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-stained
Scientific classification
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]


The phylogeny is based on 16S rRNA-based LTP release 132 by the All-Species Living Tree Project,[5] with the currently accepted taxonomy based on the List of Prokaryotic names with Standing in Nomenclature (LPSN),[6] National Center for Biotechnology Information (NCBI),[7] and some non-validated clade names from Genome Taxonomic Database.[8]

Firmicutes classification

Thermaerobacter {"Thermaerobacterales" ♥: "Thermaerobacteraceae" ♥}


Caldicellulosiruptor {"Caldicellulosiruptorales" ♥: "Caldicellulosiruptoraceae" ♥}


Dictyoglomus {Dictyoglomaceae}

Tepidanaerobacter {"Tepidanaerobacteraceae" ♥}

"Thermovenabulaceae" ♥

"Ammonificaceae" ♥ {"Ammonifexales" ♥}

Thermoanaerobacteraceae {Thermoanaerobacterales}

"Moorellaceae" ♥ {"Moorellales" ♥}

"Thermacetogeniaceae" ♥ {"Thermacetogeniales" ♥}

Carboxydothermus {"Carboxydothermales" ♥: "Carboxydothermaceae" ♥}



Symbiobacterium {"Symbiobacteriales" ♥: Symbiobacteriaceae}

Sulfobacillus {"Sulfobacillales" ♥: "Sulfobacillaceae" ♥}

Clostridia s.s.


Unassigned Clostridia s.s.

  • Desulfuribacillus Sorokin et al. 2014 emend. Abin & Hollibaugh 2017 {"Desulfuribacillia" ♥: "Desulfuribacillales" ♥: "Desulfuribacillaceae" ♥}
  • Limnochorda pilosa Watanabe, Kojima & Fukui 2015 {Limnochordia Watanabe, Kojima & Fukui 2015: Limnochordales Watanabe, Kojima & Fukui 2015: Limnochordaceae Watanabe, Kojima & Fukui 2015}

♥ Clade names not lodged at National Center for Biotechnology Information (NCBI) or listed in the List of Prokaryotic names with Standing in Nomenclature (LPSN)

The family Synergistaceae (Clostridiales Family XV Incertae Sedis) and Thermodesulfobiaceae have been removed from Firmicutes based on the fact, they do not form a monophyletic clade with it.


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

Bacilli, order Bacillales

Bacilli, order Lactobacillales



Health implications

Firmicutes make up the largest portion of the mouse and human gut microbiome.[9] The division Firmicutes as part of the gut flora has been shown to be involved in energy resorption, and potentially related to the development of diabetes and obesity.[10][11][12][13] 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.[14] 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.[15]

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.[16] 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.[17]

The presence of Christensenella (Firmicutes, in class Clostridia), isolated from human faeces, has been found to correlate with lower body mass index.[18]

Laboratory detection

The presence of Firmicutes can be reliably detected with polymerase chain reaction (PCR) techniques.[19]


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  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
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  7. Sayers; et al. "Firmicutes". National Center for Biotechnology Information (NCBI) taxonomy database. Retrieved 24 April 2019.
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  13. Komaroff AL. The Microbiome and Risk for Obesity and Diabetes. JAMA. Published online December 22, 2016. doi:10.1001/jama.2016.20099
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