Manipulation of regulators of morphogenesis is not sufficient to render a Candida albicans colonizer strain pathogenic

As a member of the microbiome, Candida albicans colonizes the oral cavity and other mucosal surfaces of the human body. While commensalism is tightly controlled by the host immune system, the fungal determinants enabling the fungus to colonize the host mucosa without causing tissue damage and inflammation remain less clear. In search of genetic determinants that may underlie the commensal lifestyle of the low-damage-inducing C. albicans isolate 101, we identified a small sequence duplication in one allele of the Brg1 hyphal morphogenesis regulator gene, resulting in a truncated loss-of-function allele (BRG1TRUNC). Replacing BRG1TRUNC by the full-length allele (BRG1FL) resulted in a modest increase in filamentation but did not alter the phenotype of the fungus in the oral mucosa of experimentally colonized mice. Analysis of a spontaneous hyperfilamentous variant of the BRG1FL/BRG1FL derivative of C. albicans strain 101 identified a Glu-to-Lys change at position 1541 in the Cyr1 adenylate cyclase (CYR1E1541K). While the CYR1E1541K mutation led to greatly increased filamentation, expression of hyphae-associated genes, and host cell damage when tested in vitro, it was insufficient to render C. albicans strain 101 more pathogenic in the oral mucosa in vivo, irrespective of the BRG1 status. Together, this highlights that the low-damage-inducing nature of strain 101 cannot be overcome by manipulating BRG1 and CYR1, two genes with known roles in C. albicans virulence.IMPORTANCEDuring homeostasis, the fungus Candida albicans establishes mutualistic interactions with its human host. It can, however, also adopt a pathogenic state and cause infections with diverse clinical manifestations that pose a significant challenge for diagnosis and therapy. Understanding the fungal determinants that underlie C. albicans colonization under steady-state conditions may thus provide new avenues for modulating the fungus-host interaction in candidiasis patients to restore homeostasis. Here, we report gene variants in key regulators of C. albicans morphogenesis and virulence that distinguish strains with distinct capacity to drive inflammation and cause disease. Gene-exchange mutants provided evidence for the impact of a BRG1 loss-of-function allele and a CYR1 gain-of-function mutation toward in vitro biomarkers of fungal pathogenicity. However, in vivo in an experimental model of C. albicans oral colonization, none of these gene variants individually or in combination was sufficient to change the pathogenic state of the fungus. These findings indicate that C. albicans mucosal colonization is regulated by a complex gene network rather than by single genetic determinants.