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Ruminant Microbiota Insight: Part 1 – Meet the microbes

Ruminant Microbiota Insight: Part 1 – Meet the microbes

The rumen hosts a very diverse community called the microbiota (made up of bacteria, protozoa, and fungi). Ruminant animals would not be able to digest feedstuffs, especially forages, without these microbial populations. Thanks to this unique symbiotic relationship, ruminants can convert plant materials into milk, meat or wool.

Here is an overview of the components of the microbiota and its role in the rumen:


Rumen bacteria account for 1010 organisms/ml of rumen fluid and several hundred species have been characterized. By volume, they compose 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.


Ciliate protozoa are larger than bacteria and account for 106 organisms/ 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.

Most of them degrade proteins very efficiently and release ammonia. These proteins represent around 25% of the microbial protein available for the animal. 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.


Rumen fungi compose 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 can facilitate access to more digestible tissues and help release polysaccharides, which are linked to lignin and increase the pool of digestible energy for the rumen microflora.

rumen bugs with legends

3D representation of the rumen microbes

Digestive comfort is directly linked to the balance of the different microbial communities in the digestive tract.

These communities are changed in their function and activity according to dietary factors, but also to management and environmental conditions. It has been demonstrated that when the rumen microbiota is challenged, a ruminal microbial imbalance occurs, which impairs fiber digestion, increases the animal’s acidosis risk, and may trigger inflammation that negatively impacts digestive comfort and well-being.


For more information on rumen microbiota, visit:

Published Nov 11, 2019 | Updated May 29, 2023