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Chapter 14: The Streptococci, Enterococci, and Related Genera

INTRODUCTION

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.

CLASSIFICATION OF STREPTOCOCCI

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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TABLE 14-1 Characteristics of Medically Important Streptococci
Name Group-Specific Substancea Hemolysisb Habitat Important Laboratory Criteria Common and Important Diseases
Pyogenic Streptococci          
Streptococcus pyogenes A β Throat, skin Large colonies (>0.5 mm), PYRc test positive, inhibited by bacitracin Pharyngitis, impetigo, deep soft tissue infections; bacteremia; rheumatic fever, glomerulonephritis, toxic shock
Streptococcus agalactiae B β Urogenital tract, lower GI tract Hippurate hydrolysis, CAMP-factor positived Neonatal sepsis and meningitis; bacteremia, UTIs,e meningitis in adults
Streptococcus dysgalactiae subspecies equisimilis; others A, C, G β (human) infections), α, none Throat Large (>0.5 mm) colonies Pharyngitis, pyogenic infections similar to group A streptococci
Viridans Streptococci          
Streptococcus bovis groupf D None Colon, biliary tree Growth in presence of bile, hydrolyze esculin, no growth in 6.5% NaCl, degrades starch Endocarditis, common blood isolate in colon cancer, biliary disease
Streptococcus anginosus group (S. anginosus, Streptococcus intermedius, Streptococcus constellatus) F (A, C, G) and untypeable α, β, none Throat, colon, urogenital tract Small (<0.5 mm) colony variants of β-hemolytic species; group A are bacitracin resistant and PYR negative; carbohydrate fermentation patterns; arginine, esculin, VPg positive Pyogenic infections, including brain, liver, lung abscesses
Mutans group Usually not typed α, none Oral cavity Carbohydrate fermentation patterns; esculin, VP positive Dental caries (Streptococcus mutans), endocarditis; abscesses (with many other bacterial species)
Mitis-Sanguinis group          
Streptococcus pneumoniae None° α Nasopharynx Susceptible to optochin; colonies soluble in bile; quellung reaction positive Pneumonia, meningitis, bacteremia, otitis media, sinusitis
Streptococcus mitis None α, none Oral cavity VP negativeg; carbohydrate fermentation patterns Endocarditis; bacteremia, sepsis ...

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