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McCallister: Anaerobic bacteria within the rumen play unique roles in digestion

by Wyoming Livestock Roundup

Beginning in 1950, a scientist named Bob Hungate developed techniques to study anaerobic bacteria, or bugs that cannot survive in oxygen.

“When he developed those techniques, a number of scientists started using it, and we started culturing all the bacteria in the rumen,” says Tim McCallister from Agriculture and Agri-Food Canada. “We began to understand the bacteria. We knew the ones that digested fiber, the ones that digested starch and the one that digested protein.” 

As microbiology and molecular biology techniques began to expand, so did the technologies used to study such bacteria. 

Growing technologies

Instead of simply collecting and growing bacteria, today, scientists utilize the genetic information of bacteria to identify the bacteria. 

“Now, we take samples, extract the genetic material and run them through a sequencer,” McCallister explains. “Then, by looking at the sequences, we can tell which bacteria are there. Each bacterial type has a specific gene which helps us identify it.” 

Additionally, each bacterium has genetic information that codes for specific functions. As an example, the genetic information that codes for starch digestion present within bacteria means the bug participates in starch digestion.

“When we did the math to compare what we cultured in the laboratory to what we found in the genetic information, we found out that 95 percent of bacteria in the rumen had never been cultured in the laboratory,” he emphasizes. “There is a huge portion of the rumen population we thought we figured out, but we only had about five percent of it.” 

McCallister says work today continues to focus on rumen microbiome, 95 percent of which is done using molecular biology today.

Rumen biome

Within the rumen, microbes work together as a team, according to McCallister.

“They communicate with each other and with the host animal, as well,” he explains. “There are 10 to 100 billion bacteria in one cubic centimeter of rumen fluid.”

Huge numbers of bacteria are present in the rumen, and they are joined by eukaryotes – multicellular organisms that include anaerobic fungi and protozoa.

“The final group in the rumen are methanogens,” McCallister says. “These are not true bacteria.” 

Small bacteria

The three principal populations of bacteria in the rumen are also segregated by their location.

“The first are the bacteria that are attached to the rumen wall. They have the greatest amount of communication with the host and account for a very small portion of the total population,” he says.

Only one to three percent of the population of bacteria fit into this category, and McCallister says they are capable of surviving in environments both with and without oxygen.

They are important for capturing any oxygen that may seep through the rumen wall from the bloodstream. These bacteria also have urease, which breaks down urea and releases nitrogen to help in amino acid synthesis. 

Largest bacteria 


The largest population of bacteria in the rumen – about 70 percent of the total microbiome – attach to the surface of feed and carry out the digestion process. 

“For bacteria to digest feed, they have to be able to attach to it,” McCallister says. “When we process feed, we are creating more surfaces for the bacteria to attach to.” 

Different bacteria attach to and digest different feed sources.

The final 30 percent of the bacteria in the rumen are free bacteria. 

“They have probably been released from a feed particle, and they are waiting for the animal to ingest more feed, so they can do the work of digestion.” says McCallister


Protozoa are also present in the rumen.

“Protozoa are capable of eating or engulfing starch granules,” McCallister says. “They account for 30 to 50 percent of the biomass in the rumen, just because they are so much larger.”

Protozoa, however, are present as a much smaller percentage of the overall rumen microbiome.

McCallister summarizes, “These microbes are all present in a community and they work together to digest feed and make it usable for cattle.”

McCallister spoke during the 2019 National Cattlemen’s Beef Association Cattlemen’s College, held during late January 2019. 

Saige Albert is managing editor of the Wyoming Livestock Roundup. Send comments on this article to

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