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Normal Flora

Typically, when one says "I have an infection" they mean to say "I have a disease", however the latter is not quite so socially acceptable. In fact, we are all "infected" with a variety of microorganisms throughout our entire lives. Incredibly, our bodies are actually composed of more bacterial cells than human cells; while the human body is made up of about 1013 human cells, we harbor near 1014 bacteria. This group of organisms, traditionally referred to as "normal flora" (although they are not plants) is composed of a fairly stable set of genera, mostly anaerobes. While each person has a relatively unique set of normal flora, members of the Streptococcus and Bacteroides make up a large percentage of the inhabitants. These organisms contribute to our existence in several ways. These normal flora may:

  • Help us by competing with pathogens such as Salmonella
  • Help us by providing vitamins or eliminating toxins (e.g. Bacteroides)
  • Harm us by promoting disease (e.g. dental caries)
  • Cause neither help nor harm (e.g. "commensals").
One of the most important functions of our normal flora is to protect us from highly pathogenic organisms. For example, in a normal (bacterially inhabited animal), about 106 Salmonella must be ingested in order to cause disease. However, when an animal has been maintained in a sterile environment all of its life (a "gnotobiotic" animal), the same level of disease can be produced by as few as 10 Salmonella. This dramatic difference is simply due to competition.

To a microorganism, the human body seems very much like the planet Earth seems to us. Just like our planet, our bodies contain numerous different environments, ranging from dry deserts (e.g. the forearm) to tropical forests (e.g. the perineum) to extremely hostile regions (e.g. the intestinal tract). Each environment possesses certain advantages and disadvantages and different microorganisms have adapted to certain regions of the body for their particular needs. This page will examine these regions and describe the types of microorganisms found in each. You may review these regions by clicking on the human body "map" shown below.

Human Body Map
Body Site
Predominant/Important Genera

Skin Flora
The surface of the skin itself comprises several distinct environments. Areas such as the axilla (armpit), the perineum (groin) and the toe webs provide typically moister regions for bacterial growth. These "tropical forest" environments often harbor the largest diversity amongst the skin flora. Typical organisms include Staphylococcus aureus, Corynebacterium and some Gram-negative bacteria. The bulk of the human skin surface, however, is much drier and is predominantly inhabited by Staphylococcus epidermidis and Propionobacterium.

Oral Cavity and Nasopharyngeal Flora
Streptococci predominate in the oral cavity and nasopharyngeal regions but one can also find other anaerobes and species of Neisseria. Many potential pathogens may also be found in the nasopharynx of a healthy individual, providing a reservoir for infection of others. These pathogens include Streptococcus pneumoniae, Neisseria meningitidis and Haemophilus influenzae.

Intestinal Flora
The intestinal tract is a rather hostile environment for microorganisms yet the bulk of our normal flora inhabit this region of the body. In fact, the colon may contain 109 to 1011 bacteria per gram of material. Most (95 - 99.9%) of these are anaerobes, represented by Bacteroides, Bifidobacterium, anaerobic streptococci and Clostridium. These organisms inhibit the growth of other pathogens but some can be opportunistic (e.g. C. difficile can produce pseudomembranous colitis).

Urogenital Flora
The urogenital tract is normally sterile with the exception of the vagina and the distal 1 cm of the urethra. Various members of the genus Lactobacillus predominate in the vagina. These organisms generally lower the pH to around 4-5, which is optimal for the lactobacilli but inhibitory for the growth of many other bacteria. Loss of this protective effect by antibiotic therapy can lead to infection by Candida ("yeast infection"). The urethra may contain predominantly skin microorganisms including staphylococci, streptococci and diphtheroids.

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