Fungaemia Caused by the Rare Yeast Starmerella sorbosivorans: A Case Report with Identification via Molecular Phylogenetic Analysis and MALDI-TOF MS.

Invasive yeast infections are associated with high mortality rates, particularly in immunocompromised patients. While most cases of fungaemia are caused by Candida species, rare yeast infections occasionally occur. Here, we present the first documented case of a Starmerella sorbosivorans (= Candida sorbosivorans, homotypic synonym) bloodstream infection in a 39 year-old man with chronic intestinal pseudo-obstruction requiring parenteral nutrition via a tunnelled central venous catheter. The patient presented with fever and was diagnosed with catheter-related bloodstream infection. Initial biochemical identification suggested S. magnoliae, but MALDI-TOF mass spectrometry (MS) failed to provide a definitive identification. Molecular phylogenetic analysis using multiple loci (18S rDNA, internal transcribed spacer [ITS] region, 26S rDNA-D1/D2, TEF1α, and RPB2), combined with proteomic analysis using MALDI-TOF MS, confirmed the isolate as S. sorbosivorans, despite a low ITS sequence similarity (91.3%) to the type strain. The patient was successfully treated with sequential administration of micafungin and liposomal amphotericin B, along with removal of the catheter. Antifungal susceptibility testing revealed low minimum inhibitory concentrations (MICs) for micafungin, amphotericin B, and voriconazole, but high MICs for fluconazole. Our findings emphasise the importance of multilocus sequence analysis for accurate species identification, especially in rare yeast infections caused by uncommon species such as S. sorbosivorans, for which conventional methods may be insufficient. However, to improve the applicability of MALDI-TOF MS in clinical settings, the reference library should be expanded to include rare and emerging yeast species.