Andrewes FW, Horder TJ 1906
Streptococcus equinus is a Gram-positive, non-hemolytic, non-pathogenic, lactic acid bacteria 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.
Streptococcus 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 cow and human feces.
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.
A prominent characteristic of Streptococcus equinus is its inability to ferment lactose and mannitol.
Morphology: Generally, 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.
Temperature of Growth: Streptococcus equinus has a high minimum temperature of growth, evidenced by little or no growth in gelatin cultures at temperatures lower than 21°C. No growth occurs at 10°C nor at 15°C, and growth is very slow at 21°C. The maximum temperature of growth takes place at 45°C, and 47°C where growth seldom occurs. No growth occurs at 48°C.
Thermal Resistance: It has a higher resistance possessed by pathogenic streptococci, but substantially lower than that of thermoduric streptococci.
Most of the other properties of Streptococcus equinus have not yet been determined.
In 1993, there was a case report of a farmer with documented aortic valve disease who developed bacterial endocarditis due to Streptococcus equinus. The case report also noted that Streptococcus equinus is a rare pathogen in man and its acquisition may be related to the subject’s occupation.
In 1998, a case of Streptococcus equinus peritonitis in a patient on continuous ambulatory peritoneal dialysis (CAPD) was reported. This case reported that Streptococcus equinus is a rare but easily treatable cause of peritonitis in CAPD patients.
In 2000, a woman with no underlying cardiac abnormalities developed S. equinus endocarditis. However, it was reported that the patient also had pulmonary histiocytosis X. While this may have been a coincidence, such patients have many abnormalities of the immune system including imbalance of the immunoregulatory cell system and a decreased production of natural antibodies. Such abnormalities can predispose the patients with histiocytosis X to the development of bacterial infections, and a similar mechanism may have taken place in this patient.
Overall, while this organism has been isolated from the human intestine, currently it has not been reported to cause endocarditis in patients without history of cardiac disease or another underlying condition.
In order to obtain a definitive discrimination between Streptococcus equinus and Streptococcus bovis, extensive further studies are required. Additional DNA-DNA hybridization studies or genomic and proteomic comparison experiments of the two species could lead to more definitive results. Also, further studies using new techniques such as MALDI-TOF may also be effective.
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