The skin is an organ that protects humans from harmful elements in the environment, including bacteria. However, there are some bacteria in the human body and on the skin that reside there and cause no disease. In fact, some of them help perform functions such as digestion. These are the "normal flora." Bacteria that cause disease are called "pathogens." Many pathogens cause different diseases of the skin, including rashes, boils and acne.
The Skin
The skin is the largest organ of the human body. It consists of three layers. The outermost layer, the epidermis, is exposed to the environment constantly. A complicated system of cells, glands and hair defends the skin from environmental threats like sunlight, heat, cold, chemicals and invading organisms such as viruses and bacteria. There are bacteria on the skin of every human that are good and some that are bad for the skin.
Bacteria
Bacteria are single-celled organisms that reproduce asexually through dividing. Bacteria do this very efficiently, and they are found everywhere in the environment. When bacteria invade the human body, they cause diseases. Examples of these diseases are pneumonia, an infection of the lungs caused sometimes by Pneumococcus bacteria, or meningitis, and infection of the lining of the brain and spinal cord caused sometimes by the Neisseria species of bacteria. However, there are some species of bacteria that perform vital functions for humans.
Normal Flora
Bacteria that are normally found in the human body and do not cause disease are called "normal flora." In the intestines, E. coli bacteria help in the digestion of food. On the skin, bacteria such as Staphylococcus epidermidis and Streptococcocus keep pathogenic ("bad" bacteria) at bay by taking up the space that pathogenic bacteria would occupy. Without normal flora, pathogenic bacteria would be free to invade the skin and possibly cause skin conditions. Other normal flora includes Micrococcus and even Lactobacillus species. The type of bacteria on the skin greatly depends on the type of skin being observed.
Pathogens
Interestingly enough, some of the bacteria that are bad for the skin and may cause disease belong to the same species as the good bacteria. This is because bacteria of different strains have different feeding and reproduction patterns. When bacteria outgrow the normal flora and begin to cause disease, they are said to be "pathogens." Pathogens can cause mild diseases like a rash that clears up in a few days, or they can cause serious diseases like leprosy (Hansen's disease) that take years to run their course and may cause disability or death.
Common Bacterial Skin Infections
Common skin infections include those caused by Staphylococcus aureus, particularly the strain resistant to penicillin (known as MRSA), when it invades the skin and causes boils and abscesses. Leprosy is caused by Mycobacterium leprae. Impetigo, a kind of rash, is caused by Streptococcus pyogenes. Folliculitis, inflammation of the hair follicles on the skin, is sometimes caused by Proteus vulgaris bacteria. And then there is acne, a skin condition familiar to every teenager. It is often caused by Propionibacterium acnes bacteria.
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