Analysis of Candida albicans gene expression using single cell profiling
Candida albicans is an important fungal pathogen of humans and shows great flexibility in adapting to diverse host niches, which present different ambient pH values, changes in O2 concentrations, various different substrates for metabolism and host defence mechanisms. Knowledge on how these environmental stimuli are sensed by the fungal cells is limited. Here, a bioinformatics approach was employed to search for putative receptors in the C. albicans genome. However, no obvious novel receptor candidates were identified that might function in environmental sensing in C. albicans. Using a GFP-based reporter system gene expression in C. albicans on the single cell level was monitored. To examine responses to ambient pH, the PHR1 and PHR2 promoters, which are regulated in an inverse pH-dependent manner, were used to construct C. albicans strains that could function as pH biosensors in vitro and in vivo. The two strains were used as indicators of ambient pH under various in vitro conditions as well as in ex vivo and in vivo models of infections. In the mouse model of systemic candidiasis the ambient pH in infected kidney tissue appeared to show local variation. The fatty acid beta-oxidation in C. albicans was also examined. Three strains were constructed containing GFP fusions with the promoters of beta-oxidation genes FAA21, POX4 and POT11. The regulation of the three promoters was first studied in vitro and later in different infection models. When introduced into the mouse model of systemic candidiasis differential expression of POX4 and POT11 was observed despite being co-regulated in vitro under the range of conditions examined. Hence, POX4 and POT11 may respond to additional as yet undefined stimuli that are relevant in vivo. Nevertheless, the data indicate that the lipid beta-oxidation is active in some fungal cells growing in the mouse kidney.