The Bacillota (synonym Firmicutes) are a phylum of bacteria, most of which have gram-positive cell wall 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, Selenomonas and Zymophilus, have a porous pseudo-outer membrane that causes them to stain gram-negative.
Many Bacillota (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. Bacillota 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. However, Bacillota as a whole is generally believed to be monophyletic, or paraphyletic with the exclusion of Mollicutes.
The currently accepted taxonomy based on the List of Prokaryotic names with Standing in Nomenclature (LPSN) and the National Center for Biotechnology Information (NCBI).
More than 274 genera were considered to be within the Bacillota phylum, notable genera of Bacillota include:
Bacilli, order Bacillales
Bacillus
Listeria
Staphylococcus
Bacilli, order Lactobacillales
Enterococcus
Lactobacillus
Leuconostoc
Streptococcus
Clostridia
Clostridioides
Clostridium
Selenomonas
Erysipelotrichia
Erysipelothrix
Infectobesity
Bacillota make up ~30% of the mouse and human gut microbiome.