In this blog post, we will take a scientific look at how microorganisms in the stomachs of ruminants break down fiber and use it to produce energy.
Carbohydrates are an essential source of energy for humans and other animals. Carbohydrates are divided into fiber and non-fiber. Humans use enzymes synthesized in the body to break down non-fiber, such as starch in grains, into glucose, which is then absorbed in the small intestine and used as an energy source. However, humans cannot synthesize enzymes that break down fiber, such as cellulose, which is the main component of grass and vegetables, into glucose, so fiber cannot be utilized in the small intestine. Despite this limitation, fiber plays an important role in the human digestive system. For example, fiber promotes peristalsis as it passes through the digestive tract, preventing constipation and helping to maintain the balance of the intestinal microbiota. This means that although fiber is not used as a direct source of energy, it is essential for maintaining a healthy digestive system.
Ruminants such as cows, sheep, and deer are also unable to synthesize enzymes that break down fiber, but they live by using both non-fiber and fiber as energy sources. These ruminants are able to do so thanks to their unique digestive structure. The stomach of ruminants is divided into four parts, each of which has a specific function. The first stomach, called the rumen, plays an important role in this digestive process. The rumen is home to various types of microorganisms that produce enzymes necessary for breaking down cellulose and non-cellulose.
The rumen of ruminants is devoid of oxygen, and the microorganisms that thrive in this environment have various physiological characteristics. Among them, Fibrobacter succinogen (F) is a representative microorganism that breaks down cellulose. In plants, cellulose is intricately intertwined with other substances surrounding it, but the enzyme complex possessed by F breaks down this structure, exposing the cellulose, which is then broken down into glucose. F metabolizes this glucose within its cells and uses it as an energy source to survive and increase its population, thereby growing. During this metabolic process, metabolites such as acetic acid and succinic acid are produced and discharged outside the cells. The acetic acid produced by microorganisms in the rumen is absorbed directly into the cells of ruminants and is mainly used to produce the energy necessary for survival and to synthesize body fat. On the other hand, succinic acid in the rumen is quickly consumed as an energy source by other microorganisms that produce propionic acid as a metabolite. The propionic acid produced in this process is used as a major ingredient in the metabolic process of glucose synthesis in the liver of ruminants.
The rumen also harbors Streptococcus bovis (S), which breaks down non-fibrous starch. These microorganisms break down the starch ingested by ruminants into glucose, which they then use as an energy source for themselves through metabolic processes within their cells. At this time, the metabolic products excreted by S into the cell exterior vary depending on the acidity within the cell. When the acidity, indicated by the hydrogen ion concentration index (pH), is neutral at around 7.0 and the growth rate is slow, metabolic products such as acetic acid and ethanol are excreted. On the other hand, when the acidity increases and the pH falls below 6.0, or when there is a sufficient amount of starch and the growth rate is fast, lactic acid is discharged as a metabolite. In the rumen, lactic acid is directly absorbed into the cells of ruminants and used to produce the energy they need, or used as an energy source for other microorganisms that discharge acetic acid or propionic acid as metabolites.
However, excessive growth of S can have adverse effects on ruminants. When ruminants ingest excessive amounts of non-fiber substances in a short period of time, the number of S increases rapidly, and excessive amounts of lactic acid are discharged, increasing the acidity of the rumen. As a result, lactic acid-producing microorganisms such as Lactobacillus rumenis (L), which grow vigorously in an acidic environment and always excrete lactic acid as a metabolite, increase in number and begin to excrete large amounts of lactic acid. Fiber-degrading microorganisms, including F, have the characteristic of maintaining a constant pH inside their cells. When the pH outside their cells decreases due to an increase in lactic acid concentration, they use energy to maintain homeostasis inside their cells, which reduces their growth. If the pH outside their cells falls below 5.8, their energy is depleted, and they enter a stage of growth cessation and death. In contrast, S and L are relatively resistant to acidity and can lower the pH inside their cells to match the external pH even if it drops to around 5.5, so they use almost none of their energy to maintain the pH inside their cells and can continue to grow. However, if the pH outside the cells of S falls below this level, they also stop growing and enter a stage of death, and lactic acid-producing microorganisms, including L, which are more resistant to acidity, come to dominate the rumen microorganisms. This causes acute ruminal acidosis, in which the pH of the rumen falls below 5.0. This situation poses a serious threat to the health of ruminants and requires proper management and prevention.
