Cytobacillus

Cytobacillus is a genus of rod-shaped bacteria that stain either Gram-positive or Gram-variable in the family Bacillaceae within the order Bacillales. The type species for this genus is Cytobacillus firmus.

Members of this genus was transferred from the Bacillus genus after comparative genomics studies have determined they were sufficiently different by phylogenetic measures than Bacillus subtilis, the type species of the genus Bacillus. The genus Bacillus has long been under close scrutiny by the scientific community due to its polyphyletic nature, displaying many distinct monophyletic groupings in various phylogenetic trees within the genus. In addition, while Bacillus species have a diverse range of biochemical characteristics, there is no unique characteristic that can be used to reliably distinguish all Bacillus species from other bacteria. Many studies have used phylogenetic and comparative genomic analyses as a means towards clarifying the complicated taxonomic relationships within Bacillus, resulting in the transfer of many species into novel genera such as Alkalihalobacillus, Virigibacillus, Brevibacillus, Solibacillus and Evansella. In addition, the genus Bacillus has been restricted to include only species closely related to Bacillus subtilis and Bacillus cereus.

The name Cytobacillus can be broken down into the prefix "cyto-" (from the Greek noun kytos, referring to hollow, vessel, jar or a cell in biology) and the suffix "-bacillus" (from the Latin noun bacillus, referring to a small staff or rod and Bacillus, the bacterial genus). Together, Cytobacillus refers to a rod-shaped cell.

Biochemical characteristics and molecular signatures
Members of Cytobacillus can be either aerobic or facultatively anaerobic. All studied species of this genus has been observed to produce endospores under adverse environmental or nutritional conditions. Cytobacillus can be isolated and found a diverse range of locations, ranging from natural locations (soil, marine sediments), living organisms (human gut, earthworm) to pharmaceutical production sites. Most species are motile, and some are able to tolerate moderate saline conditions and high alkalinity environments. While Cytobacillus species can grow in temperatures ranging from 10–45°C, optimum growth occurs in the range 25–37°C.

Three conserved signature indels (CSIs) have been identified as exclusively shared by most/all members of Cytobacillus in the following proteins: PDZ domain-containing protein, histidinol dehydrogenase and transcription-repair coupling factor. These CSIs were identified through the analysis of genome sequences from Cytobacillus species and provide novel molecular means to differentiate this genus from other Bacillaceae genera.

Taxonomy
As of May 2021, there are a total of 13 species with validly published names in the genus Cytobacillus. Members of Cytobacillus are observed to group together in a monophyletic clade in phylogenetic trees created based on different datasets of conserved proteins as well as 16S rRNA genome sequences. This branching pattern is also observed in the Genome Taxonomy Database (GTDB).

In addition to these species with published names, several non-validly published species (‘Bacillus dabaoshanensis', ‘Bacillus dafuensis’, ‘Bacillus massiliogabonensis’, and ‘Bacillus mesophilum') been observed to branch with other Cytobacillus species and share molecular markers (in the form of conserved signature indels) exclusively unique for this genus. However the lack of culture strain information prevented the transfer of these species into the genus. It is important to note as genome and strain information becomes available, it is necessary to conduct additional analyses of these classifications to account for any required amendments.