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Spaceflight and Simulated Microgravity Conditions Increase Virulence of Serratia marcescens in the Drosophila melanogaster Infection Model

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Spaceflight and Simulated Microgravity Conditions Increase Virulence of Serratia marcescens in the Drosophila melanogaster Infection Model

Rachel Gilbert et al. NPJ Microgravity.

Abstract

While it has been shown that astronauts suffer immune disorders after spaceflight, the underlying causes are still poorly understood and there are many variables to consider when investigating the immune system in a complex environment. Additionally, there is growing evidence that suggests that not only is the immune system being altered, but the pathogens that infect the host are significantly influenced by spaceflight and ground-based spaceflight conditions. In this study, we demonstrate that Serratia marcescens (strain Db11) was significantly more lethal to Drosophila melanogaster after growth on the International Space Station than ground-based controls, but the increased virulence phenotype of S. marcescens did not persist after the bacterial cultures were passaged on the ground. Increased virulence was also observed in bacteria that were grown in simulated microgravity conditions on the ground using the rotating wall vessel. Increased virulence of the space-flown bacteria was similar in magnitude between wild-type flies and those that were mutants for the well-characterized immune pathways Imd and Toll, suggesting that changes to the host immune system after infection are likely not a major factor contributing towards increased susceptibility of ground-reared flies infected with space-flown bacteria. Characterization of the bacteria shows that at later timepoints spaceflight bacteria grew at a greater rate than ground controls in vitro, and in the host. These results suggest complex physiological changes occurring in pathogenic bacteria in space environments, and there may be novel mechanisms mediating these physiological effects that need to be characterized.

Keywords: Infection; Innate immunity; Microbiology.

Conflict of interest statement

Competing interestsThe authors declare no competing interests.

Figures

Fig. 1
Fig. 1. Spaceflight-exposed Serratia marcescens show increased virulence in flies.
a Graph showing survival of w1118 flies after injection with ground or spaceflight Db11 samples. b Graph showing in vivo growth of Db11 after injection into w1118 flies. Error bars represent one standard error.
Fig. 2
Fig. 2. Increased virulence of spaceflight-exposed S. marcescens is reversible on the ground.
a Graph showing survival of w1118 flies after injection with first subcultures of ground or spaceflight Db11 samples. b Graph showing in vivo growth of Db11 after injection into w1118 flies. There was no significant difference in survival of w1118 flies after injection with first subcultures, nor was there significant difference revealed in the assay for in vivo bacterial growth. Error bars represent one standard error.
Fig. 3
Fig. 3. Simulated microgravity (SMG) can increase virulence of S. marcescens.
a Graph showing survival of w1118 flies after injection with LSMMG-treated (SMG), RWV control-treated or sham-treated Db11 samples, and b Graph showing in vivo growth of LSMMG-treated (SMG), RWV control or sham-treated Db11 after injection into w1118 flies. Error bars represent one standard error.
Fig. 4
Fig. 4. Measuring host changes in wild-type and mutant ground flies exposed to space-flown S. marcescens.
Survival curves of Imd pathway mutant flies, a and b, and of Toll pathway mutant flies, c and d, after exposure to either Ground 1 bacteria, a and c, or Space 1 bacteria, b and d. All graphs have data from two wild-type flies (y1w* and w1118) for reference. e Volcano plot of the RNA-seq data from ground-reared wild-type D. melanogaster that were infected with space bacteria relative to ground bacteria. RNA was extracted from infected flies 18 h post-infection. The x-axis represents the beta-value, an estimator of fold change, and the y-axis represents the −log10q value, a p-value adjusted for false discovery rate. Red dots represent transcripts with q-value < 0.05, which are considered significantly differentially expressed. f PCA of ground, space, and sham injected flies shows no major difference in distribution of the groups between the conditions, with as much variation occurring within conditions as between conditions.

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