The Normal Bacterial Flora of Humans

The Normal Bacterial Flora of Humans

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  • Date Submitted: 04/26/2013 12:24 PM
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iaThe Normal Bacterial Flora of Humans (page 1)

(This chapter has 5 pages)

© 2008 Kenneth Todar, PhD

The Normal Flora

In a healthy animal, the internal tissues, e.g. blood, brain, muscle, etc., are normally free of microorganisms. However, the surface tissues, i.e., skin and mucous membranes, are constantly in contact with environmental organisms and become readily colonized by various microbial species. The mixture of organisms regularly found at any anatomical site is referred to as the normal flora, except by researchers in the field who prefer the term "indigenous microbiota". The normal flora of humans consists of a few eucaryotic fungi and protists, but bacteria are the most numerous and obvious microbial components of the normal flora.

Figure 1. Gram stain of a species of Micrococcus, commonly isolated from the skin and nasal membranes of humans.

The predominant bacterial flora of humans are shown in Table 1. This table lists only a fraction of the total bacterial species that occur as normal flora of humans. A recent experiment that used 16S RNA probes to survey the diversity of bacteria in dental plaque revealed that only one percent of the total species found have ever been cultivated. Similar  observations have been made with the intestinal flora.  Also, this table does not indicate the relative number or concentration of bacteria at a particular site. If you are reading online, you can skip this table and use it as an ongoing reference. To continue this article, scroll to the bottom of the Table notes to Associations Between Humans and the Normal Flora

Table 1. Bacteria commonly found on the surfaces of the human body.
BACTERIUM | Skin | Con-
tiva | Nose | Pharynx | Mouth | Lower
GI | Ant. ure-
thra | Vagina |
Staphylococcus epidermidis (1) | ++ | + | ++ | ++ | ++ | + | ++ | ++ |
Staphylococcus aureus*  (2) | +  | +/- | + | + | + | ++ | +/- | + |
Streptococcus mitis  | | | | + | ++  | +/-  | + | + |...

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