Ene, I. V. | Search bioRxiv

bioRxiv2018

A Global Analysis of Mutations Accompanying Microevolution in the Heterozygous Diploid Pathogen Candida albicans

Candida albicans is a heterozygous diploid yeast that is a commensal of the human gastrointestinal (GI) tract and a prevalent opportunistic pathogen. Here, whole-genome sequencing was performed on multiple C. albicans isolates passaged in different niches to characterize the complete spectrum of mutations arising during microevolution. We reveal that evolution during short time-scales (<600 generations) is driven by both de novo base substitutions and short-tract loss of heterozygosity (LOH) events. In contrast, large-scale chromosomal changes are relatively rare, although chromosome 7 trisomies repeatedly emerged during passaging in one GI colonization model. Both strain background and chromosomal features affected mutational patterns, with mutation rates being greatly elevated in regions adjacent to emergent LOH tracts. Mutation rates were also elevated during host infection where genomes showed strong evidence of purifying selection. These results establish the genetic events driving C. albicans evolution and that this heterozygous diploid is extensively shaped by purifying selection.

genomics↗

bioRxiv2017

Clearing the Fungal FoG: Perseverance, a property distinct from resistance, is associated with clinical persistence

Drug susceptibility, defined by the minimal inhibitory concentration (MIC), often does not predict whether fungal infections will respond to therapy in the clinic. Tolerance at supra-MIC antifungal drug concentrations is rarely quantified and current clinical recommendations suggest it be ignored. Here, we measured and characterized drug-response variables that could influence the outcomes of fungal infections and be generalizable across major clades of Candida albicans, one of the most frequently isolated human fungal pathogens. We quantified antifungal tolerance as the fraction of growth (FoG) above the MIC and found that it is clearly distinct from susceptibility/resistance measured as MIC. Instead, tolerance is due to the slow growth of subpopulations of cells that overcome drug stress more efficiently than the rest of the population, and correlates inversely with the accumulation of intracellular drug. Importantly, many adjuvant drugs used together with fluconazole, a fungistatic drug, reduce tolerance without affecting resistance. These include inhibitors of major stress response hubs such as Hsp90, calcineurin, PKC1 and TOR. Accordingly, in an invertebrate infection model, adjuvant combination therapy was significantly more effective than fluconazole alone in treating highly tolerant isolates and did not improve the treatment of isolates with low tolerance levels. Furthermore, isolates recovered from immunocompetent patients with persistent candidemia displayed significantly higher tolerance than isolates that were readily cleared by fluconazole. Thus, tolerance correlates with the response to fluconazole therapy in patients and may help predict whether infections will respond to fluconazole alone. Similarly, measuring tolerance may provide a useful clinical parameter for choosing appropriate therapeutic strategies to overcome persistent clinical candidemia.

microbiology↗