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
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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 bacteremia, endocarditis|
|Nonenterococci: S. bovis||Usually nonhemolytic||Bacteremia, endocarditis|
|Variable or nongroupable||Viridans streptococci: S. sanguis; S. mitis||α||Endocarditis, dental abscess, brain abscess|
|Intermedius or milleri group: S. intermedius, S. anginosus, S. constellatus||Variable||Brain abscess, visceral abscess|
|Anaerobic streptococcib: Peptostreptococcus magnus||Usually nonhemolytic||Sinusitis, pneumonia, empyema, brain abscess, liver abscess|
Lancefield's group A consists of a single species, S. pyogenes. As its species name implies, this organism is associated with a variety of suppurative infections. In addition, GAS can trigger the postinfectious syndromes of ARF (which is uniquely associated with S. pyogenes infection; Chap. 322) and PSGN (Chap. 283).
Worldwide, GAS infections and their postinfectious sequelae (primarily ARF and rheumatic heart disease) account for an estimated 500,000 deaths per year. Although data are incomplete, the incidence of all forms of GAS infection and that of rheumatic heart disease are thought to be tenfold higher in resource-limited countries than in developed countries (Fig. 136-1).
Prevalence of rheumatic heart disease in children 5–14 years old. The circles within Australia and New Zealand represent indigenous populations (and also Pacific Islanders in New Zealand). (From Carapetis et al, 2005, with permission.)
GAS elaborates a number of cell-surface components and extracellular products important in both the pathogenesis of infection and the human immune response. The cell wall contains a carbohydrate antigen that may be released by acid treatment. The reaction of such acid extracts with group A–specific antiserum is the basis for definitive identification of a streptococcal strain as S. pyogenes. The major surface protein of GAS is M protein, which occurs in more than 100 antigenically distinct types and is the basis for the serotyping of strains with specific antisera. The M protein molecules are fibrillar structures anchored in the cell wall of the organism that extend as hairlike projections away from the cell surface. The amino acid sequence of the distal or amino-terminal portion of the M protein molecule is quite variable, accounting for the antigenic variation of the different M types, while more proximal regions of the protein are relatively conserved. A newer technique for assignment of M type to GAS isolates uses the polymerase chain reaction to amplify the variable region of the emm gene, which encodes M protein. DNA sequence analysis of the amplified gene segment can be compared with an extensive database [developed at the Centers for Disease Control and Prevention (CDC)] for assignment of emm type. This method eliminates the need for typing sera, which are available in only a few reference laboratories. The presence of M protein on a GAS isolate correlates with its capacity to resist phagocytic killing in fresh human blood. This phenomenon appears to be due, at least in part, to the ...