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.
PATHOGENESIS AND EPIDEMIOLOGY
Nongroup A or B streptococci that are pathogenic for humans fall into 3 categories: viridans streptococci, β-hemolytic streptococci groups C and G (Streptococcus dysgalactiae), and nonenterococcal group D Streptococcus. These organisms are normal commensal flora of the upper airway and asymptomatic colonizers of the skin (groups C and G), gastrointestinal tract, or female genital tract. Viridans streptococci, so named because of the Latin viridis, or green, comprise a group of at least 30 species, subdivided into several groups including 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 2 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 (also called S gallolyticus) is the only nonhemolytic group D species; it must be differentiated from enterococci, which also carry group D antigens.
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. 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. Members of the S anginosus group (SAG; includes S intermedius, S constellatus, and S anginosus) have a propensity for abscess formation. Large-colony groups C and G streptococci possess virulence factors in common with group A streptococci such as hemolysins, extracellular proteins, and M proteins.
Viridans streptococci are recognized as a major organism causing bacterial endocarditis. The majority of children with bacterial endocarditis (see Chapter 230) have underlying congenital heart defects and usually have undergone cardiac surgery; rheumatic heart disease also is a risk factor. Endocarditis due to viridans streptococci is typically subacute in presentation, with fever and fatigue in the background of a changing cardiac murmur. An elevated erythrocyte sedimentation rate and anemia are the most common laboratory findings. When compared to acute endocarditis, viridans streptococcal endocarditis tends to respond to therapy more quickly with more rapid defervescence and clearing of the bacteremia. Complications and the need for heart surgery are significantly less frequent with viridans streptococcal infections.
Viridans streptococci are also important pathogens in immunocompromised hosts. In cancer patients and children receiving hematopoietic stem cell transplants, viridans streptococci are a major cause of bacteremia, mainly associated with indwelling vascular catheters, mucositis, gastrointestinal toxicity, and neutropenia. Pneumonia and septic shock are common complications. Rare cases of early-onset sepsis have also been described in neonates.