| Streptococcus equinus | |
|---|---|
Scientific classification | |
| Domain: | Bacteria |
| Kingdom: | Bacillati |
| Phylum: | Bacillota |
| Class: | Bacilli |
| Order: | Lactobacillales |
| Family: | Streptococcaceae |
| Genus: | Streptococcus |
| Species: | S. equinus |
| Binomial name | |
| Streptococcus equinus Andrewes FW, Horder TJ 1906 | |
| Synonyms[1][2][3][4] | |
Streptococcus equinus is aGram-positive, nonhemolytic, nonpathogenic,lactic acid bacterium of the genusStreptococcus.[5] It is the principalStreptococcus found in thealimentary canal of a horse,[6] and makes up the majority of the bacterial flora in horse feces.[7] Equivalence withStreptococcus bovis has been contested.[4]
S. equinus, which is always abundant in the feces ofhorses, 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.[8]
In 1910, Winslow and Palmer verified the findings of Andrewes and Horder and reported further findings in both cattle and human feces.[9]
After the bacterium was discovered in 1906, the termStreptococcus equinus became a convenient “wastebasket” into which nonhemolytic streptococci that do not fermentlactose andmannitol were categorized.[9] The classification of allstreptococci that fail to ferment lactose into one large category has made the classification ofS. equinus very difficult.
However, as shown to the left, it is known thatS. equinus, a nonenterococcal, group D streptococcus, is most closely related to the speciesS. bovis.[7] In 2003,S. bovis andS. equinus were found to have a 99% 16SrRNA sequence similarity.[4] While particularly similar in phylogeny they differ in biochemical reactions and physiological characteristics.[4]
The taxonomy of the organisms designated asS. bovis andS. equinus has a very complex history.S. equinus andS. bovis were reported to be synonyms by Farrow et al. in 1984, but were listed as separate species inBergey's Manual of Systematic Bacteriology in 1986.[11] Recently, the situation has become more complex by the description of two novel species for strains originally identified asS. bovis as actuallyS. caprinus, andS. gallolyticus.[11] The taxonomy ofS. equinus has yet to be fully resolved.[12]
A prominent characteristic ofS. equinus is its inability to ferment lactose and mannitol.[8] Moreover, it is nonhemolytic and not known to bepathogenic 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.[9]
S. equinus has a high minimum temperature of growth, evidenced by little or no growth ingelatin cultures at temperatures lower than 21 °C.[8][9] No growth occurs at 10 or 15 °C, and growth is very slow at 21 °C.[9] The maximum temperature of growth takes place at 45 °C, and 47 °C where growth seldom occurs.[9] No growth occurs at 48 °C.[9]
It does not grow well in nor can it coagulate milk.[8][9] However, it has a high fermentative power inglucose broth.[9] The organism grows with vigor inglucose-peptone-litmus milk.[9]
It has a higher resistance to temperature than that possessed by pathogenic streptococci, but substantially lower than that ofthermoduric streptococci.
Most of the other properties ofS. equinus have not yet been determined.
S. equinus is one of the rareGram-positive bacteria that may causebacteremia andendocarditis in humans, but infection with this organism is very rare in humans.[13]
Among the rare published cases ofS. equinus reported are: infective endocarditis,[14] andperitonitis.[15]
In 1993, a case was reported of a farmer with documentedaortic valve disease who developed bacterial endocarditis due toS. equinus.[14] The case report also noted thatS. equinus is a rare pathogen in man and its acquisition may be related to the subject’s occupation.[14]
In 1998, a case ofS. equinus peritonitis in a patient oncontinuous ambulatory peritoneal dialysis (CAPD) was reported.[15] This case reported thatS. equinus is a rare but easily treatable cause of peritonitis in CAPD patients.[15]
In 2000, a woman with no underlying cardiac abnormalities developedS. equinus endocarditis.[16] However, the patient also had pulmonaryhistiocytosis X.[16] 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.[16] 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.[16]
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.[17]
To obtain a definitive discrimination betweenS. equinus andS. bovis, extensive further studies are required. AdditionalDNA-DNA hybridization studies or genomic and proteomic comparison experiments of the two species could lead to more definitive results.[18] Also, further studies using new techniques such asMALDI-TOF may also be effective.[18]