# Negativicutes

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.

## 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 and Acidaminococcaceae, respectively, while the order Selenomonadales 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. 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 (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 Clostridia, 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. 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 Clostridia.

## 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)
| 16S rRNA based LTP_08_2023 | 120 single copy marker proteins based GTDB 08-RS214 |
| Acidaminococcales Acidaminococcaceae Succinispira Acidaminococcus Phascolarctobacterium Phascolarctobacterium wakonense Shigeno et al. 2019 Succiniclasticum Selenomonadales Anaeromusa Anaeroarcus Sporomusaceae Anaerosporomusa Pelorhabdus Acetonema Pelosinus Methylomusa Sporomusa Lucifera Sporolituus Thermosinus Propionispora Anaerospora Dendrosporobacter Anaerosinus Strömpl et al. 1999 Veillonellaceae Veillonella Megasphaera [incl. Anaeroglobus] Negativicoccus Dialister [incl. Allisonella] Selenomonadaceae Propionispira Megamonas Pectinatus Selenomonas lacticifex Schleifer et al. 1990 Selenomastix Selenomonas species-group 2 Mitsuokella Anaerovibrio Schwartzia Selenomonas Centipeda Selenomonas species-group 3 | Acidaminococcales "Succinispiraceae" Succinispira Janssen and O'Farrell 1999 Acidaminococcaceae "Ca. Avacidaminococcus" Gilroy et al. 2021 Acidaminococcus Rogosa 1969 Phascolarctobacterium Del Dot et al. 1994 Phascolarctobacterium succinatutens Watanabe et al. 2012 Succiniclasticum van Gylswyk 1995 Veillonellales "Negativicoccaceae" Negativicoccus Marchandin et al. 2010 "Dialisteraceae" Dialister Bergey et al. 1923 ex Moore & Moore 1994 [incl. Allisonella] "Megasphaeraceae" Megasphaera paucivorans Juvonen & Suihko 2006 Anaeroglobus Carlier et al. 2002 "Caecibacter" Ricaboni et al. 2017 Megasphaera Rogosa 1971 Veillonellaceae Veillonella Prévot 1933 Sporomusales_A "Acetonemaceae" Anaerosporomusa Choi et al. 2016 Acetonema Kane and Breznak 1992 Sporomusales C DSM‑15969 Anaerospora Woo et al. 2005 "Thermosinaceae" Thermosinus Sokolova et al. 2004 [Sporolituus Ogg and Patel 2009] "Sporomusales" "Luciferaceae" Lucifera Sanchez-Andrea et al. 2018 Sporomusaceae Methylomusa Amano et al. 2018 Sporomusa Möller et al. 1985 "Anaeromusales" "Anaeromusaceae" Anaeromusa Baena et al. 1999 (incl. Anaeroarcus Strömpl et al. 1999) UMGS1260 UMGS1260 Pelorhabdus Grässle et al. 2022 "Propionisporales" "Propionisporaceae" Propionispora Biebl et al. 2001 "Pelosinaceae" Pelosinus Shelobolina et al. 2007 "Dendrosporobacterales" "Dendrosporobacteraceae" Dendrosporobacter Strömpl et al. 2000 Selenomonadales "Massilibacillaceae" "Massilibacillus" Tidjani Alou et al. 2017 Selenomonadaceae Propionispira arboris Schink et al. 1983 Megamonas Shah and Collins 1983 Pectinatus Lee et al. 1978 Selenomonas von Prowazek 1913 Anaerovibrio Hungate 1966 Schwartzia Gylswyk et al. 1997 Quinella Krumholz et al. 1993 Centipeda Lai et al. 1983 Selenomonas species-group 2 Selenomastix Woodcock & Lapage 1913 Mitsuokella Shah & Collins 1983 |