Fibrobacterota

Fibrobacterota is a phylum of bacteria known for breaking down cellulose, commonly found in the digestive systems of mammals such as ruminants. The genus Fibrobacter was originally classified under Bacteroides until it was reclassified as a separate genus in 1988.

Phylogenetic and comparative genomic analyses show that Fibrobacterota is a distinct phylum, but it shares close evolutionary ties with Bacteroidota and Chlorobiota. These three phyla form the FCB group, supported by similarities in RpoC and DNA gyrase B protein sequences, as well as shared conserved signature indels—including a small insert in RpoC and a longer one in serine hydroxymethyltransferase—and a unique signature protein (PG00081).

Phylogenetic trees based on 16S rRNA gene sequences (from LTP_10_2024) and genome-based data (from GTDB release R11-RS232) offer slightly different classifications. For instance, GTDB treats Fibrobacter succinogenes subsp. elongatus as a separate species, Fibrobacter elongatus, and moves Fibrobacter intestinalis to the genus Hallerella as Hallerella intestinalis. LPSN, however, keeps the former as a subspecies and the latter in Fibrobacter.

The accepted taxonomy follows LPSN and NCBI. It includes three classes: Chitinispirillia, Chitinivibrionia, and Fibrobacteria. The class Fibrobacteria contains the order Fibrobacterales and family Fibrobacteraceae, which includes the genera Fibrobacter (with species F. intestinalis and F. succinogenes, plus two subspecies) and Hallerella (with species H. porci and H. succinigenes). There is also a candidate order, "Candidatus Fibrimonadales," with the genus "Candidatus Fibrimonas" found in termites.

Members of Fibrobacter are commonly found in the digestive tracts of cattle and pigs, where they help break down plant fiber. However, molecular studies using 16S rRNA gene amplification suggest the phylum is more widespread than once thought. Most clones from mammalian guts belong to subphylum 1, while subphylum 2 members have been detected in termite guts and some litter-feeding cockroaches, particularly in fiber-associated communities in the hindgut of the wood-feeding termite Nasutitermes corniger.