Melanization of Candida auris Is Associated with Alteration of Extracellular pH

Abstract

Candida auris is a recently emerged global fungal pathogen, which causes life-threatening infections, often in healthcare settings. C. auris infections are worrisome because the fungus is often resistant to multiple antifungal drug classes. Furthermore, C. auris forms durable and difficult to remove biofilms. Due to the relatively recent, resilient, and resistant nature of C. auris, we investigated whether it produces the common fungal virulence factor melanin. Melanin is a black-brown pigment typically produced following enzymatic oxidation of aromatic precursors, which promotes fungal virulence through oxidative stress resistance, mammalian immune response evasion, and antifungal peptide and pharmaceutical inactivation. We found that certain strains of C. auris oxidized L-DOPA and catecholamines into melanin. Melanization occurred extracellularly in a process mediated by alkalinization of the extracellular environment, resulting in granule-like structures that adhere to the fungus’ external surface. C. auris had relatively high cell surface hydrophobicity, but there was no correlation between hydrophobicity and melanization. Melanin protected the fungus from oxidative damage, but we did not observe a protective role during infection of macrophages or Galleria mellonella larvae. In summary, C. auris alkalinizes the extracellular medium, which promotes the non-enzymatic oxidation of L-DOPA to melanin that attaches to its surface, thus illustrating a novel mechanism for fungal melanization.

Diverse mechanisms control amino acid‐dependent environmental alkalization by Candida albicans

“…The capacity to neutralize the environment in the presence of amino acids was reported previously in other members of the pathogenic Candida species complex (Vylkova et al, 2011), including the multidrug resistant C. auris (Smith et al, 2022) and C. glabrata (Kasper et al, 2014). To test whether Gdh2-dependent alkalization is conserved in these species, we created GDH2 deletions in C. auris and C. glabrata by homology-directed recombination using an HA-tagging cassette (pFA6a-3HA-SAT-flipper) derived from the original SAT1flipper cassette (Reuss et al, 2004).…”

Protein Kinase A Controls the Melanization of Candida auris through the Alteration of Cell Wall Components

“…Recent research has uncovered that C. auris , an emerging multidrug-resistant pathogen, is capable of producing melanin, akin to other Candida species [ 12 ]. Interestingly, the melanin granules in C. auris do not serve as a virulence factor, as seen in C. albicans or C. glabrata [ 12 ].…”

“…Recent research has uncovered that C. auris , an emerging multidrug-resistant pathogen, is capable of producing melanin, akin to other Candida species [ 12 ]. Interestingly, the melanin granules in C. auris do not serve as a virulence factor, as seen in C. albicans or C. glabrata [ 12 ]. Instead, they can be produced using a variety of substrates, such as DOPA, dopamine, epinephrine, and a brain mixture, with the choice of substrate varying depending on the clade [ 12 ].…”

“…Interestingly, the melanin granules in C. auris do not serve as a virulence factor, as seen in C. albicans or C. glabrata [ 12 ]. Instead, they can be produced using a variety of substrates, such as DOPA, dopamine, epinephrine, and a brain mixture, with the choice of substrate varying depending on the clade [ 12 ]. Notably, C. auris is unable to use L-tyrosine as a precursor for melanin synthesis and exhibits no laccase activity [ 12 ].…”

“…Instead, they can be produced using a variety of substrates, such as DOPA, dopamine, epinephrine, and a brain mixture, with the choice of substrate varying depending on the clade [ 12 ]. Notably, C. auris is unable to use L-tyrosine as a precursor for melanin synthesis and exhibits no laccase activity [ 12 ]. Factors such as temperature, pH, and cell density influence the formation of melanin granules, and the cell wall polysaccharide content is key to the successful attachment of these granules to the cell wall [ 12 ].…”