Bacteria in the human body

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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 (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.

Many of the bacteria in the digestive tract, collectively referred to as gut flora, are able to break down certain nutrients such as carbohydrates that humans otherwise could not digest. The majority of these commensal bacteria are anaerobes, meaning they survive in an environment with no oxygen.

Many of the bacteria of the normal flora can act as opportunistic pathogens at times of lowered immunity.

Escherichia coli is a bacterium that lives in the colon; it is an extensively studied model organism and probably the best understood organism of all.

Certain mutated strands of these gut bacteria can cause disease; an example is E. 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 consist mostly of various lactobacillus species. It was long thought that the most common of these species was Lactobacillus acidophilus, 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.