Piedra Workup

Updated: Apr 22, 2019
  • Author: Robert A Schwartz, MD, MPH; Chief Editor: Dirk M Elston, MD  more...
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Workup

Laboratory Studies

Staining

Place hair shaft nodules into a 10-15% potassium hydroxide preparation on a glass slide. A fungal stain, such as chlorazol black E stain or Parker blue-black ink may be added to highlight the hyphae. If the nodule is from black piedra, tightly packed and pigmented hyphae, asci, and ascospores are seen attached to the hair shaft. If the nodule is from white piedra, darkly stained and loosely arranged hyphae, blastoconidia, and arthroconidia are seen attached to the hair shaft.

Culturing

P hortae, the cause of black piedra, grows slowly on Sabouraud dextrose agar and is not inhibited by cycloheximide. Microscopic examination reveals septate hyphae, chlamydospores, and irregularly shaped hyphal elements. [46] These cultures are of the asexual phase of the fungus. Organisms in the sexual phase are difficult to grow in culture.

T asahii, the typical cause of white piedra, rarely grows on Sabouraud dextrose agar because of inhibition by cycloheximide, which is present in dermatophyte test medium and in Mycosel and mycobiotic agars. In addition, Trichosporon species grow best at 28-30°C.

Mycological culture on Mycosel plus molecular identification may be used to confirm the diagnosis. [47]

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Other Tests

Morphological and physiological analysis may provide inconsistent identification of Trichosporon to species level. For example, sequencing of the intergenic spacer 1 region of ribosomal DNA may be necessary accurately identify T inkin. [9] Such molecular identification is desirable for suitable epidemiological surveillance of superficial mycoses from Trichosporon species.

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Histologic Findings

In black piedra, the dark nodules are composed of different components based on the area of the body in which they are located. Black piedra is distinguished by ascospores borne in a subglobose ascus in groups of 8. The periphery of the nodule has regularly aligned hyphae and arthroconidia. [48] P hortae is one of the few pathogenic human fungi that produce sexual spores in its parasitic phase.

White piedra is characterized by discrete-to-coalesced nodules that typically are white, cream, or brown. [28, 49] Nodules on the hair shaft often appear amorphous, but may be outlined by hyaline arthroconidia, 2-4 septate hyphae, and differentiated blastoconidia that arise from loosely packed hyphae. Sexual spores are not known to be present. [28] Fungal structures stain easily with Parker blue-black ink.

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