Melanization of Cryptococcus neoformans reduces its susceptibility to the antimicrobial effects of silver nitrate

Cryptococcus neoformans is a human pathogenic fungus that is frequently found in avian feces and Eucalyptus trees. There is evidence that C. neoformans can make a melanin-like pigment in pigeon excreta, a major natural environmental niche. Silver nitrate, AgNO₃, is a highly toxic compound for bacteria and fungi. In this study we investigated the effects of melanin production by C. neoformans on the susceptibility of this fungus to AgNO₃. C. neoformans was grown in media with and without the melanin precursor, L-dopa, for various times and susceptibility to AgNO₃ was determined by measuring percentage of survival after incubation in AgNO₃. There was an inverse association between time allowed for melanization and susceptibility to Ag⁺. Addition of melanin particles to a suspension of non-melanized C. neoformans cells reduced their susceptibility to AgNO₃, consistent with metal ion chelation by melanin. Binding of Ag⁺ to melanin particles was demonstrated by atomic absorption spectroscopy. The results indicate that melanization of C. neoformans reduces susceptibility to a toxic heavy metal. This suggests a role for melanin in environmental protection against heavy metal toxicity.