Production of melanin by Aspergillus fumigatus

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Abstract
Melanins, or melanin-like compounds, may play a role in the pathogenesis of a number of human fungal infections. This study investigated the production of melanin by the important opportunistic pathogen Aspergillus fumigatus. Conidia from A. fumigatus were harvested and treated with proteolytic enzymes, denaturant and hot, concentrated acid; this yielded dark particles which were similar in size and shape to the original propagules. Electron spin resonance spectroscopy revealed that the conidial-derived particles were stable free radicals consistent with an identification as melanin. Melanin particles were used to immunize BALB/c mice in order to produce a total of five anti-melanin monoclonal antibodies (mAbs). The latter mAbs were strongly reactive both with intact conidia and with extracted melanin particles by ELISA and immunofluorescence reactivity. Immunofluorescence labelling with the novel mAbs was used to examine the temporal expression of melanin during in vitro culture of A. fumigatus –melanization was confined to conidial structures and was absent from hyphae. SDS-PAGE l-3,4-dihydroxyphenylalanine (l-DOPA) substrate analysis confirmed the presence of a laccase-type activity in conidial extracts, but not in hyphae. Melanin-binding mAbs were used to detect the presence of melanized conidia in three patients with nasal aspergilloma, indicating that in vivo melanization may occur during infection.