Butyrivibrio is a genus of bacteria in Class Clostridia. Bacteria of this genus are common in the gastrointestinal systems of many animals. Genus Butyrivibrio was first described by Bryant and Small as anaerobic, butyric acid-producing, curved rods. Butyrivibrio cells are small, typically 0.4 – 0.6 µm by 2 – 5 µm. They are motile, using a single polar or subpolar monotrichous flagellum. They are commonly found singly or in short chains but it is not unusual for them to form long chains. Despite historically being described as Gram-negative, their cell wallscontain derivatives of teichoic acid, and electron microscopy indicates that bacteria of this genus have a Gram-positive cell wall type. It is thought that they appear Gram-negative when Gram stained because their cell walls thin to 12 to 18 nm as they reach stationary phase. Butyrivibriospecies are common in the rumens of ruminant animals such as cows, deer and sheep, where they are involved in a number of ruminal functions of agricultural importance in addition to butyrate production. These include fibre degradation, protein breakdown, biohydrogenation of lipids and the production of microbial inhibitors. Of particular importance to ruminant digestion, and therefore productivity, is their contribution to the degradation of plant structural carbohydrates, principally hemicellulose. Butyrivibrio species are metabolically versatile and are able to ferment a wide range of sugars and cellodextrins. Some strains have been reported to break downcellulose, although their ability to sustain growth on cellulose appears to be lost during in vitro culturing. Most isolates are amylolytic and are able to degrade xylan by producing xylanolytic and esterase enzymes. The induction of xylanase enzymes varies between strains; in group D1 strains xylanase expression appears to be constitutively expressed, while groups B1 and C are induced only by growth on xylan, and those of group B2 are induced by growth on xylan or arabinose. A number of genes encoding glycoside hydrolases have been identified in Butyrivibrio species including endocellulase ; β-Glucosidase ; endoxylanase ; β-Xylosidase ; and α-Amylase enzymes. Several carbohydrate binding modules have also been identified that are predicted to bind glycogen ; xylan or chitin ; and starch. The genus Butyrivibrio encompasses over 60 strains that were originally confined to the species Butyrivibrio fibrisolvens based on their phenotypic and metabolic characteristics. However, phylogenetic analyses based on 16S ribosomal RNA gene sequences has divided the genus Butyrivibrio into six families. These families include the rumen isolates Butyrivibrio fibrisolvens, B. hungateii, B. proteoclasticus, Pseudobutyrivibrio xylanivorans, and P. ruminis and the human isolateB. crossotus. The families B. fibrisolvens, B. crossotus, B. hungateii as well as B. proteoclasticus all belong to the Clostridium sub-cluster XIVa.
Butyrivibrio proteoclasticus B316T was the first Butyrivibrio species to have its genome sequenced. It was first isolated and described by Attwood et al., and was originally assigned to the genus Clostridium based on its similarity to Clostridium aminophilum, a member of the Clostridium sub-cluster XIVa. Further analysis has shown that it is more appropriately placed within the genus Butyrivibrio and the organism was given its current name. Within this genus its 16S rDNA sequence is most similar to, but distinct from, that of B. hungateii. B. proteoclasticus is found in rumen contents at significant concentrations of from 2.01 x 106/ml to 3.12 x 107/mL as estimated by competitive PCR or 2.2% to 9.4% of the total eubacterial DNA within the rumen, as estimated by real time PCR. B. proteoclasticus cells are anaerobic, slightly curved rods, commonly found singly or in short chains, but it is not unusual for them to form long chains. They possess a single sub-terminal flagellum, but unlike other Butyrivibrio species, they are not motile. They are ultrastructurally Gram-positive, although as with all Butyrivibrio species, they stain Gram-negative B. proteoclasticus has been shown to have an important role in biohydrogenation, converting linoleic acid to stearic acid.
