Unrecognized azole resistance among Candida parapsilosis causing bloodstream infections

About This Grant

Project Summary Candida spp. are the 4th leading cause of bloodstream infections (BSIs) in the US. C. parapsilosis (CP), the 2nd or 3rd most common cause of candidemia, is designated as a high priority fungal pathogen for research by the World Health Organization. Mortality rates for CP BSIs are ~20% despite treatment with echinocandins (ECHs), the frontline antifungal class. ECH resistance is uncommonly identified among CP clinical strains, and most ECH treatment failures of CP BSIs are not linked to a resistant strain. Antifungal heteroresistance (a low-frequency subpopulation of resistant cells co-existing with susceptible cells) and tolerance (some cells grow better than controls in presence of drug without minimum inhibitory concentration changes) are reported in CP and other spp. The clinical relevance of ECH HR or tolerance is not broadly validated, but recent studies have implicated the phenotypes in at least some patients failing treatment for CP BSIs. The long-standing model is that almost all sterile site infections, including candidemia, stem from a single, clonal organism that passes through a bottleneck to establish disease. Our preliminary data challenge the “single organism” model by demonstrating that blood cultures (BCs) from individual patients with C. glabrata, C. albicans and CP BSIs are comprised of mixed populations of genetically and phenotypically diverse strains, including strains exhibiting differences in antifungal-resistance or tolerance that were not recognized by the clinical lab. Our objectives in this project are to characterize genetic and phenotypic diversity of CP populations in baseline and ECH treatment failure BCs from individual patients, and to implicate specific genetic variants and genes in ECH heteroresistance or tolerance. We hypothesize that certain CP genetic variants or genes enriched in BCs from ECH treatment failures will contribute to ECH heteroresistance or tolerance, and that heteroresistance or tolerant strains are present, but unrecognized in baseline BCs from some pts. We will test these hypotheses by pursuing 2 specific aims. In aim 1, we will perform deep whole genome sequencing on CP populations from baseline and ECH treatment failure BCs from 10 pts. We will prioritize CP genetic variants and genes enriched in treatment failure BCs, and assay strains containing these variants and genes for ECH heteroresistance and tolerance in vitro. In aim 2, we will validate the impact of CP genetic variants and genes on ECH treatment responses during BSIs. We will create isogenic mutant strains for genetic variants and genes. Then, we will test strains for ECH heteroresistance and tolerance in vitro and for ECH treatment responses during mouse BSIs. This study will afford new insights into CP responses to ECH exposure during BSIs and identify novel genetic determinants of ECH heteroresistance and tolerance. Our findings will provide a foundation for future studies of clinical significance of within-pt CP strain diversity during BSIs and mechanisms by which CP genes promote heteroresistance and tolerance. By challenging the single organism model, the study has potential to reshape current clinical and clinical microbiology lab practices.