The human microbiota
“The ecological community of commensal, symbiotic and pathogenic microorganisms that literally share our body space”. Joshua Lederberg coined the term, emphasizing the importance of the microorganisms inhabiting the human body in health and disease.
Extensive research during recent years shows the importance of the microbiota to normal physiology: they are absolutely essential for effective digestion (releasing energy) and strong immune function (protecting us from serious disease). Scientists believe they play an important role in many other aspects of our health and well-being.
Why the microbiota is essential for our health
The microbiota is a very important component of our primary defense mechanism. Our skin and mucosa are colonized by trillions of microbes collectively known as the microbiota. Despite the enormous microbial population inhabiting the human body, including variable levels of potential harmful microbes (pathogens), most of us harbor these organisms without any signs of disease.
Figure: All human epithelial surfaces are colonized with microbiota
In order to survive in close proximity to these vast numbers of microbes, our epithelial tissues have several barrier functions that limit direct exposure to these microbes. For the skin, the major barrier is the epidermis which consists of several layers of cells that prevent pathogens reaching deeper skin tissues. In addition, the dry and slightly acidic environment on the external surface of the skin is inhospitable and prevents microbial growth. For mucosal surfaces, the mucus layer forms a protective barrier that prevents microorganisms from infecting the underlying cells.
When things go wrong in the microbiota we know about it. Harmful microbes breach the protective layers and create imbalance (dysbiosis) in the skin and mucosal tissues.
A healthy microbiota is specifically important because the microbes:
• prevent colonization by pathogens (competition)
• attack pathogenic bacteria (biocides)
• synthesize and excrete vitamins (nutrients for host cells)
• stimulate the development of the immune system and the tissues and support the natural healing process
How harmful microbes affect human tissues and cause imbalance (dysbiosis)
Good microbes (commensal and symbiotic microbes) live quietly in the outer skin and mucus layers and do not penetrate to deeper skin or mucus layers. These microbes do not have the tendency to infect the epithelial cells. In contrast, most bad microbes (pathogens) have the ability to breach the mucus and skin barrier function and infect epithelial tissues. Examples of sites where microbes can adhere to human cells are the urogenital tract, the digestive tract, the respiratory tract, skin and the eyes. Imbalanced conditions where harmful microbes dominate the microbiota and are able to cause problems at the epithelial surfaces of their host are called ‘dysbiosis’.
Situations of dysbiosis or imbalance often result in an inflammatory response which can cause symptoms such as itch, a burning sensation, redness and swelling.
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When beneficial commensal microbiota are displaced by pathogens, a situation of dysbiosis occurs. The human body responds to this with an inflammatory reaction to attack the pathogens. This causes discomforts such as irritation, swelling or itch.
Conditions of dysbiosis are often treated with antimicrobials such as antibiotics or antimycotics, but this is not always the best option.