lecture 7 - microbial fermentation pathways

Why is the relationship between a ruminant and its microbes considered commensal?

-The microbes receive food and a suitable environment, while the ruminant receives useful fermentation products.

-Mutually beneficial relationship

What do rumen microbes do for the ruminant?

They ferment carbohydrates the animal cannot digest with its own enzymes and produce volatile fatty acids (VFAs) for energy

What do rumen microbes receive from the ruminant?

A steady supply of feed substrates, warmth, moisture, and an environment with little oxygen

What must complex carbohydrates be broken down into before fermentation?

They are broken into simple sugars, such as glucose, that microbes can take up and metabolize

What is glycolysis?

The process in which glucose is broken down into pyruvate, producing ATP and reducing equivalents such as NADH

What is pyruvate?

A key product of glycolysis that microbes use to produce fermentation end products

What are the main products of carbohydrate fermentation in the rumen?

Volatile fatty acids (mainly acetate, propionate, and butyrate), gases, and microbial cells

What are the three major volatile fatty acids?

Acetate (2 carbons), propionate (3 carbons), and butyrate (4 carbons)

Why are VFAs a good energy source for ruminants?

They are absorbed through the digestive tract and used by the animal for energy and metabolism

What benefit do microbes get from converting pyruvate into VFAs?

The pathways help regenerate NAD⁺ so glycolysis can continue producing ATP

Why must NADH be converted back to NAD⁺?

Glycolysis requires NAD⁺; regenerating it allows microbes to continue breaking down carbohydrates for energy

What is ATP?

The cell’s usable energy currency, produced during processes such as glycolysis

What happens to carbon dioxide during rumen fermentation?

CO₂ is produced during certain fermentation pathways and can be used by methanogens to produce methane

Why is methane production undesirable for the ruminant?

Methane carries away feed energy that the animal could otherwise use

Why is carbon dioxide production associated with energy loss?

Carbon in gaseous end products is not retained as an energy-yielding nutrient for the animal

What is methane?

A gas produced by methanogens that use hydrogen and carbon dioxide; its production represents an energy loss to the animal

What are methanogens?

Microorganisms that produce methane, commonly using hydrogen and carbon dioxide

Why is rumen fermentation less efficient than enzymatic carbohydrate digestion in the small intestine?

Microbial fermentation loses energy as gases and heat, while enzymatic digestion can capture more carbohydrate energy for the animal

What happens to some feed energy during rumen fermentation?

Some is lost as methane and heat, while much of the remaining usable energy is captured in VFAs

How does fiber fermentation differ from starch fermentation in its main VFA product?

Fiber fermentation generally favors acetate, while starch and other non-structural carbohydrates generally favor propionate

What is the difference between fermentation and enzymatic digestion?

-Fermentation is carried out by microbes and produces VFAs and gases

-Enzymatic digestion uses the animal’s own enzymes to break nutrients into absorbable units

Why is hydrogen balance important in the rumen?

Hydrogen must be used or transferred into other products; excess hydrogen can support methane production

What happens to the VFA products after they are formed?

They are absorbed and used by the ruminant as energy sources or metabolic building blocks