Diagnostic fungal cultures depend on selection of the most appropriate culture site and proper collection technique. Fungal cultures are mostly commonly pursued in patients suspected of having superficial fungal infections of the hair, skin, and nails (dermatophytosis).
Samples collected from patients suspected of having fungal infections of the nasal cavity (e.g., aspergillosis) or systemic (also called “deep”) mycotic infections (e.g., histoplasmosis, cryptococcosis) are usually assessed by cytopathology or serology (see Section 5) or with tissue biopsy and histopathology involving special stains.
Direct Microscopic Examination
Direct cytologic assessment of samples from patients suspected of having fungal infections is always indicated. You certainly get credit for trying! However, experience in recognizing diagnostic elements of individual fungi and spores is essential, as is the availability of special stains for wet mount (10% potassium hydroxide) cytopathology.
Collection Technique and Sample Transport
Skin and Hair
Scrapings of skin and plucked hair shafts are commonly selected for fungal culture. The area of skin and hair to be sampled should be cleaned with 70% alcohol. Iodine-based soaps and solutions should not be used. Hair shafts, particularly those immediately adjacent to the lesion, are removed from skin with a sterile hemostat. Skin scrapings can be collected with a sterile surgical blade or the edge of a clean (unused) microscope slide. Scrapings from healthy, normal-appearing skin as well as abnormal skin should be collected. Skin biopsy may be required if results of attempts to culture hair and skin scrapings are negative. Sterile cotton-tipped swabs should not be used to collect samples for fungal culture.
Hair and skin scrapings can be placed directly into a sterile, dry container without need for any type of media as long as the sample can be processed within hours. Refrigeration is generally not required. If transport times are extended, it is reasonable to place samples in a vial containing bacterial transport medium and refrigerate for up to 15 hours. Samples should never be frozen.
In-Hospital Fungal Culture
Skin and hair samples from patients suspected of having superficial fungal infections can be inoculated directly on a commercially available substrate called Dermatophyte Test Medium (DTM). Because samples can remain at room temperature and do not require special handling, the use of DTM is ideal for in-hospital use. The medium contains phenol red as a pH indicator. If a dermatophyte is present, characteristic colony morphology will be observed and the medium underlying the colonies will turn red. Vials are unreliable after 2 weeks; color change noted 2 weeks or more after inoculation of the DTM should be disregarded.
Ultraviolet light filtered through nickel oxide produces a beam called Wood light. If an animal is taken into a dark room and its hair and skin are exposed to a Wood lamp, fluorescence may occur for several reasons. Hair shafts affected by some species of Microsporum fluoresce a bright yellow-green (like the color of a fluorescing watch face). However, iodide medications, petroleum, soap, dyes, bacteria, and even keratin may produce purple-, blue-, or yellow-colored fluorescence. The positive fungal fluorescence is a valuable aid in selecting affected hairs for culture inoculation. However, a negative fluorescence does not preclude apossible diagnosis of fungal infection. False-negative and false-positive interpretations are common.