What are the differences between the three fungal culture plates (Sabouraud Dextrose Agar, Sabouraud Dextrose Agar with cycloheximide, and Dermatophyte Test Medium) used for dermatophyte isolation?

Fungal Culture Plates for Dermatophyte Isolation

The three plates you received from the microbiology lab are likely Sabouraud Dextrose Agar (SDA) without cycloheximide, Sabouraud Dextrose Agar with cycloheximide, and possibly Dermatophyte Test Medium (DTM), each serving distinct but complementary purposes in dermatophyte isolation.

Key Differences Between the Three Plates

Sabouraud Dextrose Agar (SDA) - Standard Plate

• Purpose: Broad-spectrum fungal growth medium that supports growth of dermatophytes, yeasts, and non-dermatophyte moulds
• Composition: Contains peptone and dextrose (glucose) to support fungal growth
• Use: Allows growth of ALL fungi, including contaminants
• Advantage: Won't miss slow-growing or unusual dermatophyte species

Sabouraud Dextrose Agar with Cycloheximide (SCCA/SDA+CHX)

• Purpose: Selective medium that inhibits non-dermatophyte mould growth while allowing dermatophytes to grow 1
• Composition: SDA plus cycloheximide (antifungal that blocks saprophytic moulds) and typically chloramphenicol (antibacterial)
• Use: This is the critical plate recommended by guidelines - at least one plate containing cycloheximide should always be used 1
• Advantage: Reduces contaminating moulds that can overgrow and obscure dermatophyte colonies
• Critical caveat: Some dermatophyte species are cycloheximide-sensitive and won't grow on this medium

Dermatophyte Test Medium (DTM)

• Purpose: Screening medium with pH indicator that changes from yellow to red when dermatophytes grow
• Composition: SDA with cycloheximide, chloramphenicol, gentamicin, and phenol red pH indicator
• Use: Easy visual identification - dermatophytes produce alkaline metabolites turning the medium red
• Advantage: Can be interpreted without extensive mycology training; useful for screening 2
• Disadvantage: Some saprophytic fungi can also cause color change if left too long; doesn't allow microscopic examination of specimen before culture

Why Use Multiple Plates?

The standard approach uses at least two plates: one SDA with cycloheximide and one without 1. This dual approach ensures:

• The cycloheximide plate suppresses contaminants, making dermatophyte identification easier
• The non-cycloheximide plate captures any cycloheximide-sensitive dermatophyte species
• Research shows SDA and SCCA have nearly equal efficiency (93.5% vs 95.4%) 2, 3
• DTM shows comparable isolation rates to SDA with no statistically significant difference 2, 4

Incubation Requirements

All plates should be incubated for at least 2 weeks 1. However, if you suspect Trichophyton verrucosum (particularly with cattle exposure history), extend incubation to 3 weeks and examine very carefully for slow-growing, inconspicuous colonies 1.

Clinical Bottom Line

The three-plate approach maximizes sensitivity while minimizing false negatives from either contaminant overgrowth or cycloheximide sensitivity. Research demonstrates that DTM is particularly useful as a screening tool since it's "easy to evaluate even without handling the growth of culture in the laboratory" 2, while the combination of SDA with and without cycloheximide provides the most comprehensive detection strategy endorsed by guidelines 1.