LEARNING OBJECTIVE: Recall how potassium hydroxide (KOH) preparation is used in the detection of fungi.

Fungi (sing. fungus) are chlorophyll-free, heterotrophic (not self-sustaining) of the same family of plants (i.e., Thyllophyta) as algae and lichens. They reproduce by spores that germinate into long filaments called hyphae. As the hyphae continue to grow and branch, they develop into a mat of growth called the mycelium (pl. mycelia). From the mycelium, spores are produced in characteristic patterns. These spores, when dispersed to new substances, germinate and form new growths. Reproduction is often asexual, usually by budding (as in yeast), but certain fungi have sexual reproduction.

Common superficial infections of the skin caused by fungi are athlete's foot and ringworm of the scalp.

Asimple and frequently used method of detecting fungi is the potassium hydroxide (KOH) preparation. Fungi are seen in clustered round buds with thick walls, accompanied by fragments of mycelia. Scrapings from the affected area of the skin are mounted in 10% KOH for positive laboratory diagnosis.

Figure 7-21.-Illustration of positive and negative Monosticon DRI-DOT Slide Test Results. style="mso-spacerun: yes">

To detect fungi in infected tissue using the KOH preparation, follow the steps below.

1. Place skin, hair, or nail scrapings from the affected area on a glass slide and add one drop of 10% KOH. (Dissolve 10 g of KOH in 100 ml of distilled water.)

2. Place a coverslip on the preparation.
3. Warm the preparation gently over a flame, being careful not to boil it, and allow it to stand until clear. Do not allow the preparation to dry out.

4. Examine the preparation by using the high-power objective on microscope with subdued light.

Fungi on the skin and nails appear as refractile fragments of hyphae.

Fungi in the hair appear as dense clouds around the hair stub or as linear rows inside the hair shaft.