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Nongroup A or B streptococci are a diverse group of gram-positive microorganisms that may be commensal or may be associated with severe, even life-threatening, infections.

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Epidemiology

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Nongroup A or B streptococci that are pathogenic for humans tend to fall into three categories: viridans streptococci, β-hemolytic streptococci groups C or G, and nonhemolytic group D. These organisms are normal resident flora of the mouth, gastrointestinal tract, or female genital tract. They are only occasionally found on the skin. Viridans streptococci, so named because of the Latin viridis, or green, comprise a group of at least 18 species, now subdivided into four groups: anginosis (previously milleri), mitis, salivarius, and mutans. Growth on blood agar may elicit α, γ, or occasionally β-hemolysis. β-Hemolytic groups C and G each comprise two strains, which are identifiable by colony size. Colonies greater than 0.5 mm are considered to be pathogenic, whereas colonies less than 0.5 mm are nonpathogens. Streptococcus bovis is the only nonhemolytic group D species; it must be differentiated from enterococci, which also carry group D antigens.

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Viridans streptococci do not share any of the pathogenic features of pyogenic streptococci. Their propensity to cause disease is primarily related to a high frequency of transient bacteremia following dental procedures or loss of integrity of mucosal membranes. In a study of 735 children, the rates of bacteremia following various dental procedures were: polishing teeth, 24.5%; intraligamental injection, 96.6%; and toothbrushing, 38.5%. Viridans streptococci comprised 50% of the bloodstream isolates.1 Some strains of viridans streptococci, particularly S mutans, S sanguis, and S mitis, appear to have enhanced ability to adhere to damaged heart valves and vegetations. Large-colony groups C and G streptococci possess virulence factors in common with group A streptococci such as hemolysins, extracellular proteins, and M proteins.

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Clinical Manifestations

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Viridans streptococci have long been recognized as a major organism causing 22% to 38% of cases of bacterial endocarditis. The majority of children with bacterial endocarditis have underlying congenital heart defects and usually have undergone cardiac surgery (see Chapter 235).2 Rheumatic heart disease is the second most common cardiac abnormality predisposing to endocarditis. Fever and fatigue in the background of a changing cardiac exam are the most common presenting symptoms of endocarditis. An elevated erythrocyte sedimentation rate and anemia are the most common laboratory findings. When compared to infections with S aureus, those caused by viridans streptococci respond to therapy more quickly with more rapid defervescence and clearing of the bacteremia. Complications and need for heart surgery are significantly less frequent with viridans streptococcal infections.

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Viridans streptococci are also important pathogens in some immunocompromised hosts. In cancer patients and children receiving hematopoietic stem cell transplants, viridans streptococci account for 5% to 30% of cases of bacteremia, mainly associated with indwelling vascular catheters, mucositis, gastrointestinal toxicity, and neutropenia.3,4 Pneumonia and septic shock are common complications. Rare cases ...

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