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Bacillus subtilis, Gram stained
Scientific classification


The Firmicutes are a phylum of bacteria, most of which have the Gram-positive type of cell walls.


The group is typically divided into the Clostridia, which are anaerobic, the Bacilli, which are aerobic, and the Mollicutes, a class of bacteria which do not have cell walls.

On phylogenetic trees, the first two groups show up as paraphyletic or polyphyletic, as do their main genera, Clostridium and Bacillus.[1]


Firmicutes make up the largest portion of the mouse and human gut microbiome.[2] The division Firmicutes as part of the gut flora has been shown to be involved in energy resorption and obesity.[3][4][5]


  1. Wolf M. (2004). "Phylogeny of Firmicutes with special reference to Mycoplasma (Mollicutes) as inferred from phosphoglycerate kinase amino acid sequence data". Int. J. Syst. Evol. Microbiol. 54 (Pt 3): 871–5. doi:10.1099/ijs.0.02868-0 . PMID 15143038 . 
  2. Ley R.E; Peterson D.A. & Gordon J.I. (2006). "Ecological and evolutionary forces shaping microbial diversity in the human intestine". Cell 124 (4): 837–48. doi:10.1016/j.cell.2006.02.017 . PMID 16497592 . 
  3. Ley R.E. (2006). "Microbial ecology: human gut microbes associated with obesity". Nature 444 (7122): 1022–3. PMID 17183309 . 
  4. Henig, Robin Marantz (2006-08-13). Fat Factors. New York Times Magazine. Retrieved 2008-09-28. 
  5. Ley R.E. (2005). "Obesity alters gut microbial ecology". Proc. Natl. Acad. Sci. U.S.A. 102 (31): 11070–5. doi:10.1073/pnas.0504978102 . PMC 1176910 . PMID 16033867 . Retrieved 2008-09-28.