Characterization of virulence-related phenotypes of Candida parapsilosis and Rhodotorula mucilaginosa isolated from the International Space Station (ISS)

There is increased interest in characterizing and describing the effects of space stressors on human microflora. This study describes virulence-related phenotypes of two human yeast commensals, Rhodotorula mucilaginosa and Candida parapsilosis, isolated from the International Space Station (ISS). The strains were compared with ATCC control strains to provide insights into adaptation and phenotypic switching of fungal species in spacecraft environments. Strains were grown in media that induce filamentation and capsule production. Antimycotic susceptibility was determined after exposure of liquid cultures to fluconazole, amphotericin B, and caspofungin. Biofilm formation was quantified using the crystal violet assay, and autoinducer (AI) production was detected by activation of a reporter fluorescent gene present in biosensor bacterial strains. In vivo infection studies were conducted using a C. elegans killing model. Results indicated increased filamentation production patterns in ISS Candida parapsilosis and increased capsule production in ISS Rhodotorula. Additionally, there was increased resistance to antifungal activity, biofilm formation, long-chain autoinducer production, and heightened nematode virulence detected in the ISS isolates. These results suggest that space conditions might enhance adaptation and phenotypic plasticity in yeast, leading to increased virulence-related phenotypes.