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The Negativicutes are a subclass of firmicute bacteria, whose members have a peculiar cell wall with a lipopolysaccharide outer membrane which stains Gram-negative, unlike most other members of the Firmicutes.^[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 subclass 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]

Taxonomy

The subclass 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]

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.^[8][9] 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.^[10][11]

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.^[10][11] 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 Firmicutes, with Clostridia as their closest phylogenetic neighbours.^{[9][11][12][13]} 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]

Phylogeny

The currently accepted taxonomy is based on the List of Prokaryotic names with Standing in Nomenclature (LPSN) ^[14] and National Center for Biotechnology Information (NCBI)^[15]

• 
  Selenomonadales Marchandin et al. 2010 emend. Campbell et al. 2015
  
  • 
    Selenomonadaceae Campbell et al. 2015
    
    • 
      Anaerovibrio Hungate 1966
      
    
    
    • 
      Centipeda (bacteria) Lai et al. 1983
      
    
    
    • 
      Megamonas Shah and Collins 1983
      
    
    
    • 
      Mitsuokella Shah and Collins 1983
      
    
    
    • 
      Pectinatus Lee et al. 1978 emend. Juvonen and Suihko 2006
      
    
    
    • 
      Propionispira Schink et al. 1983
      
    
    
    • 
      Schwartzia van Gylswyk et al. 1997
      
    
    
    • 
      Selenomonas von Prowazek 1913
      
    
    
    • 
      Zymophilus Schleifer et al. 1990
      
    
    
  
  
  • 
    SporomusaceaeCampbell et al. 2015
    
    • 
      Acetonema Kane and Breznak 1992
      
    
    
    • 
      Anaeroarcus Strömpl et al. 1999
      
    
    
    • 
      Anaeromusa Baena et al. 1999
      
    
    
    • 
      Anaerosinus Strömpl et al. 1999
      
    
    
    • 
      Anaerospora ♠ Woo et al. 2005
      
    
    
    • 
      Dendrosporobacter Strömpl et al. 2000
      
    
    
    • 
      Desulfosporomusa ♠ Sass et al. 2004
      
    
    
    • 
      Pelosinus Shelobolina et al. 2007
      
    
    
    • 
      Propionispora Biebl et al. 2001
      
    
    
    • 
      Psychrosinus ♠ Sattley et al. 2008
      
    
    
    • 
      Sporolituus Ogg and Patel 2009
      
    
    
    • 
      Sporomusa Möller et al. 1985
      
    
    
    • 
      Thermosinus Sokolova et al. 2004
      
    
    
  
  


• 
  Acidaminococcales Campbell et al. 2015
  
  • 
    Acidaminococcaceae Marchandin et al. 2010 emend. Campbell et al. 2015
    
    • 
      Acidaminococcus Rogosa 1969 emend. Jumas-Bilak et al. 2007
      
    
    
    • 
      Phascolarctobacterium Del Dot et al. 1994
      
    
    
    • 
      Succiniclasticum van Gylswyk 1995
      
    
    
    • 
      Succinispira Janssen and O'Farrell 1999
      
    
    


• 
  VellionellalesCampbell et al. 2015
  
  • 
    VellionellaceaeRogosa 1969 emend. Campbell et al. 2015
    
    • 
      Allisonella Garner et al. 2003
      
    
    
    • 
      Anaeroglobus Carlier et al. 2002
      
    
    
    • 
      Dialister (ex Bergey et al. 1923) Moore and Moore 1994 emend. Morotomi et al. 2008
      
    
    
    • 
      Megasphaera Rogosa 1971 emend. Marchandin et al. 2003
      
    
    
    • 
      Negativicoccus Marchandin et al. 2010
      
    
    
    • 
      Veillonella Prévot 1933 emend. Mays et al. 1982
      
    
    


• 
  Unclassified
  
  • • 
      Quinella ♪ Krumholz et al. 1993
      

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

References