Only a few species of bacteria commonly invade the intact skin directly, which is not surprising in view of the anatomical and physiological features discussed earlier. Hair follicle infections exemplify direct invasion.
Infections originating in hair follicles commonly clear up without treatment. In some instances, however, they progress into severe or even life-threatening disease.
Folliculitis, furuncles, and carbuncles represent different outcomes of hair follicle infections. In folliculitis, a small red bump, or pimple, develops at the site of the involved hair follicle. Often, the hair can be pulled from its follicle, accompanied by a small amount of pus, and then the infection goes away without further treatment. If, however, the infection extends from the follicle to adjacent tissues, causing localized redness, swelling, severe tenderness, and pain, the lesion is called a furuncle or boil. Pus may drain from the boil along with a plug of inflammatory cells and dead tissue. A carbuncle is a large area of redness, swelling, and
22.3 Bacterial Skin Diseases pain punctuated by several sites of draining pus. Carbuncles usually develop in areas of the body where the skin is thick, such as the back of the neck. Fever is often present, along with other signs of a serious infection.
Most furuncles and carbuncles, as well as many cases of folliculitis, are caused by Staphylococcus aureus, a staphylococcus that produces coagulase and is therefore called "coagulase-positive." It is much more virulent than the staphylococci normally found on the skin. The name derives from staphyle, "a bunch of grapes," referring to the arrangement of the bacteria as seen on stained smears, and aureus, "golden," referring to the color of the S. aureus colonies. This bacterium is an extremely important pathogen and is mentioned frequently throughout this text as the cause of a number of medical conditions (table 22.2). ■ coagulase, p. 694
Infection begins when virulent staphylococci attach to the cells of a hair follicle, multiply, and spread downward to involve the follicle and sebaceous glands. The infection induces an inflammatory response with swelling and redness, followed by attrac-
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