Streptococci can be classified by several schemes, for example, by the hemolytic properties of the organisms, and according to the presence of specific surface antigens determined by immunologic assays.

A. Hemolytic properties on blood agar

α-Hemolytic streptococci cause a chemical change in the hemoglobin of red cells in blood agar, resulting in the appearance of a green pigment that forms a ring around the colony (see Figure 9.16).β-Hemolytic streptococci cause gross lysis of red blood cells, resulting in a clear ring around the colony (see Figure 9.16). γ-Hemolytic is a term applied to streptococci that cause no color change or lysis of red blood cells. The traditional division of streptococci based on the ability of the bacterial colony to hemolyze erythrocytes in the blood agar medium is still considered the first step in the classification of streptococci.

B. Serologic (Lancefield) groupings

Many species of streptococci have a polysaccharide in their cell walls known as C-substance, which is antigenic and easily extractable with dilute acid. The Lancefield scheme classifies primarilyβ-hemolytic streptococci into groups A through U on the basis of their C-substance. The clinically most important groups of β-hemolytic streptococci are Types A and B (Figure 9.2). Commercial kits in which group- specific antisera are coupled to latex beads are now widely used for identification of β-hemolytic streptococci.

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