## 3 Results
### Cell Morphology and Filamentation  
The propensity of _C. albicans_ to form filaments when confronted with environmental stresses, and the association of this morphology with increased virulence, led us to evaluate cell size and shape as first line analyses to determine whether and how the yeast adapt to a novel environment. Indeed, we and others have demonstrated differences in cell size, cell clumping, and filamentation of _C. albicans_ cultivated in simulated or true microgravity [29; 35; 36; 45]. Initial analyses of the Micro-6 payload suggested that more of the samples cultured in microgravity contained filamented cells than did the corresponding terrestrial control samples. A more thorough analysis of each payload found substantial variability, but did not support the hypothesis that microgravity stimulates a filamentous morphology (Figure 5A). Interestingly, although the overall level of filamented cells was low in each of the payloads, cells cultured in FEP bags had a significantly lower level of filamented cells than those in FPAs, regardless of whether they were cultured in microgravity or on Earth (Figure 5A). Cell size was also variable, with large cells measuring over 7.5 \(\upmu\)m and smaller cells measuring 3.1 \(\upmu\)m, although the average size of yeast form cells was not significantly different between flight (4.8 \(\upmu\)m) and terrestrial controls (4.6 \(\upmu\)m). Filaments were observed within the populations, whether cultivated in microgravity or terrestrially, with some filaments reaching lengths of over 142 \(\upmu\)m (Figure 5C). In contrast to previous studies [28; 35], cell clumping was not observed. Figure 4: Passive seeding of biofilm communities on siliconized rubber membrane coupons. Siliconized rubber membrane coupons were included in the medium (chamber A) of selected FPAs in Micro-14/FPA and allowed to freely seed during the 24 h growth period of the experiment. Samples were fixed in microgravity and following sample return and hardware de-integration, the membranes were recovered and stained with Crystal Violet as an indicator of cells, cellular debris, and extracellular matrix. Stain that is retained on the membrane through a series of three water washing steps (wash 1, 2 and 3) and a final acetic acid wash (aa wash) is suggestive of a tighter integration of the cells with the substrate material. The stain removed by each wash was quantified by optical density at 595 nm. Black line = flight; green line = terrestrial control. Figure 5: Quantification of yeast filamentous cell morphology. Aliquots from each growth conditionin the three payloads were fixed and retained at 4 \({}^{\circ}\)C for microscopic analyses. Some samples were fixed in microgravity, but for those samples returned frozen or viable, the aliquot was fixed immediately upon sample return to the home laboratory. (**A**) Filaments were quantified as a percentage of the total population. In this analysis, samples were aggregated by hardware, thus the FPA data represents samples cultured in YPD from the Micro-6 and Micro-14/FPA payloads. Similarly, the FEP data represents the aggregate from samples cultured in YPD in Large FEP bags and Small FEP bags from the Micro-14/FEP payload. * \(p\) < 0.003 for FPA vs. FEP. (**B-E**) Calcofluor White stained cells were visualized to evaluate cell morphology from Micro-14/FPA, scale bar = 35 \(\upmu\)m; (**B,C**) are from terrestrial control samples and (**D,E**) are from flight samples.