Phylogeny
After the bacteria was discovered in 1906, the term Streptococcus equinus became a convenient “wastebasket” into which non-hemolytic 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 Streptococcus equinus very difficult.
However, as shown to the left, it is known that Streptococcus equinus, a non-enterococcus, group D streptococci, is most closely related to the species S. bovis. In 2003, it was found that S. bovis and S. equinus 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 Streptococcus bovis and Streptococcus equinus has a very complex history. S. equinus and S. bovis were reported 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 Streptococcus caprinus, and Streptococcus gallolyticus. The taxonomy of S. equinus has yet to be fully resolved.
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