Role of Microbes in Ruminants

 Microbe-Animal Interaction - Role of Microbes in Ruminants 

Ruminants 

Ruminants, are mammals of the suborder Ruminantia (order Artiodactyla), which includes the Cow, sheep, giraffe, deer, antelope, moose, goat etc. Most ruminants have four-chambered stomach. The four chambers of ruminant stomach are named as Rumen, Reticulum, Omasum and 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 center, 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 is filled with billions of tiny microorganisms that are able to break 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 into 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. 

Digestive system of Ruminants 

The primary difference between ruminants and non-ruminants is that ruminants' stomachs have four compartments: Rumen, Reticulum, Omasum, 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. 

  Internal anatomy of Rumen showing 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. 

Internal anatomy of 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 back  large substances back to rumen and reticulum while allowing smaller, well-broken down  substances to pass through into abomasum. The materials entering omasum is made up of  90 to 95 % water. The primary function of omasum is to remove some of this water and to further grind and breakdown the feed. The laminae are covered in papillae which direct  the flow of food particles towards the next chamber, abomasum.

Internal anatomy of 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. 

Internal anatomy of 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 (100 to 108 °F) 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. 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.

Bacteria: 

Rumen bacteria account for 10¹⁰ organism/mL of rumen fluid and several hundred  species have been characterized to date. By volume, they comprise up to 50% of the total microbial biomass. Bacteria species 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: 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, Methanosphaera stadtmanae, Butyrivibrio sp., Eubacterium sp.,  Lactobacillus sp. 

 

Protozoa: 

Ciliate protozoa are organisms larger than bacteria and account for 10⁶organisms / mL of  rumen fluid, however they still make up to 50% of the total microbial biomass.  They have various activities: 

✓ Cellulolytic and hemicellulolytic protozoa can digest plant particles. 

✓ Different protozoa have a positive role 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 -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 and are strictly anaerobic. They  play an essential role in fiber digestion due to the production of  filamentous rhizoids which invade plant tissues, and their efficient enzymatic activities.  This physical action to plant cell walls, can facilitate access to more digestible tissues and  help release polysaccharides, which are linked to lignin increasing the pool of digestible  energy for the other rumen microflora. 

Examples - Neocallimastix sp., Caecomyces sp., Piromyces sp., Anaeromyces sp., Orpinomyces sp., Cyllamyces sp 

Microbial digestion 

 

Rumen microbes can decompose complex plant material, such as cellulose, starch, hemicellulose, 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.