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 doues 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 skin, especially the moist areas, such as the groin and between the toes.
The respiratory tract, particularly the nose.
Urinary tract.
The digestive tract, i.e. the mouth, the terminal ileum and the colon.
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

Mutualism. In a mutualist relationship, both the host and the microbe benefit from the relationship. A good example if this is E.Coli. These organisms live in the intestines, where they receive nourishment, and in turn produce Vitamin K, which the human body requires for the process of blood clotting.
Commensalism. A commensalist relationship is where one partner of the relationship benefits, and the other partner is neither benefitted nor harmed.
Parasitism. A parasitic relationship is where one organism benefits at the expense of the host. The cost to the host can vary from slight to fatal. An external parasite (ectoparasite) is said to cause infestation, an internal parasite (endoparasite) is said to cause infection.
Pathogenic. A pathogenic relationship is where an organism causes damage to the host during infection. An Opportunistic Pathogen causes disease in a host that is physically impaired or debilitated. Normally the opportunist organism is harmless, but it takes advantage when the hosts defences are impaired, for example when the normal flora have been destroyed by antibiotics, or when the immune system has been suppressed by drug treatment or by other illnesses.
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

The hosts ability to nourish itself is increased. The bacteria may produce vitamins (such as B and K), and may break down food stuffs that are normally indigestible by the host into components that can be digested.
The host is protected against infection by pathogenic organisms. This happens in several ways.
The normal flora pre-occupy the favorable ecological niches for bacteria, e.g. the intestinal villi, thus presenting the invading pathogen with the problem of finding somewhere to anchor itself.
The normal flora may out-compete the invader for available nutrients, thus starving the invader and preventing it from multiplying.
Some members of the normal flora produce anti-bacterial chemicals (bacteriocins) as a side product of their metabolism, thus generating a local antibiotic effect which hinders the invader.
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.