Skin flora
Skin flora, also called skin microbiota, refers to microbiota (communities of microorganisms) that reside on the skin, typically human skin.
Many of them are bacteria of which there are around 1,000 species upon human skin from nineteen phyla.[1][2] Most are found in the superficial layers of the epidermis and the upper parts of hair follicles.
Skin flora is usually non-pathogenic, and either commensal (are not harmful to their host) or mutualistic (offer a benefit). The benefits bacteria can offer include preventing transient pathogenic organisms from colonizing the skin surface, either by competing for nutrients, secreting chemicals against them, or stimulating the skin's immune system.[3] However, resident microbes can cause skin diseases and enter the blood system, creating life-threatening diseases, particularly in immunosuppressed people.[3]
A major non-human skin flora is Batrachochytrium dendrobatidis, a chytrid and non-hyphal zoosporic fungus that causes chytridiomycosis, an infectious disease thought to be responsible for the decline in amphibian populations.[4]
Skin microflora can be commensals, mutualistic or pathogens. Often they can be all three depending upon the strength of the person's immune system.[3] Research upon the immune system in the gut and lungs has shown that microflora aids immunity development: however such research has only started upon whether this is the case with the skin.[3] Pseudomonas aeruginosa is an example of a mutualistic bacterium that can turn into a pathogen and cause disease: if it gains entry into the circulatory system it can result in infections in bone, joint, gastrointestinal, and respiratory systems. It can also cause dermatitis. However, P. aeruginosa produces antimicrobial substances such as pseudomonic acid (that are exploited commercially such as Mupirocin). This works against staphylococcal and streptococcal infections. P. aeruginosa also produces substances that inhibit the growth of fungus species such as Candida krusei, Candida albicans, Torulopsis glabrata, Saccharomyces cerevisiae and Aspergillus fumigatus.[24] It can also inhibit the growth of Helicobacter pylori.[25] So important is its antimicrobial actions that it has been noted that "removing P. aeruginosa from the skin, through use of oral or topical antibiotics, may inversely allow for aberrant yeast colonization and infection."[3]
Another aspect of bacteria is the generation of body odor. Sweat is odorless however several bacteria may consume it and create byproducts which may be considered putrid by humans (as in contrast to flies, for example, that may find them attractive/appealing).
Several examples are:
Skin defenses[edit]
Antimicrobial peptides[edit]
The skin creates antimicrobial peptides such as cathelicidins that control the proliferation of skin microbes. Cathelicidins not only reduce microbe numbers directly but also cause the secretion of cytokine release which induces inflammation, angiogenesis, and reepithelialization. Conditions such as atopic dermatitis have been linked to the suppression in cathelicidin production.[29] In rosacea abnormal processing of cathelicidin cause inflammation. Psoriasis has been linked to self-DNA created from cathelicidin peptides that causes autoinflammation. A major factor controlling cathelicidin is vitamin D3.[30]
Acidity[edit]
The superficial layers of the skin are naturally acidic (pH 4–4.5) due to lactic acid in sweat and produced by skin bacteria.[31] At this pH mutualistic flora such as Staphylococci, Micrococci, Corynebacterium and Propionibacteria grow but not transient bacteria such as Gram-negative bacteria like Escherichia and Pseudomonas or Gram positive ones such as Staphylococcus aureus.[31] Another factor affecting the growth of pathological bacteria is that the antimicrobial substances secreted by the skin are enhanced in acidic conditions.[31] In alkaline conditions, bacteria cease to be attached to the skin and are more readily shed. It has been observed that the skin also swells under alkaline conditions and opens up allowing bacterial movement to the surface.[31]
Immune system[edit]
If activated, the immune system in the skin produces cell-mediated immunity against microbes such as dermatophytes (skin fungi).[32] One reaction is to increase stratum corneum turnover and so shed the fungus from the skin surface. Skin fungi such as Trichophyton rubrum have evolved to create substances that limit the immune response to them.[32] The shedding of skin is a general means to control the buildup of flora upon the skin surface.[33]
Comparison with other flora[edit]
The skin flora is different from that of the gut which is predominantly Bacillota and Bacteroidota.[63] There is also low level of variation between people that is not found in gut studies.[5] Both gut and skin flora however lack the diversity found in soil flora.[1]