A small subpopulation of blastospores in Candida albicans biofilms exhibit resistance to amphotericin B associated with differential regulation of ergosterol and ß-1,6-glucan pathway genes

Abstract

The resistance of Candida albicans biofilms to a broad spectrum of antimicrobial agents has been well documented. Biofilms are known to be heterogeneous, consisting of microenvironments that may induce formation of resistant subpopulations. In this study we characterized one such subpopulation. C. albicans biofilms were cultured in a tubular flowcell (TF) for 36 h. The relatively large shear forces imposed by draining the TF removed most of the biofilm which consisted of a tangled mass of filamentous forms with associated clusters of yeast. This portion of the biofilm exhibited the classic architecture and morphological heterogeneity of a C. albicans biofilm, and was only slightly more resistant than either exponential or stationary phase planktonic cells. A submonolayer fraction of blastospores that remained on the substratum was resistant to 10 times the AmB dose that eliminated the activity of the planktonic populations. A comparison between planktonic and biofilm populations of transcript abundance for genes encoding for enzymes in the ergosterol (ERG1, 3, 5, 6, 9, 11, and 25) and ß-1,6-glucan (SKN1, KRE1, 5, 6, and 9) pathways was performed by quantitative RT-PCR. The results indicate a possible association between the high level of resistance exhibited by the blastospore subpopulation and differential regulation of ERG1, ERG25, SKN1 and KRE1. We hypothesize that the resistance originates from a synergistic effect involving changes in both the cell membrane and cell wall.