Streptococcus equinus

Streptococcus equinus is a Gram-positive, nonhemolytic, nonpathogenic, lactic acid bacterium of the genus Streptococcus. It is the principal Streptococcus found in the alimentary canal of a horse, and makes up the majority of the bacterial flora in horse feces. S. equinus is seldom found in humans. S. equinus, which is always abundant in the feces of horses, was first isolated from the air in 1906 by Andrewes and Horder due to the presence of dried horse manure, common in most cities at the time. In 1910, Winslow and Palmer verified the findings of Andrewes and Horder and reported further findings in both cattle and human feces. After the bacterium was discovered in 1906, the term Streptococcus equinus became a convenient “wastebasket” into which nonhemolytic streptococci that do not ferment lactose and mannitol were categorized. The classification of all streptococci that fail to ferment lactose into one large category has made the classification of S. equinus very difficult. However, as shown to the left, it is known that S. equinus, a nonenterococcal, group D streptococcus, is most closely related to the species S. bovis. In 2003, S. bovis and S. equinus were found to have a 99% 16S rRNA sequence similarity. While particularly similar in phylogeny they differ in biochemical reactions and physiological characteristics. The taxonomy of the organisms designated as S. bovis and S. equinus has a very complex history. S. equinus and S. bovis were reported to be synonyms by Farrow et al. in 1984, but were listed as separate species in Bergey's Manual of Systematic Bacteriology in 1986. Recently, the situation has become more complex by the description of two novel species for strains originally identified as S. bovis as actually S. caprinus, and S. gallolyticus. The taxonomy of S. equinus has yet to be fully resolved. A prominent characteristic of S. equinus is its inability to ferment lactose and mannitol. Moreover, it is nonhemolytic and not known to be pathogenic for animals. Generally, it appears as short chains of spherical or ovoid cells. These chains are somewhat longer in broth cultures than milk. Some cultures form extremely long chains in broth.