Differential Media

Differential media contain a substance that certain bacteria change in a recognizable way. For example, blood agar, in addition to being nutritious, is differential; it is used to detect bacteria that produce a hemolysin, a substance that lyses red blood cells (figure 4.6). The lysis appears as a zone of clearing around the colony growing on the blood agar plate. The type of hemolysis is used as an identifying characteristic. For example, species of Streptococcus

Colony

Colony

Zone of clearing

Figure 4.6 Blood Agar This complex medium is differential for hemolysis. (a) A zone of complete clearing around a colony growing on blood agar is called beta hemolysis. (b) A zone of greenish clearing is called alpha hemolysis.

Figure 4.6 Blood Agar This complex medium is differential for hemolysis. (a) A zone of complete clearing around a colony growing on blood agar is called beta hemolysis. (b) A zone of greenish clearing is called alpha hemolysis.

that reside harmlessly in the throat often cause a type of hemoly-sis called alpha hemolysis, which is characterized by a zone of greenish clearing around the colonies. In contrast, Streptococcus pyogenes, which causes strep throat, causes beta hemolysis, which is characterized by a clear zone of hemolysis. Still other bacteria have no effect on red blood cells. ■ Streptococcus pyogenes, p. 565

MacConkey agar, which is selective, is also differential (figure 4.7). In addition to peptones and other nutrients, it contains lactose and a pH indicator. Bacteria that ferment the sugar produce acid, which turns the pH indicator pink. Thus, those lactose-fermenting bacteria that can grow on MacConkey agar, such as E. coli, form pink colonies. Lactose-negative bacteria form tan or colorless colonies. ■ lactose, p. 30

Providing Appropriate Atmospheric Conditions

To cultivate bacteria in the laboratory, appropriate atmospheric conditions must be provided. For example, broth cultures of obligate aerobes grow best when tubes or flasks containing the media are shaken, providing maximum aeration. Special methods create atmospheric environments such as increased CO2, microaerophilic, and anaerobic conditions.

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