Many varieties of streptococci are found as part of the normal flora colonizing the human respiratory, gastrointestinal, and genitourinary tracts. Several species are important causes of human disease. Group A Streptococcus (GAS, S. pyogenes) is responsible for streptococcal pharyngitis, one of the most common bacterial infections of school-age children, and for the postinfectious syndromes of acute rheumatic fever (ARF) and poststreptococcal glomerulonephritis (PSGN). Group B Streptococcus (GBS, S. agalactiae) is the leading cause of bacterial sepsis and meningitis in newborns and a major cause of endometritis and fever in parturient women. Viridans streptococci are the most common cause of bacterial endocarditis. Enterococci, which are morphologically similar to streptococci, are now considered a separate genus on the basis of DNA homology studies. Thus, the species previously designated as S. faecalis and S. faecium have been renamed Enterococcus faecalis and E. faecium, respectively. The enterococci are discussed in Chap. 137.
Streptococci are gram-positive, spherical to ovoid bacteria that characteristically form chains when grown in liquid media. Most streptococci that cause human infections are facultative anaerobes, although some are strict anaerobes. Streptococci are relatively fastidious organisms, requiring enriched media for growth in the laboratory. Clinicians and clinical microbiologists identify streptococci by several classification systems, including hemolytic pattern, Lancefield group, species name, and common or trivial name. Many streptococci associated with human infection produce a zone of complete (β) hemolysis around the bacterial colony when cultured on blood agar. The β-hemolytic streptococci can be classified by the Lancefield system, a serologic grouping based on the reaction of specific antisera with bacterial cell-wall carbohydrate antigens. With rare exceptions, organisms belonging to Lancefield groups A, B, C, and G are all β-hemolytic, and each is associated with characteristic patterns of human infection. Other streptococci produce a zone of partial (α) hemolysis, often imparting a greenish appearance to the agar. These α-hemolytic streptococci are further identified by biochemical testing and include S. pneumoniae (Chap. 134), an important cause of pneumonia, meningitis, and other infections, and the several species referred to collectively as the viridans streptococci, which are part of the normal oral flora and are important agents of subacute bacterial endocarditis. Finally, some streptococci are nonhemolytic, a pattern sometimes called γ hemolysis. Among the organisms classified serologically as group D streptococci, the enterococci are classified as a distinct genus (Chap. 137). The classification of the major streptococcal groups causing human infections is outlined in Table 136–1.
Table 136–1 Classification of Streptococci |Favorite Table|Download (.pdf)
Table 136–1 Classification of Streptococci
|Lancefield Group||Representative Species||Hemolytic Pattern||Typical Infections|
|A||S. pyogenes||β||Pharyngitis, impetigo, cellulitis, scarlet fever|
|B||S. agalactiae||β||Neonatal sepsis and meningitis, puerperal infection, urinary tract infection, diabetic ulcer infection, endocarditis|
|C, G||S. dysgalactiae subsp. equisimilis||β||Cellulitis, bacteremia, endocarditis|
|D||Enterococcia: E. faecalis; E. faecium||Usually nonhemolytic||Urinary tract infection, nosocomial ...|