Selenomonas ruminantium
1. Classification
a. Higher order taxa
Bacteria; Bacillota; Negativicutes; ; Selenomonadales; Selenomonadaceae [1].
Species
Selenomonas ruminantium2. Description and significance
Selenomonas ruminantium are rod-shaped, anaerobic bacteria that aids ruminants such as giraffes, deer, cattle, sheep, and goats in their digestion of food [2,3]. S. ruminantium plays an important role in fermenting carbohydrates into volatile fatty acids such as propionate and acetate, which are critical energy sources for ruminants [4,5]. Recent studies address the way in which S. ruminantium can be beneficial to the ruminant's nutritional health and energy use. Although genetic information about this microbe is available, further genomic research is needed for better understanding of its potential, specifically for the amplification of its role in the rumen and biotechnological applications to improve ruminant nutrition.
3. Genome structure
Scientists have sequenced the genome of various strains of S. ruminantium. The genome consists of a single circular chromosome made of double-stranded DNA that is about 3.4 million base pairs long, while some subspecies have acquired plasmids [6]. For instance, the complete genome of S. ruminantium subspecies lactilytica TAM6421 has been sequenced, consisting of a single circular chromosome of 3,003,680 base pairs with an average GC content of 50.7%, along with nine plasmids. The chromosome contains 2,826 protein-coding genes, 69 tRNA genes, and seven copies of ribosomal RNA operons6. The S. ruminantium genome encodes proteins such as glycoside hydrolases, which break down complex carbohydrates to produce energy via glycolysis, and fumarate reductase, which is used in the anaerobic succinate fermentation pathway to produce energy [7,8. The bacterium also has genes that allow it to produce propionate, a vital energy source for ruminants [9]. One unique feature of S. ruminantium's genome is the presence of genes involved in vitamin B12 synthesis, such as the cobA gene, indicating that S. ruminantium could play a role in producing this essential vitamin within the rumen [10]. Another important group of proteins encoded in the genome are those involved in nitrate reduction; S. ruminantium has genes for nitrate reductase enzymes (narG and narH), which allow it to convert nitrate to ammonia [11]. The genome also includes genes that allow S. ruminantium to utilize nucleic acids as an energy source. It can ferment RNA, providing an alternative energy source when carbohydrates are scarce [7].
4. Cell structure
S. ruminantium is a Gram-negative, motile bacterium found in the rumen of ruminant animals, such as cows and sheep, and contributes to fermentation processes essential for digestion9. It has a unique curved rod shape, with cells measuring 0.8 – 1.0 μm (micrometers) in width and 2–7 μm in length, helping differentiate it from other rumen bacteria[7]. The bacterium achieves motility within the ruminal environment through flagella positioned on the concave side of the cell, which supports its ability to actively participate in fermentation processes11. S. ruminantium is non-spore-forming and it relies on its adaptive metabolic pathways to thrive in the anaerobic, nutrient-rich rumen environment [2,3]. The cell wall structure is typical of Gram-negative bacteria, with an outer membrane containing lipopolysaccharides, which help protect the cell from external stressors in the rumen [12]. The cell envelope contains several enzymes involved in carbohydrate metabolism, including multiple lactate dehydrogenases, which play a role in fermenting sugars into propionate and other volatile fatty acids [13,3]. Additionally, S. ruminantium has been identified to express nitrate reductase, an enzyme critical for nitrate metabolism, allowing it to reduce nitrate to nitrite and support anaerobic respiration when conditions require it [9,11]. There are three clades of S. ruminantium; The first clade primarily metabolizes soluble sugars and produces acetate and propionate as fermentation products[14]. These cells tend to be smaller in size, averaging 0.4 µm in diameter and 1.5 µm in length. The outer membrane of Clade 1 strains shows enhanced expression of carbohydrate transport proteins, facilitating sugar uptake [2]. Bacteria within the second clade specialize in fermenting lactate into volatile fatty acids. These strains express multiple lactate dehydrogenase enzymes and exhibit slightly larger cells, around 2.0–2.5 µm in length, with increased flagellar density to enhance motility [12,13]. This motility allows them to relocate to regions in the rumen with higher lactate concentrations, maintaining metabolic efficiency. The third clade includes strains capable of reducing nitrate to nitrite, a feature uncommon in other clades. Their cell walls contain enzymes involved in nitrate metabolism embedded in the cytoplasmic membrane, reflecting their specialization in anaerobic respiration [11]. These cells are often found in association with other nitrate-utilizing organisms, suggesting a role in cooperative metabolism within the rumen [11].
5. Metabolic processes
S. ruminantium is an anaerobic chemoorganoheterotroph because it utilizes a wide variety of carbohydrate sources, like glucose, to produce energy [2]. After glucose is fermented to lactate, S. ruminantium uses the “succinate” pathway to make propionate. This involves adding a carbon monoxide molecule to a lactate molecule to make succinate, and then removing a carbon dioxide molecule from succinate to make propionate [15]. S. ruminantium can also use ribose—a component of RNA—as an energy source when glucose is scarce [7]. While propionate is the main end product of lactate fermentation, other byproducts, such as acetate, carbon dioxide, and hydrogen molecules are also produced [5]. These metabolic capabilities help stabilize the rumen environment and maintain microbial balance [9].
6. Ecology
S. ruminantium lives in the rumen of livestock, where it helps break down and ferment carbohydrates, aiding in digestion [4]. Sheep are frequently studied as a source of S. ruminantium. For example, specific strains of this bacterium have been isolated from sheep rumen and evaluated for their role in breaking down fibrous plant material [3]. Selenomonas ruminantium has a worldwide distribution, reflecting the global presence of ruminants across nearly all continents. However, it is especially prevalent in areas abundant in livestock, such as China, South America, and Africa [16,17].
S. ruminantium thrives best when ruminants are fed grain-rich diets because grains provide fermentable carbohydrates that the bacteria utilize [4]. High-grain feeding also lowers the rumen’s pH and promotes the formation of lactate, both of which create favorable conditions for the growth of S. ruminantium [4]. The resulting acidic environment allows this bacterium to flourish, as it can efficiently convert lactate and sugars into volatile fatty acids beneficial to the host animal [12].
7. Pathology
Selenomonas ruminantium is not considered a primary pathogen for humans, animals, or plants. Instead, its presence and metabolic activities are primarily associated with ruminal digestion, supporting the breakdown of fiber and sugars in the host animal's digestive tract [3,14]3,14.
Disruptions to the rumen microbiome, such as through dietary changes, can influence microbial composition. In some contexts, shifts in microbial populations within the rumen, including S. ruminantium, could influence fermentation outcomes and, subsequently, the animal's metabolism and health status [12].
8. Current Research
Recent studies have advanced our understanding of S. ruminantium and its role in ruminant digestion and nutrition.
A 2024 study investigated how S. ruminantium affects feed efficiency in sheep. Researchers analyzed sheep that were allowed to eat freely for 60 days and grouped them based on their weight gain relative to feed intake. They discovered that sheep with higher feed efficiency had greater concentrations of propionate in their stomachs compared to those with lower efficiency [12]. Propionate is a type of volatile fatty acid that serves as an important energy source for ruminants. Since S. ruminantium is known to convert lactate and cellulose into propionate, this suggests that having more of this bacterium could enhance energy availability for the animals, which further implies that promoting S. ruminantium could improve weight gain and feed utilization in livestock [12].
In a 2022 study, researchers studied how different dietary ingredients affect rumen fermentation. The study tested various diets on wethers (castrated male sheep) to see how these diets affected the fermentation process in the rumen [14]. The study found that while the type of diet did not significantly change overall feed intake or rumen pH, it did affect the synthesis and absorption of certain fatty acids, particularly butyrate, when beet pulp was included. Butyrate plays a role in energy production and gut health [14]. These findings are important because they show how dietary composition can influence the activity of beneficial microbes like S. ruminantium in the rumen.
References
[4] [R. B. Hespell, B. J. Paster, and F. E. Dewhirst. 2006. “The Genus Selenomonas.” In M. Dworkin, S. Falkow, E. Rosenberg, K. H. Schleifer, and E. Stackebrandt (Eds.), The Prokaryotes (Vol. 4, pp. 982-990). Springer.]
Edited by Absa Gueye, Jessica Lee, Krishi Shah, Trijal Thakkar, and Hailey Wiscott, students of Jennifer Bhatnagar for [http://www.bu.edu/academics/cas/courses/cas-bi-311/ BI 311
General Microbiology], 2024, Boston University.
[[Category:Pages edited by students of Jennifer
Bhatnagar at Boston University]]
