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Skin flora, also called skin microbiota, refers to microbiota (communities of ) that reside on the skin, typically human skin. Many of them are bacterium of which there are around 1,000 species upon human skin from nineteen Phylum. Most are found in the superficial layers of the epidermis and the upper parts of .
Skin flora is usually non-pathogenic, and either commensalism (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. However, resident microbes can cause and enter the blood system, creating life-threatening diseases, particularly in immunosuppressed people.
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.
Staphylococcus epidermidis and Staphylococcus aureus were thought from cultural based research to be dominant. However 16S ribosomal RNA research finds that while common, these species make up only 5% of skin bacteria. However, skin variety provides a rich and diverse habitat for bacteria. Most come from four phyla: Actinomycetota (51.8%), Bacillota (24.4%), Pseudomonadota (16.5%), and Bacteroidota (6.3%).
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 , the spaces between , axillae, and umbilical cord stump. Most similarly were beside the nostril, nares (inside the nostril), and on the back.
| + Frequency of the best studied skin microbes | ||
| Staphylococcus epidermidis | Common | occasionally pathogenic |
| Staphylococcus aureus | Infrequent | usually pathogenic |
| Staphylococcus warneri | Infrequent | occasionally pathogenic |
| Streptococcus pyogenes | Infrequent | usually pathogenic |
| Streptococcus mitis | Frequent | occasionally pathogenic |
| Cutibacterium acnes | Frequent | occasionally pathogenic |
| Corynebacterium spp. | Frequent | occasionally pathogenic |
| Acinetobacter johnsonii | Frequent | occasionally pathogenic |
| Pseudomonas aeruginosa | Infrequent | occasionally pathogenic |
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.
The Belly Button Biodiversity Project began at North Carolina State University in early 2011 with two initial groups of 35 and 25 volunteers. Volunteers were given sterile cotton swabs and were asked to insert the cotton swabs into their navels, to turn the cotton swab around three times and then return the cotton swab to the researchers in a vial that contained a 0.5 ml 10% phosphate saline buffer. Researchers at North Carolina State University, led by Jiri Hulcr, 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). These samples then were analyzed using 16S rDNA libraries so that strains that did not grow well in cultures could be identified.
The researchers at North Carolina State University discovered that while it was difficult to predict every strain of bacteria in the microbiome of the navel that they could predict which strains would be prevalent and which strains of bacteria would be quite rare in the microbiome. It was found that the navel microbiomes only contained a few prevalent types of bacteria ( Staphylococcus, Corynebacterium, Actinobacteria, Clostridiales, and Bacilli) and many different types of rare bacteria. Other types of rare organisms were discovered inside the navels of the volunteers including three types of Archaea, two of which were found in one volunteer who claimed not to have bathed or showered for many years.
Staphylococcus and Corynebacterium were among the most common types of bacteria found in the navels of this project's volunteers and these types of bacteria have been found to be the most common types of bacteria found on the human skin in larger studies of the skin microbiome (of which the Belly Button Biodiversity Project is a part). (In these larger studies it has been found that females generally have more Staphylococcus living in their skin microbiomes (usually Staphylococcus epidermidis) and that men have more Corynebacterium living in their skin microbiomes.)
According to the Belly Button Biodiversity Project at North Carolina State University, there are two types of microorganisms found in the navel and surrounding areas. Transient bacteria (bacteria that does not reproduce) forms the majority of the organisms found in the navel, and an estimated 1400 various strains were found in 95% of participants of the study.
The Belly Button Biodiversity Project is ongoing and has now taken swabs from over 500 people. The project was designed with the aim of countering that misconception that bacteria are always harmful to humans and that humans are at war with bacteria. In actuality, most strains of bacteria are harmless if not beneficial for the human body. Another of the project's goals is to foster public interest in microbiology. Working in concert with the Human Microbiome Project, the Belly Button Biodiversity Project also studies the connections between human microbiomes and the factors of age, sex, ethnicity, location and overall health.
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:
Current treatment includes topical and systemic antibacterial drugs which result in decreased C. acnes colonisation and/or activity. 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.
Current treatments include combinations of topical or systemic antibiotics, corticosteroids, and diluted bleach baths. 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.
Low Gut flora 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. (See gut microbiota)
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.
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. Populations of S. epidermidis have also been isolated from pustules of rosacea patients. However it is possible that they were moved by Demodex to areas that favour growth as Demodex has shown to transport bacteria around the face.
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.
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.
| + Skin flora upon two hospital groups in colony-forming units per millilitre. | ||
| Health care workers healthy | 3.47 | 3.15 |
| Health care workers damaged | 3.33 | 3.29 |
| Students/technicians healthy | 4.39 | 3.54 |
| Students/technicians damaged | 4.58 | 4.43 |
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.
One study of soap in dentist clinics found they all had their own flora and on average from two to five different genera of microorganisms with those used most more likely to have more species varieties. 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.
Hand washing can damage skin because the stratum corneum top layer of skin consists of 15 to 20 layers of keratin disks, , each of which is each surrounded by a thin film of skin which can be removed by alcohols and .
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.
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