Purpose of review
Production of melanin has been associated with virulence in diverse microorganisms. Melanization of fungi has been noted for many years in predominantly subcutaneous infections such as chromoblastomycosis and more recently most extensively studied in a yeast causing systemic infection, Cryptococcus neoformans. Pigmented fungi are increasingly important human pathogens and currently available antifungals are often sub-optimal for serious infections. This review focuses on recent publications on melanin in fungi with particular reference to the role of melanin in virulence, protection against antifungal drugs, and promoting survival in the environment.
Inhibition of melanin production by C. neoformans can prolong survival of lethally infected mice. In contrast, melanin in C. neoformans and Histoplasma capsulatum yeast cells can bind amphotericin B and caspofungin, thereby reducing the fungicidal affects of these drugs. H. capsulatum and Paracoccidioides brasiliensis have only recently been shown to produce melanin in vitro and during infection. Additionally, melanin derived from melanized C. neoformans yeast and Aspergillus niger conidia can activate complement, which may modify immune responses to infection. Studies on C. neoformans laccase have revealed that the enzyme is located on the cell wall, which may allow for interactions with the host. Melanization reduces the susceptibility of C. neoformans to enzymatic degradation and toxicity from a heavy metal, which may afford protection to the fungus against similar insults in the environment.
Melanin has been referred to as ‘fungal armor’ due to the ability of the polymer to protect microorganisms against a broad range of toxic insults. Recent publications continue to reveal important contributions of melanin to survival of fungi in the environment and during infection.