What are you aiming for?

Our studies are aiming to contribute to milk and meat production via maximization of the inherent ability of “plant fiber utilization” of herbivorous livestock.

To start with, how can cattle gain up to several hundred kilograms by eating only grass? Their secret lies in their gut microorganisms. To get energy from grass, cellulose, the main component of plant body, has to be degraded. However, as with humans, cattle cannot degrade cellulose with their own digestive enzymes. So cattle harbor cellulolytic microorganisms in their gastrointestinal tracts to carry out cellulose degradation.

Fig. 1 Cattle raised eating grass / Rumen microorganisms that support it.

Cattle are polygastric animals with four stomachs, from the first stomach to the fourth stomach. The first stomach (called the rumen) is an especially large organ with capacity of 150-200 liters (comparable to typical household bathtubs) in adult cattle. In the rumen, there is no gastric acid secretion, the pH ranges from 5.5-7.0, and the temperature is kept about 39°C. Huge numbers of microorganisms live in the rumen, where they degrade and ferment the feed ingested by cattle to convert it to an energy source for the host (Fig. 1).

Fig. 2 Aiming at improving livestock productivity by controlling gut microorganism

The microorganisms living in the rumen are broadly divided into bacteria, protozoa, and fungi. They are distributed in high densities, with populations of 10^10-11, 10^5-6, and 10^4-7 per gram of contents, respectively. These rumen microorganisms coexist through cooperating or competing for the degradation and fermentation of feed ingested by cattle, resulting in a complex microorganism ecosystem, or a “microcosmos.” The main subject of our study is exploring the microcosmos composed of wide varieties of microorganisms to determine the types and functions of microorganisms that play important roles in the rumen. Ultimately, we are aiming to contributing to improved livestock productivity by maximizing the ability of key microorganisms (Fig. 2).

 

What kind of research are you carrying out?

In order to determine the function of microorganisms, the pure culture technique has generally been employed. For example, microorganisms that can grow in a culture medium containing cellulose as the sole carbon source can be identified as cellulolytic. However, pure culturing under an anoxic environment like in a rumen requires special equipment (Fig. 3, left). Although 65 years have passed since the culture method for anaerobic microorganisms was established, more than 90% of rumen microorganisms seem to have not yet been cultured. Therefore, our research includes two approaches: 1, trying to culture these not-yet-cultured bacteria, and 2, using an experimental method that does not require culturing. Our research group pursues its research by combining these two approaches.

All microorganisms possess ribosomal RNA genes in their genomic DNA. As the nucleic acid sequence of these genes varies depending on the microorganism, it can be used to identify the microbial species. In other words, microorganisms contained in the sample can be identified by examining the nucleic acid sequence of a microorganism’s ribosomal RNA genes in the sample (DNA analysis). As a result of DNA analysis, it was found that several thousand different microorganisms are living in the rumen, and most of them are not-yet-cultured microorganisms. We narrowed down the targets from these numerous types of microorganisms based on the results of the DNA analysis. Specifically, we tried to cultivate the previously-uncultured bacteria that were detected from cattle rumen all over the world and in large quantities. For cultivation, we used various materials as the nutrient sources and monitored the increase and decrease of the target microorganisms by DNA analysis (Fig. 3, middle). Finally, we employed rice straw as a nutrient source as it offered the most active growth of target microorganisms, and succeeded in the cultivation of previously-uncultured bacteria (Fig. 3, right). It is becoming clear that these new microorganisms contribute to fiber digestion via helping cellulolytic bacterium.

Fig. 3 Trying isolation culture of previously-uncultured bacteria under anoxic conditions (left). Analysis of key microorganisms at the molecular level using DNA/RNA (middle). Pure culture of microorganisms identified only by DNA data has succeeded (right).

 

What is your next goal?

Among the several thousand microorganisms living in the rumens of cattle, we consider those found in large quantity as important microorganisms and are working on the cultivation of them. Due to remarkable innovations in DNA analysis technology (DNA sequencers), it is becoming possible to sequence a genome faster and cheaper than ever before. We intend to employ functional analysis of microorganisms that makes full use of genomic information in the future. We will continue this research in the belief that elucidation of the micro-scale phenomena of microorganisms will promote better understanding of rumen function and eventually lead to improvement of the macro-scale phenomenon of cattle productivity.