Skin flora

Bacteria image of Staphylococcus epidermidis, one of roughly a thousand bacteria species present on human skin. Though usually not pathogenic, it can cause skin infections and even life-threatening illnesses in those that are immunocompromised. The estimate of the number of bacteria species present on skin has been radically changed by the use of 16S ribosomal RNA to identify bacterial species present on skin samples direct from their genetic material. Previously such identification had depended upon microbiological culture upon which many varieties of bacteria did not grow and so were hidden to science. There are three main ecological areas: sebaceous, moist, and dry. Propionibacteria and Staphylococci species were the main species in sebaceous areas. In moist places on the body Corynebacteria together with Staphylococci dominate. In dry areas, there is a mixture of species but Betaproteobacteria and Flavobacteriales are dominant. Ecologically, sebaceous areas had greater species richness than moist and dry ones. The areas with least similarity between people in species were the spaces between fingers, the spaces between toes, axillae, and umbilical cord stump. Most similarly were beside the nostril, nares (inside the nostril), and on the back. A study by the National Human Genome Research Institute in Bethesda, Maryland, researched the DNA of human skin fungi at 14 different locations on the body. These were the ear canal, between the eyebrows, the back of the head, behind the ear, the heel, toenails, between the toes, forearm, back, groin, nostrils, chest, palm, and the crook of the elbow. The study showed a large fungal diversity across the body, the richest habitat being the heel, which hosts about 80 species of fungi. By way of contrast, there are some 60 species in toenail clippings and 40 between the toes. Other rich areas are the palm, forearm and inside the elbow, with from 18 to 32 species. The head and the trunk hosted between 2 and 10 each. Umbilical microbiome The umbilicus, or navel, is an area of the body that is rarely exposed to UV light, soaps, or bodily secretions that contained a 0.5 ml 10% phosphate saline buffer. then grew the samples in a culture until the bacterial colonies were large enough to be photographed and then these pictures were posted on the Belly Button Biodiversity Project's website (volunteers were given sample numbers so that they could view their own samples online). (of which the Belly Button Biodiversity Project is a part). The Belly Button Biodiversity Project is ongoing and has now taken swabs from over 500 people. and that humans are at war with bacteria. In actuality, most strains of bacteria are harmless Another of the project's goals is to foster public interest in microbiology. == Relationship to host ==

Skin microflora can be commensals, mutualistic or pathogens. Often they can be all three depending upon the strength of the person's immune system. It can also inhibit the growth of Helicobacter pylori. 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." • Staphylococcus epidermidis creates body odor by breaking sweat into isovaleric acid (3-methyl butanoic acid). • Bacillus subtilis creates strong foot odor. ==Skin defenses==

Antimicrobial peptides 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. 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. Acidity The superficial layers of the skin are naturally acidic (pH 4–4.5) due to lactic acid in sweat and produced by skin bacteria. 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. 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. ==Skin diseases==

Microorganisms play a role in noninfectious skin diseases such as atopic dermatitis, rosacea, psoriasis, and acne Damaged skin can cause nonpathogenic bacteria to become pathogenic. The diversity of species on the skin is related to later development of dermatitis. Acne vulgaris Acne vulgaris is a common skin condition characterised by excessive sebum production by the pilosebaceous unit and inflammation of the skin. Affected areas are typically colonised by Cutibacterium acnes; a member of the commensal microbiota even in those without acne. High populations of C. acnes are linked to acne vulgaris although only certain strains are strongly associated with acne while others with healthy skin. The relative population of C. acnes is similar between those with acne and those without. Potential probiotic treatment includes the use of Staphylococcus epidermidis to inhibit C. acnes growth. S. epidermidis produces succinic acid which has been shown to inhibit C. acnes growth. Lactobacillus plantarum has also been shown to act as an anti-inflammatory and improve antimicrobial properties of the skin when applied topically. It was also shown to be effective in reducing acne lesion size. Atopic dermatitis Individuals with atopic dermatitis have shown an increase in populations of Staphylococcus aureus in both lesional and nonlesional skin. Potential probiotic treatments include using the commensal skin bacteria, S. epidermidis, to inhibit S. aureus growth. During atopic dermatitis flares, population levels of S. epidermidis has been shown to increase as an attempt to control S. aureus populations. Regular use of moisturisers (emollients) helps maintain skin hydration and barrier integrity, which may reduce irritation, allergen penetration, and disease flares. Various emollient types, such as lotions, creams, and ointments, are similarly effective, with choice guided by individual preference and skin condition. Low gut microbial diversity in babies has been associated with an increased risk of atopic dermatitis. Infants with atopic eczema have low levels of Bacteroides and high levels of Bacillota. Bacteroides have anti-inflammatory properties which are essential against dermatitis. While another study using biopsies associate increased levels of Bacillota and Actinomycetota with healthy skin. However most studies show that individuals affected by psoriasis have a lower microbial diversity in the affected areas. Treatments for psoriasis include topical agents, phototherapy, and systemic agents. Current research on the skin microbiota's role in psoriasis is inconsistent therefore there are no potential probiotic treatments. Rosacea Rosacea is typically connected to sebaceous sites of the skin. The skin mite Demodex folliculorum produce lipases that allow them to use sebum as a source of food therefore they have a high affinity for sebaceous skin sites. Although it is a part of the commensal skin microbiota, patients affected with rosacea show an increase in D. folliculorum compared to healthy individuals, suggesting pathogenicity. Bacillus oleronius, a Demodex associated microbe, is not typically found in the commensal skin microbiota but initiates inflammatory pathways whose starting mechanism is similar to rosacea patients. Current treatments include topical and oral antibiotics and laser therapy. As current research has yet to show a clear mechanism for Demodex influence in rosacea, there are no potential probiotic treatments. ==Clinical==

Infected devices Skin microbes are a potential source of infected medical devices such as catheters. ==Hygiene==

The human skin is host to numerous bacterial and fungal species, some of which are known to be harmful, some known to be beneficial and the vast majority unresearched. The use of bactericidal and fungicidal soaps will inevitably lead to bacterial and fungal populations which are resistant to the chemicals employed (see drug resistance). Contagion Skin flora do not readily pass between people: 30 seconds of moderate friction and dry hand contact results in a transfer of only 0.07% of natural hand flora from naked with a greater percentage from gloves. Removal The most effective (60–80% reduction) antimicrobial washing is with ethanol, isopropanol, and n-propanol. Viruses are most affected by high (95%) concentrations of ethanol, while bacteria are more affected by n-propanol. Unmedicated soaps are not very effective as illustrated by the following data. Health care workers washed their hands once in nonmedicated liquid soap for 30 seconds. The students/technicians for 20 times. ::: An important use of hand washing is to prevent the transmission of antibiotic resistant skin flora that cause hospital-acquired infections such as methicillin-resistant Staphylococcus aureus. While such flora have become antibiotic resistant due to antibiotics there is no evidence that recommended antiseptics or disinfectants selects for antibiotic-resistant organisms when used in hand washing. However, many strains of organisms are resistant to some of the substances used in antibacterial soaps such as triclosan. Another study of bar soaps in public toilets found even more flora. Another study found that very dry soaps are not colonized while all are that rest in pools of water. However, one experiment using soaps inoculated with Pseudomonas aeruginosa and Escherichia coli that washing with inoculated bar soap did not transmit these bacteria to participants hands. Damaged skin Washing skin repeatedly can damage the protective external layer and cause transepidermal loss of water. This can be seen in roughness characterized by scaling and dryness, itchiness, dermatitis provoked by microorganisms and allergens penetrating the corneal layer and redness. Wearing gloves can cause further problems since it produces a humid environment favoring the growth of microbes and also contains irritants such as latex and talcum powder. Hand washing can damage skin because the stratum corneum top layer of skin consists of 15 to 20 layers of keratin disks, corneocytes, each of which is each surrounded by a thin film of skin lipids which can be removed by alcohols and detergents. Damaged skin defined by extensive cracking of skin surface, widespread reddening or occasional bleeding has also been found to be more frequently colonized by Staphylococcus hominis and these were more likely to be methicillin resistant. Though not related to greater antibiotic resistance, damaged skin was also more like to be colonized by Staphylococcus aureus, gram-negative bacteria, Enterococci and Candida. ==Comparison with other flora==

The skin flora is different from that of the gut which is predominantly Bacillota and Bacteroidota. There is also low level of variation between people that is not found in gut studies. Both gut and skin flora however lack the diversity found in soil flora. ==See also==