Fibrobacterota
Cellulose-degrading bacteria in mammalian digestive tracts.
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*.
- phylum
- Fibrobacterota
- type
- Bacterial phylum
- key_genera
- Fibrobacter, Hallerella, Chitinispirillum, Chitinivibrio
- known_for
- Cellulose degradation in mammalian digestive tracts
- phylogenetic_group
- FCB group (with Bacteroidota and Chlorobiota)
- first_described_genus
- Fibrobacter (separated from Bacteroides in 1988)
Quick Facts
- Taxon
- Fibrobacterota
- Subdivision
- Chitinispirillia / Chitinivibrionia / Fibrobacteria
Facts from the source article.
Lore & Background
Fibrobacterota is a bacterial phylum that includes cellulose-degrading bacteria associated with the digestive tracts of mammals, including ruminants. The genus Fibrobacter was separated from Bacteroides in 1988. Phylogenetic studies based on RpoC and DNA gyrase B protein sequences indicate that it is related to the phyla Bacteroidota and Chlorobiota, and species from these three phyla cluster together based on conserved signature indels in several proteins.
Reader's Guide
Fibrobacterota is significant as a distinct bacterial phylum whose members play a key role in breaking down plant fiber in the digestive tracts of mammals such as cattle and pigs. The two described species in the genus Fibrobacter, Fibrobacter succinogenes and Fibrobacter intestinalis, are members of fibrolytic communities and have been studied for their ability to degrade plant fibre. Molecular evidence based on 16S rRNA gene amplification suggests the phylum is more widespread than initially recognized, with subphylum 1 found in mammalian environments and subphylum 2 found in the guts of termites and some litter-feeding cockroaches, where they are abundant in fibre-associated bacterial communities. The placement of Fibrobacterota in the FCB group alongside Bacteroidota and Chlorobiota is supported by conserved signature indels and a shared signature protein, highlighting its evolutionary relationships.
Did You Know?
- Fibrobacterota is part of the FCB group, which also includes Bacteroidota and Chlorobiota.
- The genus Fibrobacter was separated from Bacteroides in 1988.
- Members of subphylum 2 of Fibrobacterota are found in the guts of termites and some litter-feeding cockroaches.
- Comparative genomic studies identified two conserved signature indels and one signature protein shared by Fibrobacterota, Bacteroidota, and Chlorobiota.
Frequently Asked Questions
What is Fibrobacterota?
Fibrobacterota is a phylum of bacteria whose members are best known for their ability to break down cellulose. Key genera within the phylum include Fibrobacter, Hallerella, Chitinispirillum, and Chitinivibrio, all of which play a specialized ecological niche in degrading tough plant polysaccharides.
Where do Fibrobacterota live and what do they do there?
These bacteria are most commonly found in the digestive tracts of mammals, particularly ruminants, where they help ferment and degrade cellulose that the host animal cannot process on its own. Their cellulolytic activity makes them a critical component of the gut microbiome in herbivorous and mixed-diet mammals.
How is Fibrobacterota related to other bacterial phyla?
Fibrobacterota belongs to the FCB group, a clade that also includes Bacteroidota and Chlorobiota. This close evolutionary relationship is supported by shared conserved signature indels and sequence similarities in housekeeping proteins such as RpoC and DNA gyrase B.
When was Fibrobacterota first recognized as its own group?
The type genus Fibrobacter was originally lumped under Bacteroides until 1988, when researchers reclassified it as a separate genus. That split eventually led to the recognition of Fibrobacterota as a distinct phylum rather than a subgroup of Bacteroidota.
Why do fans and researchers care about Fibrobacterota?
Because these organisms are among the few bacteria capable of directly degrading cellulose in mammalian guts, they are central to understanding how herbivores extract energy from plant material. Their placement in the FCB group also makes them a useful reference point for tracing deep bacterial evolutionary relationships.
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