Bovis ISSN 2398-2993

Fungal culture

Synonym(s): Dermatophyte, ringworm

Contributor(s): Catherine Fraser , Tammy Hassel

Overview

  • Dermatophyte test medium (DTM) is a Sabouraud’s dextrose agar with cyclohexamide, gentamycin, chlortetracycline and phenol red as a pH indicator.
  • It is suitable for use in diagnosis of cattle dermatophytosis. Ringworm
  • The principle of the test is that pathogenic fungi utilize the protein in the medium and the resulting metabolites change the pH to alkaline and the medium from yellow to red.
  • Saprophytic fungi use carbohydrates first and no colour change is seen.
  • Once the carbohydrate is depleted, saprophytes will then utilise protein and a late colour change is seen.
  • Therefore it is important to observe the DTM daily and a colour change either before or in association with the early visible growth of the colonies is positive.

Sampling

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Tests

Methodologies

  • Culture is best at room temperature and a positive result may be seen in as little as 5 days.
  • Leave the culture for up to 14 days before treating as negative. Increased humidity is helpful for fungal growth and a pan of water in the incubator will provide this.
  • Identifying particular dermatophytes is not often possible with DTM as macroconidia are not produced. Macroconidia are more commonly produced by colonies older than seven days on BCP-milk solids-yeast extract agar.
  • Setting up a subculture on Sabouraud’s dextrose agar will culture colonies that sporulate and mature macroconidia can then be identified using a fungal guide (experience required).
  • The most common dermatophytosis of cattle is Tricophyton verrucosum. This fungus has a requirement for thiamine and some varieties also for inositol. It is also very slow growing and may need to be incubated at 37° and for up to 1 month.
  • Colonies on Sabouraud’s agar are white to cream-colored sometimes ochre, they are initially waxy, generally small and raised showing folds, and they rarely show macroconidia without vitamin enrichment.
  • Once colonies are seen, take a small square of acetate tape e.g. Scotch pressure sensitive tape® (3M), holding it in forceps touch it lightly to the surface of the culture then place it on a glass slide with a drop of lactophenol cotton blue and examine under x100 magnification. Chains of chlamydospores are seen described as like fir trees with club-shaped tips.

Availability

  • Fungal culture can easily be carried out in the practice situation. DTM is commercially available eg DermaphytR (Kruuse).
  • Alternatively, all commercial laboratories will offer the test.

Validity

Sensitivity

  • As alluded to before, choosing sample material carefully remains the single most important determinant of success.
  • Trichophyton verrucosum can be fastidious in its cultural requirements and may not always grow.

Specificity

  • Visual analysis of gross cultural morphology and microscopic analysis of hyphal form and macroconidial morphology is necessary for species identification and requires experience

 

Technique (intrinsic) limitations

  • Fastidious cultural requirements.
  • Overgrowth by non-pathogenic fungi or bacteria on Sabouraud's dextrose agar plates.

Technician (extrinsic) limitations

  • Veterinarian or laboratory technician experience required.

Result Data

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Further Reading

Publications

Other sources of information

  • van Cutsem, J & Rochette, F: Mycoses in Domestic Animals Janssen Research Foundation 1991


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