Dermatophyte growth on SDA: Introduction, culture, microscopic exam

Dermatophyte Growth on SDA: Introduction, Culture, Microscopic Examination and Lab Diagnosis

Introduction of Dermatophyte

Dermatophyte is a group fungus of three genera Trichophyton, Microsporum, and Epidermatophyton. This one is Trichophyton mentagrophyte powdery pigmented colonies on Sabouraud dextrose agar (SDA). It causes ringworm.

Culture of Dermatophyte

Common solid media are as SDA, or DTM with antimicrobial agent and cycloheximide. Small fragments of keratinous material are planted or scattered on SDA containing chloramphenicol and cycloheximide and incubated at room temperature. Growth is slow and colonies may appear in 1-3 weeks. Specialized media may be necessary to stimulate sporulation such as cornmeal agar, potato flake agar, lacrimal rice grains, or PDA so that sporulation will occur. Culture from suspected superficial mycoses is incubated at 30ºC. DTM incorporates gentamicin and chloramphenicol to inhibit the bacteria and a phenol red indicator that changes from yellow to red when the medium becomes alkaline as the result of the growth of dermatophytes.

Identification of Culture

Colony characteristics(macroscopy)- Gross colony features observed on SDA include the color of the surface, the color of reverse, texture of the surface(powdery, granular, velvety, or fluffy) type of folding (radial, cerebriform), and the rate of growth.

Microscopic examination of culture for dermatophyte

Microscopic characteristics that should be observed are the following:
Septate or aseptate hyphae, hyaline or dematiaceous hyphae, fruiting structures, the types, size, shape, and arrangement of conidia. Microscopic examinations can be studied in teased mounts, slide cultures, or cellophane tape preparation.

Tease Mount

With the help of two teasing needles, a portion of the mycelium is removed. An LPCB preparation is prepared on a slide. LPCB does not work well with the dematiaceous fungi because they retain their dark color. The major disadvantage of this procedure is the disruption of conidia during the teasing process. The preparations may be mounted in Poirier’s blue or lactophenol aniline blue or carbol fuchsin.

Cellophane Tape Preparation of Dermatophyte

Cellophane tape preparations involve gently touching a piece of clear tape, sticky side down to the surface of the colony, and then removing it. The tape is placed onto a drop of LCB on a slide and examined. With this procedure, the conidial arrangement is retained.

Slide Culture of Dermatophyte

Slide cultures are useful for demonstrating the natural morphology of the fungal structures and for encouraging conidiation in some poorly fruiting fungi. Slide culture is particularly useful when the slide culture from a known isolate is stored in a collection for future comparison against isolates awaiting identification.

Slide Culture Technique

Culture character and microscopy of dermatophytes

Physiological And Biochemical Tests For Fungal Infections Laboratory Diagnosis

Hair perforation test
Trichophyton rubrum, which may be morphologically similar to T. mentagrophytes, usually causes only surface erosion of hair shafts in this test, whereas T. mentagrophytes usually form perpendicular pegs in the hair shafts. This test can also be used to distinguish penetration capable Microsporum canis from M. equinum which does not penetrate the hair.
Urease test
5 days urease test help to differentiate T. mentagrophytes from T. rubrum. Tubes of Christensen urea agar are very lightly inoculated with the dermatophytes and held for 5 days at room temperature. Most isolates of T. mentagrophytes demonstrate urease production resulting in a color change. Most T. rubrum isolates are negative or require more than 5 days to give a positive reaction.
Growth on rice grains
This test is useful to differentiate poorly sporulating isolates of M. canis from M. audouinii. Sterile nonfortified rice is inoculated lightly with a portion of a colony. After 10 days of incubation at room temperature, the medium is observed for growth. M. canis and other dermatophytes grow well and usually form many conidia whereas M. audouinii does not grow but rather turns the rice grains brown.

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