Trichophyton mentagrophytes spores differ from mycelia in their ability to induce pustules and activate complement.

To determine whether the morphology of dermatophytes plays a role in the clinical manifestation of dermatophytosis, we isolated spores and mycelia from colonies of killed Trichophyton mentagrophytes and tested their ability to induce gross and microscopic cutaneous changes when applied to guinea pig skin. The skin of normal guinea pigs failed to develop any cutaneous changes after inoculation with either dead spores or mycelia. In contrast, guinea pigs that had recovered from a T. mentagrophytes infection reacted to topically applied spores by producing pustules but failed to develop pustules either grossly or microscopically when inoculated with mycelia. Because differences in the activation of C' and the subsequent generation of the polymorphonuclear leukocyte (PMN) chemoattractant, C5a, might account for our in vivo findings, we measured the amount of C5a activated in vitro when serum was incubated with equivalent amounts of spores or mycelia. Spores consistently activated more C' to C5a than mycelia, although each could activate serum C'. The results support previous studies that show a direct correlation between the clinical manifestations of a dermatophytic infection and the development of immunity against the dermatophyte. Furthermore, the data suggest that a dermatophyte's predominant morphology during an infection may play a critical role in producing pustules by activating C' and chemoattracting PMNs into the fungus-laden stratum corneum.