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 1000 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 (1 quadrillion (1015) versus 100 trilli ...

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