The Negativicutes are a

class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Class (philosophy), an analytical concept used differently ...

of bacteria in the phylum

Bacillota The Bacillota (synonym Firmicutes) are a phylum of bacteria, most of which have gram-positive cell wall structure. The renaming of phyla such as Firmicutes in 2021 remains controversial among microbiologists, many of whom continue to use the ear ...

, whose members have a peculiar cell wall with a

lipopolysaccharide Lipopolysaccharides (LPS) are large molecules consisting of a lipid and a polysaccharide that are bacterial toxins. They are composed of an O-antigen, an outer core, and an inner core all joined by a covalent bond, and are found in the outer me ...

outer membrane which stains gram-negative, unlike most other members of the

. Although several neighbouring ''

Clostridia The Clostridia are a highly polyphyletic class of Bacillota, including ''Clostridium'' and other similar genera. They are distinguished from the Bacilli by lacking aerobic respiration. They are obligate anaerobes and oxygen is toxic to them. Sp ...

'' 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 anaerobe Obligate anaerobes are microorganisms killed by normal atmospheric concentrations of oxygen (20.95% O2). Oxygen tolerance varies between species, with some species capable of surviving in up to 8% oxygen, while others lose viability in environme ...

s, and occur in habitats such as rivers, lakes, and the intestines of vertebrates. They range from spherical forms, such as ''Megasphaera'' and ''

Veillonella ''Veillonella'' are Gram-negative bacteria (Gram stain pink) anaerobic cocci, unlike most Bacillota, which are Gram-positive bacteria. This bacterium is well known for its lactate fermenting abilities. It is a normal bacterium in the intestin ...

'', to curved rods, as typified by the

selenomonad Members of the genus ''Selenomonas'' (motile crescent-shaped bacteria in general) are referred to trivially as selenomonads. The genus ''Selenomonas'' constitutes a group of motile crescent-shaped bacteria and includes species living in the gast ...

s. ''

Selenomonas Members of the genus ''Selenomonas'' (motile crescent-shaped bacteria in general) are referred to trivially as selenomonads. The genus ''Selenomonas'' constitutes a group of motile crescent-shaped bacteria and includes species living in the gast ...

'' 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 A banana is an elongated, edible fruit – botanically a berry – produced by several kinds of large herbaceous flowering plants in the genus ''Musa''. In some countries, bananas used for cooking may be called "plantains", distinguis ...

Taxonomy

The class currently consists of 32 validly named genera across three orders and four families. The orders Veillonellales and Acidaminococcales each contain a single family,

Veillonellaceae The Veillonellaceae are a family of the Clostridia, formerly known as Acidaminococcaceae. Bacteria in this family are grouped together mainly based on genetic studies, which place them among the Bacillota. Supporting this placement, several s ...

and

Acidaminococcaceae 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 neighbou ...

, respectively, while the order

Selenomonadales The Selenomonadales are an order of bacteria within the class Negativicutes; unlike most other members of Bacillota, they are Gram-negative. The phylogeny of this order was initially determined by 16S rRNA comparisons. More recently, molecular ...

contains two families, Selenomonadaceae and Sporomusaceae.

Molecular signatures

Historically, the Negativicutes consisted of a single order, the Selenomonadales, and two families, Veillonellaceae and Acidaminococcaceae based on 16S rRNA gene sequence similarity.Ludwig W, Schleifer K-H, Whitman, WB (2009) Revised road map to the phylum Firmicutes. In: Bergey’s Manual of Systematic Bacteriology, vol. 3, 2nd edn. pp. 1–13. Eds P. De Vos, G. M. Garrity, D. Jones, N. R. Krieg, W. Ludwig, F. A. Rainey, K. H. Schleifer & W. B. Whitman Springer-: New York. However, these groupings did not include several members within the Negativicutes that branched outside of the two families. The current taxonomic view is inclusive of these members who have been validly assigned to the families Selenomonadaceae and Sporomusaceae within the emended Selenomonadales order. Molecular markers in the form of

conserved signature indels Conserved signature inserts and deletions (CSIs) in protein sequences provide an important category of molecular markers for understanding phylogenetic relationships. CSIs, brought about by rare genetic changes, provide useful phylogenetic markers ...

(CSIs) and proteins (CSPs) justify the present taxonomic divisions. These molecular markers are found at each taxonomic rank, and their distribution is in agreement with the observed phylogenetic branching. Many works have implicated that the Negativicutes should be reclassified as an order within the class

, based on close phylogenetic branching, and the observation that the spore-forming members of the Negativicutes share similar sporulation genes as the Clostridia, and that both stain gram-negative. However, the heterogeneity of members within the Negativicutes, as well as the distribution of molecular signatures, supports the view that the Negativicutes are in fact an independent class within the Bacillota, with Clostridia as their closest phylogenetic neighbours.Merchandin H, Jumas-Bilak E (2014) The family Veillonellaceae. In: The Prokaryotes, vol. 7, 4th edn. pp. 433–453. Eds E. Rosenberg, E. DeLong, S. Lory, E. Stackebrandt & F. Thompson Springer-: Berlin, Heidelberg. Additionally, several CSIs and CSPs have been found to be uniquely shared among all Negativicutes, while no CSIs have been found to be shared by both Negativicutes and

Phylogeny

The currently accepted taxonomy is based on the List of Prokaryotic names with Standing in Nomenclature (LPSN) and National Center for Biotechnology Information (NCBI)

References

{{Taxonbar, from=Q623144 Bacteria classes

Taxonomy

Molecular signatures

Phylogeny

See also

References