Molecular and biological characteristics of two rare bloodstream Candida isolates: Candida nonsorbophila and Candida sonorensis.

Background: The incidence of new infections caused by rare Candida species has been steadily increasing, particularly in immunocompromised patients. This study investigates two rare Candida species responsible for Candida bloodstream infections and explores their molecular characteristics.

Methods: Clinical Candida strains were continuously isolated from the lower respiratory tract and blood specimens of a patient. Identification was performed using conventional culture techniques, ITS sequencing, and whole-genome sequencing. Additionally, antifungal susceptibility testing, phylogenetic analysis, macrophage survival assays, and in vivo survival experiments were conducted to evaluate the antifungal resistance, infection source, and pathogenicity of the isolates.

Results: Molecular identification confirmed that the RP (pinkish-purple colonies from respiratory specimens), RW (pinkish-white colonies from respiratory specimens), and BP (pinkish-purple colonies from peripheral blood) strains were Candida nonsorbophila, while the BW (pinkish-white colonies from peripheral blood) strain was identified as Candida sonorensis. Phylogenetic analysis revealed that the RP strain from the lower respiratory tract and the BP strain from the bloodstream belonged to the same clonal lineage, suggesting that the pulmonary isolate entered the bloodstream, resulting in candidemia. Antifungal susceptibility testing showed that C. nonsorbophila RW strain exhibited significant resistance to fluconazole, likely due to the E70D mutation in the ERG11 gene. Both C. sonorensis and C. nonsorbophila exhibited relatively weak virulence, with no significant differences in pathogenicity between single-strain infections and mixed infections of both species (P > 0.05).

Conclusion: This study successfully isolated C. nonsorbophila and C. sonorensis from clinical specimens, providing detailed microbiological and molecular characterization. Rare fungal infections in immunocompromised patients require careful consideration.