The
human body contains a large number of
bacteria, most of them performing tasks that are useful or even essential to human survival. Those that are expected to be present, and that under normal circumstances do not cause disease, are termed normal
flora.It is estimated that 500 to 100000 different
species of bacteria live in the human body (Sears, 2005). Bacterial cells are much smaller than human cells, and there are about ten times as many bacteria as human
cells in the body (1000 trillion (1015) versus 100 trillion (1014); Sears, 2005). Though normal flora are found on all surfaces exposed to the environment (on the skin and eyes, in the mouth, nose,
small intestine, and
colon), the vast majority of bacteria live in the
large intestine.
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