The Negativicutes is a class of firmicutebacteria, whose members have a peculiar cell wall composition which stains Gram negative, unlike most other members of the Firmicutes. The family Veillonellaceae, formerly known as Acidaminococcaceae, were once considered members of the class Clostridia. The name Selenobacteria also refers to some members of this group. The description of this family was emended in 2010 and, together with the Acidaminococcaceae (familia nova, i.e. a newly coined taxon), it was placed in the order Selenomonadales (ordo novus) in the class Negativicutes (classis novus) of the phylum Firmicutes.
Members of this family are all obligate anaerobes, and occur in habitats such as rivers, lakes, and the intestines of vertebrates. They range from spherical forms, such as Megasphaera and Veillonella, to curved rods, as typified by the Selenomonads. Selenomonas has a characteristic crescent-shape, with flagella inserted on the concave side, while Sporomusa is similar, but non-motile. Their names refer to this distinctive morphology: selene means moon, and musa means banana.
Further examination of members of this group suggests that the members of this clade are actually Clostridia.
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^Yutin N, Galperin MY (2013) A genomic update on Clostridial phylogeny: Gram-negative spore formers and other misplaced Clostridia. Environ Microbiol doi: 10.1111/1462-2920.12173