The fecal streptococci and enterococci are members of the genera Streptococcus and Enterococcus. These bacteria are spherical, gram positive and grow in chains. Most are facultative, “aerotolerant” anaerobes and have no aerobic metabolism. They must ferment sugars and are unable to synthesize heme, a necessary component of cytochromes. The enterococci and many fecal streptococci are normal flora of the gastrointestinal tract, where they are present at high concentrations. However, they can sometimes spread from the gastrointestinal tract and cause frank disease. Bacteremia, wound infections, urinary tract infections and endocarditis are known to occur. These illnesses are more likely in newborns and in the elderly. Enterococci are now a concern in nosocomial infections as well as infections of non-hospitalized populations because of growing antibiotic resistance, especially to antibiotics such as vancomycin.
The streptococci and enterococci are relatively fastidious in their growth requirements because they have lost the ability to synthesize many essential nutrients. Therefore, they require several amino acids and the B vitamins to be grown in the laboratory. The fecal streptococci/enterococci will grow: (1) in high NaCl (6.5%), i.e., salt tolerant; (2) in 1% methylene blue, (3) in bile‑esculin medium (hydrolyze esculin in the presence of bile); and (4) at pH 9.6.
The streptococci/enterococci can be classified on the basis of:
(i) Lancefield (cell wall) antigens (A, B, C, D, E-T)
(ii) colony morphology and hemolysis
(iii) biochemical reactions
(iv) resistance to physical and chemical agents
(v) antigenic composition and serological reactions;
- (vi) ecology
- (vii) Molecular genetic properties
The fecal streptococci and enterococci belong to serogroup D. Serogrouping of the streptococci is based on the group carbohydrate of the cell wall. Most group D streptococci are nonhemolytic or alpha hemolytic on sheep RBCs. (beta hemolysis = complete hemolysis; alpha hemolysis = partial hemolysis). Not all group D streptococci are enterococci; they differ from enterococci by inability to grow in 6.5% NaCl and inhibition by 40% bile. There are more than 2 dozen species of enterococci, some of which colonize or infect humans and other animals and some of which occur naturally in the environment (some do both). The important enterococci with respect to human fecal contamination are E. faecalis, E. faecium and E. durans. Non‑enterococci, esp. S. bovis and S. equinus are also in the gut.