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Overview
Eubacterium eligens is a motile, obligate anaerobic, Gram-positive, rod-shaped mesophilic bacteria that live in the human gut microbiome. In 1974, Holdeman and Moore isolated and identified over 100 bacterial species from human feces, including E. eligens. The genus Eubacterium inhabits normal gut microbiota and contributes to anti-inflammatory secretions. E. eligens is unlike most Firmicutes species in that it can degrade pectin, which is usually more common among Bacteroidetes.

Discovery Process
E. eligens was first identified among 113 species that Moore and Holdeman isolated from the feces of twenty Japanese-Hawaiian males. In this study, they analyzed the bacteria using [https://www.urmc.rochester.edu/encyclopedia/content.aspx?contenttypeid=167&contentid=anaerobic_culture#:~:text=An%20anaerobic%20culture%20means%20the,%2C%20infected%20bites%2C%20and%20gangrene. anaerobic culture]. DNA homology and genetic differences along with metabolic and chemical variances were used to differentiate the species. Isolation methods were also practiced and infections were taken into consideration. It is estimated that the 113 isolated organisms represent 94 percent of the viable cells in the fecal flora. Several species of Eubacterium were detected, using morphology and hydrogen production as differentiation methods.

Physiology
E. eligens is a motile Gram-positive species of obligate anaerobe rods. E. eligens is mesophilic and lives in the normal human gut microbiota. According to Duttaroy, Firmicutes such as E. eligens metabolize polysaccharides such as pectin and the major fermentation products include acetate and formate. BacDive establishes that E. eligens performs arginine and urea hydrolysis, nitrate reduction, and utilizes tryptophan as an energy source.

Taxonomy and Phylogeny
The genus Eubacterium is highly heterogeneous and its definition continues to be updated. According to BioCyc, E. eligens belongs to the phylum Firmicutes, class Clostridia, order Eubacteriales, family Eubacteriaceae, and genus Eubacterium. A 2019 type-strain genome analysis by García-López et al. classifies it as Lachnospira eligens instead, which is published on the Genome Taxonomy Database (GTDB) and National Center for Biotechnology Information (NCBI). Mukherjee et al. acknowledges that because E. eligens is extremely similar to Lachnospira pectinoschiza in terms of phylogeny and phenotype, reclassification may be inevitable given future studies. L. pectinoschiza and E. eligens are the only members of Firmicutes that can utilize pectin. However, Eubacterium as a genus is very diverse so it has been difficult to determine taxonomic relatives. Because NCBI Taxonomy states that L. eligens is not published in the International Code of Nomenclature of Prokaryotes, this article will be referring to the microorganism as the basotype E. eligens.

The closest species to E. eligens based on G+C content include Eubacterium rectale and Eubacterium limosum. NCBI Taxonomy uses hierarchical orthologous groups (HOGs) which are sets of genes that share a common ancestor. ​​Using a proteome of 2761 proteins, NCBI establishes that the closest neighbors of E. eligens based on HOGs are Agathobacter rectalis (1115 relations), Roseburia hominis (1087 relations), and Lachnoclostridium phytofermentans (1070 relations). A. rectalis was recently reclassified from Eubacterium rectale, displaying the commonality of taxonomic reassignment within gut microbiota.

Genomics
Eubacterium eligens has a total of 2,723 genes which constitute 2,613 protein genes, 66 RNA genes, and 44 pseudogenes. It consists of 2.83 Mb and can undergo 959 enzymatic reactions. According to taxonomical data and classification under NCBI, whole genome sequencing was done in part by Mahowald et al. The closest neighbor to E. eligens is E. rectale, and both genome sequences are used together to sequence newly identified microorganisms. The Eubacterium species E. rectale, E. hallii, E. ventriosum, E. eligens, E. coprostanoligenes, and E. limosum are the most closely related to one another with DNA G+C content of the genus varying from 30 to 57 percent. It is difficult to identify E. rectale, E. eligens, and E. biforme using traditional culture methods. Kageyama and Benno found success in using nested PCR, a universal primer pair, and subsequent amplification of the bacterial sample using species-specific primers.

Scientific interest has been spurred by the isolation and genetic sequencing of Eubacterium eligens, which has revealed the significance of this organism for human health. The genome of E. eligens has been sequenced and analyzed by researchers using modern methods. To produce high-quality genome drafts, one study concentrated on isolating and sequencing uncultured microbes, such as E. eligens, observed in metagenomic shotgun sequencing of 16s rRNA. The scientists used an assembler known as Athena, which builds high-quality genome drafts from a single shotgun sequencing operation by using barcode information from read clouds.

Ecological Role in the Gut
Not much is known about the role of E. eligens, but experiments have shown that it is prolific in the colon. It has been reported that several human health conditions are associated with gut dysbiosis, which can modify the abundance of Eubacterium in the gut microbiome. Maintaining gut homeostasis and systemic health requires understanding the diversity and makeup of the gut microbiota. The population changes of Eubacterium in the gut depend on several factors, including age and nutrition. Studies have indicated a significant correlation between the prevalence of E. eligens in the gut microbiome and specific dietary fibers. Zhang et al. state that fiber is fermented by normal microbiota to make beneficial metabolites such as short-chain fatty acids (SCFA), which support mucosal intestine linings and reduce inflammation. These findings highlight the relationship between E. eligens and dietary practices. Despite gut microbiota performing many of the same functions, using taxon-centric analysis to classify species leads to high dimensionality because of the vast complexity of strains. Eubacterium is one of the many possible gut microbial taxa that degrade dietary fiber, though E. eligens is one of the only Firmicutes to degrade pectin.

Relevance of Eubacterium eligens
People outside the field should care about Eubacterium eligens because it is present in the normal human gut microbiome and has the potential to maintain health. E. eligens keeps the colon healthy by generating butyrate which helps prevent “leaky gut”. E. eligens can also stimulate the synthesis of anti-inflammatory substances such as interleukin-10, demonstrating its potential to lower inflammation and improve gut health. There has been increasing interest in the relationship between the microbiome and health, though correlation does not equal causation. A study done by Harvard showed links between E. eligens and less severe post-traumatic stress disorder (PTSD) symptoms; there may be confounding effects that independently influence these observations. Positive eating practices have been linked to E. eligens, emphasizing its significance in preserving a balanced gut microflora. In general, human health may be greatly impacted by promoting the existence of E. eligens in the gut through dietary fiber, prebiotics, and probiotics. This encompasses the framework of One Health, a newly recognized approach that unites several perspectives on the interactions of people, animals, and the environment to enhance their co-evolution and general welfare.