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The streptococci 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 by streptococci or in other cases to an immunologic response to them. Streptococci elaborate a variety of extracellular substances and enzymes.

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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.

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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. Molecular genetics have also been used to study the streptococci. Combinations of these methods have permitted the classification of streptococci for purposes of clinical and epidemiologic convenience, but as the knowledge evolved, new methods have been introduced with the result that several classification systems have been described. In some cases, different species names have been used to describe the same organisms; in other instances, some members of the same species have been included in another species or classified separately. The genus Enterococcus, for example, now includes some species previously classified as group D streptococci. The classification of streptococci described in the following paragraphs and summarized in Table 14-1 is one logical approach.

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Table 14–1 Characteristics of Medically Important Streptococci

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