Six-month surveillance of Candida parapsilosis in Tyrol, Austria: high-risk ST11 lineage and early, heterogeneous fluconazole resistance.

INTRODUCTION

Candida parapsilosis is an emerging cause of invasive candidiasis, driven by nosocomial transmission, environmental persistence, and rising azole resistance. Following an increase in detections at our institution, we investigated clinical C. parapsilosis isolates collected in Tyrol, Austria (February-July 2024).

METHODS

Susceptibility to fluconazole and anidulafungin was assessed by CLSI broth microdilution (BMD) and gradient diffusion (Etest) methods. Clonal relatedness was evaluated by whole-genome sequencing (WGS) with SNP-based phylogeny and multilocus sequence typing (MLST); azole-associated genes were analyzed in fluconazole non-susceptible isolates.

RESULTS

WGS demonstrated a genetically diverse population comprising six clusters, arguing against a single clonal outbreak. The most concerning cluster contained closely related ST11 isolates linked to previously described epidemic ST11 lineages and included both fluconazole-susceptible and non-susceptible isolates; non-susceptibility occurred in only one patient. Non-susceptible isolates carried multiple predominantly heterozygous non-synonymous variants across azole-associated pathways, consistent with an early, genetically heterogeneous stage of resistance development. Marked method-dependent discrepancies were observed for anidulafungin (all susceptible by CLSI BMD, but a substantial fraction non-susceptible by gradient diffusion method), while fluconazole showed higher concordance with minor misclassification.

DISCUSSION: Combining WGS with standardized susceptibility testing differentiated increased prevalence from clonal transmission, identified a high-risk ST11 lineage with a complex, likely polygenic resistance architecture, and highlighted limitations of gradient diffusion testing for echinocandin categorization. These findings support molecular surveillance and cautious interpretation of gradient diffusion susceptibility testing results to guide infection control and antifungal stewardship.