Deibel, R. H. (American Meat Institute Foundation, Chicago, Ill.), Donald E. Lake, and C. F. Niven, Jr. Physiology of the enterococci as related to their taxonomy. J. Bacteriol. 86:1275-1282. 1963-Studies on a collection of enterococci isolated from diverse sources have confirmed the existence of two distinct species, namely, Streptococcus faecalis and Streptococcus faecium. In contrast with S. faecium, S. faecalis characteristically ferments melezitose, sorbitol, glycerol (anaerobically), citrate, and gluconate; fails to ferment arabinose and melibiose; has strong reducing capacities as demonstrated by prompt reduction of litmus in milk or tetrazolium in an agar medium; initiates growth on an agar medium containing 0.04% potassium tellurite; does not require folic acid for growth; and does not produce strong greening in blood agar. S. liquefaciens and S. zymogenes differ from S. faecalis only in their proteolytic capacities and, therefore, deserve only varietal status. Some S. faecalis and S. faecium strains are proteolytic when tested on agar media. S. durans differs from S. faecium only in its inability to ferment arabinose and mannitol, and the failure of most strains to ferment sucrose. Therefore, it should be considered as a variety of S. faecium. It is recommended that the term "enterococcus" be used only when referring to S. faecalis and S. faecium, and their respective varieties.