The Genus Eubacterium and Related Genera
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The Genus Eubacterium and Related Genera WILLIAM G. WADE
Phylogeny The genus Eubacterium, because of its loose definition (see Taxonomy section below), includes species with a diverse range of phenotypes. Not surprisingly, therefore, 16S rRNA gene sequence comparison has revealed that the species of this genus are widely distributed within the phylogenetic tree. At present, 16S rRNA sequences have been determined for 45 of the 46 species and subspecies. These all fall within the phylum Firmicutes (Fig. 1). The type species, Eubacterium limosum belongs to a cluster with Eubacterium callanderi and Eubacterium barkeri, and it has been suggested that the genus Eubacterium sensu stricto should be restricted to these species (Willems and Collins, 1996), with the possible addition of Eubacterium aggregans, which also belongs to this phylogenetic group. Other major clusters of Eubacterium species within the Firmicutes include one consisting of Eubacterium biforme, Eubacterium cylindroides, Eubacterium dolichum and Eubacterium tortuosum within the family Erysipelotrichaceae, and a group of asaccharolytic species isolated from the oral cavity including Eubacterium brachy, Eubacterium infirmum, Eubacterium minutum, Eubacterium nodatum, Eubacterium saphenum and Eubacterium sulci, which are related to Mogibacterium (formerly Eubacterium) timidum. Other Eubacterium species are scattered among the Firmicutes and frequently have Clostridium species as close relatives. This is another heterogeneous genus of Gram-positive anaerobes; spore formation is the principal characteristic by which strains are placed within Clostridium rather than Eubacterium. The intermingling of species of these two genera throughout the phylogenetic tree suggests either that sporulation has emerged frequently throughout evolution or that it is a feature that has been lost from some species over time. Other species recently transferred from Eubacterium such as Eggerthella lenta (Wade et al., 1999), Collinsella aerofaciens (Kageyama et al., 1999a), Atopobium fossor (Kageyama et al., 1999b), Slackia exigua (Wade et al., 1999) and the recently described genera Cryptobacterium
(Nakazawa et al., 1999) and Olsenella (Dewhirst et al., 2001) are found in the phylum Actinobacteria (Fig. 2), a phylum of Gram-positive organisms with DNA of mainly high G+C content.
Taxonomy The genus definition for Eubacterium is rather unsatisfactory as it is defined by default; it includes those Gram-positive, nonsporeforming, obligately anaerobic rods that do not produce: 1) propionic acid as major acid (in contrast to Propionibacterium); 2) lactic acid alone (in contrast to Lactobacillus); 3) more acetic acid than lactic acid with and without formic acid (in contrast to Bifidobacterium); and 4) succinic acid (in the presence of carbon dioxide) and lactic acid with small amounts of acetic or formic acid (in contrast to Actinomyces; Moore and Holdeman Moore, 1986). Because of this loose definition, the genus is highly
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