The Firmicutes (Latin: firmus, strong, and cutis, skin, referring to the cell wall) are a phylum of bacteria, most of which have gram-positive cell wall structure.[3] A few, however, such as Megasphaera, Pectinatus, Selenomonas and Zymophilus, have a porous pseudo-outer membrane that causes them to stain gram-negative. Scientists once classified the Firmicutes to include all gram-positive bacteria, but have recently defined them to be of a core group of related forms called the low-G+C group, in contrast to the Actinobacteria. They have round cells, called cocci (singular coccus), or rod-like forms (bacillus).

Bacillus subtilis Gram.jpg
Bacillus subtilis, Gram-stained
Scientific classification e
Domain: Bacteria
(unranked): Terrabacteria
Phylum: "Firmicutes"
Gibbons and Murray 1978,[1] Murray, 1984[2]
  • Endospora
  • Endobacteria Cavalier-Smith 2002
  • "Endobacteria" Cavalier-Smith 1998
  • "Mollifirmicutes"
  • Mollicutes Edward & Freundt 1967
  • Mollicutaeota Oren et al. 2015
  • "Halanaerobiaeota"
  • Tenericutes Murray 1984

Many Firmicutes produce endospores, which are resistant to desiccation and can survive extreme conditions. They are found in various environments, and the group includes some notable pathogens. Those in one family, the heliobacteria, produce energy through anoxygenic photosynthesis. Firmicutes play an important role in beer, wine, and cider spoilage.


The group is typically divided into the Clostridia, which are anaerobic, and the Bacilli, which are obligate or facultative aerobes.

On phylogenetic trees, the first two groups show up as paraphyletic or polyphyletic, as do their main genera, Clostridium and Bacillus.[4] However, Firmicutes as a whole is generally believed to be monophyletic, or paraphyletic with the exclusion of Mollicutes.[5]


A phylogeny by Annotree[6] and GTDB release 05-RS95 (17 July 2020).[7]

Firmicutes G


Firmicutes E






Firmicutes B










Firmicutes A




Clostridiia s.s.

Firmicutes D









"Bacillia" s.s.

This second phylogeny is based on 16S rRNA-based LTP release 132 by the All-Species Living Tree Project,[8] with the currently accepted taxonomy based on the List of Prokaryotic names with Standing in Nomenclature (LPSN),[9] National Center for Biotechnology Information (NCBI),[10] and some non-validated clade names from Genome Taxonomy Database.[11]


Thermaerobacter {"Thermaerobacterales": "Thermaerobacteraceae"}


Caldicellulosiruptor {"Caldicellulosiruptorales": "Caldicellulosiruptoraceae"}


Dictyoglomus {Dictyoglomaceae}

Tepidanaerobacter {"Tepidanaerobacteraceae"}


"Ammonificaceae" {"Ammonifexales"}

Thermoanaerobacteraceae {Thermoanaerobacterales}

"Moorellaceae" {"Moorellales"}

"Thermacetogeniaceae" {"Thermacetogeniales"}

Carboxydothermus {"Carboxydothermales": "Carboxydothermaceae"}



Symbiobacterium {"Symbiobacteriales": Symbiobacteriaceae}

Sulfobacillus {"Sulfobacillales": "Sulfobacillaceae"}

Clostridia s.s.



More than 274 genera were considered as of 2016 to be within the Firmicutes phylum,[citation needed] notable genera of Firmicutes include:

Bacilli, order Bacillales

Bacilli, order Lactobacillales



Clinical significanceEdit

Firmicutes make up ~30% of the mouse and human gut microbiome.[12][failed verification] The division Firmicutes as part of the gut microbiota has been shown to be involved in energy resorption, and potentially related to the development of diabetes and obesity.[13][14][15][16] Within the gut of healthy human adults, the most abundant bacterium: Faecalibacterium prausnitzii (F. prausnitzii), which makes up 5% of the total gut microbiome, is a member of the Firmicutes phylum. This species is directly associated with reduced low-grade inflammation in obesity.[17] F. prausnitzii has been found in higher levels within the guts of obese children than in non-obese children.

In multiple studies a higher abundance of Firmicutes has been found in obese individuals than in lean controls. A higher level of Lactobacillus (of the Firmicutes phylum) has been found in obese patients and in one study, obese patients put on weight loss diets showed a reduced amount of Firmicutes within their guts.[18]

Diet changes in mice have also been shown to promote changes in Firmicutes abundance. A higher relative abundance of Firmicutes was seen in mice fed a western diet (high fat/high sugar) than in mice fed a standard low fat/ high polysaccharide diet. The higher amount of Firmicutes was also linked to more adiposity and body weight within mice.[19] Specifically, within obese mice, the class Mollicutes (within the Firmicutes phylum) was the most common. When the microbiota of obese mice with this higher Firmicutes abundance was transplanted into the guts of germ-free mice, the germ-free mice gained a significant amount of fat as compared to those transplanted with the microbiota of lean mice with lower Firmicutes abundance.[20]

The presence of Christensenella (Firmicutes, in class Clostridia), isolated from human faeces, has been found to correlate with lower body mass index.[21]


  1. ^ Gibbons, N. E. & Murray, R. G. E. 1978. Proposals concerning the higher taxa of bacteria. Int J Syst Bacteriol 28:1–6, (PDF)
  2. ^ Murray, R. G. E. (1984). The higher taxa, or, a place for everything...?. In: N. R. Krieg & J. G. Holt (ed.) Bergey's Manual of Systematic Bacteriology, vol. 1, The Williams & Wilkins Co., Baltimore, p. 31–34.
  3. ^ "Firmicutes" at Dorland's Medical Dictionary
  4. ^ Wolf M, Müller T, Dandekar T, Pollack JD (May 2004). "Phylogeny of Firmicutes with special reference to Mycoplasma (Mollicutes) as inferred from phosphoglycerate kinase amino acid sequence data". Int. J. Syst. Evol. Microbiol. (Comparative Study). 54 (Pt 3): 871–5. CiteSeerX doi:10.1099/ijs.0.02868-0. PMID 15143038. Archived from the original on 2012-12-09.
  5. ^ Ciccarelli, FD (2006). "Toward automatic reconstruction of a highly resolved tree of life". Science. 311 (5765): 1283–1287. Bibcode:2006Sci...311.1283C. CiteSeerX doi:10.1126/science.1123061. PMID 16513982. S2CID 1615592.
  6. ^ Mendler, K; Chen, H; Parks, DH; Hug, LA; Doxey, AC (2019). "AnnoTree: visualization and exploration of a functionally annotated microbial tree of life". Nucleic Acids Research. 47 (9): 4442–4448. doi:10.1093/nar/gkz246. PMC 6511854. PMID 31081040.
  7. ^ "GTDB release 05-RS95". Genome Taxonomy Database.
  8. ^ "16S rRNA-based LTP release 132 (full tree)". All-Species Living Tree Project. Silva Comprehensive Ribosomal RNA Database. Retrieved 2013-03-20.
  9. ^ J. P. Euzéby. "Firmicutes". List of Prokaryotic names with Standing in Nomenclature (LPSN). Archived from the original on January 27, 2013. Retrieved 2013-03-20.
  10. ^ Sayers; et al. "Firmicutes". National Center for Biotechnology Information (NCBI) taxonomy database. Retrieved 24 April 2019.
  11. ^ "GTDB taxonomy". Genome Taxonomy Database. Archived from the original on 2018-07-29. Retrieved 2018-07-20.
  12. ^ Ley RE, Peterson DA, Gordon JI (2006). "Ecological and evolutionary forces shaping microbial diversity in the human intestine". Cell (Review). 124 (4): 837–848. doi:10.1016/j.cell.2006.02.017. PMID 16497592. S2CID 17203181.
  13. ^ Ley RE, Turnbaugh PJ, Klein S, Gordon JI (2006). "Microbial ecology: human gut microbes associated with obesity". Nature (Clinical Trial). 444 (7122): 1022–1023. Bibcode:2006Natur.444.1022L. doi:10.1038/4441022a. PMID 17183309. S2CID 205034045.
  14. ^ Henig, Robin Marantz (2006-08-13). "Fat Factors". New York Times Magazine. Retrieved 2008-09-28.
  15. ^ Ley RE, Bäckhed F, Turnbaugh P, Lozupone CA, Knight RD, Gordon JI (August 2005). "Obesity alters gut microbial ecology". Proc. Natl. Acad. Sci. USA (Research Support). 102 (31): 11070–11075. Bibcode:2005PNAS..10211070L. doi:10.1073/pnas.0504978102. PMC 1176910. PMID 16033867.
  16. ^ Komaroff AL. The Microbiome and Risk for Obesity and Diabetes. JAMA. Published online December 22, 2016. doi:10.1001/jama.2016.20099
  17. ^ Chakraborti, Chandra Kanti (15 November 2015). "New-found link between microbiota and obesity". World Journal of Gastrointestinal Pathophysiology. 6 (4): 110–119. doi:10.4291/wjgp.v6.i4.110. PMC 4644874. PMID 26600968.
  18. ^ Million, M.; Lagier, J.-C; Yahav, D.; Paul, M. (April 2013). "Gut bacterial microbiota and obesity". Clinical Microbiology and Infection. 19 (4): 305–313. doi:10.1111/1469-0691.12172. PMID 23452229.
  19. ^ Turnbaugh, Peter J. (17 April 2008). "Diet-Induced Obesity Is Linked to Marked but Reversible Alterations in the Mouse Distal Gut Microbiome". Cell Host & Microbe. 3 (4): 213–223. doi:10.1016/j.chom.2008.02.015. PMC 3687783. PMID 18407065.
  20. ^ Million, M. (April 2013). "Gut bacterial microbiota and obesity". Cell Microbiology and Infection. 19 (4): 305–313. doi:10.1111/1469-0691.12172. PMID 23452229.
  21. ^ Goodrich, Julia K.; Waters, Jillian L.; Poole, Angela C.; Sutter, Jessica L.; Koren, Omry; Blekhman, Ran; Beaumont, Michelle; Van Treuren, William; Knight, Rob; Bell, Jordana T.; Spector, Timothy D.; Clark, Andrew G.; Ley, Ruth E. (2014). "Human Genetics Shape the Gut Microbiome". Cell. 159 (4): 789–799. doi:10.1016/j.cell.2014.09.053. ISSN 0092-8674. PMC 4255478. PMID 25417156. 

External linksEdit