Enterococci were originally classified as enteric gram-positive cocci and later included in the genus Streptococcus. The term enterococcus was first used by Thiercelin in a paper from France published in 1899; the name was proposed to emphasize the intestinal origin of this new gram-positive diplococcus. In the same year, MacCallum and Hastings reported a case of endocarditis caused by an organism they called Micrococcus zymogenes; later papers suggest that this organism was actually a hemolytic enterococcus.
The name Streptococcus faecalis (faecalis, relating to feces) was first coined in 1906s by Andrewes and Horder, who isolated this organism from a patient with endocarditis and considered that this streptococcus was "so characteristic of the human intestine that the term 'streptococcus faecalis' may justly be applied to it".
In 1919s, Orla-Jensen described a second organism of this group, Streptococcus faecium, which differed from the fermentation patterns of S. fecalis. A third species, streptococcus durans, proposed by Sherman and Wing, was similar to S. faecium but of less fermentation activity.
In the 1930s, with the establishment of the Lancefield serological typing system, enterococci were classified as group D streptococci and were differentiated from the non enterococcal group D streptococci such as Streptococcus bovis by distinctive biochemical characteristics.
In an excellent review in 1937, Sherman emphasized that the term enterococcus had been used to mean different things ranging from the broad definition of any fecal streptococcus to a restricted definition of organisms that appeared to be identical to S. faecalis. Sherman proposed a classification scheme which separated streptococci into four divisions: pyogenic, viridans, lactic, and enterococcus.
Sherman further recommended that the term “enterococcus” should be used specifically for streptococci that grow at both 10 and 45°C, at pH 9.6, and in 6.5% NaCl and survive at 60°C for 30 min. These organisms were also noted to hydrolyze esculin in the presence of bile.
A number of studies in the 1940s and 1950s showed that organisms referred to as S. faecium had biochemical characteristics that distinguished them from S. fecalis. Such differences included inhibition by potassium tellurite, fermentation reactions, and failure to reduce tetrazolium to formazan.
Although S. faecium was not officially recognized as a separate species in the 1957 Bergey's Manual of Determinative Bacteriology, the species status of these organisms was nonetheless widely accepted and was incorporated into official nomenclature by the mid-1960s. During this period, S. durans was sometimes listed as a separate species and sometimes referred to as a variant of S. faecium.
In 1967s, Nowlan and Deibel added Streptococcus avium to the Enterococcal group. In 1970 Kalina proposed that a genus for the Enterococcal streptococci be established and suggested that, based on cellular arrangement and phenotypic characteristics, S. faecalis and S. faecium and the subspecies of these two taxons be named Enterococcus.
In the 1980s, based on genetic differences, enterococci were removed from the genus Streptococcus and placed in their own genus, Enterococcus. Genetic evidence that S.fecalis and S. faecium were significantly different from the other members of the genus to merit a separate genus was provided by Schleifer and Kilpper-Balz. Since then it has been generally accepted that the genus enterococcus is valid.
Although a dozen Enterococcus species have been identified, only two are responsible for the majority of human infections. Until recently, Enterococcus faecalis had been the predominant enterococcal species, accounting for 80 to 90% of all clinical isolates, and Enterococcus faecium had accounted for 5 to 15%. Other Enterococcus species (E. gallinarum, E. casseliflavus, E. durans, E. avium, and E. raffinosis) are isolated much less frequently and account for less than 5% of clinical isolates.
Until now, DNA-DNA, DNA - rRNA hybridizations and 16S rRNA sequencing studies have so far resulted in many species included in the genus Enterococcus.
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