Media

To detect or isolate an organism that is part of a mixed bacterial population, it is often necessary to make it more prevalent or more obvious. For these purposes selective and differential media are used. These media can be either complex or chemically defined, depending on the needs of the microbiologist.

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