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Animals are hosts to a great number of microorganisms. In some cases, they are even essential to the animal's health. Without the 1011 microbes per millilitre present in their stomachs, ruminants could not digest the grass they eat.
The stomach of ruminants is made of four compartments: the rumen, the reticulum, the omasum, and the abomasum or rennet bag.
Energy in plant cells is stored in the form of long molecules of sugar called cellulose. In the rumen, the bacterial flora is particularly efficient in digesting cellulose. This process is called cellulolysis. The champion cellulolysis bacteria are Fibrobacter succinogenes, Ruminococcus flavefaciens and Ruminococcus albus. Other bacteria, such as Lachnospira multipara, Prevotella ruminicola, Butyrivibrio fibrisolvens and Streptococcus bovis are responsible for degradation of pectin, another plant compound
It is unclear whether ciliate protozoa participate in the digestion of cellulose in the rumen. If they do, it might actually be due to the work of some of their intracellular bacteria. Among cellulolytic protozoa, there are Eudiplodinium maggii, Epidinium ecaudatum and Ostracodinium dilobum.
Fungi represent 8 % of the microbial mass in the rumen. Most of the anaerobic rumen fungi are cellulolytic and their cellulases (proteins carrying out cellulolysis) are very active. In addition, the enzymatic activity of microorganisms that digest plant particles is increased when Neocallimastix frontalis and Piromyces communis are present.