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( Human flora)
The human flora is the microrganisms that constantly inhabit the human body. They include bacteria, fungi and archaea. Some of these organisms are known to perform tasks that are useful for the human host, while the majority have no beneficial or harmful effect. Those that are expected to be present, and that under normal circumstances do not cause disease, are termed normal flora,[1] or microbiota. An effort to better describe the microflora of humans has been initiated; see Human microbiome project. It is estimated that 500 to 100,000 species of bacteria live in the human body [2]. 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 or 1 quadrillion (1015) versus 100 trillion (1014)).[2] 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. Bacteria of the normal flora can act as opportunistic pathogens at times of lowered immunity. [1] A number of types of bacteria, such as Actinomyces viscosus and A. naeslundii, live in the mouth, where they are part of 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 tooth enamel, causing tooth decay.
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