A complex medium contains a variety of ingredients such as meat juices and digested proteins, making what might be viewed as a tasty soup for microbes. Although a specific amount of each ingredient is in the medium, the exact chemical composition of these ingredients can be highly variable. One common ingredient is peptone. This is protein taken from any of a variety of sources that has been hydrolyzed to amino acids and short peptides by treatment with enzymes, acids, or alkali. Extracts, which are the water-soluble components of a substance, are also used. For example, beef extract is a water extract of lean meat and provides vitamins, minerals, and other nutrients. A commonly used complex medium, nutrient broth, consists of only 5 grams of peptone and 3 grams of beef extract per liter of distilled water. If agar is added, then nutrient agar results.
Many medically important bacteria are fastidious, requiring a medium that is even richer than nutrient agar. One rich medium commonly used in clinical laboratories is blood agar. This contains red blood cells, which supply a variety of nutrients including hemin, in addition to other ingredients. A medium used to culture even more fastidious bacteria is chocolate agar, named for its appearance rather than the ingredients. Chocolate agar contains lysed red blood cells and additional nutrients.
Additional ingredients are often incorporated into complex media to counteract compounds that may be toxic to some exquisitely sensitive bacteria. For example, cornstarch is included in some types of media used to culture Neisseria species because it binds fatty acids, which may be toxic to these organisms.
Several biological supply companies manufacture hundreds of different types of media, each one specially formulated to permit the plentiful growth of one or several groups of organisms. Even with the availability of all of these different media, however, many organisms, including Treponema pallidum, the spirochete that causes syphilis, have yet to be successfully grown on culture media. ■ syphilis, p. 648
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