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.