ABSTRACT
The genus Streptococcus, of the family Streptococcaceae, is comprised of a diverse group of facultatively anaerobic to strictly anaerobic species. The genus is divided into three genera, Streptococcus, Enterococcus and Lactococcus. Other species of human importance include the nutritionally variant streptococci, Abiotrophia defectiva, Leuconostoc, Pediococcus, Aerococcus and S. bovis. Streptococci have been classified on the basis of their interaction with erythrocytes in agar media as α-hemolytic (greening due to hemoglobin conversion), βhemolytic (clear zone of hemolysis around colonies) and γ-hemolytic (no zone of hemolysis or discoloration around colonies). Of the numerous species in the genus Streptococcus and Enterococcus, only several members account for the majority of human infections: S. pyogenes (Group A), S. agalacteae (Group B), S. pneumoniae, S. bovis, and E. faecalis and E. faecium. Viridans (α-hemolytic) streptococci are an important cause of subacute bacterial endocarditis. The Lancefield classification scheme serologically groups β-hemolytic streptococci on the basis of their carbohydrate cell wall antigens. Subtyping of hemolytic strains is based on M protein surface antigens, and greater than 90 M serotypes have been recognized. M protein confers resistance to phagocytosis and is cross-reactive with host tissues, accounting for post-streptococcal rheumatic fever and acute glomerulonephritis. Streptococcal pharyngitis may lead to rheumatic fever, a major cause of heart disease, and acute glomerulonephritis, depending on M protein serotype. Group A streptococcal M protein types 2, 49, 57, 59, 60, and 61 are associated with streptococcal pyoderma and acute glomerulonephritis, but are not associated with rheumatic fever.
