Phenotypic switching in Candida lusitaniae on copper sulfate indicator agar: Association with amphotericin B resistance and filamentation

Candida lusitaniae is an opportunistic yeast pathogen that has the ability to develop resistance to amphotericin B (AmB). The mechanism(s) for this resistance is not well understood, although there are data supporting mutations in sterol pathways and other data supporting phenotypic switching (PS). The goal of this study was to determine whether C. lusitaniae has a PS system and to characterize any phenotypes, including any changes in AmB MICs. When 10 ⁴ CFU of an AmB-resistant (MIC of 16 to 32 μg/ml) clinical strain was plated on yeast-peptone-dextrose (YPD) agar with 1 mM CuSO₄, three colony colors were observed: light brown (LB) ≫ dark brown (DB) > white (W), similar to the result for Candida glabrata. Switching did occur with high AmB resistance (MIC of 256 μg/ml) being associated with W, whereas LB and DB colonies had MICs of 2 to 8 μg/ml and 2 to 16 μg/ml, respectively. Filamentation (pseudohyphae) was associated with DB colonies. All phenotypes occurred spontaneously with greater frequency (∼10^-2 to 10 ^-4) than spontaneous mutations, and all phenotypes were reversible, fulfilling the two PS criteria. High AmB MICs were always associated with W colonies but not with all W colonies. Detection of PS on YPD-CuSO₄ is also similar to that in Candida glabrata, and we hypothesize that this is due to similarities in metallothionein gene expression. Phenotypic switching represents a key strategy in C. lusitaniae that confers a selective advantage during environmental challenges, including the ability to switch to AmB resistance.