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Fungal culture
Synonym(s): Dermatophyte test medium (DTM)
Overview
- Dermatophyte test medium (DTM) is a selective medium for dermatophytes and is suitable to diagnose dermatophytosis in rabbits.
- It contains Sabouraud dextrose agar with added gentamycin, chlortetracycline and cyclohexamide which inhibit non-dermatophyte growth. Phenol red is included as a pH indicator.
- Dermatophytes utilize protein in the medium and create an alkaline pH which changes the medium from yellow to red.
- The medium will change color soon as dermatophyte mycelium growth is noted.
- Saprophytic fungi utilize carbohydrates first, so no color change occurs initially. Once carbohydrates are depleted and protein is utilized the medium will then change color after several days.
- It is important to observe the DTM daily as a color change either before colonies are seen or during early visible growth is positive for dermatophytes.
- DTM inhibits conidia formation and makes colony morphology less distinctive. Species identification requires additional culture on plain Sabouraud’s dextrose agar (SDA).
Uses
Alone
- To aid in the diagnosis of dermatophytosis Ringworm in rabbits (and other species).
- To identify the species of fungal pathogen Dermatology: practical guide to tests.
- To monitor response to antifungal therapy Therapeutics: antimicrobials.
- To determine when treatment for dermatophytosis can be stopped.
In combination
- To differentiate dermatophytosis as the cause of skin disease in combination with appropriate clinical history, examination, bacteriology, cytology, microscopy, etc.
- To definitively diagnose dermatophytosis in combination with cytological and/or histopathological findings – definite diagnosis requires identification of dermatophyte organisms (by culture or the presence of DNA by PCR) in addition to identification of affected hairs or tissues.
Sampling
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Tests
Methodologies
- Culture can be performed at room temperature, but 30°C/86°F is considered optimal; a small water dish should be added to the incubator to prevent insufficient humidity.
- Avoid direct light as UV light inhibits fungal growth. Placing samples and cultures in a dark area is advised.
- Positive results can be obtained after 5 days but culture must be continued for 14 days before a sample can be considered negative.
- Species identification requires additional culture on plain Sabouraud’s dextrose agar:
- Gross colony morphology is less distinctive on DTM.
- Microscopic examination of conidia is required for definitive species identification and DTM inhibits growth.
- On SDA species morphology is very distinctive:
- Trichophyton mentagrophytes Trichophyton spp colonies are flat with a powdery to granular surface and may have a raised central tuft or folding. Color varies from white to buff with a yellow-brown underside.
- Microsporum canis Microsporum canis colonies appear white and resemble cotton wool. They often have an orange-yellow undersurface.
- Once colonies are observed, forceps can be used to gently touch the colony surface with a small section of acetate tape, eg Scotch pressure sensitive tape® (3M). Place samples on a slide with lactophenol cotton blue and examine under x100.
- T. mentagrophytes conidia are long and cigar-shaped
whereas M. canis are spindle-shaped, thick-walled and have a terminal knob 
.
Availability
- Fungal culture can be performed in-house as long as samples are examined daily.
- Commercial point-of-care test kits are available, eg Dermaphyt® (Kruuse).
- Many commercial laboratories will offer the test, but samples should be submitted in paper to avoid false-negative results.
Validity
Sensitivity
- Careful selection of sample material has a significant impact on successful culture.
Specificity
- Colonies are more easily identified on plain SDA.
- Microscopic analysis of hyphae and macroconidia structure is required to identify species in addition to colony morphology.
- False results can occur if interpretation relies on assessing only medium color change.
Technique intrinsic limitations
- Fastidious cultural requirements.
- Plain SDA plates are prone to overgrowth by bacteria and saprophytes.
- DTM limits ability to identify species morphology.
Technician extrinsic limitations
- Interpretation is dependent on experience.
Result Data
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Further Reading
Publications
Refereed Papers
- Recent references from PubMed and VetMedResource.
- Coyner K S (2010) How to perform and interpret dermatophyte cultures: Use this guide – put together by a veterinary dermatologist – to maximize your success with this indispensable in-house test. Vet Med 105 (7), 304-307.
- Borman A M, Szekely A, Campbell C K & Johnson E M (2006) Evaluation of the viability of pathogenic filamentous fungi after prolonged storage in sterile water and review of recent published studies on storage methods. Mycopathologia 161 (6), 361-368 PubMed.
- Canny C J & Gamble C S (2003) Fungal diseases of rabbits. Vet Clin North Am Exotic Anim Pract 6 (2), 429-433 PubMed.
- Sinski J T, Wallis B M & Kelley L M (1979) Effect of storage temperature on viability of Trichophyton mentagrophytes in infected guinea pig skin scales. J Clin Microbiol 10 (6), 841-843 PubMed.
Other sources of information
- Miller W H, Griffin C E & Campbell L (2013) Diagnostic Methods. In: Muller & Kirk’s Small Animal Dermatology. 7th edn. Elsevier Inc, USA. pp 86-91.