For most of the time, the population of bacteria in the large intestine lives in balance within its confines.

The relationship that exists between the body and its resident bacterial population is a symbiotic one where both parties benefit from the relationship.

The bacteria play important roles in nutrient digestionvitamin synthesis, energy metabolism and immune responses. In return, the bacteria are provided with steady growth conditions and a constant stream of nutrients.

Types of bacteria present

Huge numbers of bacteria are to be found in the large intestine. It has been estimated that there are more bacteria in the large intestine than there are cells in the human body, estimated to be in the order of 100 trillion (1014

Most of these bacteria fall into only 4 of the 50 larger bacterial groupings, with the Firmicutes and Bacteroidetes being the most common. Conservative estimates put the number of bacterial species present at over 500.

Recent research has shown that the Firmicutes/Bacteroidetes ratio in the large intestine changes as we age. It undergoes an increase from birth to age 2 and remains virtually stable until being further altered with advanced age. The fat content of the human body also seems to play a part in determining this ratio. It has been found that lean and obese people have very distinct bacterial population profiles.

Maintaining gut health

Not all of the bacterial types found in the large intestine are of the helpful variety. Some bacterial types like E. coli and C. difficile are opportunistic pathogens.

This means that, under normal conditions, they live alongside other bacteria in the gut. However, once the host’s immune system is weakened or after a course of antibiotics or if there is physical damage to the gut lining, they can get a foothold and multiply rapidly, causing harm to the host.

The delicate balance that exists between the communities of bacteria that live in the large intestine can be maintained through a well balanced diet, rich in fresh fruit and vegetables. This ensures that the healthy gut bacteria have sufficient fibre to ferment into short-chain fatty acids. The production of these maintains the pH of the large intestine at a level supportive of the growth of healthy bacteria.

The high population of good bacteria will ensure that most of the attachment sites on the surface of the cells lining the gut will be filled with these good bacteria. preventing pathogenic bacteria from gaining a foothold.

Bacterial fermentation

Conditions in the large intestine are mostly anaerobic. This means that there is little oxygen around, and as a consequence, most of the bacterial types are anaerobic – they do not need oxygen as a component in the energy-releasing reactions that occur in the cytoplasm. Instead, they release the energy locked up in nutrients by means of fermentation reactions.

In these fermentation reactions, short-chain fatty acids are produced along with gases such as hydrogen, methane and carbon dioxide. The short-chain fatty acids are mostly used as an energy source, and the gases are removed in the breath or as flatus. It has been estimated that up to 10% of the body’s energy requirement is obtained through this fermentation process.

Recent research findings seem to indicate that increased production of butyrate can have an influence on satiety (a feeling of fullness) as well as an inhibitory effect on the development of colonic cancer cells.

Other functions performed

Bacteria in the large intestine perform a wide range of useful functions apart from fermenting undigested food material. They have a role to play in stimulating and activating the immune system, preventing growth of pathogenic bacteria, producing vitamins (such as biotin and vitamin K) and producing hormones that assist with the storage of fats.

As a result of this wide and varied activity, some scientists believe that gut bacteria function as a body organ in their own right.

Nature of science

Science is a constantly changing body of facts and explanations about the world around us. Our understanding of how the large intestine and its resident bacterial population functions has moved well beyond the idea that it is only a body waste storage facility.

    Published 13 July 2011