Ruminants have only one stomach separated into four compartments, despite the fact that they are typically stated to have four stomachs. The reticulum, omasum, rumen and abomasum are among these compartments. Most of the fermentation and nutrient absorption takes place in the rumen and reticulum (commonly referred to as the reticulorumen).
The rumen, often known as a paunch, is the largest component of the reticulorumen, which is the first chamber in ruminant animals' alimentary canal. It is the major location where ingested feed is fermented by microbes. The reticulum is the smaller component of the reticulorumen, which is totally continuous with the rumen but differs in texture of its lining.
In cattle, the rumen wall is lined with little fingerlike projections called papillae, which are flattened and measure around 5 mm long and 3 mm wide. Horses and other animals (such as rabbits, rats, guinea pigs, and some large birds) use a combination of glandular digestion and fermentation to digest their food.
Because fermentation occurs in the cecum and colon of these animals, they are known as hindgut fermenters, whereas ruminants are known as forestomach fermenters. The hexagonal honeycomb pattern of the reticulum (derived from the Latin meaning net) is bordered with ridges. The ridges are about 0.1–0.2 mm wide and 5 mm above the reticulum wall. In cattle, the hexagons in the reticulum are around 2–5 cm broad.
These characteristics enhance the surface area of the reticulorumen wall, making volatile fatty acid absorption easier. Microorganisms can be found everywhere, including inside our bodies. Some, like bacteria and fungus, are well-known, while others, like archaea, are not. As a result, the vast majority of bacteria on the planet offer no significant threat to humans, plants, or animals; instead, they assist humans in making the world go round by facilitating decomposition, decay, and even digestion.
Bacteria, fungus, viruses, algae, archaea, and protozoa are among the creatures classified as microbes. Bacteria are very diverse, and they are by far the most successful organism on the planet in terms of numbers. Bacteria are the only microbes that can survive in the human body without harming it, and they often help with biological functions like digestion.
Fungi are classified as eukaryotes, meaning they have a nucleus and organelles. Prokaryotes, such as bacteria, have smaller cells. Once a fungus colony has grown to a particular size, it can be seen with the naked eye. Many specialists believe that viruses are not biological entities. Nucleic acid (DNA or RNA) and a protein covering make up the majority of them. To replicate, a virion (a virus particle) needs a host cell. A virus enters the human body and hijacks a human cell, which it then uses to proliferate.
Algae are a more challenging group of creatures to define, as some classifications include both prokaryotes and eukaryotes. Algae, unlike other microbes, are photosynthesizers and can be found in maritime habitats. In the rumen of ruminants, bacteria, fungus, archaea, protozoa, and viruses live alongside bacteria, fungi, archaea, protozoa, and viruses.
These microbes can break down plant cell walls and fibrous components, converting them to absorbable molecules like proteins and volatile fatty acids. In young, nursing calves, rumen microorganisms play a vital role in rumen development. Although a few facultative anaerobes occur, most rumen bacteria are stringent anaerobes (no tolerance for oxygen), and they play an important role in rapidly eliminating oxygen from the rumen environment.
The bacterial population is diverse, ranging from those that digest carbohydrates (cellulose, hemicelluloses, pectin, starch, and sugars) to those that rely on acids or hydrogen for energy. Protein in the rumen is'sacrificed,' meaning it is broken down into ammonia (NH3) and amino acids, which are used by bacteria for growth and reproduction.
The microorganisms themselves become the host animal's protein source (referred to as microbial protein), with just a small amount of food protein available directly to the animal (referred to as escape or'bypass' protein). The bacteria' capacity to use protein fractions is heavily dependent on carbohydrate availability, so maintaining a healthy diet is essential. Primary fermentation breaks down carbohydrates into volatile fatty acids, or VFAs (the major three are propionate, acetate, and butyrate).
Some of these compounds undergo secondary fermentation and metabolism, which does not happen in the rumen. B vitamins, ammonia (NH3), carbon dioxide, and VFAs are all important to the bacteria. Given that fiber digestion (cellulose and hemicelluloses) is usually regarded to be the rumen's major function, fiber digesting bacteria attract the greatest attention.
The third component of fiber, lignin, is not digested. They also play a vital part in the host's nutritional (feeding and digestion), physiological, and immunological processes. They frequently collaborate to ferment plant sugars and proteins, both structural and nonstructural. Bacteria rule the gut and are primarily responsible for the synthesis of volatile fatty acids.
In the gut, certain bacteria species (Eubacteriumruminantium) convert unsaturated fatty acids to more saturated fatty acids, which is required for fiber breakdown. These are more common in different animals. To lower the prevalence of acidosis in the rumen, some species (Megasphaeraelsdenii) take up lactic acid. The concentration varies from species to animal.
The major task of Archaea (about 90%) is to bind the excess hydrogen in the rumen and convert it to methane synthesis. Many processes take place in the rumen, including fermentation, mixing, conversion, growth, and physiology.
In the liquid phase of digesta contents, rumen contains a vast variety of microorganisms, mostly bacteria, protozoa, and fungi, in connection with plant fragments and as a lining on the rumen epithelium. According to a research on a fistulized cow, there were approximately 10^8-10^10 bacteria per gram, which can vary depending on the time of day, time between feedings, different cows, diets, and other variables(Bovine Rumen - Microbewiki, n.d.).
Bacteria in the rumen fluids are found at a concentration of 10^9-10^10 ml and make up the majority of the microbial mass in the rumen, protozoa are found at a concentration of 105 to 106 ml, and fungi are found at a concentration of 10^3 to 10^5 ml.(Dijkstra et al., 2005). Furthermore, the role of rumen microorganisms varies depending on the types of microorganisms.
Microorganisms found in the rumen along with their roles are described as-
A. Bacteria in rumen
More than half of the work in the rumen is performed by the bacteria. They are classified as-
1. Cellulose degrading bacteria-
The principal components of the plant cell, cellulose, is digested in the rumen mostly by ruminococcus albus, ruminococcus flavefaciens, and bacteriodes succinogenes, which are the most important cellulose-degrading bacteria. Cellulose-degrading bacteria belonging to the phylum fibrobacteres, such as fibrobacter succinogenes and fibrobacter intestinali, aid in the breakdown of lignocellulosic materials in the ruminant's stomach (Ransom-Jones et al., 2012)
According to Zhang's research, the ability to digest cellulose is influenced by the kind of forage, crop maturity, and members of cellulolytic bacterial communities, as well as ph. It was discovered that the largest decomposition of cellulose present in straw occurs at pH 6.5, with no methane formation and the highest hydrogen yield.Castillo-González et al. (2014) found that attachment of microbial cells to the substrate and synergistic interactions with non-cellulolytic bacteria improve it.
Plant fiber is broken down by Ruminococcus bacteria into monosaccharide glucose, which can then be broken down further by glycolysis. These microorganisms are similarly acid-sensitive, requiring a pH of less than 6.0. When consumed by an animal, storage polysaccharide, which works as a food reserve in the plant, is rapidly destroyed by rumen microbes.
2. Streptococcus bovis Bacteria:
It's also known as "rumen weed." It only appears when animals are fed a lot of carbohydrates or sweets and the pH is low. It creates lactic acid, which is more powerful than many other VFAs made in the rumen. When the conditions are favorable, it grows rapidly ( doubling in every 13 minutes)
3. Lactate degrading bacteria
Lactobacilli, streptococci, and bifidobacteria are the most common lactic acid bacteria in the rumen (Stewart, 1992). When the food contains roughly 70% concentration, this type of bacteria increases. It's also known as a rumen maid.
A rumen bacteria, Magasphaera elesdini, metabolizes lactate and relieves rumen acidosis caused by a heavy grain diet. It has a critical function in preventing acidosis in ruminants fed high-concentrate diets throughout the adaption period.
It also helps to clean up the rumen and elevate the ph, allowing acid-intolerant fiber digesters to proliferate. Megasphaera Elsdenii Lactate Degradation Pattern Shifts in Rumen Acidosis Models, n.d. ((PDF) Megasphaera Elsdenii Lactate Degradation Pattern Shifts in Rumen Acidosis Models, n.d.)
4. Pectin Degrading Bacteria
Lachnospira multiparus, prevotella ruminicola, and butyrivibrio fibrisolvens are the main pectinolytic organisms in the rumen. Pectinolytic enzymes are released, which degrade pectin into oligogalacturonides (Duková & Marounek, 2001). Pectin bacteria also produce hydrogen gas and pectin. Other bacteria primarily use the end products of pectin fermentation, such as succinate to propionate.(Bovine Rumen - Microbewiki, n.d.)
5. Proteolytic Bacteria
The most prevalent proteolytic bacteria are Bacterioides amylophilus, Bacterioides rutminicola, and Butyrivibrio fibrisolvens. Protein hydrolysis, peptide degradation, and amino acid deamination are all activities carried out by ruminal microbes that produce enzymes (Cotta & Hespell, 1986)
6. Hydrogen Using or Methane Bacteria:
H2 does not build in the rumen under normal conditions because it is used by H2 utilizing bacteria such as Methanobacterium ruminantium.
4H2+CO2=CH4+2H2O
B. Protozoa in rumen
Protozoa are the second most numerous microbe in the rumen, accounting for 50% of the microbial mass, however they have a little role in comparison to bacteria. The bacteria in the rumen are preyed upon by protozoa ( they eat bacteria for dinner).
Epidinium, Entodinium, Diplodinium, and holotrich ciliates are the most abundant protozoans in the rumen. The substance can be digested by protozoa in the rumen, which a mammal cannot perform on its own. It makes it easier for the animal to digest plant nutrients, lipids, and proteins.
Protozoa are 40 times larger than rumen bacteria. Rumen protozoa may consume a considerable amount of starch at once and store it in their bodies, which may aid to limit the creation of acid, which lowers the rumen's favorable PH. They are also known to use tiny precursor chemicals found in the rumen fluid to create long-chain fatty acids (Emmanuel, 1974) The second type of microbial species found in the rumen is protozoa. Due to the difficulties to adequately culture them in laboratories, their specific purpose is unknown. Some are thought to be engaged in starch and sugar digestion, while others are involved in substantial ammonia generation (although bacteria have been shown to create ammonia) and hence play a key role in protein metabolism and availability to the cow.
The presence of ciliated protozoa in the rumen of adult animals speeds up the deamination process, resulting in an increase in ammonia content in the rumen fluids. Ciliated protozoa are responsible for around 35% of the rumen's plant digestion. Sugars, hemicelluloses, and pectin are all broken down by ciliates (Michaiowski, 2005)
In vitro investigations with cultivated protozoa (Castillo-González et al., 2014) found that protozoa of the genera Polyplastron and Eudiplodinium, and to a lesser extent Epidinium, breakdown crystalline cellulose. Protozoa also contribute to methanogenesis by extracting oxygen from the rumen's liquid portions and then going to the rumen's reticulum.
Because protozoa are frequently eaten by the animal's digestive system, they also serve as a source of sustenance for the host. So that they don't be washed out before they have a chance to multiply, rumen protozoa hide in the slower flowing fiber mat of the rumen. Holotrich protists are thought to account for up to 27% of the lipids in the rumen digesta (Bovine Rumen - Microbewiki, n.d.).
C. Fungi in Rumen
Though fungus make only a minor portion of the ruminal ecosystem's biomass, they play an important function in ruminant digestion. It can make up to 8% of the overall mass of the rumen, although nothing is known about it. When an animal consumes more lignified substrates, they appear to be more significant.
Rumen fungi aid fiber-digesting bacteria by performing some of the initial effort of tearing fibrous materials apart and making them more accessible to the bacteria, as well as having the ability to destroy more refractory plant walls. They attach themselves to feed particles and proliferate slowly. They may aid fiber-digesting bacteria by assisting with the early task of splitting fibrous material apart and making it more accessible to the bacteria. Animals fed very poorly digested subtropical fodder had a higher number of fungus in their rumen.
VFAs, gas, and minor amounts of other chemicals, including lactate, are produced by fungi. They play an important part in fiber digestion, but they don't work in the same way as fiber-digesting bacteria. zoospores adhere to fiber particles and form sporangia and rhizoid filaments that pierce fiber cell walls. While the filaments are unable to digest lignin, they can penetrate lignified surfaces.
For more than two decades, it has been widely acknowledged that inoculating the rumen with fungi helps increase fiber digestion. They make a lot of cellulases and hemicellulases, and they're especially good at making xylanases (Akin & Borneman, 1990).
In monoculture, some fungal species like Neocallimastix frontalis, promising joyonii, and orpinomyces communis may digest structural polysaccharides more efficiently than cellulolytic bacterial species (Castillo-González et al., 2014).
According to (Gilbert et al., 2020), certain viruses found in the rumen, such as bacteriophages and archaeophages, play an important role in microbial population balance, intra-ruminal microbial lysis, fiber breakdown, nutrient cycling, and genetic transfer.
D. VFA Metabolism:
Ruminants get a lot of their metabolizable energy from the VFAs generated by microbial fermentation. Acetate, propionate, butyrate, iso-butyrate, valeric acid, and iso-valeric acid are the major VFAs, in order of abundance.
Diet and microbial populations have a big impact on the proportions of acetic, propionic, and butyric acids. Other organic acids, like as lactic acid, which is fermented to acetate, propionate, and butyrate under normal rumen conditions, may show as metabolic products. In the most basic terms, and with regard to dairy cow nutrition: Fibre (cellulose and hemicelluloses) is fermented to produce acetate, a precursor to milk fat.
> Because starch is digested into propionate, a precursor to glucose, higher propionate-producing diets support increased milk output and/or body condition. The concentrations of VFAs in the rumen are controlled by a balance between production and absorption. Due to a differential gradient, acids are absorbed in free form across the rumen wall, with the pH of the blood being higher than the pH of the rumen. It's worth noting that VFA absorption in the lower intestine is similar to that in the rumen.
The rumen is the largest of the four compartments of the rumen's stomach, and it is where fermentation, absorption, mixing, conversion, growth, and physiology occur, with various microorganisms like as bacteria, protozoa, and fungi assisting in this process. Rumen and rumen microorganisms have a mutualism relationship in which they are both interconnected and useful to one other. The role of microorganisms in the conversion process is critical in the formations, as well as for ruminant digestion, as stated above.
Microbes make ruminant protein, VFA, ammonia, Methane, and CO2 from basic materials. The movement, or wash, of microbial species from the reticulorumen into the omasum is a risk for all of them. They are protected for a while by their adhesion to feed particles and the rumen wall, but their contribution as 'meal' in the abomasum is significant. As previously stated, they play a critical role in supplying microbial protein to the host animal. Ruminants would not survive if microorganisms did not appear in the rumen of animals with similar physiology and structure.