Viridans Streptococci

These streptococci, which cause 30 to 65 per cent of NVE case unrelated to drug abuse, are normal inhabitants of the oropharynx, characteristically produce alpha-hemolysis when grown on sheep blood agar, and are usually nontypable using Lance-field's system. The species causing streptococcal NVE were distributed as follows: Streptococcus mitior (31 per cent of cases).  Streptococcus sanguis (24 per cent). S. bovis (27 per cent), Streptococcus faecalis (now  Enterococcus faecalis (7 per cent) and streptococcus salivarius and other species (2 per cent).  S bovis  NVE is frequently associated with coexistent colonic polyps or malignancy.

Sreptococcus Pneumoniae

Although pneumococcal bacteremia occurs frequently, S. pneumoniae accounts for only 1 to 3 per cent of NVE cases. When causing IE, S. pneumoniae frequently involves a previously normal aortic valve and progresses rapidly with valve destruction, myocardial abscess formation and acute congestive heart failure (CHF).     

Enterococci

E. Faecalis and Enterococcus faecium cause 85 per cent and 10 per cent of cases of enterococcal IE, respectively. Enterococci are part of the normal gastrointestinal flora and cause genitourinary tract infection. Enterococci account for 5 to 15 per cent of cases of NVE and a similar percentage of PVE cases. Cases occur in young women as a consequence of genitourinary tract manipulation or infection and in older predominantly male patients, who have the urinary tract as a likely portal of entry. Enterococci infect either normal or previously abnormal valves and present as either acute or subacute IE.

Staphylococci

The coagulase-positive staphylococci are a single species, s. aureus. Of the 13 species of coagulase-negative staphylococci that colonize humans, one s. epidermidis has emerged as an important pathogen in the setting of implanted devices and hospitalized patients. S. aureus is a major cause of IE in all population groups. S. aureus IE is characterized by a highly toxic febrile illness, frequent  focal  metastatic infection  and  a 30 to 50 per cent rate of  central nervous system complications. Among addicts, left-sided S. aureus IE resembles that in nonaddicts. In contrast, in patients with IE limited to the tricuspid valve, complications are rare and mortality rates are only 2 to 4 per cent.