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.

Candida species rank high among nosocomial causes of bloodstream infection and are associated with higher rates of acute mortality compared to leading bacterial pathogens. 1,2 Clinical trials and meta-analyses have demonstrated the superiority of echinocandins over fluconazole for the treatment of invasive candidiasis, 3,4 driving the adoption of echinocandins as front-line agents. 5,6 There is concern, however, that this shift may promote the emergence of echinocandin-resistant Candida strains, a trend already apparent in some hospitals. 7 Current Infectious Diseases Society of America directives consider fluconazole an acceptable alternative to echinocandins for patients who are not critically ill and are considered unlikely to harbour fluconazole-resistant

24Drug resistance, defined by the minimal inhibitory concentration (MIC), often does not predict 25 whether fungal infections will respond to drug treatment in the clinic. Here we define and (which was not peer-reviewed) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity.