Streptococci

The viridans group streptococci remain among the most common causes (30-40% in most series) of community-acquired native valve IE in individuals without a history of IV drug use. The most common species include S. oralis/S. mitis, S. sanguis, S. mutans, and S. salivarius. These alpha hemolytic organisms are members of the normal flora of the gastrointestinal tract, and they usually cause a subacute presentation of IE. They are the most common causative agents among children and young women with mitral IE. Members of the S. angi-nosus (or S. milleri) group most often grow as pinpoint alpha hemolytic colonies on sheep blood agar. Although often considered to be part of the viridans group, the distinction is important because infections with these organisms are associated with abscess formation, possibly impacting the duration of therapy [9]. The organisms previously designated as nutritionally variant streptococci are now classified as Abiotrophia defectiva and Granulicatella species. These organisms can be challenging because media supplemented with pyridoxal hydrochloride or L-cysteine are required to support growth, and treatment success requires more aggressive therapy than for viridans streptococci.

Non-enterococcal group D streptococci, the Streptococcus bovis/S. equinus complex, are an important cause of IE in certain geographical areas. A recent analysis of the ICE merged database yielded 136 IE cases caused by these organisms, of which 109 (80.1%) involved native valves [10]. When compared to cases caused by viridans group streptococci, patients were older with more co-morbidities, and multiple valve involvement was more common. When data from two decades of the database were compared, the proportion of streptococcal IE caused by S. bovis/equinus increased from 10.9% (1979-89) to 23.3% (1990-99). This proportion was particularly high in France (58%) compared to other sites in Europe (9.4%) and the US (16.7%). Previous studies have shown a strong association between S. bovis bacteremia and col-orectal cancer.

S. pneumoniae was an important agent of IE in the pre-antibiotic era. Its incidence has decreased to 1-3% in recent series, though a recent small Scandinavian study showed a fourfold increase from 1981 to 1996 [11]. S. pyogenes and Lancefield groups B, C, and G streptococci are also rare causes of native valve IE.

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