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.
Overall, there are about ten times as many bacteria as human cells in the body, 1 quadrillion (1015) versus 100 trillion (1014), with bacterial cells being much smaller than human cells. 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. It is estimated that 500-1000 different species of bacteria live in the human body. Many of the bacteria of the normal flora can act as opportunistic pathogens at times of lowered immunity.
Many of the bacteria in the digestive tract are able to break down certain nutrients (often carbohydrates) that humans otherwise couldn't digest. The majority of these commensal bacteria are anaerobes.
E. coli is a bacterium that lives in the colon; it is an extensively studied model organism and probably the best understood organism of all (if one disregards viruses).
Certain mutated strands of these gut bacteria can cause disease; an example is Escherichia coli O157:H7
A number of types of bacteria, such as Actinomyces viscosus and A. naeslundii, live in the mouth, and constitute a sticky substance called plaque. If this is not removed by brushing, it hardens into calculus (also called tartar). The same bacteria also secrete acids that dissolve the tooth enamel, causing tooth decay.
The vaginal microflora consists mostly of various lactobacilli species. It was long thought that Lactobacillus acidophilus was the most common, but it has later been shown that the most common one is L. iners followed by L. crispatus. Other lactobacilli found in the vagina are L. delbruekii and L. gasseri. Disturbance of the vaginal flora can lead to bacterial vaginosis.
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