Increased filamentous growth of Candida albicans in simulated microgravity

Abstract

Knowledge of simulated microgravity (SMG)-induced changes in the pathogenicity of microorganisms is important for success of long-term spaceflight. In a previous study using the high aspect ratio vessel bioreactor, we showed that the yeast species Saccharomyces cerevisiae underwent a significant phenotypic response when grown in modeled microgravity, which was reflected in the analysis of gene expression profiles. In this study, we establish that Candida albicans responds to SMG in a similar fashion, demonstrating that there is a conserved response among yeast to this environmental stress. We also report that the growth of C. albicans in SMG results in a morphogenic switch that is consistent with enhanced pathogenicity. Specifically, we observed an increase in filamentous forms of the organism and accompanying changes in the expression of two genes associated with the yeast-hyphal transition. The morphological response may have significant implications for astronauts' safety, as the fungal pathogen may become more virulent during spaceflight.

Fig. 1

C. albicans budding patterns under normal and SMG conditions. Panels 1–4: Cells cultured in shaker (Control) exhibit either bipolar (1 and 2) or axial (3 and 4) budding; Panels 5–8: Cells cultured in HARVs under SMG conditions (Experimental) exhibit bipolar (5), axial (6), or random budding (7 and 8). The arrows in panels 1 and 3 point to bipolar and axial budding sites, respectively. Scale bar = 2 µm (1000× magnification).

Fig. 2

Cell morphology of C. albicans under normal and SMG conditions. All samples were grown at 30°C in YPD media for continuous log growth. A1–3: Cell morphology at the beginning of growth (T=0) under normal gravity. B1–3: Cell morphology after 25 generations of growth in shaker under normal gravity. C1–3: Cell morphology after 25 generations of growth in HARV under normal gravity. D1–3: Cell morphology after 25 generations of growth in HARV under SMG conditions. Examples of filamentous morphology are noted with white arrows. Scale bar = 6 µm (630× magnification).

Fig. 3

Comparison of hyphal forms induced under normal and SMG conditions. A1–3: Epifluorescence visualization of Calcofluor white staining of control (temperature-induced) filamentation. B1–5: Epifluorescence visualization of Calcofluor white staining of SMG filamentation. Scale bar = 4 µm (1000× magnification).

Fig. 4

Gene expression changes in C. albicans correlated with morphology. The relative expression of HWP1 and YWP1 under normal gravity (Control) and SMG conditions after 40 generations is shown as determined by qRT-PCR analysis (n= 4, *p= 0.04, **p= 0.0002).