Normal Flora of the Skin

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The skin represents a distinct ecological habitat, analogous to a cool desert, compared to the warm, moist tropical conditions that exist in other body systems. Large numbers of microorganisms live on and in the various components of the normal skin. For example, depending on the body location and amount of skin moisture, the number of bacteria on the skin surface may range from only about 1,000 organisms per square centimeter on the back to more than 10 million in the groin and armpit, where moisture is more plentiful. The numbers actually increase after a hot shower because of increased flow from the skin glands where many reside. Most of the microbial skin inhabitants can be categorized in three groups: diphtheroids, staphylococci, and yeasts (table 22.1, and see figure 22.1). Although generally harmless, skin organisms are opportunistic pathogens, meaning that they can only cause disease in people with impaired body defenses.

Table 22.1 Principal Members of the Normal Skin Flora

Name

Characteristics

Diphtheroids

Variably shaped non-motile, Gram-positive rods of the Corynebacterium and Propionibaderium genera

Staphylococci

Gram-positive cocci arranged in packets or clusters; coagulase negative; facultatively anaerobic

Fungi

Small yeasts of the genus Malassezia that require oily substances for growth

22.2 Normal Flora of the Skin 535

AIDS patients and others with impaired immunity are especially vulnerable. ■ normal flora, pp. 375,461

Diphtheroids

Diphtheroids are a group of bacteria named for their resemblance to the diphtheria bacillus, Corynebacterium diphtheriae. Their distinctive characteristics are Gram-positive staining, variation in shape, and low virulence. Unlike C. diphtheriae, they do not produce exotoxin. Diphtheroids are responsible for body odor, caused by their breakdown of substances in sweat, which is odorless when it is first secreted. A diphtheroid found on the skin in large numbers is Propionibacterium acnes, which is present on virtually all humans. Surprisingly, most strains of P. acnes are anaerobic, although some strains are aerotolerant. This bacterium grows primarily within the hair follicles, where conditions are anaerobic. Growth of P. acnes is enhanced by the oily secretion of the sebaceous glands, and the organisms are usually present in large numbers only in areas of the skin where these glands are especially well developed—on the face, upper chest, and back. These are also the areas of the skin where acne most commonly develops, and the frequent association of P. acnes with acne inspired its name, even though most people who carry the organisms do not have acne. ■ aerotolerance, p. 89

Acne in its most common form begins at puberty in association with a rise in sex hormones, enlargement of the sebaceous glands, and enhanced secretion of sebum. The hair follicle epithelium thickens and sloughs off in cohesive clumps, causing increasing obstruction to the flow of sebum to the skin surface. Continued sebum production by the gland can force a plug of material to the surface, where it is visible as a blackhead. With complete obstruction the follicle becomes distended with sebum, which causes the epidermis to bulge outward, producing a whitish lesion called a whitehead. The P. acnes that normally reside in the gland multiply to enormous numbers in the trapped sebum. Lipases of the bacteria degrade the sebum, releasing fatty acids and glycerol, a growth requirement of the organisms. The metabolic products of the bacteria cause an inflammatory response, attracting leukocytes (white blood cells) whose enzymes damage the wall of the distended follicle. The inflammatory process can cause the follicle to rupture, releasing the follicle contents into the surrounding tissue. The result is an abscess, a collection of white blood cells, bacteria, and cellular debris, which eventually heals and leaves a scar. Squeezing acne lesions is ill-advised, because it promotes rupture of the inflamed follicles and therefore more acne scars. Usually acne can be controlled until it goes away by itself, by using medications such as antibiotics and benzoyl peroxide that inhibit the growth of P. acnes, or by those such as azelaic acid (Azelex) and isotretinoin (Accutane) that act primarily to reverse the hair follicle abnormalities. The latter medication is reserved for the most serious cases of acne because it has potentially serious side effects. ■ inflammation, p. 385 ■ leukocytes, p. 376

Staphylococci

The second group of microorganisms universally present on the normal skin is composed of members of the genus Staphylococcus. They are salt-tolerant organisms and grow well on the salty skin surface. As with the diphtheroids, most of these bacteria have little virulence, although they certainly can cause serious disease

Microscopic Arachnid Skin Causes Acne

Figure 22.2 Tinea Versicolor

Appearance in (a) a fair-skinned individual and (b) a dark-skinned individual. (c) Microscopic appearance of skin scraping showing Malassezia furfur yeast and filamentous forms.

Figure 22.2 Tinea Versicolor

Appearance in (a) a fair-skinned individual and (b) a dark-skinned individual. (c) Microscopic appearance of skin scraping showing Malassezia furfur yeast and filamentous forms.

as short filaments called pseudohyphae. Unknown factors, probably relating to the host, are important in these diseases since most people carry Malassezia sp. on their skin without any disease. AIDS patients often have a severe rash with pus-filled pimples caused by Malassezia yeasts, and the organisms may even infect internal organs in patients receiving fat-containing intravenous feedings. ■ yeasts, p. 309

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