Beyond resistance: insights into subpopulation responses to antifungals

Treatment of fungal infections in animals, or fungal infestations of crops, requires an understanding of how much of an antifungal agent needs to be applied, how often to use it and what types of responses to expect. In general, clinical studies have focused on the minimum inhibitory concentration: the concentration of drug at which 50% or more of the cells exposed to the drug do not grow. Above this concentration, cells are said to be susceptible. However, many patients infected with susceptible isolates suffer persistent or recurrent infections despite being treated with drug concentrations above the MIC for the infecting organisms. Phenomena known as tolerance, trailing growth, heteroresistance or persistence have been noted and are due to subpopulations of cells that grow slowly in the presence of the drug. These subpopulations, together with the selective pressure of drug treatment acting upon them, have the potential to drive the evolution of new mechanisms for survival of the pathogen. We have been studying tolerance and heteroresistance in Candida albicans and Candida glabrata, two of the most prevalent human fungal pathogens. This talk will highlight the growth dynamics, physiological responses and evolutionary responses of subpopulations that survive and grow, albeit slowly, in the presence of severe antifungal stress. Mechanisms of survival in drug appear to involve: changes in chromosome stoichiometry, usually via aneuploidy and its effects on the expression of specific genes; heritabe shifts in protein states (e.g., prion-like aggregates); and shifts in cellular metabolic states.