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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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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 ...