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Medicine LibreTexts

22.11C: Bacterial Flora

The largest bacteria ecosystem in the human body is in the large intestine, where it plays a variety of important roles.

 

LEARNING OBJECTIVES

 

Describe other factors about gut/bacterial flora

 

KEY TAKEAWAYS

Key Points

 

  • The large intestine absorbs some of the products formed by the bacteria that inhabit this region, such as short-chain fatty acids that are metabolized from undigested polysaccharides (fiber).
  • Other bacterial products of undigested polysaccharide fermentation include gas (flatus), which consists primarily of nitrogen and carbon dioxide.
  • These bacteria also produce large amounts of vitamins, especially vitamin K and biotin (a B vitamin), for absorption into the blood.
  • Factors that disrupt the microorganism population of the large intestine include antibiotics, stress, and parasites.

 

Key Terms

 

  • passive diffusion: The net movement of material from an area of high concentration to an area of low concentration without any energy input.
  • bacterial flora: A community of bacteria that exists on or in the body, and possesses a unique ecological relationship with the host.
  • colitis: An inflammation of the colon or the large intestine.

 

Bacterial Flora

 

The large intestine houses over 700 species of bacteria that perform a wide variety of functions; it is the largest bacterial ecosystem in the human body. The large intestine absorbs some of the products formed by the bacteria that inhabit this region.

For example, undigested polysaccharides (fiber) are metabolized to short-chain fatty acids by the bacteria in the large intestine, and then are absorbed by passive diffusion. The bicarbonate that the large intestine secretes helps to neutralize the increased acidity that results from the formation of these fatty acids.

 

Bacteria and Vitamins

 

This is a photograph of a microscope slide of Escherichia coli, one of the many species of bacteria present in the human gut.

Bacterial flora: Escherichia coli is one of the many species of bacteria present in the human gut.

These bacteria also produce large amounts of vitamins, especially vitamin K and biotin (a B vitamin), for absorption into the blood. Although this source of vitamins, in general, provides only a small part of the daily requirement, it makes a significant contribution when dietary vitamin intake is low.

An individual who depends just on the absorption of vitamins formed by bacteria in the large intestine may become vitamin deficient if treated with antibiotics that inhibit other species of bacteria, as well as the disease-causing bacteria.

Other bacterial products include gas (flatus), which is a mixture of nitrogen and carbon dioxide, with small amounts of hydrogen, methane, and hydrogen sulphide. These are produced as a result of the bacterial fermentation of undigested polysaccharides. The normal flora is also essential for the development of certain tissues, including the cecum and lymphatics.

 

Bacteria and Antibodies

 

Bacterial flora is also involved in the production of cross-reactive antibodies. These are antibodies produced by the immune system against the normal flora, that are also effective against related pathogens, and prevent infection or invasion.

The most prevalent bacteria are the bacteroides, which have been implicated in the initiation of colitis and colon cancer. Bifidobacteria are also abundant, and are often described as friendly bacteria.

A mucus layer protects the large intestine from attacks from colonic commensal bacteria. Some factors that disrupt the microorganism population of the large intestine include antibiotics, stress, and parasites.