Friendly Bacteria in the Digestive System

Friendly bacteria in the digestive system occur mainly in the colon, also called the large intestine, and in the part of the small intestine furthest away from the stomach. The oesophagus, stomach and first section of the small intestine are mostly bacteria-free because of the deterrent effect of the strong stomach acid and the high level of digestive enzymes.

Most of the bacteria that live in the small intestine and the colon are friendly – they do the body no harm and many of them carry out useful functions. The primary benefit that they provide is protection against potentially harmful bacteria that try to establish and infection and invade the cells that line the internal intestinal wall.

Which Bacteria In the Gut Are Friendly?

A wide range of bacterial species in the gut are present there as friendly commensals. About a third of all bacteria in the gut are members of the Bacteroides species. Other species that are represented include Lactobacillus, the bacteria commonly used in probiotic foods such as live yoghurt, Escherichia species, such as E. coli, some Clostridium bacteria, the Fusobacteria, the Eubacteria, members of the Bifidobacteria group and lesser known species such as Peptostreptococcus, Ruminococcus and Peptococcus.

How Many Bacteria Live in the Gut?

The best estimates put the actual number of bacterial cells in the digestive system at any one time at about 100 trillion (1014 cells). The number of individual species is thought to be somewhere between 300 and 1000. The majority probably belong to about 50 species. If you could remove all of the bacteria in one person's digestive system, the whole lot would probably weigh about 1 kg (2.2 lbs).

Why Aren't Gut Bacteria Digested?

Friendly bacteria may not harm the digestive system, but it is a surprise that the digestive system does not manage to harm them. It is full of powerful enzymes and the mechanism of peristalsis constantly moves the contents of the intestines forwards towards the rectum and anus. Recent research has suggested that the friendly bacteria of the gut appear to the body's immune system as cells of the digestive system. Bacteroides species appear to coat themselves with an outer layer composed of sugar molecules that the bacteria remove from the surface of cells that line the large intestine.

This special coating helps disguise the bacteria and hide them from the immune system, which would otherwise see them as foreign invading bacteria and try to get rid of them. This mechanism allows Bacteroides bacteria to live in the digestive system unchallenged by antibodies or immune cells.

What Benefits Do Gut Bacteria Provide?

Bacteroides species, in particular, are very useful to the human digestive system because they produce enzymes that digest the polysaccharides in plant cell walls. When we eat plant material, some of it contributes to the fibre in our diet, which is good for a healthy colon, but without the enzymes released by friendly bacteria, most of their nutritional value of vegetables would be wasted. Humans do not make the enzymes that degrade many plant structures and so need the help that bacteria supply. We are still unable to digest high-cellulose plant material such as grass. The enzymes required to do this are produced only by the bacteria that exist in the guts of ruminants such as cattle.

Some species of bacteria in the digestive system make vitamin K and vitamins from the B group – which are difficult to obtain from food and cannot be manufactured by humans. As well as their helpful effects in maintaining an optimum pH and stimulating the immune system, friendly gut bacteria may also help in breaking down drugs, hormones that are not needed any more, and environmental substances that have the potential to be cancer-causing.

As the bacteria that live in the human gut are beneficial to their host, and the bacteria enjoy a safe environment to live, the relationship that we have with these tiny organisms is described as symbiosis or mutualism.

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