The Firmicutes (Latin: firmus, strong, and cutis, skin, referring to the cell wall) are a phylum of bacteria, most of which have gram-positivecell wall structure. 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).
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.
Firmicutes make up the largest portion of the mouse and human gut microbiome. 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. 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.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.
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. 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.
^Gibbons, N. E. & Murray, R. G. E. 1978. Proposals concerning the higher taxa of bacteria. Int J Syst Bacteriol 28:1–6, (PDF)
^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.