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.[1] 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 Proteobacteria.[1][2][3][4] 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.[5]
The class currently consists of 32 validly named genera across three orders and four families.[6][7] The orders Veillonellales and Acidaminococcales each contain a single family, Veillonellaceae and Acidaminococcaceae, respectively, while the order Selenomonadales contains two families, Selenomonadaceae and Sporomusaceae.[7]
Historically, the Negativicutes consisted of a single order, the Selenomonadales, and two families, Veillonellaceae and Acidaminococcaceae based on 16S rRNA gene sequence similarity.[8][5] 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.[7] 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.[9][10]
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.[9][10] 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.[5][10][11][12] 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.[7]
The currently accepted taxonomy is based on the List of Prokaryotic names with Standing in Nomenclature (LPSN) [13] and National Center for Biotechnology Information (NCBI)[14]
Succinispira mobilis Janssen and O'Farrell 1999
Acidaminococcus Rogosa 1969 emend. Jumas-Bilak et al. 2007
Phascolarctobacterium faecium Del Dot et al. 1994 (type sp.)
Phascolarctobacterium succinatutens Watanabe et al. 2012
Succiniclasticum ruminis van Gylswyk 1995
Veillonellaceae?Anaerospora hongkongensis ♠ Woo et al. 2005
?Desulfosporomusa polytropa ♠ Sass et al. 2004
?Psychrosinus fermentans ♠ Sattley et al. 2008
?Quinella ovalis ♪ Krumholz et al. 1993 (Quin’s Oval)
?Sporotalea colonica ♠ Ezaki et al. 2008
Sporomusa Möller et al. 1985
Sporolituus thermophilus Ogg and Patel 2009
Thermosinus carboxydivorans Sokolova et al. 2004
Veillonella Prévot 1933 emend. Mays et al. 1982
Megasphaera [incl. Anaeroglobus geminatus]
Negativicoccus succinicivorans Marchandin et al. 2010
Dialister [incl. Allisonella histaminiformans]
Anaeromusa acidaminophila Baena et al. 1999
Anaeroarcus burkinensis (Ouattara et al. 1992) Strömpl et al. 1999
Acetonema longum Kane and Breznak 1992
Pelosinus Shelobolina et al. 2007
Propionispora Biebl et al. 2001
Dendrosporobacter quercicolus (Stankewich et al. 1971) Strömpl et al. 2000
Anaerosinus glycerini (Schauder and Schink 1996) Strömpl et al. 1999
Propionispira arboris Schink et al. 1983
Zymophilus Schleifer et al. 1990
Megamonas Shah and Collins 1983
Pectinatus Lee et al. 1978 emend. Juvonen and Suihko 2006
Anaerovibrio lipolyticus Hungate 1966
Selenomonas [incl. Schwartzia succinivorans, Centipeda periodontii & Mitsuokella]
Notes:
♠ Strains found at the National Center for Biotechnology Information (NCBI), but not listed in the List of Prokaryotic names with Standing in Nomenclature (LPSN)
♪ Prokaryotes where no pure (axenic) cultures are isolated or available, i. e. not cultivated or can not be sustained in culture for more than a few serial passages
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 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.
