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 ||Download (.pdf) 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 ...|