Melanization of Candida auris is Associated with Alteration of Extracellular pH

 Candida auris is a recently emerged global fungal pathogen, which causes life-threatening infections, often in healthcare settings and with immunocompromised patients. C. auris infections are particularly worrisome because the fungus is often resistant to multiple classes of antifungal drugs. Furthermore, the fungus forms durable biofilms that are difficult to remove from surfaces. Due to the relatively recent, resilient, and resistant nature of this fungal pathogen, we investigated whether C. auris made the common melanin fungal virulence factor. Melanin is a black-brown pigment that promotes virulence in many pathogenic fungal species by contributing to their ability to resist oxidative stress, evade the mammalian immune response, inactivate antimicrobial peptides, and inactivate antifungal drugs. Melanin is typically produced following enzymatic oxidation of aromatic precursors.  We found that certain strains of C. auris, predominantly from Clades I and IV, oxidized L-DOPA and catecholamines into melanin. The melanin formation occurred extracellularly, resulting in granule-like melanin structures that adhere to the external surface of the fungal cells. This melanization was chemically driven and mediated by alkalinization of the extracellular environment. The C. auris strains had relatively high cell surface hydrophobicity, but we did not find a correlation between hydrophobicity and ability to melanize. Melanin protected the fungus from oxidative damage, but we did not observe a protective role in vitro or in vivo during infection of macrophages or Galleria mellonella, respectively. In summary, C. auris alkalinizes the extracellular medium, which promotes the non-enzymatic chemical autooxidation of exogenous L-DOPA to melanin that then attaches to its surface, thus illustrating a novel mechanism for fungal melanization.