Fibrobacter succinogenes

Fibrobacter succinogenes: A Comprehensive Reference

1. Identity and Taxonomy

Classification

Fibrobacter succinogenes is a cellulolytic bacterium species in the genus Fibrobacter. It is a Gram-negative, rod-shaped, strictly anaerobic bacterium belonging to the phylum Fibrobacteres, renowned for its specialized role in degrading crystalline cellulose and other plant cell wall polysaccharides. The organism is nonmotile, strictly anaerobic, Gram-negative, and rod-shaped. The phylum Fibrobacteres currently comprises one formal genus, Fibrobacter, and two cultured species, Fibrobacter succinogenes and Fibrobacter intestinalis, that are recognised as major bacterial degraders of lignocellulosic material in the herbivore gut.

Synonymy and Reclassification

When first isolated, the organism was classified as Bacteroides succinogenes, a species that was later renamed Fibrobacter succinogenes. Although originally classified as Bacteroides, phylogenetic analyses subsequently established its membership to a deeply rooted clade within the Bacteria designated as the phylum Fibrobacteres. The phylum Fibrobacterota (Fibrobacteres) represents a deep-branching, monophyletic group within the bacterial domain, initially proposed in 2001 through 16S rRNA gene sequence analysis that separated it from the phylum Bacteroidetes, where F. succinogenes had formerly been placed.

Evolutionary Relationships

Comparative genomic studies have identified two conserved signature indels and one signature protein that are uniquely shared by Fibrobacter succinogenes and all of the species from the Bacteroidetes and Chlorobi phyla, providing compelling evidence that F. succinogenes shared a common ancestor with Bacteroidetes and Chlorobi species exclusive of all other bacteria; these species have been proposed as part of a single "FCB" superphylum.

Reference Strain and Genome

Strain S85 became a model strain for research and continues to be representative of the wild-type species. To better understand the biology of F. succinogenes, the genome of the type strain S85 was sequenced to completion; a total of 3,085 open reading frames were predicted from its 3.84 Mbp genome. Analysis of sequences predicted to encode carbohydrate-degrading enzymes revealed an unusually high number of genes classified into 49 different families of glycoside hydrolases, carbohydrate binding modules (CBMs), carbohydrate esterases, and polysaccharide lyases.

Natural Habitat and Distribution

Fibrobacter succinogenes, Ruminococcus flavefaciens, and Ruminococcus albus are the predominant cellulolytic bacteria in rumen fluid. Fibrobacter succinogenes is an anaerobic bacterium naturally colonising the rumen and cecum of herbivores. Since their original isolation from the rumen, culture-independent studies have suggested the presence of Fibrobacter populations in diverse herbivore gastrointestinal tracts. As part of the Fibrobacterota, the genus Fibrobacter is the only well-defined group; it embraces two rumen-resident species with distinctive features, Fibrobacter succinogenes and Fibrobacter intestinalis, which have gained widespread attention in the ruminant digestive system due to their remarkable cellulolytic capacity. The abundance of F. succinogenes, determined by species-specific real-time PCR assay, is significantly higher in the rumen mat than in the rumen fluid and feces in cattle.

2. Historical and Scientific Discovery

Initial Isolation

Fibrobacter succinogenes was isolated in 1954 by M.P. Bryant and R.N. Doetsch from bovine rumen at the University of Maryland; they isolated 8 different strains — S23, S61, S85, S111, S121, C2, M13, and M34 — all of which belonged to one species, Bacteroides succinogenes. Representatives were first isolated and described by Hungate during studies of cellulose digestion in the bovine rumen (Hungate, 1950). Since its discovery in the 1950s, it has been studied for its role in herbivore digestion and cellulose fermentation, which can be utilized in biofuel production.

Context: No Traditional Human Use

Fibrobacter succinogenes is not an herb, plant extract, or traditional medicinal preparation. It is a microorganism discovered through twentieth-century laboratory microbiology and has no documented history of intentional human use in any traditional medicine system. Its study has focused on its taxonomy, phylogeny, ecology, and potential as a source of novel glycosyl hydrolases of biotechnological importance. The organism is therefore treated in this article exclusively within the framework of scientific and applied research, and no traditional-use claims can be substantiated.

Development as a Research Model

The S85 type strain of Fibrobacter succinogenes, a major ruminal fibrolytic species, was isolated from a bovine rumen and has been used since then as a model for extensive studies. The type strain, Fibrobacter succinogenes S85, has been extensively investigated and is among the most actively cellulolytic of all strictly anaerobic, mesophilic bacteria known. Comparative studies of cellulase- and xylanase-degrading activities across several other F. succinogenes strains originating from different ruminants, including recently isolated strains, have been undertaken to assess the validity of this model.

3. Key Constituents and Active Compounds

Carbohydrate-Active Enzymes (CAZymes)

The F. succinogenes S85 genome has been predicted to encode 104 different glycoside hydrolases (GHs), representing 3.37% of all S85 genes, which is among the highest percentage of GHs in any bacterial species. Among these, 31 genes are predicted to function as cellulases, distributed primarily in GH5 (10 genes, including endoglucanases such as celA and celD), GH9 (9 genes), GH8 (6 genes), GH45 (4 genes), and GH51 (2 genes). Notably, none of these cellulase genes contain dockerin domains or are associated with scaffoldin homologs, confirming the absence of a cellulosomal system; instead, many (21 of 31) possess signal peptides for secretion, and most lack carbohydrate-binding modules (CBMs) typically involved in binding crystalline cellulose.

The gene encoding a major endoglucanase (endoglucanase 1) of F. succinogenes S85 was identified as cel9B from the genome sequence; Cel9B and two other glucanases, Cel5H and Cel8B, were cloned and overexpressed and their proteins purified and characterized, and these were assayed in various combinations with two other predominant cellulases, Cel10A and Cel51A, to assess synergistic interactions using ball-milled cellulose. The degree of synergism among these enzyme combinations ranged from 0.6 to 3.7.

Outer Membrane Vesicles (OMVs)

Outer membrane vesicles (OMVs) released by F. succinogenes are enriched with carbohydrate-active enzymes, and intact OMVs were able to depolymerize a broad range of linear and branched hemicelluloses and pectin, despite the inability of F. succinogenes to utilize non-cellulosic (pentose) sugars for growth. The CAZymes carried in these vesicles were mostly glycoside hydrolases (16 GHs, including xylanases and cellulases) and included 3 polysaccharide lyases and one carbohydrate esterase. Expression profiles from in vivo studies showed that F. succinogenes relied upon outer membrane vesicles and a diverse repertoire of CAZymes.

Fibro-Slime Proteins and Adhesion Machinery

The discovery that most anaerobic cellulose-degrading bacteria rely upon strict binding of the cell to the cellulose fiber led to the proposal of a class of binding proteins termed "fibro-slime" proteins that are specific to F. succinogenes and thought to be localized to the outer membrane; these fibro-slime proteins were also shown to be involved in adhesion to and/or degradation of cellulose. Analysis of the F. succinogenes genome sequence led to a proposed mechanism for cellulose deconstruction that involves both fibro-slime and type IV pilin proteins as a means of attaching the outer membrane to the cellulose fiber, with individual cellulose chains transported through the outer membrane via ABC transporters and degraded in the periplasmic space.

Fermentation End Products

F. succinogenes converts cellulose polymers into intracellular glycogen and the fermentation metabolites succinate, acetate, and formate. Phosphoenolpyruvate (PEP), formed during glycolysis, is carboxylated to oxaloacetate by PEP carboxykinase; oxaloacetate is then reduced to malate by malate dehydrogenase, dehydrated to fumarate by fumarase, and finally reduced to succinate by a membrane-bound fumarate reductase that accepts electrons from menaquinone. This pathway allows for the disposal of reducing equivalents, with succinate yields reaching up to 1.15 mol per mol of anhydroglucose fermented under optimal continuous culture conditions. Although F. succinogenes lacks genes encoding hydrogenase enzymes and does not produce molecular hydrogen, it generates formate during fermentation, which can be utilized by syntrophic partners in the rumen microbiome to maintain intracellular redox balance through interspecies electron transfer.

Glycogen Storage

F. succinogenes is able to store intracellular glycogen which can represent up to 70% of the dry weight of the bacterium; this storage could allow bacteria to remain in the rumen in the absence of metabolizable substrates, but the intracellular glycogen is simultaneously stored and degraded, suggesting a futile cycling. F. succinogenes uses ammonia as the sole source of nitrogen, and several steps in the ammonia assimilation pathway have been identified.

4. Mechanisms of Action

Cellulose Adhesion and Surface Degradation

F. succinogenes has a particularly high activity against crystalline cellulose that requires close physical contact with this substrate; unlike other known cellulolytic microbes, it does not degrade cellulose using a cellulosome or by producing high extracellular titers of cellulase enzymes. Bacterial attachment to cellulose is the first step during lignocellulose degradation; translocation of cellulases to the surface occurs using a T9ss-dependent pathway, and on the cell surface, cellulases are attached via serine residues and form a multi-protein complex along with OmpA, TPR, and fibro-slime proteins; released products of cellulolysis (cellodextrins) are then imported to the periplasm via beta barrel proteins and the TonB/ExbB/ExbD active transport system, and cellodextrin phosphorylases cleave cellodextrin into glucose-1-phosphate and cellobiose, which are then transported to the cytoplasm via solute binding proteins and ABC transporters.

Unique Cellulolytic Strategy

The genome sequence of F. succinogenes rumen strain S85 suggested that within this group of organisms a "third" way of attacking cellulose has evolved; the superior efficiency of anaerobic cellulose hydrolysis by Fibrobacter spp., in comparison to other cellulolytic rumen bacteria that typically utilise membrane-bound enzyme complexes (cellulosomes), may be explained by this novel cellulase system. Polysaccharide hydrolysis and utilization assays showed that F. succinogenes was able to hydrolyze a number of polysaccharides but could only utilize the hydrolytic products of cellulose, suggesting that it uses its array of hemicellulose-degrading enzymes to remove hemicelluloses to gain access to cellulose; this is reflected in its genome, as F. succinogenes lacks many of the genes necessary to transport and metabolize the hydrolytic products of non-cellulose polysaccharides.

Adaptive Gene Expression

A large proportion of cellulase genes were constitutively expressed, including the gene encoding for Cel51A, the major cellulose-binding endoglucanase produced by this bacterium; other cellulase genes displayed elevated expression during growth on cellulose relative to growth on soluble sugars. Depending on the type of available cellulose, this bacterium makes a different set of proteins and enzymes necessary to degrade each type. Results indicate that the cell envelope proteome undergoes extensive rearrangements to accommodate the cellulolytic degradation machinery, as well as associated proteins involved in adhesion to cellulose and transport and metabolism of cellulolytic products; molecular features of the lignocellulolytic enzymes suggest that the Type IX secretion system is involved in the translocation of these enzymes to the cell envelope.

Ecological Cross-Feeding

F. succinogenes plays a key role in the rumen by degrading cellulose into metabolic products that are available to noncellulolytic species such as Streptococcus bovis, Selenomonas ruminantium, or Treponema bryantii, which have been shown to grow on cellulose in the presence of F. succinogenes; for example, S. bovis can be co-cultured with F. succinogenes S85 on cellulose as sole carbon substrate, and this cellodextrin release could have important ecological implications since cellodextrins can be used as substrate by other rumen microorganisms.

5. Scientific Evidence by Area of Study

5.1 Rumen Fiber Digestion and Ruminant Nutrition

Fibrobacter succinogenes is the most important microbe in terms of fiber degradation in the rumen of ruminants and is known to be a phylogenetically diverse group of species. Fibrobacter succinogenes has a major role in biodegradation of plant cell wall polymers in the rumen, based on its predominance in the rumens of animals ingesting a forage diet and its capacity to digest plant cell walls during growth. In the rumen, a complex and diverse microbial community of bacteria, archaea, fungi, and protozoa catalyzes the enzymatic breakdown of lignocellulosic substrates despite their recalcitrant nature, leading in some cases to the hydrolysis of 60–65% of cellulose within 48 hours.

Although populations of Fibrobacter succinogenes are directly affected by diet composition, in most cases it is the most dominant cellulolytic microorganism in the rumen of cattle. The number of the bacterium is much higher in the rumen of animals fed a high forage diet. Fibrobacter succinogenes, the major Fibrobacter species in the rumen, is especially adept at solubilizing highly ordered forms of cellulose; moreover, strains of F. succinogenes have demonstrated a greater ability to digest cellulose from intact forages than other species of fibrolytic rumen bacteria.

Evidence strength: This area of research is supported by decades of well-replicated in vitro and in vivo animal studies, including genomic, transcriptomic, and metabolomic analyses. No human clinical evidence exists for the direct role of F. succinogenes in human digestive physiology.

5.2 In Vivo Gnotobiotic Animal Studies

A gnotobiotic sheep study provided the first evidence of in vivo competitions between F. succinogenes and the two Ruminococcus species, showing that a simple disequilibrium within the cellulolytic community has repercussions on the rumen metabolome and fermentation end products. The changes in cellulolytics also affected the rumen metabolome, including an increase in acetate and butyrate at the expense of propionate. In this multi-period gnotobiotic sheep experiment, dry matter disappearance and polysaccharidase specific activities were compared between a period with F. succinogenes only (weeks 22 to 27) and a period after inoculation of cellulolytic ruminococci (weeks 28 to 33), with differences considered significant at P < 0.05.

Evidence strength: Animal model (gnotobiotic sheep); no human clinical trials have been conducted on F. succinogenes as a dietary intervention. These findings are mechanistic and ecological, not directly translatable to human health outcomes.

5.3 Diet-Induced Population Dynamics

The 16S rRNA gene libraries of hay-fed animals contained a significantly higher number of bacteria belonging to the phylum Fibrobacteres, whereas libraries of grain-fed animals contained a significantly higher number of Bacteroidetes; real-time PCR analysis detected significant fold increases in Megasphaera elsdenii, Streptococcus bovis, Selenomonas ruminantium, and Prevotella bryantii during adaptation to the high-concentrate diet, whereas Butyrivibrio fibrisolvens and Fibrobacter succinogenes populations gradually decreased as the animals were adapted to the high-concentrate diet. The 40-fold decrease in the numbers of Fibrobacter isolates detected was consistent with previous observations that reported a 20-fold decrease in population size.

Using quantitative PCR, a significant decrease in F. succinogenes, Ruminococcus albus and R. flavefaciens 16S rrs gene copy numbers per gram of rumen contents was measured in sheep fed 50% concentrate / 50% hay, compared with a 100% hay diet.

Evidence strength: Controlled ruminant trials using quantitative molecular methods. Consistent and replicated across multiple studies, but entirely in herbivore animal models.

5.4 Acid Tolerance and Subacute Ruminal Acidosis

Major rumen pH decline can reduce populations of cellulolytic bacteria and increase bacterial endotoxins, leading to inflammatory responses and a negative impact on animal health. In one study, the F. succinogenes type strain S85 was adapted for steady-state growth in continuous culture at pH 5.75 and confirmed to grow in the range of pH 5.60–5.65, which is lower than had been reported previously; wild type and acid tolerant strains digested corn stover with equal efficiency in batch culture at low pH, and RNA-seq analysis revealed 268 and 829 genes were differentially expressed at pH 6.10 and 5.65 compared to pH 6.70, respectively. Fibrolytic bacteria are unable to maintain the pH inside their cells when ruminal pH is low. They cannot grow at a low intracellular pH, and an increase in pH gradient leads to an entry of undissociated VFAs into the cells; an accumulation of dissociated anions in the intracellular compartment induces severe toxicity for the bacteria.

Evidence strength: Controlled in vitro continuous culture experiments combined with transcriptomic analysis. Mechanistically informative but limited to laboratory and animal contexts; no human-relevant clinical data.

5.5 Methane Emission and Environmental Relevance

Results from one study showed that methane production was clearly reduced when the dominant fibrolytic species was a non-H₂-producing species such as Fibrobacter succinogenes, without significantly impairing fiber degradation and fermentations in the rumen; the addition of fumarate to rumen contents stimulated H₂ utilization only by the ruminal microbiota inoculated with F. succinogenes, suggesting that these communities could play an important role in fumarate reduction in vivo. Although F. succinogenes lacks genes encoding hydrogenase enzymes and does not produce molecular hydrogen, it generates formate during fermentation, which can be utilized by syntrophic partners in the rumen microbiome to maintain intracellular redox balance through interspecies electron transfer.

Evidence strength: In vitro incubation experiments and controlled animal inoculation studies; this area has environmental and livestock production relevance but no direct human health application.

5.6 Biotechnological Applications: Biofuel Production

In addition to its interest for ruminant nutrition, F. succinogenes has received much attention from the biotechnology sector because this species produces an original cellulolytic system that includes membrane vesicles as vehicles of CAZymes; deciphering this system could help in the design of novel Consolidated Bioprocessing (CBP) for the production of cost-effective and sustainable lignocellulosic biofuels. The genome sequence of F. succinogenes rumen strain S85 suggested that within this group of organisms a "third" way of attacking cellulose has evolved, with the superior efficiency of anaerobic cellulose hydrolysis by Fibrobacter spp. compared to other cellulolytic rumen bacteria that typically utilise cellulosomes possibly explained by this novel cellulase system.

F. succinogenes S85 is a cellulose-degrading and succinate-producing bacterium; such functions are central for the rumen ecosystem and are of special interest for several industrial applications.

Evidence strength: Preliminary to intermediate-stage biotechnological research. Genome-scale metabolic modelling, in vitro fermentation experiments, and transcriptomics have been published in peer-reviewed journals, but no commercial biofuel processes using F. succinogenes have been validated at scale as of available literature.

5.7 Yeast–Bacterium Interactions and Feed Additives

One study analysed in vitro the metabolic interactions between the cellulolytic bacterium Fibrobacter succinogenes S85 and the saccharolytic bacterium Selenomonas ruminantium PC18, as well as the impact of Saccharomyces cerevisiae CNCM I-1077, a strain used as a feed additive for ruminants, on their metabolism and interactions. The presence of bacteria was found to influence yeast metabolism, and crossfeeding between bacteria and yeast was suggested; the findings provide deeper insight into the in vitro interactions between F. succinogenes, S. ruminantium, and S. cerevisiae, highlighting possible modes of action that may explain some of the observed positive effects of yeast on rumen function in vivo, and underscoring mutual interactions between rumen bacteria and live yeast.

Evidence strength: In vitro transcriptomic and metabolomic study; the findings are exploratory and mechanistic. No human clinical studies have been conducted.

6. Body Systems and Health Areas of Association

Gastrointestinal System (Ruminants)

The rumen microbiota plays an essential role in ruminant nutrition by breaking down and fermenting plant-based feed, transforming it into a source of energy and protein for the host. The F. succinogenes genome reveals a bacterium that specializes in cellulose as its sole energy source and provides insight into a novel strategy for cellulose degradation. In the rumen, the microbial community catalyzes the enzymatic breakdown of lignocellulosic substrates, leading in some cases to the hydrolysis of 60–65% of cellulose within 48 hours.

Metabolic Outputs Relevant to Host Nutrition

F. succinogenes S85 ferments the breakdown products of cellulose to primarily succinic acid and lesser amounts of acetic acid, and the production of these fermentation products, along with an apparent inability to ferment pentoses, appears to be a conserved feature among Fibrobacter spp. Succinate produced in the rumen is absorbed and can be metabolized by the host, contributing to the volatile fatty acid pool. Acetate and formate serve as minor end products relative to succinate, contributing to the overall volatile fatty acid pool in ruminal ecosystems.

Animal Health: Rumen Acidosis

Major rumen pH decline can reduce populations of cellulolytic bacteria and increase bacterial endotoxins, leading to inflammatory responses and a negative impact on animal health. A decrease of in situ fibre digestion following sub-acute ruminal acidosis was found, likely due to the negative effect of low pH on the microorganisms involved in fibre digestion. After the rumen pH falls below 6.0, the environment for rumen microorganisms changes, resulting in a decrease in the growth of fiber-degrading bacteria such as Fibrobacter succinogenes and Ruminococcus flavefaciens, which are highly sensitive to pH fluctuations.

Hindgut Fermenting Herbivores

Fibrobacter succinogenes and R. flavefaciens were selected as representative fibrolytic bacteria in horses, and F. succinogenes was found to be present at the highest relative levels in all individuals and during feeding of all diets studied.

7. Dosage Forms and Preparations Reported in the Literature

Fibrobacter succinogenes is not available as a commercial human dietary supplement and has no established human dosage. The organism is studied exclusively in laboratory, animal, and biotechnological contexts. The following preparations and conditions have been described in the published scientific literature:

• Fibrobacter succinogenes S85, a strictly anaerobic Gram-negative bacterium, has been grown in continuous culture in a bioreactor at different dilution rates (0.02 to 0.092 h⁻¹) on a fully synthetic culture medium with glucose as carbon source.
• In published fermentation experiments, Fibrobacter strains were grown on medium containing 30% rumen fluid and 0.5% filter paper cellulose.
• In acid tolerance studies, the F. succinogenes type strain S85 was adapted for steady-state growth in continuous culture at pH 5.75 and confirmed to grow in the range of pH 5.60–5.65.
• At a pH of 6.5, F. succinogenes has a greater presence in the rumen.

No peer-reviewed source documents a formulated human supplement, tablet, capsule, or dosage form containing Fibrobacter succinogenes. All dosage-related information in the literature pertains to in vitro culture conditions or animal model inoculations.

8. Safety Considerations

Not a Human Pathogen

F. succinogenes is a Gram-negative, rod-shaped, obligate anaerobe that is a major contributor to cellulose digestion. Its obligate anaerobic nature means it cannot survive in the presence of oxygen. There is no peer-reviewed evidence characterizing F. succinogenes as a human pathogen, and it is not listed as a risk group agent by major health authorities. It has no established role in human infectious disease.

pH and Environmental Sensitivity

Fibrolytic bacteria are unable to maintain the pH inside their cells when ruminal pH is low. They cannot grow at a low intracellular pH, and an increase in the pH gradient leads to an entry of undissociated VFAs into the cells; an accumulation of dissociated anions in the intracellular compartment induces severe toxicity for the bacteria. This extreme sensitivity to acidic conditions and obligate anaerobiosis would substantially limit any potential survival in the human gut, though this has not been studied directly.

Endotoxin Considerations in Animal Models

Major rumen pH decline can reduce populations of cellulolytic bacteria and increase bacterial endotoxins, leading to inflammatory responses and a negative impact on animal health. As a Gram-negative bacterium, F. succinogenes possesses lipopolysaccharide (LPS) in its outer membrane, which is a structural feature of all Gram-negative bacteria. No peer-reviewed literature documents direct human exposure risk from this bacterium.

Absence of Human Safety Data

There are few bacterial phyla with potential functional importance for which there is such a paucity of phenotypic and functional data. The current scientific literature contains no human clinical trials, safety pharmacology studies, or toxicological assessments of F. succinogenes in a human context. Any application to human health, whether as a probiotic or therapeutic agent, would require characterization under applicable regulatory frameworks, which has not occurred as of the available literature.

Interactions with Rumen Microbiota (Animal-Relevant)

A simple disequilibrium within the cellulolytic community has repercussions on the rumen metabolome and fermentation end products, a finding that must be considered when determining strategies aimed at directing rumen fermentations for animal production. There is interest in better understanding and preventing adverse outcomes associated with sub-acute ruminal acidosis.

9. Current Research Directions and Limitations of Evidence

Work illustrating how genomic information of F. succinogenes can be translated to develop predictive dynamic models of rumen fermentation processes is expected to be applicable to other rumen microbes for producing a model of the rumen microbiome that can be used for studying microbial manipulation strategies aimed at enhancing feed utilization and mitigating enteric emissions.

The availability of the F. succinogenes genome will serve to increase understanding of its unique cellulose degrading properties and provide insight into the peculiar biology of this bacterium and its phylum; furthermore, with the increasing number of ruminal bacterial genomes becoming available, this data can be leveraged to begin understanding how these microbes interact within the rumen and their impact on ruminant health and animal performance.

A fundamental limitation across all existing research is that essentially all evidence is derived from in vitro biochemical experiments, genomic and transcriptomic analyses, or ruminant animal models. Cultured representatives from most hosts are rare, and as a result, understanding of their physiology is limited. There are no human clinical studies, no randomized controlled trials, and no systematic reviews pertaining to F. succinogenes as a human dietary supplement or probiotic agent. Claims that might arise in commercial supplement marketing cannot be substantiated by the peer-reviewed literature.

References

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• ScienceDirect Topics: Fibrobacter Succinogenes overview (Biochemistry, Genetics and Molecular Biology)
• ScienceDirect Topics: Fibrobacter Succinogenes overview (Agricultural and Biological Sciences)