The Bacillota (synonym Firmicutes) are a

phylum In biology, a phylum (; plural: phyla) is a level of classification or taxonomic rank below kingdom and above class. Traditionally, in botany the term division has been used instead of phylum, although the International Code of Nomenclature f ...

of bacteria, most of which have gram-positive

cell wall A cell wall is a structural layer surrounding some types of cells, just outside the cell membrane. It can be tough, flexible, and sometimes rigid. It provides the cell with both structural support and protection, and also acts as a filtering mech ...

structure. The renaming of phyla such as Firmicutes in 2021 remains controversial among microbiologists, many of whom continue to use the earlier names of long standing in the literature. The name "Firmicutes" was derived from the Latin words for "tough skin," referring to the thick cell wall typical of bacteria in this phylum. 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 Actinomycetota. They have round cells, called cocci (singular coccus), or rod-like forms (bacillus). A few Firmicutes, such as '' Megasphaera'', ''

Pectinatus ''Pectinatus'' is a genus of Bacillota bacteria classified within the class Negativicutes. Phylogeny The currently accepted taxonomy is based on the List of Prokaryotic names with Standing in Nomenclature (LPSN) and National Center for Biotechno ...

'', ''

Selenomonas Members of the genus ''Selenomonas'' (motile crescent-shaped bacteria in general) are referred to trivially as selenomonads. The genus ''Selenomonas'' constitutes a group of motile crescent-shaped bacteria and includes species living in the gast ...

'' and ''

Zymophilus ''Zymophilus'' is a genus of Bacillota bacteria classified within the class Negativicutes. See the NCBI The National Center for Biotechnology Information (NCBI) is part of the United States National Library of Medicine (NLM), a branch of th ...

'', have a porous pseudo-outer membrane that causes them to stain gram-negative. Many Bacillota (Firmicutes) produce

endospore An endospore is a dormant, tough, and non-reproductive structure produced by some bacteria in the phylum Bacillota. The name "endospore" is suggestive of a spore or seed-like form (''endo'' means 'within'), but it is not a true spore (i.e., no ...

s, which are resistant to

desiccation Desiccation () is the state of extreme dryness, or the process of extreme drying. A desiccant is a hygroscopic (attracts and holds water) substance that induces or sustains such a state in its local vicinity in a moderately sealed container. ...

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. Bacillota play an important role in beer, wine, and cider spoilage.

Classes

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

phylogenetic tree A phylogenetic tree (also phylogeny or evolutionary tree Felsenstein J. (2004). ''Inferring Phylogenies'' Sinauer Associates: Sunderland, MA.) is a branching diagram or a tree showing the evolutionary relationships among various biological spec ...

s, the first two groups show up as

paraphyletic In taxonomy (general), taxonomy, a group is paraphyletic if it consists of the group's most recent common ancestor, last common ancestor and most of its descendants, excluding a few Monophyly, monophyletic subgroups. The group is said to be pa ...

or polyphyletic, as do their main genera, ''

Clostridium ''Clostridium'' is a genus of anaerobic, Gram-positive bacteria. Species of ''Clostridium'' inhabit soils and the intestinal tract of animals, including humans. This genus includes several significant human pathogens, including the causative ag ...

'' and '' Bacillus''. However, Bacillota as a whole is generally believed to be monophyletic, or paraphyletic with the exclusion of Mollicutes.

Phylogeny

The currently accepted taxonomy based on the List of Prokaryotic names with Standing in Nomenclature (LPSN) and the National Center for Biotechnology Information (NCBI).

Genera

More than 274 genera were considered to be within the Bacillota phylum, notable genera of Bacillota include: Bacilli, order Bacillales * '' Bacillus'' * '' Listeria'' * ''

Staphylococcus ''Staphylococcus'' is a genus of Gram-positive bacteria in the family Staphylococcaceae from the order Bacillales. Under the microscope, they appear spherical (cocci), and form in grape-like clusters. ''Staphylococcus'' species are facultative ...

'' Bacilli, order Lactobacillales * '' Enterococcus'' * '' Lactobacillus'' * '' Leuconostoc'' * '' Streptococcus'' Clostridia * ''

Clostridioides ''Clostridioides'' is a genus of Gram-positive bacteria, which includes ''Clostridioides difficile'', a human pathogen causing an infectious diarrhea. __TOC__ Taxonomy The genus ''Clostridioides'' was created to describe a few species formerly in ...

'' * ''

Erysipelotrichia The Erysipelotrichia are a class of bacteria of the phylum Bacillota. Species of this class are known to be common in the gut microbiome, as they have been isolated from swine manure and increase in composition of the mouse gut microbiome for mi ...

* ''

Erysipelothrix ''Erysipelothrix'' is a genus of bacteria containing four described species, ''Erysipelothrix rhusiopathiae'', ''Erysipelothrix tonsillarum'', ''Erysipelothrix inopinata'' and ''Erysipelothrix larvae''.Verbarg, S., et al. (2004)''Erysipelothrix i ...

Clinical significance

Bacillota make up ~30% of the mouse and human gut microbiome. The phylum Bacillota as part of the

gut microbiota Gut microbiota, gut microbiome, or gut flora, are the microorganisms, including bacteria, archaea, fungi, and viruses that live in the digestive tracts of animals. The gastrointestinal metagenome is the aggregate of all the genomes of the gut m ...

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 ''Faecalibacterium'' is a genus of bacteria. Its sole known species, ''Faecalibacterium prausnitzii'' is gram-positive, mesophilic, rod-shaped, anaerobic and is one of the most abundant and important commensal bacteria of the human gut microbiota ...

prausnitzii (F. prausnitzii)'', which makes up 5% of the total gut microbiome, is a member of the Bacillota 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 Bacillota has been found in obese individuals than in lean controls. A higher level of ''Lactobacillus'' (of the Bacillota phylum) has been found in obese patients and in one study, obese patients put on weight loss diets showed a reduced amount of Bacillota within their guts. Diet changes in mice have also been shown to promote changes in Bacillota abundance. A higher relative abundance of Bacillota 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 Bacillota was also linked to more adiposity and body weight within mice. Specifically, within obese mice, the class ''Mollicutes'' (within the Bacillota phylum) was the most common. When the microbiota of obese mice with this higher Bacillota 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 Bacillota abundance. The presence of ''

Christensenella ''Christensenella'' is a genus of non- spore-forming, anaerobic, and nonmotile bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitu ...

'' (Bacillota, in class Clostridia), isolated from human faeces, has been found to correlate with lower

body mass index Body mass index (BMI) is a value derived from the mass (weight) and height of a person. The BMI is defined as the body mass divided by the square of the body height, and is expressed in units of kg/m2, resulting from mass in kilograms and he ...

References

External links