The Negativicutes are a class of bacteria in the phylum Bacillota, whose members have a peculiar cell wall with a lipopolysaccharide outer membrane which stains gram-negative, unlike most other members of the Bacillota. Although several neighbouring Clostridia species (firmicute bacteria) also stain gram-negative, the proteins responsible for the unusual diderm structure of the Negativicutes may have actually been laterally acquired from Pseudomonadota (formerly Proteobacteria). Additional research is required to confirm the origin of the diderm cell envelope in the Negativicutes.
Most members of this class are 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 nonmotile. Their names refer to this distinctive morphology: selene means moon, and musa means banana.
