The Genus Enterococcus
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The Genus Enterococcus: Taxonomy LUC DEVRIESE, MARGO BAELE AND PATRICK BUTAYE
Introduction The genus Enterococcus contains bacterial species associated with animals and plants. Only species from humans and domestic animals have been studied in some detail. Limited information is available on plant-associated species and this has been mainly derived from the study of strains transiently associated with humans or animals. The enterococci are most often considered as components of the intestinal flora of humans and animals acting as opportunistic pathogens in different extra-intestinal compartments of the body. They have received considerable attention in medical bacteriology because of their increasing role in hospital-acquired (nosocomial) infections. An important factor contributing to this phenomenon undoubtedly has been their natural (intrinsic) and acquired resistance to frequently used antibiotics. Numerous studies have been devoted in recent years to this topic. Genetic studies (not treated in the present contribution), except those undertaken for taxonomic purposes, have largely concerned plasmids and transposons in connection with antibiotic resistance, and two genetic systems that have been described in Enterococcus faecalis: conjugative plasmids and sex pheromone plasmids.
Phylogeny Relation to Other Genera The enterococci have been separated from the streptococci, first based on DNA-DNA and DNA-rRNA hybridization studies (Schleifer and Kilpper-Bälz, 1984; Schleifer et al., 1985; Schleifer and Kilpper-Bälz, 1987). This separation was confirmed by 16S rRNA sequence analysis (Ludwig et al., 1985) which showed that the enterococci also differed from the lactococci and certain other Gram-positive cocci. The enterococci belong to the Firmicutes with low G+C content, the so-called clostridial branch. Phylogenetically the closest relative of the enterococci, but well separated from the latter,
is the genus Vagococcus and next Carnobacterium, Tetragenococcus, Aerococcus, Alloiococcus, Dolosigranulum, Facklamia, Globicatella and Abiotrophia (Collins et al., 1997). The streptococci and the lactococci to which the enterococci have been linked in the past, are more distantly related, as are the lactobacilli.
Species Groups 16S rRNA Reverse Transcriptase Sequence Analysis. Within the genus certain groups of species (Table 1) have been shown by 16S rRNA reverse trancriptase sequence analysis to be more closely related to each other than to others (Williams et al., 1991). Enterococcus faecalis forms a distinct lineage, as do E. saccharolyticus, E. sulfureus and E. dispar. The intraspecies group distances between E. cecorum and E. columbae are larger than the distances seen within other species groups. Patel et al. (1998) produced by the same technique a distance matrix tree that was nearly identical except for the fact that E. sulfureus and E. saccharolyticus appeared to form still another group with its two distantly related members.
Relation to Phenotypic Characteristics Most in
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