The skin surface of all animal organisms, including humans, is colonized with a skin flora of bacteria and fungi. In this context, the normal flora consists only of nonpathogenic microorganisms. As commensals or mutualisms, many bacteria or fungi have a beneficial effect on skin health.
What is the skin flora?
The skin surface of all animal organisms, including humans, is colonized with a skin flora of bacteria and fungi. Every human has a skin flora of microorganisms on its skin surface. These are apathogenic bacteria or fungi, which often even play a major role in the health of the skin and the entire organism. The normal flora of the skin also includes neutral microorganisms, which feed on the substances of the skin surface, but otherwise have no further significance. Pathogenic microorganisms have a chance only in case of weakened immune system or strongly deviating surface conditions of the skin. The skin flora is divided into resident and transient flora. While the resident skin flora describes the permanent colonization of certain microorganisms, the transient flora describes the colonization with transient bacterial or fungal species. Healthy skin also harbors opportunistic microorganisms that are normally nonpathogenic. However, they can take on pathogenic characteristics when the immune system is weakened or when the skin is injured. Normal skin flora differs from person to person and depends on age, genetic predisposition, sex, skin area, and environmental conditions.
Function and task
The skin flora has a great importance for the health of the skin and the whole organism. The normal colonization of the skin includes resident microorganisms that, as commensals or mutualisms, are essential in protecting the body from pathogens. The existing colonization of non-pathogenic bacteria or fungi prevents the invasion of pathogenic germs into the existing biotope. Certain environmental conditions have developed which only benefit the respective existing skin flora. However, the different skin areas are also populated differently. For example, different microorganisms settle near sweat glands than on dry skin areas. Sebaceous glands in turn favor lipophilic fungal and bacterial species. The PH value of the skin is in the acidic PH range of 5.4 to 5.9, which is referred to as the protective acid mantle of the skin. In this range, apathogenic microorganisms are favored. Studies have also shown that the growth of pathogenic species such as Propionibacterium acnes is inhibited under these conditions. At the same time, the pathogenic germs compete with the non-pathogenic microorganisms. Under normal conditions, the pathogenic germs cannot establish themselves. The resident permanent colonizers of the skin include Staphylococcus (coagulase-negative), Micrococcus or the Corynebacterium. Unlike Staphylococcus aureus, the coagulase-negative Staphylococcus does not produce coagulase. Coagulase is a protein complex that plays a major role in the pathogenesis of abscesses. Due to this fact, coagulase-negative Staphylococcus is not pathogenic. Micrococcus is also apathogenic and is part of the normal human skin population. Corynebacteria are also found everywhere. Many of them are harmless and colonize the skin. Colonization of the skin with these bacteria reduces the chance of pathogenic germs. Transient transient microorganisms include bacterial species such as Pseudomonas or Enterobacteria. Furthermore, fungi or viruses can also settle temporarily on the skin. Under normal conditions, these microorganisms pose no danger. However, there are also germs that are referred to as temporary residents. Although they basically belong to the transient flora, they do not form any symptoms for a long time. Only under special conditions do they become pathogenic. A classic example of this type is Staphylococcus aureus.
Diseases and symptoms
When the balance of skin flora is disturbed for any reason, pathogenic germs spread with different symptoms. Already with change in the hormonal system during puberty, changes occur in the milieu of the skin. Especially boys often suffer from acne during this time. One reason for this is the spread of the bacterium Propionibacterium acnes. The bacterium Staphylococcus aureus often triggers folliculitis.This is an inflammation of the outer part of a hair follicle. This condition occurs particularly in hairy areas and is promoted by frequent sweating. Under certain circumstances, an abscess can develop, which must be surgically removed. Folliculitis can also be caused by the yeast Candida albicans. In this case, however, there is usually a serious immune weakening disease. The change of the PH value to higher values damages the acid mantle of the skin. Various pathogenic microorganisms cannot tolerate the normal acidic PH of the skin and are inhibited from growing. In addition, some enzymes involved in building a skin barrier function only in this PH range. The loss of this acidic protective layer and skin barrier is often the starting point for infectious skin diseases. Thus, excessive skin hygiene through frequent hand washing and showering with alkaline soaps can destroy the protective acid mantle of the skin. In addition, this also degreases the skin and thus promotes the penetration of pathogenic germs. Increased moisture of the skin due to increased perspiration sometimes promotes peptostreptococci, which may be the cause of sweat gland abscesses. This frequently affects armpits, interdigital spaces, groins or the anal crease. However, bacterial skin diseases or fungal diseases of the skin and mucosa can also be the result of serious underlying diseases. Thus, non-infectious skin diseases such as eczema or psoriasis can also be the starting point for infectious skin diseases. Immune-weakening diseases such as diabetes, cancer, or AIDS, as well as therapies such as chemotherapy or antibiotic treatment, can also destroy the normal skin flora.
