Skin is the outermost covering of the human body. It is the largest organ with an estimated area of 20 square feet. It forms the first line of defense, protecting the body against pathogens, regulates the body temperature and enables the sensation of heat, touch and cold. It is divided into three layers: epidermis– upermost waterproof protective layer, followed by dermis-consists of connective tissues, hair follicles and sweat glands, followed by subcutaneous layer– lowermost layer composed of connective tissues. The pH and temperature of the skin depends on the area of the body it covers. In general, the pH ranges from 5.6 to 6.4 and temperature from 25 to 35 degree Celsius.
The microbes residing on the skin constitute the skin microbiota. In general, the microbes are found all over the body but dermatologists have classified the three anatomical sites on the skin for their microbiological differences.These regions are distinguised based on the pH, temperature , moisture and concentration of skin lipids. The regions are:
- The Dry site: Includes forearm, legs, hands and feet.Since the area is exposed to the outer environment, it has most diverse microbial population.
- The Moist site: Includes axilla, toe webs, groins and beneath the breast areas.
- The Oily site: Includes head, neck and face. This area has sebaceous glands that secret an oily susbstance called sebum.
The oily site is more species rich than the dry and moist sites.The microflora inhabiting the skin include both bacteria and fungi e.g.Staphylococcus, Proprionibacteria, Cornybacterium, Malasezzia, Dermabacter and Micrococcus.
The Bacterial Inhabitants
According to a research the human skin is a home to 113 phylotypes that belong to six bacterial divisions. Some of the common species found are described below.
- Staphylococcus epidermidis : It is a Gram-positive bacterium, non-motile and facultative anaerobe found around sebaceous glands and moist areas. In general , it is not pathogenic but in immunosuppresed individuals it causes infections. The infections acquired are mostly nosocomial. It forms biofilms on catheters and other surgical implants implanted within the body. The biofilms formed are resistant to antibiotic treatments, therefore, taking the infection to an advanced level. Vancomycin have been , so far, the prefeered antibiotic that effectively helps fighting the infection.
- Staphylococcus aureus: It is Gram-positive, immobile and facultative anerobe found around sebaceous glands and moist areas. It is pathogenic in nature and is the most common cause of staph infections. It is also the causative agent of mild skin infections, invasive diseases like bacteremia, wound infection, etc. and toxin-mediated diseases like food poisioning, scaled skin syndrome, etc. It also forms biofilms and is resistant to antibiotics. The emergence of its antibiotic resistant strain Methicillin resistant Staphylococcus aurues has now come in the hit-list for the researcher s and doctors as it is increasly victimising the people.
- Propionibacteria: It is a Gram positive bacteria. They share a commensal relationship with humans and inhabit on the skin, majorily concentrated around sebaceous glands. In general they are non-pathogenic but often cause infections when come in contact with blood or any other body fluid. P. acnes is reponsible for acne vulgaris and is also associated with anaerobic arthritis and in some cases with osteomylitis and endocarditis.Antibiotics such as penicillin, carbapenems and clindamycin are generally used for the treatment.
- Cornybacteria: It is Gram-positive aerobic bacteria found in moist area of the human skin.Generally, they are non-pathogenic but some species are known to be infective. C.diptheriae is the causative agent of dipetheria, a respiratory disease mainly affecting the children.Its other pathogenic species in humans are C.amicolatum, C.striatum, C.xerosis that mostly affect the immunosuppressed patients. The bacteria is also responsible for skin infections and endocarditis. C.striatum causes axillary odor while C.minutissimum cause erythrasma.
- Proteobacteria: It is a Gram-negative , facultative or obligatory anaerobic. They are concentrated in the dry areas of the skin. They are mostly pathogenic and includes species like Escheresia, Salmonella, Vibrio, Helicobacter and Yersenia.
The Fungal Inhabitants
According to a study around 14 genera of fungi have been found to colonise human skin. Majority of the species are concentrated around heels, toenails and beween the toes. Other areas include palm, forearm and elbows. Some of the common species are Candida albicans, Torulopsis, Trichopytom rubrum, Alternaria alternata, Fusarium, Rhizopus stolonifer, Penicillum etc. Some of the common fungal infections are:
- Athelete’s Foot: It is caused by a microscopic fungi Trichopytom rubrum that lives on dead skin, hair , toenails and outer skin layers. The disease may be interdigital, mocassin (entire sole of the foot and may extend to the sides of foot) and Vesicular (between toes, on the heel and top of the foot). It is treated with antifungal medication.
- Jock Itch: It is a skin infection caused by a fungus Tinea. Since the fungus resides in warm moist area, therefore, the infection majorily occurs in genital area, inner thigh regions and buttocks. It is treated with antifungal sprays and creams.
- Ringworm: It is a skin inection caused by fungus Tinea. It can occur anywhere on the body and is contagious. Treatment generally includes antifungal creams like Lmicil, Mycelex, etc.
- Yeast Infections: It is caused by yeast Candida albicans. Yeast infections are not contagious. They mostly occur in warm, moist, oily area, like armpits and groin.It is alo responsible for diaper rashes in infants and vaginal yeast infections. They are treated with anti-yeast medications.
As described above the skin is inhabited by both bacterial and fungal species, of which some are pathogenic. In defensive mode, the skin protects itself by secreting some antimicrobial peptides like cathelicidins that keep a check on the proliferation of skin microbes. These peptides reduce the microbes and also help release cytokines which induces inflammation, angiogenesis and reepitheliazation. The acidic pH of the skin due to lactic acid found in the sweat also helps to control on the microbe number.