Bacteria in the human body
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. Most of the bacteria live in the mouth, the small intestine, the colon, and on the skin. It is estimated that 500-1000 different species of bacteria live in the human body. 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.