Dwight A. Powell
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.
EPIDEMIOLOGY
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.
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.
CLINICAL MANIFESTATIONS
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
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 of early-onset sepsis have been described in neonates. While large-colony group C and group G streptococci are implicated as a cause of pharyngitis in children and adults, they are most commonly associated with food-borne outbreaks. In a recent Delaware study, conducted over a 1-year period, of 2085 children with pharyngitis and 195 controls, large-colony group C or G streptococci were isolated from 3.1% of patients and 1.5% of controls, a difference that was not statistically significant.5 These organisms are not a cause of acute rheumatic fever. Infrequently, group C and group G streptococci cause a variety of pyogenic infections: pneumonia, epiglottitis, osteomyelitis, septic arthritis, pyomyositis, brain abscess, meningitis, cellulitis, endocarditis, sinusitis, urinary tract infections, bacteremia; and toxic shock syndrome.6-8
Group D streptococci (S bovis) are a rare cause of disease in children. There are few reports of endocarditis (seen mainly in adults), meningitis, or bacteremia, particularly in newborns.9
DIAGNOSIS
Streptococci are readily recovered from standard blood culture media. Complex media containing 5% sheep’s blood are usually recommended for subculture of blood isolates and for the culture of streptococci from throat swabs. Colony size and hemolytic pattern determine the initial classification.
TREATMENT
Group C and G streptococci remain susceptible to penicillin; first-generation cephalosporins and erythromycin are treatment alternatives for the penicillin-allergic individual. An increasing incidence of penicillin resistance is emerging among viridans streptococci and group D streptococci. Therapy must be guided by susceptibility testing, which is best performed with the E test or agar dilution. In general, these organisms remain vancomycin susceptible. In institutions where penicillin resistance is frequent, vancomycin should be included in the broad-spectrum empiric antibiotic coverage of febrile neutropenic hosts pending antibiotic susceptibility data. Refer to Chapter 490 for treatment guidelines.11