Human Normal Flora

It may or may not surprise you to find out that your body is host to billions of bacteria of many different kinds. These bacteria exist in many different parts of the body, and their presence usually does not cause any problems for the host body. The following is a list of the main sites for bacteria that consitute the normal flora.

The normal flora of the digestive tract

The stomach contains few bacteria because of its high acidity. Still there are some bacteria that survive there. Perhaps the most important of these is a bacterium whose human interaction was discovered in the 1980's, Helicobacter Pylori, now proven to be the cause of most (greater than 95%) cases of gastritis and peptic ulcers.

The small intestine usually contains small numbers of Streptococci, Lactobacilli, and yeasts, particularly Candida Albicans. Larger numbers of these bacteria are found in the terminal ileum, the section of the small intestine just before the colon.

The large intestine, or colon, is the main site for bacteria in the body. Roughly twenty percent of the faeces of a normal person consists of bacteria, most of which have come from the colon. The main bacteria in the colon are Bacteroides, Bifidobacterium, Eubacterium, Coliforms (e.g. E. coli), Streptococcus, Lactobacillus and Clostridium.

There are many different types of relationship that the body can have with the normal flora. These are

The relationships between the human host and most normal flora usually fall under the the category of Mutualist relationships. The benefit to the bacteria is that they have a place to survive and multiply. The benefits to the human host are as follows

Are the normal flora always beneficial?

The answer to this question is: definitely not! If they remain in the site with which they are usually associated, the normal flora are usually beneficial. However, some members of the normal flora are also opportunistic pathogens, or are pathogenic if they turn up at a site with which they are not normally associated. For example, Bacteroides bacteria, which normally reside in the intestines, may produce abscesses if they penetrate into deeper tissues via traumatic or surgical wounds. E. coli, a normal inhabitant of the gastrointestinal tract, is the most common cause of urinary tract infections.

Why do the normal flora not induce an immune response from the host?

The short answer to this question is: the normal flora do induce an immune response. Antibodies to the normal flora exist in our bodies, but at lower concentrations than would exist for pathogenic bacteria. They provide a "sparring partner" for the human body that keeps the immune system in tune. The precise role that the human immune system takes in regulating the populations of the normal flora is not known.