Abstract
The human spaceflight environment is notable for the unique factor of microgravity, which exerts numerous physiologic effects on macroscopic organisms, but how this environment may affect single-celled microbes is less clear. In an effort to understand how the microbial transcriptome responds to the unique environment of spaceflight, the model Gram-positive bacterium Bacillus subtilis was flown on two separate missions to the International Space Station in experiments dubbed BRIC-21 and BRIC-23. Cells were grown to late-exponential/early stationary phase, frozen, then returned to Earth for RNA-seq analysis in parallel with matched ground control samples. A total of 91 genes were significantly differentially expressed in both experiments; 55 exhibiting higher transcript levels in flight samples and 36 showing higher transcript levels in ground control samples. Genes upregulated in flight samples notably included those involved in biofilm formation, biotin and arginine biosynthesis, siderophores, manganese transport, toxin production and resistance, and sporulation inhibition. Genes preferentially upregulated in ground control samples notably included those responding to oxygen limitation, e.g., fermentation, anaerobic respiration, subtilosin biosynthesis, and anaerobic regulatory genes. The results indicated differences in oxygen availability between flight and ground control samples, likely due to differences in cell sedimentation and the toroidal shape assumed by the liquid cultures in microgravity.
Highlights
In certain respects, human spaceflight habitats resemble other confined built environments, such as submersible vehicles, aircraft, hospital isolation wards, or remote research installations.[1]
From the Biological Research in Canister (BRIC)-21 experiment we have previously reported in detail measurements of the growth, antibiotic resistance, frequency and spectrum of mutagenesis exhibited by B. subtilis flight (FL) samples in comparison to matched ground control (GC) samples.[16,17]
RNA-seq was used to characterize the transcriptomic response of B. subtilis cultures exposed to the human spaceflight environment of the International Space Station (ISS) (FL samples) vs. matched GC samples on two separate missions, BRIC-21 (n = 3) and BRIC-23 (n = 9)
Summary
Human spaceflight habitats resemble other confined built environments, such as submersible vehicles, aircraft, hospital isolation wards, or remote research installations.[1]. Extensive investigations conducted in spaceflight on macroscopic organisms have resulted in a relatively good understanding of the biological effects of microgravity and radiation at levels ranging from the whole body down to the organ, cellular, and molecular level in humans,[2] animals,[3] and plants.[4] While microorganisms have been the subject of focused research in the spaceflight environment, it has proven more difficult to understand their responses to spaceflight stress.[5,6,7] From a theoretical perspective, exposure to microgravity results in a number of alterations in a microbial cell’s immediate surroundings, such as loss of convective mass and heat transfer, reduction in mechanical shear forces, and alterations in the way liquids behave at air and solid interfaces. It has been proposed that upon perception of these alterations in their environment, microbes mount a complex set of stress responses (the so-called “spaceflight syndrome”[8])
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