Infection of skin is due to viruses, fungus, or bacteria. Also many diseases that affect other organs may be due to skin manifestation. To really understands skin infection, there is need to understands skin anatomy and physiology. The skin is divided into 3 major layers: the epidermis, the dermis and the fat layer. The skin is only sterile at birth, after that is inhabited by flora which includes both anaerobic and aerobic bacteria.
Two factors made the skin difficult to bacteria growth, exfoliation and dryness. The constant removal of the top layer (stratum corneum), removes many of the bacteria that stick to it. According to research, bacteria growths were higher in area of the skin that is moist. Also the pH of the skin also help in reducing the bacteria growth on the skin. The average pH of the skin is about 5.5, which is acidic. The low temperature of the skin (33 centigrade) as well as salty nature of the skin due to evaporation of sweat. But this saltiness may encourage the growth of certain bacteria that are salt resistant. Some organisms are affected by the lipid content of the skin.
Infections of the skin occurs in three main way: from the outside (exogenous infections) through cuts, wounds, insects bites, skin disease. The second way is through within (endogenous infections) the skin, from underlying tissue or carried by blood or lymph. The third way is by toxin, caused by toxin release at a distant site.
Once the skin barrier is broken, small numbers of bacteria is needed for infection to occur. These factors tends to predispose the skin to infections,
Excessive moisture: This may be as a result of occlusive dressing, wet diapers in babies. Also obese people accumulate water in their body folds.
Trauma: This the popular factor leading to skin and soft tissue infections. The major form of trauma that expose the skin to infection is surgery, gunshot, wounds, crush injuries, or burns, in which large areas of the skin damaged and left open. Any condition that infringes on the blood supply, exposes the skin to invasion by causing barrier breakdown and limiting defences. Blood supply infringement could occur as a result of peripheral vascular disease, as in diabetics, elderly parents as well as patients with vasculities. In diabetic patients, compromise of the blood supply often lead to peripheral sensory neuropathy, in which case they may not be aware of the damage to their skin.
Invasion From Within
This infection occurs as a result of microorganisms that spread form another infected sites. This could be direct form underlying focus or through blood stream. These type of secondary infection occurs in people whose immunity is strong as well as in those whose immunity is weak, but in different ways and degree.
Toxin-induced Skin Disease
The skin responds to toxin produced during infections that occur in another place at a distant site. Example is streptococci that produced toxin called erythrogenic factor, which spread through the blood stream. These toxin have been linked to red rash, strawberry tongue. Streptococci causes two main type of skin disease namely: scalded skin syndrome and toxic shock syndrome.
Scalded skin syndrome: is a disease of children, as a result of the action of toxin exfoliatin, which separate the epidermis by damaging the intracellular connections. The appearance looks like skin scalded with hot water.
Toxic shock syndrome: This disease is caused by an exotoxin release by a bacteria called staphylococcus aureus. This disease can be fatal if left untreated.
The skin responds to invading microorganisms in a three major ways:
Spreading infections: this is called impetigo when is limited to epidermis, erysipelas when confined to dermal lymphatics and cellulitis when found in the subcutaneous fat layer.
Abscess formation: This is known as folliculitis, boils (furuncles), and carbuncles.
Necrotising Infections: This includes fasciitis and gas gangrene (myonecrosis).