February 09, 2007

Bacterial Skin Infections


The skin provides a remarkably good barrier against bacterial infections. Although many bacteria come in contact with or reside on the skin, they are normally unable to establish an infection. When bacterial skin infections do occur, they can range in size from a tiny spot to the entire body surface. They can range in seriousness as well, from harmless to life threatening.

Many types of bacteria can infect the skin. The most common are Staphylococcus and Streptococcus. Skin infections caused by less common bacteria may develop in hospitals or nursing homes or while gardening or swimming in a pond, lake, or ocean.

Some people are at particular risk of contracting skin infections—for example, people with diabetes because they are likely to have poor blood flow, especially to the hands and feet, and because high levels of sugar in the blood decrease the ability of white blood cells to fight infections. People with AIDS or other immune disorders and those undergoing chemotherapy are at higher risk as well, because they have a weakened immune system. Skin that is inflamed or damaged by sunburn, scratching, or other trauma is more likely to be infected. In fact, any break in the skin predisposes a person to infection.

Prevention involves keeping the skin undamaged and clean. When the skin is cut or scraped, the injury should be washed with soap and water and covered with a sterile bandage. Antibiotic creams and ointments may be applied to open areas to keep the tissue moist and to try to prevent bacterial invasion. If an infection develops, small areas may be treated with antibiotic creams. Larger areas require antibiotics taken by mouth or given by injection. Abscesses (pus-filled pockets) should be cut open by the doctor and allowed to drain, and any dead tissue must be surgically removed.

Cellulitis is a spreading bacterial infection of the skin and the tissues immediately beneath the skin.

Cellulitis may be caused by many different bacteria; the most common are those of the Streptococcus species. Streptococci spread rapidly in the skin because they produce enzymes that hinder the ability of the tissue to confine the infection. Staphylococcus bacteria can also cause cellulitis, as can many other bacteria, especially after bites by humans or animals or after injuries in water or dirt.

Bacteria usually enter through small breaks in the epidermis that result from scrapes, punctures, burns, and skin disorders such as dermatitis. Areas of the skin that become swollen with fluid (edema) are especially vulnerable. However, cellulitis can also occur in skin that is not overtly injured.

Symptoms and Complications

Cellulitis most commonly develops on the legs but can occur anywhere. The first symptoms are redness, pain, and tenderness over an area of skin. These symptoms are caused both by the bacteria themselves and by the body's attempts to halt the infection. The infected skin becomes hot and slightly swollen and may look slightly pitted, like an orange peel. Fluid-filled blisters, which may be small (vesicles) or large (bullae), sometimes appear on the infected skin. Erysipelas is one form of streptococcal cellulitis in which the skin is bright red and noticeably swollen and the edges of the infected area are raised. The swelling occurs because the infection blocks the lymphatic vessels in the skin.

Most people with cellulitis feel only mildly ill, but some may have a fever, chills, rapid heart rate, headache, low blood pressure, and confusion.

As the infection spreads, nearby lymph nodes may become enlarged and tender (lymphadenitis). Other complications—lymphangitis, skin abscesses, and spread through the blood (sepsis) are also possible.

When cellulitis affects the same site repeatedly, especially the leg, lymphatic vessels may be damaged, causing permanent swelling of the affected tissue.

Diagnosis and Treatment

A doctor usually diagnoses cellulitis based on its appearance and symptoms. Laboratory identification of the bacteria from blood, pus, or tissue specimens usually is not necessary unless a person is seriously ill. Sometimes, doctors need to perform tests to differentiate cellulitis from a blood clot in the deep veins of the leg (deep vein thrombosis), because the symptoms of these disorders are similar.

Prompt treatment with antibiotics can prevent the infection from spreading rapidly and reaching the blood and organs. Antibiotics, such as dicloxacillin, that are effective against both streptococci and staphylococci are used. People with mild cellulitis may take antibiotics by mouth; those with rapidly spreading cellulitis, high fever, or other evidence of serious infection often receive intravenous antibiotics. Also, the affected part of the body, when possible, is kept immobile and elevated to help reduce swelling. Cool, wet dressings applied to the infected area may relieve discomfort.

Symptoms of cellulitis usually disappear after a few days of antibiotic therapy. However, symptoms often get worse before they get better, probably because with the death of the bacteria, substances that cause tissue damage are released. When this occurs, the body continues to react even though the bacteria are dead. Antibiotics are continued for 10 days or longer even though the symptoms may disappear earlier.

Adapted from: Merck & Co., Inc