Abstract
The major factor influencing the behavior of microbes growing in liquids in space is microgravity. We recently measured the transcriptomic response of the Gram-positive bacterium Bacillus subtilis to the microgravity environment inside the International Space Station (ISS) in spaceflight hardware called Biological Research in Canisters-Petri Dish Fixation Units (BRIC-PDFUs). In two separate experiments in the ISS, dubbed BRIC-21 and BRIC-23, we grew multiple replicates of the same B. subtilis strain in the same hardware, growth medium, and temperature with matching ground control samples (npj Micrograv. 5:1.2019, doi: 10.1038/s41526-018-0061-0). In both experiments we observed similar responses of the transcriptome to spaceflight. However, we also noted that the liquid cultures assumed a different configuration in microgravity (a toroidal shape) compared with the ground control samples (a flat disc shape), leading us to question whether the transcriptome differences we observed were a direct result of microgravity, or a secondary result of the different liquid geometries of the samples affecting, for example, oxygen availability. To mitigate the influence of microgravity on liquid geometry in BRIC canisters, we have designed an insert to replace the standard 60-mm Petri dish in BRIC-PDFU or BRIC-LED sample compartments. In this design, liquid cultures are expected to assume a more disk-like configuration regardless of gravity or its absence. We have: (i) constructed a prototype device by 3D printing; (ii) evaluated different starting materials, treatments, and coatings for their wettability (i.e., hydrophilicity) using contact angle measurements; (iii) confirmed that the device performs as designed by drop-tower testing and; (iv) performed material biocompatibility studies using liquid cultures of Bacillus subtilis and Staphylococcus aureus bacteria. Future microgravity testing of the device in the ISS is planned.
Highlights
Effective handling of liquids in microgravity and low-acceleration environments is essential for optimal performance of numerous systems in spaceflight such as propulsion, heating/cooling, water purification, and life support
Biological Research in Canister (BRIC) hardware samples are contained within five separate compartments called Petri Dish Fixation Units (PDFUs), each of which encloses the bottom of one 60-mm Petri dish, which in turn contains the sample culture (Wells et al, 2001). Designed for organisms such as plant seedlings cultivated on semisolid agar media (Wells et al, 2001; Kiss et al, 2007; Kruse et al, 2020), we subsequently introduced modifications in BRICPDFU protocols to enable cultivation of microorganisms in liquid media (Fajardo-Cavazos and Nicholson, 2016b), and to date have successfully cultivated the Gram-positive bacteria Staphylococcus epidermidis, S. aureus, and Bacillus subtilis on three separate missions to the International Space Station (ISS), designated BRIC-18, BRIC-21 and BRIC-23 (Fajardo-Cavazos and Nicholson, 2016b; Fajardo-Cavazos and Nicholson, 2016a; Morrison et al, 2017; Fajardo-Cavazos et al, 2018; Morrison et al, 2019)
We noted that among the differentially expressed transcripts in the FL vs. ground control (GC) samples were a number of oxygen-responsive genes, indicating that GC cultures were receiving less oxygen than FL cultures (Morrison et al, 2019). These results suggested to us that the differential expression of some genes in FL vs. GC samples may be a secondary effect of liquid geometry, O2 availability, and not a primary effect of microgravity (Morrison et al, 2019). To remove this potentially confounding factor from future BRICPDFU experiments, in this communication we describe the design, fabrication and testing of a novel Petri dish insert designed to mitigate the effect of liquid geometry in microgravity
Summary
Effective handling of liquids in microgravity and low-acceleration environments is essential for optimal performance of numerous systems in spaceflight such as propulsion, heating/cooling, water purification, and life support. We designed and constructed a modified Petri dish insert intended to mitigate the effect of microgravity on liquid configuration in BRIC hardware (Figure 2).
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