Laboratory Identification of Dermatophytes
Media and Tests Used:

BCP

Bromocresol purple - milk solids glucose is a differential media useful in the characterization of dermatophyte species. The growth pattern of restricted or profuse is determined by comparison to a tube of nutrient media such as SDA. Some species produce an alkaline reaction (change media to purple), others do not produce a pH change (leave the media a sky blue color). Hydrolysis of the milk solids results in a zone of clearing around the colony.

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Hair perforation

Many dermatophytes have the ability to degrade hair. The hair perforation test determines whether the organism simply erodes the hair shaft or produces an enzyme that will penetrate and invade the shaft resulting in perforating bodies or cones.

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PDA

Potato dextrose agar is a media useful for the production of pigment.

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PYE

Phytone yeast extract agar is a nutritionally enriched media that supports luxurious growth of most fungi. It contains antibiotics to inhibit bacterial growth.

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Rice grain test

Microsporum audouinii grows poorly on rice grains and produces a dark discoloration useful in differentiating it from atypical M. canis strains. The rice grains also enhance the production of macroconidia in some species.

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Scotch Tape Mount

The scotch tape mount is used for examining the microscopic structures of filamentous fungi. With forceps pick up a piece of clear, transparent tape and touch the surface of the colony. Place the tape onto a drop of mounting media on a slide; add another drop and a coverslip. Examine with a phase contrast microscope.

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SDA

Sabouraud dextrose agar (Emmon's modification) is a non-selective media which supports the growth of most fungi.

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Slide culture

The slide culture is a method of examining the microscopic structures of a fungus. The organism is grown on a glass coverslip placed on a block of agar. When sufficient growth has occurred the coverslip is placed on a drop of mounting media on a microscope slide and examined by phase contrast microscopy.

Urea

Christensen's urea broth indicates the presence of the enzyme urease, which splits urea into ammonia, resulting in an alkaline environment. The phenol red indicator turns the media from a straw yellow to fuschia at pH 8.4.

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Vitamin requirements

Certain species of dermatophytes have distinctive nutritional requirements that may be beneficial to differentiate from similar species. The agar base is vitamin free and various vitamins are added to the basal media. The growth on the vitamin-enriched media is compared to the vitamin free media to determine enhancement of aerial mycelium.

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