Symbiosis Regulation in Microgravity

Whilst space flight’s microgravity is no good for animals, causing bone loss, muscle atrophy and immune dysfunction, microbes seem to flourish in it. At least, pathogens do, with increased growth rates, antibiotic resistance and gene transfer. How microgravity affects symbiotic microbes has not yet been considered.

Microgravity is demonstrated to alter the expression of the global regulator Hfq, an RNA-binding protein that regulates the translation of mRNA according to environmental cues.

Half of all bacteria have an Hfq homologue, including members of prominent symbiont groups. The hfq gene in pathogenic species is down-regulated by microgravity, by mechanisms unknown. To determine the role of hfq in symbioses and how microgravity influences this regulator, the symbiosis between Euprymna scolopes and Vibrio fischeri was used as a model.

V. fischeri colonization of the host topples a sequence of developmental dominoes, leading to the controlled apoptosis of the ciliated epithelial appendages (CEA), once cilial beating has gathered enough bacteria for bioluminescence. These apoptotic cells normally show condensed nuclei after bacterial colonization and CEA regression can be observed within 24 hours, making it an ideal study model.

Microgravity conditions were created using rotating wall vessel bioreactors and Hfq deficient mutants were compared to the wild-type to assess colonization and CEA apoptosis-inducing abilities. These phenotypic characters were also tested in the presence or absence of microgravity.

Expression of hfq in V. fischeri was confirmed to be down-regulated by microgravity, causing acceleration of CEA apoptosis. It was also found to serve no role in colonization of the host light organ. Mutants lacking Hfq showed that the complete absence of this regulator reduced the apoptosis-accelerating effect of microgravity, but yet sped up CEA regression.

Aspects of future research in this topic will need to investigate the changes in transcription between the wild-type and the Hfq-mutants, in order to explore the full role of Hfq. In addition, there is a need for identification of the stimulon (the collection of genes under regulation by the same stimulus) influenced by gravity in both pathogenesis and symbiosis. Hfq’s role in symbiosis development indicates a research gap in which regulators control the diel cycle of this squid-microbe symbiosis, as microgravity appeared to have no effect on the regrowth of V. fischeri populations following expulsion. Faster CEA regression highlights the need to investigate the role of Hfq in quorum sensing; there is also an undetermined possibility that microgravity can regulate genes for extracellular matrix molecules, which may compromise membrane integrity and hasten apoptosis.

This is also an excellent example of how understanding of a marine microbe and its symbiosis is an effective tool for addressing fundamental questions which may apply to many other taxa. The apoptosis shown here to be encouraged by Hfq down-regulation in V. fischeri is part of the bobtail squid’s normal life cycle, but it is not unreasonable to posit that regulation of apoptotic effects may also be something to consider for pathogenic microbes.

Grant, K. C., Khodadad, C. L., & Foster, J. S. (2014). Role of Hfq in an animal–microbe symbiosis under simulated microgravity conditions. International Journal of Astrobiology, 1-9.