1. Taxonomy and Principles
In 1903, the genus name Enterococcus was proposed for Gram-positive, catalase-negative, coccoid-shaped bacteria of intestinal origin.1 Several years later, it was suggested2 that the genus name be changed to Streptococcus because the bacteria form chains of coccoid-shaped cells. Subsequently, the species was given the name faecalis because of their fecal origin. The genus name Streptococcus was adopted for the next 78 years and applied to coccoid-shaped, chain-forming, Gram-positive bacteria of intestinal origin. In 1984, the genus name Enterococcus was revived, and it was suggested that Streptococcus faecalis and Streptococcus faecium be transferred to the genus Enterococcus on the basis of genotypic similarity to microbial taxonomy.3 Shortly after the acceptance of the revived Enterococcus genus, Streptococcus avium and Streptococcus gallinarum also were transferred to the genus. Currently, more than 50 Enterococcus species are recognized.4
Analysts can differentiate the various species of enterococci and streptococci biochemically, but it is impractical to perform the tiered testing required to identify specific species of interest in water. Although enterococci are broadly used as an indicator of fecal pollution in water and are commonly found in the feces of human and other warm-blooded animals, some species can persist and grow in the natural environment and do not appear to be related to fecal pollution.5
Enterococci are extremely hardy and can tolerate a wide variety of growth conditions. The culture media used to isolate enterococci utilize inhibitors such as sodium azide, cycloheximide, and 2,3,5-triphenyltetrazolium chloride (TTC) to eliminate background organisms. Enterococci are differentiated from streptococci by their ability to grow in 6.5% sodium chloride (NaCl), on bile esculin azide (BEA) agar, and at 45 ± 0.5 °C in brain-heart infusion (BHI) broth.6,7