VMC 311: Systematic Veterinary Bacteriology and Mycology (2+1)

MORPHOLOGY

• In their nonparasitic phase, including culture, dermato­phytes produce septate, branching hyphae collectively called mycelium.
• The asexual reproductive units (conidia) are found in the aerial mycelium. These units may be either macroconidia or microconidia.
• Shape, size, structure, arrangement and abundance of conidia are diagnostic criteria.
• A general rule that can be applied (but not always) is that most species in the genus Microsporum produce predominately macroconidia, and most Trichophyton species produce predominately microconidia with few or no macroconidia.
• Hyphal peculiarities — spirals, nodules, rackets, chandeliers and chlamydoconidia — are more common in some species than others, but they are rarely diagnostic.
• Pigmentation is useful in dermatophyte differentiation.
• In tissue sections, arthroconidia can often be identified. This parasitic phase arthrospore can remain infectious for years.
• Except in size ranges, which overlap among dermatophyte species, arthroconidia are indistinguishable from species to species.
• Chlamydoconidia are also commonly seen in some dermatophytes in vitro and their presence may be of use diagnostically in the absence of other conidia.
• Sexual spores (ascospores) are absent in the parasitic phase.