Causative Agent

Streptococcus pyogenes, the cause of strep throat, is a Gram-positive coccus that grows in chains of varying lengths (figure 23.2). It can be differentiated from other steptococci that normally inhabit the throat by its characteristic colonial morphology when grown on blood agar. Streptococcus pyogenes produces hemolysins, enzymes that lyse red blood cells, which result in the colonies being surrounded by a zone of ^-hemolysis (see figure 22.6). Because of their characteristic hemolysis, S. pyogenes and other streptococci that show a similar phenotype are called ^-hemolytic streptococci. In contrast, species of Streptococcus that are typically part of the normal throat flora are either non-hemolytic, or they produce a-hemolysis, characterized by a zone of incomplete, often greenish clearing around colonies grown on blood agar. ■ streptococcal hemolysis, p. 94

Streptococcus pyogenes is commonly referred to as the group A streptococcus. The group A carbohydrate in the cell wall of S. pyogenes differs antigenically from that of most other streptococci and serves as a convenient basis for identification. Lancefield grouping uses antibodies to differentiate the various

10 mm

Figure 23.2 Streptococcus pyogenes Chain formation in fluid culture as revealed by fluorescence microscopy.

species of streptococci based on their cell wall carbohydrate. These antibodies are also the basis for many of the rapid diagnostic tests done on throat specimens in a physician's office. Other rapid tests utilize a DNA probe.

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