Twelve-year surveillance identifies parent B Candida orthopsilosis causing breakthrough candidemia during caspofungin therapy with echinocandin heteroresistance

Background Echinocandin breakthrough infection can occur despite conventional susceptibility results that do not indicate resistance, suggesting that population-level minimum inhibitory concentration (MIC) testing may miss clinically relevant within-isolate heterogeneity. Most clinical Candida orthopsilosis isolates are hybrids, whereas parent B has previously been represented only by marine isolates despite evidence of pathogenic potential and echinocandin fitness. It remains unknown whether parent B can contribute to invasive infection under echinocandin exposure and whether conventional susceptibility testing adequately captures its antifungal response. Results We analysed C. orthopsilosis bloodstream isolates recovered at a tertiary hospital in Guangzhou, within the coastal Pearl River Delta of southern China, from 2010 to 2021. Among 197 C. parapsilosis species-complex isolates, 17 were confirmed as C. orthopsilosis , including six bloodstream isolates. Nuclear and mitochondrial genome analyses assigned one highly homozygous isolate, BS03, to parent B, a lineage previously represented only by marine isolates. A blood culture yielding BS03 was collected after 12 days of ongoing caspofungin therapy. The patient developed septic shock and died two days later. In a Galleria mellonella model, BS03 showed pathogenic potential without significantly greater virulence than the tested fungal comparators. Its caspofungin MIC was 0.5 mg/L, within the wild-type range. Population analysis profiling nevertheless detected resistant subpopulations at 2, 4, and 8 mg/L caspofungin, corresponding to 4×, 8×, and 16× MIC, respectively. BS03 thus fulfilled the predefined criterion for caspofungin heteroresistance. Comparison of GSC1 ( FKS1 ), GSL1 , and GSL2 identified no BS03-specific canonical Fks1 hot spot resistance substitution. Conclusions These findings extend the known clinical spectrum of C. orthopsilosis parent B beyond its previously recognised marine context. The coexistence of a wild-type-range caspofungin MIC with a resistant minority population further highlights within-isolate heterogeneity as an additional dimension of echinocandin susceptibility that may be missed by population-level MIC testing. Clinical trial number Not applicable.