There are three common types of skin infections: cellulitis, erysipelas, and impetigo (pyoderma).
Cellulitis is usually an easily treated, but potentially dangerous skin infection. Cellulitis sometimes results when a cut, scrape, insect bite, splinter, or other break in the skin allows bacteria to enter underlying tissues. While a scrape or a cut will not usually result in cellulitis, a tender, firm, painful, and rapidly expanding area of redness on the skin surrounding broken skin should be a cause for concern. Often, lymph nodes close to the affected skin will be enlarged, and there may be red streaks running through the skin away from the infection.
Erysipelas is a form of cellulitis that affects only the top layers of the skin. It is more common among children and the elderly. Erysipelas differs from cellulitis in that the inflamed area is distinct from surrounding skin—it is raised, firm, and the redness is sharply marked off. Erysipelas is often found on the face; however, it can also develop on the arms and legs. Sometimes the skin will have what is called a 'peau d'orange,' or orange peel, look to it.
Impetigo (pyoderma) is the most common bacterial infection of the skin. It is contagious and can happen at any age, but is most common in young children. Impetigo appears in the form of reddish sores that blister to produce a yellow-brown fluid. This fluid dries to form a thick, honey-colored crust. The sores are not usually painful, but can itch. They are usually found on exposed parts of the body like the hands, feet, and legs. Bullae, which look like blisters, may also be present, and spread rapidly. Bullous impetigo is most often found on the face, arms, and legs.
Warm temperatures, humidity, poor hygiene, and crowded living conditions make the spread of impetigo easier. When the sores of impetigo are deep, causing larger ulcers, the infection is called ecthyma. As with cellulitis and erysipelas, impetigo is not difficult to treat, but requires the attention of a physician.
The most common cause of cellulitis is a family of bacteria called Group A streptococcus, the same bacteria that cause strep throat. The streptococcus bacteria exist on the skin and in the throats of large numbers of people without any ill effects. However, when the bacteria penetrate the protective barrier of the skin, they can cause serious disease. Streptococci make proteins called enzymes that break down the skin and allow bacteria to spread. They also produce toxins that cause the body to start a protective response, leading to more inflammation.
Another organism, called Staphylococcus aureus, also causes cellulitis, and is second in occurrence only to Group A streptococcus.
In some cases, other, more rare types of bacteria cause cellulitis:
- Steptococcus galactiae (Group B streptococcus) attacks those with impaired immune systems, the elderly, and people with diabetes.
- Hemophilis influenzae can cause cellulitis in the skin surrounding the eye (periorbital cellulites) in children. These bacteria are often associated with sinus infection (sinusitis), ear infection (otitis media), or a serious infection of the throat called epiglottitis. Childhood vaccination for hemophilis has made this infection less common.
- Pasteurella multocida is associated with dog and cat bites.
- Pseudomonas aeruginosa can cause infection of a hair follicle after bathing in contaminated water (hot tub folliculitis), as well as cellulitis. Cellulitis may also occur after a person has stepped on a nail or had some other type of penetrating injury.
- Erysipelothrix rhusiopathiae can cause cellulitis in people who work as meat workers, bone renderers, and fishmongers.
- Vibrio vulnificus (and other Vibrio species) can enter the skin directly (usually from contaminated seawater) and cause cellulitis or, rarely, be ingested in the form of undercooked seafood. From the gut, Vibrio vulnificus can enter the bloodstream and “seed” the skin.
Group A strep usually causes erysipelas. Other streptococcus types and rarely, staphylococcus, another type of bacteria, may be involved. Unlike many other organisms, streptococci have infrequently developed drug resistance. As a result, most of these infections, if treated early, do not require expensive new or broad-spectrum antibiotics. They can generally be treated with oral forms of penicillin, or agents like erythromycin for those allergic to penicillin.
A mixture of streptococcus and staphylococcus usually causes impetigo. Because staphylococcus is almost universally resistant to basic penicillin, advanced penicillins or other types of oral antibiotics are used to treat this infection.
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