Interaction of Candida albicans Biofilms with Antifungals: Transcriptional Response and Binding of Antifungals to Beta-Glucans

Abstract

Candida albicans can form biofilms that exhibit elevated intrinsic resistance to various antifungal agents, in particular azoles and polyenes. The molecular mechanisms involved in the antifungal resistance of biofilms remain poorly understood. We have used transcript profiling to explore the early transcriptional responses of mature C. albicans biofilms exposed to various antifungal agents. Mature C. albicans biofilms grown under continuous flow were exposed for as long as 2 h to concentrations of fluconazole (FLU), amphotericin B (AMB), and caspofungin (CAS) that, while lethal for planktonic cells, were not lethal for biofilms. Interestingly, FLU-exposed biofilms showed no significant changes in gene expression over the course of the experiment. In AMB-exposed biofilms, 2.7% of the genes showed altered expression, while in CAS-exposed biofilms, 13.0% of the genes had their expression modified. In particular, exposure to CAS resulted in the upregulation of hypha-specific genes known to play a role in biofilm formation, such as ALS3 and HWP1. There was little overlap between AMB-or CAS-responsive genes in biofilms and those that have been identified as AMB, FLU, or CAS responsive in C. albicans planktonic cultures. These results suggested that the resistance of C. albicans biofilms to azoles or polyenes was due not to the activation of specific mechanisms in response to exposure to these antifungals but rather to the intrinsic properties of the mature biofilms. In this regard, our study led us to observe that AMB physically bound C. albicans biofilms and beta-glucans, which have been proposed to be major constituents of the biofilm extracellular matrix and to prevent azoles from reaching biofilm cells. Thus, enhanced extracellular matrix or beta-glucan synthesis during biofilm growth might prevent antifungals, such as azoles and polyenes, from reaching biofilm cells, thus limiting their toxicity to these cells and the associated transcriptional responses.

In Vitro Interaction between Alginate Lyase and Amphotericin B against Aspergillus fumigatus Biofilm Determined by Different Methods

“…Although the FIC index assessment could be not fully appropriate in this situation, in which one of the agents is an enzyme with no effect on its own, some conclusions can be drawn. Data emerging from the C. albicans biofilm field demonstrate that AMB, facilitated by the amphiphilic nature, physically binds to ␤-1,3-glucans, a structural component of the fungal cell wall as well as the C. albicans biofilm ECM (41), and such a drug-sequestering ECM might hamper the polyene from reaching biofilm cells. So, it is plausible that the lowering of antifungal MICs for A. fumigatus biofilms in the presence of AlgL detected here could be ascribed to a nonspecific degradation of the ECM by the enzyme, thus enabling both AMB and LAMB to exert their antifungal action.…”

Inactivation of genes TEC1 and EFG1 in Candida albicans influences extracellular matrix composition and biofilm morphology

“…Furthermore, it protects against phagocytic cells and works as a scaffold for preserving biofilm integrity by limiting the diffusion of noxious substances into the biofilm [32]. Although biofilm resistance is multifactorial [33], the protection exerted by the ECM is a key factor in the high levels of antifungal drug resistance displayed by C. albicans biofilms [13,14,34]. It has been demonstrated that transcriptional regulatory genes in C. albicans, including TEC1 and EFG1, regulate biofilm formation [23,35,36].…”

“…Antifungal resistance in Candida biofilms is multifactorial and is associated with the physiological state of the cells, the activation of drug efflux pumps, and the protective effect of the ECM performed by β-glucans [alkali-soluble polysaccharides (ASPs)], which bind to fluconazole [13] and amphotericin B, preventing the penetration of drugs into the biofilm [14]. In addition to the protective effects of the ECM due to β-glucan, it has been shown that the extracellular DNA (eDNA) is another key component contributing to the structural integrity of C. albicans biofilms [15].…”

Fluconazole impacts the extracellular matrix of fluconazole-susceptible and -resistant Candida albicans and Candida glabrata biofilms

“…Antifungal resistance of Candida biofilms is multifactorial and involves the stimulation of drug efflux pumps, the physiological state of cells, and the protection exerted by the ECM via β-glucans that bind to FLZ and amphotericin B [22], which limits diffusion of these antifungals through biofilms [23]. C. albicans biofilms that grow under constant flow produce higher quantities of ECM than statically cultured biofilms.…”