Digestion: The Fermentative processes
Digestion is the physiochemical breakdown of food particles and molecules into subunits suitable for absorption in gastrointestinal tract especially in glandular stomach. But in case of herbivores the digestion is the fermentative process (anaerobic breakdown of food). The herbivores signify the ruminant animals and non ruminant animals. The non ruminants bear a glandular stomach whereas ruminants bear four chambered stomach viz. rumen, reticulum, omasum and abomasum. The rumen is largest stomach acts as a fermentative chamber and abomasums resembles glandular stomach of non ruminants.
Fermentation is the metabolic action of microbes as enzymes responsible for the digestion of food molecular substrates are microbial in origin rather than from host animals (carnivores,non-ruminants). The sites of fermentative digestion are positioned either before or after stomach (abomasums in ruminants) and small intestine, which must be suitable for microbial growth. Fermentative compartments positioned before stomach are called forestomach developed in ruminants(cattle,buffalo,sheep,deer) and cameloids, while that of positioned distal to small intestine are caeca and colon collectively known as hindgut, developed in non ruminant herbivores(equine,swine,elephants). These both contribute in fermentative digestion as their pH (close to neutral~6.8), ionic strength and oxidation-reduction conditions are maintained in a range compatible for the growth of microbes. Due to such complexity the fermentative digestion is comparatively slower than that of normal enzymatic digestion of glandular stomach. Well, this process has significant variations among species.
Here, the fermentative digestion is totally dependent upon the microbial enzymatic actions so that the microbial ecosystem is well balanced in fore stomach and hindgut which includes the populations of bacteria (ranging from 10^10 to 10^11 cells per gram of ingesta), protozoa (ranging from 10^5 to 10^6 cells per gram of rumen content) and significantly fungi.
Grouping of rumen bacterial species according to the type of substrates that are fermented:
Major Cellulolytic species
Bacteroides succinogens, Ruminococcus flavefaciens, Ruminococcus albus, etc.
Major hemicellulolytic species
Butyrovibrio fibrisolvens, Bacteriodes ruminicola, Ruminicoccus spp., etc.
Major methane-producing species
Methanobacter ruminantium, Methanobacter formicicium, Methanomicrobium mobile.
Major proteolytic species
Bacteroides amylophilus, B. ruminicola, Butyrivibrio fibrisolvens, Streptococcus bovis.
There are many more bacterial species in rumen contributing in fermentation of ingesta. Most of them are anaerobes.Bacteria are major source which produces respective enzymes like cellulose,hemicellulase,pectinase,etc for digestion of food molecular substrates.
Most protozoa are ciliated and belong to the Genus Isotricha or Entodinium. The digestive abilities of protozoa and bacteria are similar performing most of fermentative functions. The protozoa is large in size and is equal to bacterial mass. It engulfs the large number of bacteria and hold the bacterial population in check hence maintaining the balance of inhabitant microbes and reducing risk of many diseases completely, as the above optimum level of microbial load results disturbances in normal digestive physiology. However, protozoal role in fermentation is not found to be more significant.
Recent researches have found that fungi play an important role in breakdown of plant cell walls and fibers.
The fermentative digestion is simply the cooperation and interplay between above mentioned ecosystem of rumen or hindgut. It is extremely complex,with the waste product of one microbial species serving as substrate for another. For e.g. the cellulolytic bacteria can digest cellulose but can’t ferment protein which is digested by proteolytic bacteria. The cellulolytic bacteria and hemicellulolytic bacteria are synergistic to each other i.e. they will provide energy to each other when grown together. Here, each bacteria produces respective enzymes for hydrolysis of ingested substrates.
In case of herbivores the plant is major source of nutrients i.e. plant cell walls are major substrates for fermentation in rumen or hindgut. The plant cell walls along with other parts contains cellulose, hemicellulose, pectin,etc, which are non branching chains of glucose monomers joined by beta-glycosidic linkages. Each dietary sources is subjected to fermentative digestion in herbivores.
When these materials or let’s say carbohydrates enter the rumen or colon, it is attacked by hydrolytic microbial enzymes, but in case of insoluble carbohydrates attack requires the physical attachment of the bacteria to the surface of the plant particle, the enzymes themselves being part of the surface coating of the bacteria. Enzymes like cellulase, hemicellulase, pectinase,etc are involved in liberating glucose,other monosaccharides and short chain Polysachharides into the fluid phase of rumen, outside the microbial cell bodies, which are immediately subjected to further metabolism by bacteria. The further metabolism in bacteria is similar to that of mammalian cells (Embden-Meyerhof pathway) resulting 2 molecule of pyruvate. Since there is anaerobic environment the pyruvate molecule undergoes reduction to provide NAD (nicotinamide adenine dinucleotide) with an additional yield of ATPs. These pathways ultimately leads to the production of heavy amount of methane gas and volatile fatty acids (acetic>propionic>butyric,) which are important source of energy to host animals. The end products of Fermentative digestion is VFAs and its chemical reactions and products which is absorbed from rumen and undergoes beta-oxidation liberating abundant ATPS. The whole process is governed by enteric nervous system.
Recent research have found that the methane gas released by ruminants especially by cattle, contributes in ozone layer depletion.
The interesting fact about fermentative digestion is that ruminal fermentative digestion supplies abundant protein nutrient to animals even with no supplements of proteins in diet. The requirement is met from the dead microbial population washed out of rumen into lower tract (abomasum & small intestine) where the microbial protein is synthesized for host animals. Really Inexpensive rearing of ruminants is possible.
In this way fermentative digestion is the process of digestion developed in polygastric and monogastric herbivores. The beauty of fermentative digestion is symbiotic relation in ecosystem of rumen pr hindgut resulting VFAs as end products. The efficacy of digestion is directly related to performance of farm animals hence we must know about them a little bit!
Cunningham Textbook of Veterinary Physiology, second edition,
1997 by W.B. Saunders company.
This article is contributed by Ranjita Bastola. Bastola is a student of Veterinary Science and Animal Husbandry at Agriculture and Forestry University (AFU, Chitwan)