ASM conference on Candida and Candidiasis edition:8 location:Denver, USA date:12-17 March 2006
C. albicans biofilms are often formed on medical devices, including cathethers and silicone voice prostheses. This constitutes a significant risk for patients as these biofilms form a persistent source of infection and result in the deterioration of the material. Biofilm formation has been described as a possible response to stress, e.g. in response to poor nutrient conditions. The trehalose metabolism is known to play an important role in stress resistance in fungi. Trehalose synthesis is induced under a variety of stress conditions and is part of the general stress response in S. cerevisiae. We investigated the role of the trehalose-6-phosphate phosphatase (encoded by TPS2), one of the two enzymes involved in trehalose synthesis, in biofilm formation. Strains of C. albicans deleted for TPS2 showed diminished virulence in a mouse model of systemic infection. In order to assess whether the reduced virulence could (partially) be attributed to reduced biofilm formation, the in vitro biofilm-forming capacities were determined. We also determined the MIC of antimycotic agents against planktonic and biofilm cells, in order to determine if the mutant strain had an altered susceptibility.
Biofilms were grown on silicone disks in a static (microtiter plate), as well as in a dynamic flow-through system (CDC reactor). MIC values for planktonic cells were determined using the EUCAST microdilution method. MIC values for biofilm cells were determined using a microdilution method with biofilms formed on the wells of microtiter plates and various fluoro- and colorimetric assays. Besides the tps2 mutant, we also included strains SC5314, CAI-4 and CAI-2 in our assays.
Compared to SC5314, CAI-2 and CAI-4 planktonic cells, cells of the tps2 mutant showed a reduced growth rate and also showed a different temperature- and pH-dependent floculation behaviour. Planktonic cells of the tps2 mutant were significantly more sensitive to fluconazole than SC5314, CAI-2 and CAI-4 cells. When grown in the biofilm systems, the tps2 mutant formed significantly less biofilms than SC5314. Surprisingly, biofilm formation in strain CAI-2 was also affected. However, biofilm cells of the tps2 mutant were not more susceptible to fluconazole than wild-type biofilm cells.
In conclusion, our results show that C. albicans can behave differently in various biofilm model systems. Further research will be required to establish whether differences in in vitro biofilm-forming capacities are reflected in differences in virulence in in vivo models. The unexpected differences between strains CAI-2 and SC5314 show that the interpretation of mutant phenotypes should be done with care and again highlights the necessity of appropriate control experiments.