Extracellular alkalinization drives the development of pathogenic Candida albicans biofilms

Candida albicans is the fourth most common cause of nosocomial bloodstream infections that are often initiated by adherence of host cells as well as medical indwelling devices. The formation of fungal biofilms goes along with development of multilayered and polymorphic cell communities surrounded by an extracellular matrix, which promotes high resistance against antifungal treatments.

Biofilm formation is essentially linked to robust filamentation, since filamentation-defective C. albicans strains display thinner biofilms. When grown on host-relevant alternative carbon sources, such as amino acids, the fungus can raise of environmental pH. This process leads to hyphal morphogenesis and can be observed in vitro and within immune cells. Here, the transcription factor Stp2 was shown to be the key regulator, because its loss led to an impaired alkalinization. In contrast to planktonic cells, the growing biofilm develops nutrient and gas gradients, with increasing abundance of alternative carbon sources. Thus, we propose that C. albicans actively alkalinizes the environment during biofilm maturation. Therefore, our goal is to develop methods to measure extracellular pH in biofilms.

In vitro tracing of pH development was facilitated by using pH micro-electrodes and preliminary results identified a diminished pH rise in biofilms of C. albicans stp2Δ deletion mutants in comparison to the wild type strain SC5314. This pH defect correlated with thinner biofilms and a reduction in biomass under static and shear flow conditions. Thus, we predict that the transcription factor Stp2 interconnects amino acid metabolism and biofilm formation. As heterogeneous structures, biofilms comprise environmental microniches and we applied pH-sensitive fluorescent dyes to follow in a non-invasive manner the distribution of pH microscopically. Three-dimensional profiling revealed large neutralized areas in a mature biofilm that was grown in an unbuffered, initially acid medium.

The study of pH-dependent biofilm formation will reveal potential novel targets to combat this cause of severe infections.