Molecular Mechanisms of Fluconazole Tolerance and Heteroresistance in Candida tropicalis Isolates Causing Invasive Candidiasis in Immunocompromised Patients in Southeast Nigeria

Candida tropicalis is the third most common cause of invasive candidiasis globally and demonstrates an alarming capacity for fluconazole heteroresistance, a phenotype in which a subpopulation of cells survives drug concentrations above the MIC. This mechanistic molecular study characterized fluconazole tolerance and heteroresistance mechanisms in C. tropicalis isolates from immunocompromised patients at the University of Nigeria Teaching Hospital Enugu, Federal Medical Centre Owerri, and Ebonyi State University Teaching Hospital, representing the South East geopolitical zone of Nigeria. Forty C. tropicalis bloodstream isolates from cancer, transplant, and HIV patients were subjected to population analysis profiling to detect heteroresistance, disk diffusion and broth microdilution for susceptibility characterization, and whole-genome sequencing for resistance mechanism elucidation. Transcriptomics by RNA-seq was performed under fluconazole stress. Heteroresistance was detected in 37.5 percent of isolates classified as fluconazole-susceptible by standard MIC testing. ERG11 point mutations (Y132F, K143R) were identified in 55 percent of isolates with elevated MICs. UPC2 gain-of-function mutations were present in 27.5 percent of strains. RNA-seq demonstrated significant upregulation of CDR1, CDR2, and MDR1 efflux pump transcripts in heteroresistant isolates under drug stress. Heteroresistant isolates demonstrated 4.2-fold higher mortality in a Galleria mellonella infection model. These findings reveal that fluconazole heteroresistance is prevalent and clinically relevant in South East Nigerian C. tropicalis and support treatment escalation to echinocandins, routine population analysis profiling, and sequential antifungal susceptibility monitoring. Keywords: Candida tropicalis, fluconazole heteroresistance, ERG11, invasive candidiasis, immunocompromised.