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The streptococci, enterococci, and related organisms are Gram-positive spherical bacteria that characteristically form pairs or chains during growth. They are widely distributed in nature. Some are members of the normal human microbiota; others are associated with important human diseases attributable to the direct effects of infection or in other cases to an immunologic response to them. Streptococci elaborate a variety of extracellular substances and enzymes.

The streptococci are a large and heterogeneous group of bacteria, and no one system suffices to classify them. Yet, understanding their taxonomy is key to understanding their medical importance.


The classification of streptococci into major categories has been based on a series of observations over many years: (1) colony morphology and hemolytic reactions on blood agar, (2) serologic specificity of the cell wall group-specific substance (Lancefield antigens) and other cell wall or capsular antigens, (3) biochemical reactions and resistance to physical and chemical factors, and (4) ecologic features. More recently, molecular genetics have replaced phenotypic methods in the taxonomic assignment of these organisms. The classification of streptococci of medical importance is summarized in Table 14-1.

TABLE 14-1Characteristics of Medically Important Streptococci

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