Role of Microbes in Ruminants

 Microbe-Animal Interaction

Ruminants 

Ruminants, are any mammal of the suborder Ruminantia (order Artiodactyla), which  includes the cow, sheep, giraffe, deer, antelope, moose, goat etc. These herbivores have a symbiotic relationship with the microorganisms in their forestomach (rumen) to exploit fibrous feeds as a source of energy and nutrients.  Most ruminants have  four-chambered stomach. The four chambers of ruminant stomach are named as Rumen, Reticulum, Omasum and Abomasum. 

The specialized rumen forestomach allows colonization of billions of  bacteria, archaea, protozoa, and fungi, collectively called the rumen microbiome. Bacteria predominate in the rumen and convert energy stored in plant biomass to microbial protein and short-chain fatty acids required to manufacture food products. 

Ruminal content (fluid and feed material) contains 10¹⁰ to 10¹¹  microorganisms per milliliter, including prokaryotic (bacterial and archaeal) and eukaryotic species.

Microbial fermentation of ingested plant materials is a crucial step in the digestion of feed by the host animal. Rumen microorganisms usually adhere to feed particles and form biofilms to degrade plant material.

Most microorganisms have different roles in feed digestion and act synergistically to ferment plant carbohydrates and proteins, but antagonistic relationships can develop if different microbes occupy a similar niche. Microbial populations change with feed type as well as with other environmental influences such as the inclusion of antibiotics in the diet.

Rumen microorganisms can also detoxify many feeds but occasionally they also produce end products (e.g., nitrate, cyanide derivatives, lactic acid) that may be detrimental to the host.

Manipulation of rumen fermentation through proper diet formulation or through the use of additives can  alter microbial populations and alleviates these problems.

Digestive system of Ruminants 

The primary difference between ruminants and non-ruminants is that ruminants' stomach has four compartments: Rumen, Reticulum, Omasum, Abomasum.

Ruminant animals do not completely chew the grass or vegetation they eat. The partially chewed grass goes into the large rumen, which is the largest section and the main digestive centre, where it is stored and broken down into balls of “cud”.

Cud is a portion of food that returns from a ruminant's stomach to the mouth to be chewed for the second time. It is a bolus of semi-degraded food regurgitated from the rumen. The rumen  filled with billions of  microorganisms play a major role in breaking down grass and other coarse vegetation that animals with one stomach (including humans, chickens and pigs)  cannot digest. e.g. Cellulose, which can be digested by the cellulase enzyme produced by the microbes. 

When the animal has eaten its fill, it will rest and “chew its cud”. That means, they later  regurgitate the cud, and chew it again to further break down in to smaller particle size and  mix thoroughly with saliva. The cud is then swallowed once again where it will pass into the next three compartments—the reticulum, the omasum and the true stomach, the  abomasum. 

Rumen 

It is the largest compartment. it can hold as much as 50 gallons of food and other ingested  substances at a time. It contains huge number of different microbes, including bacteria,  fungi and protozoa. Its internal surface is covered with tiny projections, papillae, which  increase the surface area and allow better absorption of digested nutrients. 

                            Rumen with papillae

 

Reticulum / Honeycomb 

Reticulum is separated from the rumen by a ridge of tissue. Its lining has a raised honey  comb like pattern, also covered with papillae. It traps hard, indigestible substances like  rocks, nails, or wires that may be ingested by accident while the bovine is grazing. 

  

                    Reticulum showing honey comb projections and papillae

 

Omasum 

It is also known as “many-piles”, with leaf-like fold shaped compartment. Large plate like folds are known as laminae, which extend from the walls of the omasum. Omasum lies between the reticulum and abomasum and act as a gateway to the abomasum. It sends large substances back to rumen and reticulum while allowing smaller, well-broken down  substances to pass through into abomasum.  The laminae are covered in papillae which direct  the flow of food particles towards the next chamber, abomasum.

                            Omasum showing laminae and papillae 

 

Abomasum / true stomach 

It connects the omasum to the small intestine. It is much same as the human stomach.  The acid and enzyme digestion takes place here. The lining of the abomasum is folded in  to ridges, which produce gastric juices containing hydrochloric acids and enzymes  (Pepsins). The pH of these gastric juices varies from 1 to 1.3 making the abomasum very acidic, with an average pH of about 2. The acidity in the abomasum kills the rumen microbes. The pepsins carry out the initial digestion of microbial and dietary proteins in  the abdomen. 

                                                                                                            

                                                                                                                  

Abomasum showing ridges

Process of Rumen digestion 

Once the food has been ingested by the animal, it is briefly chewed and mixed with saliva,  swallowed and then moved down the oesophagus in to the rumen. The rumen is adapted  for the digestion of fibre. The microbes breakdown the feed through the process of  fermentation. the rumen and the reticulum, make up the fermentation vat, which is the  major site of microbial activity.

Fermentation is crucial to digestion because it breaks down complex carbohydrates, such as cellulose, and enables the animal to utilize them. Microbes function best in a warm, moist, anaerobic environment with a temperature range of 37.7 to 42.2 °C and a pH between 6.0 and 6.4. Without the help of microbes, ruminants would not be able to utilize nutrients from forages. 

The breakdown of food starts in the mouth itself due to the mechanical action of chewing. The chemical breakdown starts in the rumen by the action of microbial enzymes. The  walls of the rumen and reticulum moves continuously, churning and mixing the ingested  feed with the rumen fluid and microbes. The feed is returned to the mouth for cud chewing,  which further breaks the feed in to smaller pieces. Cud chewing increase the rate of  microbial digestion in the rumen. The contraction of the rumen and reticulum help the flow  of finer food particles in to the next chamber, the omasum. Omasum controls what is able  to pass into the abomasum. It keeps the particle size as small as possible in order to pass  into the abomasum. Abomasum is the gastric compartment of the ruminant stomach. This  compartment releases acids and enzymes that further digest the material passing through.  This is also where the ruminant digests the microbes which may reach from rumen.  

 

Microbes of Rumen and their role in Digestion 

 

The microbes in the rumen include, Bacteria, Protozoa and Fungi. 1 ml of rumen is estimated to contain 10–50 billion bacteria and 1 million protozoa, as well as several  yeasts and fungi. Since the environment inside a rumen is anaerobic, most of these   microbial species are obligate or facultative anaerobes. They can decompose complex  plant material, such as cellulose, hemicellulose, starch, and proteins.  

 

 

The major end products of microbial fermentation are; 

1. Volatile fatty acids, including acetate, propionate and butyrate, which are the major  energy source of cow. 

2. Ammonia, which is used to manufacture microbial proteins. Bacteria are made up of  60 % protein. These bacteria are digested in the abomasum and become the major  source of protein for the cow. 

3. Gases, like carbon dioxide and methane, which are wasted energy, as they are belched  out regularly. 

 

Bacteria: 

Rumen bacteria account for 10¹⁰ organism/mL of rumen fluid and several hundred  species have been characterized to date. They comprise up to 50% of the total  microbial biomass. Rumen bacteria are classified into fiber digesters, starch and sugar digesters, lactate using bacteria, and hydrogen-using bacteria. The bacteria exist in co-operation with each other. Some breakdown certain carbohydrates and proteins which are then used by others. Some require certain growth factors, such as B-vitamins, which are made by others. Some bacteria help to clean up the rumen of others’ end products, such as hydrogen ions, which could otherwise accumulate and become toxic to other organisms. This is called “cross-feeding”.

Bacteria are an important source of microbial protein, which  supply the ruminant with 75-80% of its metabolizable protein. Bacteria are also important for producing enzymes that digest fiber (cellulose, hemicellulose), starch and sugars.

 

Examples of Rumen Bacteria

Ruminococcus flavefacians,  Ruminococcus albus,  Bacteriodes succinogenes,  Butyrivibrio fibrisolvens,  Bacteriodes ruminocola, Bacteriodes amylophilus,  Methanomicrobium sp., Methanobacterium sp.,  Methanosarcina sp.,  Selenomonas ruminantium, Streptococcus bovis,  Succinomonas amylolytica, Methanobrevibacter ruminatium, Butyrivibrio sp., Eubacterium sp., Lactobacillus

 

Protozoa: 

As much as 50% of the microbial mass in the rumen can be made up of protozoa. However, their role, as compared to the rumen bacteria, is not as significant. The protozoa are predators to the bacteria in the rumen. Protozoa are about 40 times the size of rumen bacteria. The rumen protozoa produce acetate, butyrate, and hydrogen as fermentation end-products similar to those made by the bacteria. Rumen methane bacteria actually attach and live on the surface of rumen protozoa for immediate access to hydrogen.

Rumen protozoa eat large amounts of starch at one time and can store it in their bodies. This may help to slow down the production of acids that lower rumen pH, benefiting the rumen.

Rumen protozoa multiply very slowly in the rumen --- over 15-24 hours – as opposed to the bacteria that may take as little as 13 minutes to multiply. The rumen protozoa hide in the slower moving fiber mat of the rumen and are not washed out before they multiply. Low roughage diets reduce the retention of fiber in the rumen and may decrease the number of protozoa in a cow’s rumen.

Protozoa account for 10⁶organisms / mL of  rumen fluid, and have various activities: 

✓ Cellulolytic and hemicellulolytic protozoa can digest plant particles. 

✓ Different protozoa have a role in digesting starch (more slowly than bacteria)

✓ Other protozoa can consume lactic acid, thereby limiting the risk of acidosis. 

✓ Some types of protozoa are able to remove oxygen so they have a stabilizing effect upon anaerobiosis. 

✓ Most of them degrade proteins very efficiently and release ammonia, so they  can waste dietary protein.  These proteins represent around 25% of the microbial protein available for the animal.  

✓ Ciliate protozoa produce large amounts of hydrogen, which is a substrate for  methanogens. 

✓ The ciliate species are predators of other rumen microbes.  A single protozoal cell can swallow up to several thousand bacteria in an hour so  they play a very important role in rumen microbial population stability. 

 

Examples of Rumen Protozoa 

Ophryoscolex monoacanthus, Entodinium exiguum, Eudiplodinium maggii, Isotricha intestinalis, Epidinium sp., Entodinium sp., Diplodinium sp., Sarcodina sp. 

Fungi: 

Rumen fungi comprise up to 8-10% of microbial biomass. They attach to feed particles and they reproduce very slowly. They assist the fiber-digesting bacteria by the initial splitting of fibrous material thus making it more accessible for the bacteria. They  play an essential role in fiber digestion due to the production of  filamentous rhizoids which invade plant tissues, and secrete enzymes.  This digests plant cell walls and increase access to more digestible tissues, release polysaccharides, which are linked to lignin. In effect, this increases the pool of digestible  energy for the other rumen microflora.

 

Examples of Rumen Fungi 
Neocallimastix sp., Caecomyces sp., Piromyces sp., Anaeromyces sp., Orpinomyces sp., Cyllamyces sp 

Ruminant microbiology deals with the different microbial groups present in rumen of ruminants.  Microbial populations change with feed type as well as with other environmental influences such as the inclusion of antibiotics in the diet. 

 Application of the tools of modern molecular biotechnology is enhancing our understanding of this extremely complex microbial ecosystem in a manner that should provide opportunities for further optimization of rumen fermentation.