Microcheck 222

The normal skin flora are important because they help protect against colonization by pathogens. Occasionally, they cause disease when body defenses are impaired. They are responsible for body odor, and probably contribute to acne.

■ Name and describe the three groups of organisms generally present on normal skin.

■ Under what circumstances is Malassezia furfur most likely to be pathogenic?

■ Would frequent showering tend to increase or decrease the numbers of Staphylococcus on the surface of the skin? Why?

if host defenses are impaired. Generally, staphylococci are the most common of the skin bacteria able to grow aerobically. The principal species is Staphylococcus epidermidis. ■ staphylococci, p. 290 Important functions of the skin's staphylococci are to prevent colonization by pathogens and to maintain a balance among the microbial inhabitants of the skin ecosystem. These Gram-positive cocci compete for nutrients with other potential skin colonizers, and they also produce antimicrobial substances highly active against P. acnes and other Gram-positive bacteria.

Fungi

Tiny lipophilic, meaning oil-requiring, yeasts almost universally inhabit the normal human skin from late childhood onward. Their shape varies with different strains, being round, oval, or sometimes short, rods. These yeasts can be cultivated on laboratory media containing fatty substances such as olive oil. They belong to the genus Malassezia, formerly Pityrosporum, and are generally harmless. In some people, however, they cause skin conditions such as a scaly face rash, dandruff, or tinea versicolor (figure 22.2). The latter is a common skin disease that causes a patchy scaliness and increased pigment in light-skinned persons, or a decrease in pigment in dark-skinned people. Scrapings of the affected skin show large numbers of Malassezia furfur both in its yeast form and

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