Authors: Jacqueline M. Acres, Myka Jaap Youngapelian, Jay Nadeau
Institutions: Portland State University; JPL
Our review article entitled “The influence of spaceflight and simulated microgravity on bacterial motility and chemotaxis” was published in the journal npj Microgravity. Here, we would like to offer our thoughts and motivation for submitting a literature review on this subject.
Why did we think a review on this subject was necessary?
As humans devote more resources to space exploration and potentially colonization, we face fundamental questions about the nature of life. Using knowledge of how microbes have evolved and adapted on Earth, we continue to try to understand how life might continue to evolve on places like the weightless environment of the International Space Station, or even exist on the low gravity environment of Earth’s moon, or Jovian and Saturnian satellites such as Enceladus.
One way to understand microbial adaptation is through motility. Motility, a fundamental signature of life, is essential for organisms to seek out nutrients, avoid dangers and perhaps even build communities (i.e., biofilm). There are two different routes to pursue in understanding how bacterial motility has been impacted by an experimental alteration in conditions. One way is to study bacterial phenotypes using various imaging techniques such as phase contrast microscopy or digital holographic microscopy. These techniques impart knowledge of motility patterns, swimming speeds and biofilm formation. Advances in these techniques--such as non-invasive cell labeling, high throughput of imaging data, and high-speed cameras--provide unprecedented access to a world that we are only beginning to understand and categorize. Phenotypic characterizations of bacterial motility remain rare, as imaging in simulated microgravity chambers and on the ISS is very limited. A more common approach, which we have focused on in this article, is to study genomic/transcriptomic/proteomic changes that are associated with alterations in motility and chemotaxis. While we do not yet have a precise correlation between genetic or transcriptional changes and phenotype, these studies provide an excellent starting point for future design of imaging instrumentation and experiments.
What results stood out from our review?
One result that stands out from our review are the changes in response to motility genes after spaceflight and simulated microgravity. While the downregulation of hfq expression is the most consistent finding, motility and chemotaxis gene expression did not seem to have a standard response across bacterial genera or between simulated microgravity and spaceflight. On the surface, this difference can be explained by the fact that while simulated microgravity represents certain aspects of the microgravity environment--including low fluid shear and lack of sedimentation--these experiments are still conducted on Earth and microbes are put into a moving fluid environment. In short, while simulated microgravity mimics aspects of the spaceflight environment, it is not the exact same as that environment. What is interesting, however, is that motility gene expression also differed between simulated microgravity instruments. This difference could indicate that there are other factors involved with these instruments that are not fully understood.
An important consideration of simulated microgravity devices are the fluid environments they create. Recent work has shown that under confinement, liquid environments and bacteria can form active suspensions. The interdisciplinary nature of active suspensions combines such fields as fluid mechanics and statistical physics and is still under development. Future work would involve a combined approach of analyzing these so-called active suspensions using simulation, experiment, and the quantifying the ways in which gene expression in bacteria is impacted.
How does our review factor into the bigger picture of space research?
As of the publication of this article, the question of whether bacteria sense gravity, and if they do, by what mechanism, is still open. It is generally understood that eukaryotes sense gravity. Certainly, astronauts are subject to health and safety concerns such as reduced bone mass and suppressed immune systems after prolonged stays on the International Space Station due to the lack of normal gravity. An article from the 1960s posed that bacteria are too small to sense gravity. Recently, a review article published in 2021 detailed analogues between mechanosensing in both eukaryotes and prokaryotes, comparing various cell structures. They hypothesized that mechanosensing is a way in which bacteria might, in fact, sense gravity.
Whether microbes sense gravity by mechanosensing or some other means, genomic/transcriptomic/proteomic changes are quantifiable ways in which bacteria adapt to their environment. Determining what other environmental cues might also prompt these responses remains to be explored.