Candida inconspicua and Candida norvegensis: new insights into identification in relation to sexual reproduction and genome organization

Abstract

Candida inconspicua and Candida (Pichia) norvegensis are two emerging pathogenic species that exhibit reduced susceptibility to azole derivatives. Conventional (biochemical) approaches do not readily differentiate between the two species. The first aim of this work was to analyze the performance of biochemical, proteomic (matrix-assisted laser desorption ionization-time of flight [MALDI-TOF]), and molecular approaches in the precise identification of these species. These results then led us to sequence 3 genomic loci, i.e., the internal transcribed spacer (ITS) region of the ribosomal DNA (rDNA), the D1/D2 domain of the 28S rDNA, and the elongation factor 1α (EF-1α) gene, either directly or following cloning, of 13 clinical isolates and 9 reference strains belonging to the 5 species included in the Pichia cactophila clade, namely, Pichia cactophila, Pichia insulana, C. inconspicua, C. norvegensis, and P. pseudocactophila. Finally, isolates of C. inconspicua were challenged for sexual reproduction on the appropriate medium. Our results show that EF-1α sequencing and proteic profiling by MALDI-TOF are the two most efficient approaches to distinguish between C. norvegensis and C. inconspicua. As a characteristic of the P. cactophila clade, we found multiple alleles of the rDNA regions in certain strains belonging to the tested species, making ITS or D1/D2 sequencing not appropriate for identification. Whatever the method of identification, including MALDI-TOF and EF-1α sequencing, none could differentiate C. inconspicua from P. cactophila. The results of phylogenetic analysis and the generation of asci from pure cultures of all C. inconspicua strains both support the identification of P. cactophila as the teleomorph of C. inconspicua.