Human microbial flora

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,or microbiota. An effort to better describe the microflora of humans has been initiated; see Human microbiome project.
Bacterial flora
It is estimated that 500 to 100,000 species of bacteria live in the human body . 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.
Escherichia coli (a.k.a. E. coli) is a bacterium that lives in the colon; it is an extensively studied model organism and probably the best understood bacterium of all Certain mutated strains of these gut bacteria do 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, 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.
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.
Human bacterial flora and human health
Bacteria are vital for the maintenance of human health, but some pathogenic bacteria also pose a significant health threat by causing diseases. Large numbers of bacteria live on the skin and in the digestive tract. Their growth can be increased by warmth and sweat. Large populations of these organisms on humans are the cause of body odor and thought to play a part in acne. There are more than 500 bacterial species present in the normal human gut and are generally beneficial: they synthesize vitamins such as folic acid, vitamin K and biotin, and they ferment complex indigestible carbohydrates. Other beneficial bacteria in the normal flora include Lactobacillus species, which convert lactose and other sugars to lactic acid in the gut.[6] The presence of such bacterial colonies also inhibits the growth of potentially pathogenic bacteria (usually through competitive exclusion) and some beneficial bacteria are consequently sold as probiotic dietary supplements.Archaea are present in the human gut, but in contrast to the enormous variety of bacteria in this organ, the number of archaeal species are much more limited.The dominant group is the methanogens, particularly Methanobrevibacter smithii and Methanosphaera stadtmanae. However, colonization by methanogens is variable and only about 50% of humans have easily-detectable populations or these organisms.
Archaea and human healthAs of 2007, no clear examples of archaeal pathogens are known, although a relationship has been proposed between the presence of some methanogens and human periodontal disease.
Fungal floraFungi, particularly yeasts are present in the human gut. The best studied of these are Candida species, due to their ability to become pathogenic in immunocompromised hosts.[14] Yeasts are also present on the skin, particularly Malassezia species, where they consume oils secreted from the sebaceous glands.