Microbial flora of the stomach and gastrointestinal tract

The stomach and gastrointestinal tract are not sterile and are colonized by microorganisms that perform functions beneficial to the host, including the manufacture of essential vitamins, and the prevention of colonization by undesirable microbes.

The benefits of the close relationship between the microorganisms and the host also extends to the microbes. Microorganisms are provided with a protected place to live and their environment—rich in nutrients—and is relatively free from predators.

This mutually beneficial association is always present. At human birth, the stomach and gastrointestinal tract are usually sterile. But, with the first intake of food, colonization by bacteria commences. For example, in breast-fed babies, most of the intestinal flora consists of bacteria known as bifidobacteria. As breast milk gives way to bottled milk, the intestinal flora changes to include enteric bacteria, bacteroides, enterococci, lactobacilli, and clostridia.

The flora of the gastrointestinal tract in animals has been studied intensively. These studies have demonstrated that bacteria are the most numerous microbes present in the stomach and gastrointestinal tract. The composition of the bacterial populations varies from animal to animal, even within a species. Sometimes the diet of an animal can select for the dominance of one or a few bacteria over other species. The situation is similar in humans. Other factors that influence the bacterial make up of the human stomach and gastrointestinal tract include age, cultural conditions, and the use of antibiotics . In particular, the use of antibiotics can greatly change the composition of the gastrointestinal flora.

Despite the variation in bacterial flora, the following bacteria tend to be present in the gastrointestinal tract of humans and many animals: Escherichia coli , Clostridium perfringens, Enterococci, Lactobacilli, and Bacteroides.

The esophagus is considered to be part of the gastrointestinal tract. In this region, the bacteria present are usually those that have been swallowed with the food. These bacteria do not normally survive the journey through the highly acidic stomach. Only bacteria that can tolerate strongly acidic environments are able to survive in the stomach. One bacterium that has been shown to be present in the stomach of many people is Helicobacter pylori. This bacterium is now known to be the leading cause of stomach ulcers. In addition, very convincing evidence is mounting that links the bacterium to the development of stomach and intestinal cancers.

In humans, the small intestine contains low numbers of bacteria, some 100,000 to 10 million bacteria per milliliter of fluid. To put these numbers into perspective, a laboratory liquid culture that has attained maximum bacterial numbers will contain 100 million to one billion bacteria per milliliter. The bacterial flora of this region consists mostly of lactobacilli and Enterococcus faecalis. The lower regions of the small intestine contain more bacteria and a wider variety of species, including coliform bacteria such as Escherichia coli.

In the large intestine, the bacterial numbers can reach 100 billion per milliliter of fluid. The predominant species are anaerobic bacteria, which do not grow in the presence of oxygen. These include anaerobic lactic acid bacteria , Bacteroides, and Bifidobacterium bifidum. The bacteria numbers and composition in the large intestine is effectively that of fecal material.

The massive numbers of bacteria in the large intestine creates a great special variation in the flora. Sampling the intestinal wall at different locations will reveal differences in the species of bacteria present. As well, sampling any given point in the intestine will reveal differences in the bacterial population at various depths in the adherent growth on the intestinal wall.

Some bacteria specifically associate with certain cells in the gastrointestinal tract. Gram-positive bacteria such as streptococci and lactobacilli often adhere to cells by means of capsules surrounding the bacteria. Gram-negative bacteria such as Escherichia coli can adhere to receptors on the intestinal epithelial cells by means of the bacterial appendage called fimbriae.

The importance of the microbial flora of the stomach and gastrointestinal tract has been demonstrated by comparison of the structure and function of the digestive tracts of normal animals and notobiotic animals. The latter animals lack bacteria. The altered structure of the intestinal tract in the notobiotic animals is less efficient in terms of processing food and absorbing nutrients. Additionally, in animals like cows that consume cellulose, the fermentation activity of intestinal microorganisms is vital to digestion. Thus, the flora of the stomach and intestinal tract is very important to the health of animals including humans.

See also Enterobacteriaceae; Probiotics; Salmonella food poisoning