ENDOCARDITIS
Viridans streptococci have a strong association with bacterial endocarditis (see site) (Box 1). A defectiva and A adjacens were once an important cause of culture-negative endocarditis. However, current laboratory media and techniques enable these “nutritionally variant streptococci” to be identified more readily.
BACTEREMIA
Viridans streptococci account for 2.6% of positive blood cultures reported from clinical laboratories; however, of these, only about one-fifth are thought to be clinically significant (the remainder are attributed to contamination or transient bacteremia). Viridans streptococci are, however, one of the leading causes of bacteremia in febrile neutropenic patients. Viridans streptococcal bacteremia in neutropenic patients usually occurs in association with aggressive cytoreductive therapy for acute leukemia or allogeneic bone marrow transplantation. The prophylactic administration of trimethoprim-sulfamethoxazole or the quinolones, the presence of mucositis, and the presence of indwelling central venous catheters are associated with viridans streptococcal bacteremia in these populations.
Patients may present with fever alone; however, neurologic, pulmonary, and cardiovascular manifestations may be seen, and a fulminant shock syndrome characterized by hypotension, rash, palmar desquamation, and the adult respiratory distress syndrome may be present. Notably, clinically apparent endocarditis is seldom present in neutropenic patients with viridans streptococcal bacteremia.
Bacteremia associated with S milleri group isolates is often associated with deep-seated abscesses in visceral organs.
MENINGITIS
When viridans streptococci are recovered from cerebrospinal fluid, they are most often contaminants. Viridans streptococci, however, are rare causes of meningitis, which may occur in patients of all ages, including neonates. Clinical manifestations are typical of acute pyogenic meningitis with evidence of meningeal irritation, neurologic deficits, seizures, and altered sensorium.
OTHER VIRIDANS STREPTOCOCCI INFECTIONS
Viridans streptococci may be associated with a variety of other infections, including pneumonia, pericarditis, peritonitis, acute bacterial sialadenitis, orofacial and odontogenic infections, endophthalmitis, otitis media, sinusitis, liver abscesses, pelvic abscesses, subphrenic abscesses, appendicitis, abdominal wound infections, cholangitis, mediastinitis, brain abscesses, subcutaneous abscesses, and cellulitis.
Diagnosis
The diagnosis of viridans streptococcal infection is made by isolating viridans streptococci from typically sterile sites. As mentioned above, communityacquired viridans streptococcal bacteremias are strongly associated with bacterial endocarditis. This diagnosis may be further confirmed by the presence of clinical findings consistent with endocarditis as well as by the use of echocardiography. A defectiva and A adjacens require pyridoxal or thiol group supplementation for growth. There is sufficient pyridoxal in human blood to support the growth of A defectiva and A adjacens in most blood culture media (with the notable exception of unsupplemented tryptic soy broth). For subculture, however, solid media must be supplemented with 0.001% pyridoxal or 0.01% L-cysteine to sustain growth.
As an alternative, the culture plate may be cross-streaked with Staphylococcus aureus to provide these factors and permit the growth of the streptococci as satellite colonies. In addition, as noted above, members of the S milleri group are associated with deep-seated abscesses in visceral organs. Therefore infections caused by these organisms should alert the clinician to initiate an appropriate investigation for the detection of a possible subclinical focus of infection.
Treatment
Viridans streptococci with a minimum inhibitory concentration (MIC) of = 0.12 ug/mL to penicillin are defined as penicillin susceptible. Those with a penicillin MIC of 0.25 to 2.0 ug/mL are intermediately susceptible to penicillin. Those with a penicillin MIC of > 2 ug/mL are resistant to penicillin. A high frequency of penicillin-resistant viridans streptococcal infections may be noted in febrile neutropenic patients. For serious infections, such as endocarditis, with these resistant or intermediately susceptible organisms, combination therapy consisting of a penicillin plus an aminoglycoside is recommended. Viridans streptococci are usually resistant to aminoglycosides when traditional breakpoint concentrations for these agents are applied. However, in vitro studies in experimental models of endocarditis have demonstrated synergistic bactericidal activity between combinations of penicillin and aminoglycosides.
Many other ß-lactam antibiotics have in vitro activity similar to penicillin against streptococci. In particular, ceftriaxone is an alternative agent to penicillin for the outpatient treatment of viridans streptococcal endocarditis. Other agents with consistently good in vitro activity against viridans streptococci are cefazolin, vancomycin, and imipenem. A defectiva and A adjacens are less susceptible in vitro to penicillin than are most other streptococci. It is recommended that all patients with A defectiva and A adjacens endocarditis be treated with combination therapy consisting of a penicillin plus an aminoglycoside.
Prevention & Control
Endocarditis prophylaxis, as discussed in site on bacterial endocarditis, is used to prevent viridans group streptococcal endocarditis.