Effect of glucose and thiamine concentrations on the formation of macroconidia in dermatophytes. Occurrence of dysgonic Microsporum canis strains in Athens, Greece.

Data collected from multiple trials with 110 fresh and preserved clinical isolates of Trichophyton mentagrophytes var. mentagrophytes, T. violaceum, T. rubrum, T. verrucosum, Microsporum canis and Epidermophyton floccosum revealed that production of macroconidia depends on glucose and thiamine concentrations in the medium. Optimal macroconidia production was obtained at the critical concentrations of 5 g l-1 glucose and 0.6 g l-1 thiamine when the two compounds were used in combination. The same conditions also encouraged macroconidia production in aconidial strains of T. verrucosum. Cutaneous inoculation in immunocompetent laboratory rabbits further enhanced the macroconidia producing capacity of the tested strains. Emphasis was placed on the occurrence of dysgonic/atypical strains of M. canis, which readily reverted to their typical phenotypes after growth on medium supplemented with 0.6 g l-1 thiamine, a process greatly augmented after cutaneous animal inoculation. It was verified that selective exogenous factors affect macroconidial production and that the dysgonic group of M. canis constitutes an epidemiologically significant group in the Greater Athens area. This is the first report of the occurrence of such M. canis strains from Greece.