Antifungal tolerance is a subpopulation effect distinct from resistance and is associated with persistent candidemia

Abstract

Tolerance to antifungal drug concentrations above the minimal inhibitory concentration (MIC) is rarely quantified, and current clinical recommendations suggest it should be ignored. Here, we quantify antifungal tolerance in Candida albicans isolates as the fraction of growth above the MIC, and find that it is distinct from susceptibility/resistance. 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 intracellular drug accumulation. Many adjuvant drugs used in combination with fluconazole, a widely used fungistatic drug, reduce tolerance without affecting resistance. Accordingly, in an invertebrate infection model, adjuvant combination therapy is more effective than fluconazole in treating infections with highly tolerant isolates and does not affect infections with low tolerance isolates. Furthermore, isolates recovered from immunocompetent patients with persistent candidemia display higher tolerance than isolates readily cleared by fluconazole. Thus, tolerance correlates with, and may help predict, patient responses to fluconazole therapy.

Puf4 Mediates Post-transcriptional Regulation of Caspofungin Resistance inCryptococcus neoformans

“…Microbial cultures show molecular and phenotypical heterogeneity that may be important within the scope of antimicrobial resistance (33). Histogram graphs of the calcofluor staining showed that the increase in chitin content is at a sub-population level (Fig 7B).…”

Candida albicans exhibits distinct cytoprotective responses to anti-fungal drugs that facilitate the evolution of drug resistance

“…That is, the source of the cells physiological ability to survive in the presence of the inhibitory compound is not due to stable genomic modifications, but rests upon post-transcriptional programs such as position in the cell cycle, expression of stress genes, or random fluctuation in key cellular defense genes. In general, tolerance is not nearly as well understood as drug resistance 37,38 . Even when grown under standard laboratory conditions, isogenetic (or near isogenic) Candida populations respond differentially to antifungal drugs, with some fraction of the population exhibiting drug tolerance, defined as the ability of individuals to survive and grow at drug concentrations above the minimal inhibitory concentration (MIC) 37,39 .…”

Candida albicansgenetic background influences mean and heterogeneity of drug responses and genome stability during evolution to fluconazole

“…Antifungal drug tolerance, a property distinct from drug resistance (2,3) , is the ability of some cells in a strain to grow slowly in the presence of a drug at concentrations above the MIC. In tolerant strains, the subpopulation of cells that grow (generally from 10-90% of cells, depending on the strain) is usually evident when growth is assessed after 48 h or longer in the drug (reviewed in 4) .…”

“…Clinically, the failure to clear infections is more likely when the infecting strain is drug-resistant. High levels of drug tolerance also may influence infection clearance (3,7) , although more studies that use quantitative criteria to definitively measure tolerance levels and distinguish tolerance from resistant isolates are needed (reviewed in 4) .…”

“…A mechanistic link between drug resistance and specific aneuploidies exists in multiple pathogenic fungal species (e.g., C. albicans, 22, 25-28 ; Cryptococcus neoformans , 29, 30 ; suggested in Candida auris, 31,32) . Because fluconazole tolerance and drug resistance are distinct and tolerance is sensitive to inhibitors that do not affect resistance (2,3) , it follows that antifungal tolerance likely evolves via a different subset of genes than resistance, although it is also possible that there is overlap between genes involved in tolerance and resistance.…”