Figure 1. Astronaut reaching for D. radiodurans in outer space
People always say to “shoot for the moon,” but what about the rest of the solar system? All you need to do is imagine yourself with the ability to withstand radiation, cold, vacuum, dormancy, oxidative damage, and other factors and you’re halfway there! For Deinococcus radiodurans however, this imagination is not a necessity, but rather its reality. We as humans are always discovering and investigating new and extreme environments, from the deepest, darkest parts of the ocean to the Milky Way and beyond. Oftentimes, these areas are not readily accessible to humans even for small periods of time, let alone expected to be the next place of settlement. Outerspace is one of those places, but in spite of it all, humans have walked on the moon, and it’s only a matter of time before we’re living our best life on Mars! Traversing such a location requires immense skill and preparation and can be incredibly dangerous and taxing on the body, but nonetheless, human inquisition surpasses all. The question of sustaining life outside of Earth is, afterall, at hand. This is where D. radiodurans comes in! Being known as the toughest bacterium in the world isn’t just for show! Its ability to withstand a multitude of pressures can help us narrow down how to take a bigger leap for mankind by helping scientists better understand the effects of microgravity. This condition of people and objects appearing weightless is a well known characteristic of the limits beyond our home planet! Its ability to adapt in response to stressors such as reduced gravity is a characteristic that can help bridge the gap between life on Earth and beyond.
The force of gravity has always been a fact of life, even long before we learned of its presence. Gravity is the force that allows us to walk and jump, all of which is essentially falling. Without it, we would all float away! As a result of its presence on Earth, all organisms have developed within its parameters, which makes its absence in places like space so dangerous. Studies have found that when gravity is reduced or absent, it affects the proteins in our cells, which can lead to motion sickness, impaired eyesight, suppressed immune responses, and decreased muscle, and bone density which may be easily remedied on Earth, but can be fatal beyond our planet.Traveling outside Earth’s gravity can do a serious number on the human body. It’s crucial to study the impact of microgravity so that appropriate safety measures can be implemented.
Figure 2. D. radiodurans flexing its radiation resistance capabilities
Scientists have long studied D. radiodurans' ability to resist radiation. In fact, it is this exact capacity that brought it into the limelight! It’s not that it doesn’t get damaged from extreme conditions, but rather that it can repair the damage that is caused. It is able to do this because of its genetic makeup. Within itself, D. radiodurans’ has a series of redundancies, meaning it basically holds many copies of its genome instead of just one, which allows it to fix itself quite rapidly – often in just a few hours. If this ability could be isolated and better understood, the radioactive barriers to space travel could be eliminated! However, the gravitational barrier (or lack thereof) still remains a cause for concern. Emanuel Ott and his team are looking to change that!
Due to the unique resistances to ionizing radiation, UV radiation, and desiccation, this microbe makes for a great model to study in space. In a previous experiment, after D. radiodurans were exposed to harmful radiation, they were observed to self-repair at high levels when inserted into an environment without gravity. After seeing this rapid recovery, scientists wanted to further research what molecular mechanisms it possessed in order to survive. In a paper, “Molecular response of Deinococcus radiodurans to simulated microgravity explored by proteometabolomic approach,” Emanuel Ott et al. performed an experiment which involved putting the microbe, D. radiodurans into simulated microgravity to test its reaction to a stressful environment. Most microbes would perish with so much stress, but this one found a way to adapt and survive.
Creating realistic micro-gravitational scenarios doesn’t come cheap! Due to the high cost, and extraordinary detail that goes into performing experiments with real microgravity on Earth, Ott and his colleagues decided to use a device that simulates microgravity, called a 2-D clinostat (Figure 3). They grew single cell colonies inside of this device as it rotated very quickly, creating a mostly gravity-free environment to incubate the microbe in. To the researchers’ amazement, D. radiodurans underwent a number of incredible adaptive processes within itself to deal with the newly presented stress. Data showed that there were new features on the outside of the cell that weren’t there in the control group! To better understand how this organism went from lacking these extracellular features to presenting them as a stress-induced response, Ott et al. looked closer at its molecular mechanisms using state-of-the-art technology. They sequenced, measured, plotted, and analyzed tons of proteins in the cell in order to understand these adaptations.
Figure 3. Principle of a 2-D Clinostat: Growing D. radiodurans cells are placed on the axis of rotation attached to the center of a TGB-agar petri dish
Their results showed them that there were a variety of molecules activated in stressed D. radiodurans that weren’t present in the normally incubated microbe. One feature they noticed was a new group of “signal proteins.” Signal proteins are responsible for stimulating additional protein production associated with the envelope of the cell and extracellular features.
Figure 4. Model of D. radiodurans’ primary molecular responses to growth under simulated microgravity
As seen in the figure above, these unique developments begin with a PAS domain – a series of proteins that work as a sensor of the external environment. In this experiment, the domain is able to recognize the lack of gravity and send signals to GGDEF, another protein domain. This domain assists in the transformation of an energy molecule called GTP into c-di-GMP. This final protein is a key supporter of gene expression in bacteria. At the top of Figure 4 you can see a strand of novel DNA being replicated in response to stress that it usually wouldn’t have! These new genes encode important proteins that tell D. radiodurans to modify and create unique extracellular structures that help deal with external stressors. This is shown in the figure on the bottom of the cell. Different features that these genes promote include an increase in extracellular trafficking, transport, and cell to cell interactions. All of these modifications allow the microbe to respond to stress in a productive and self-regulatory way that promotes repair to damaged genes and further induction of transcriptional regulation. Thanks to these mechanisms, it’s able to survive in highly stressful environments!
Studying D. radiodurans has given scientists an insight into how certain microbes survive in stressful environments. There’s still a lot to be discovered about this microbe and other extremophiles like it, but Ott et al. gave a valuable insight into some of the specific molecular mechanisms that allow them to exist in unusual environments, such as microgravity. Future studies have a stable foundation to further assess cell growth under decreased gravity. Additional research that incorporates transcriptomic approaches and electron microscopy techniques have the potential to reveal the inner workings of the molecular mechanisms that allow D. radiodurans to “defy” simulated microgravity. With this information, we can go beyond the moon and shoot for the rest of the universe!
About the authors:
Mahmuda Alam ‘20 is a Biology major from Hudson, New York. She is looking forward to enjoying post-graduate life as she prepares to pursue a career in optometry. Outside of academics, she plays for the Mount Holyoke Ultimate Frisbee Team and is a part of an ongoing documentary named The Hudson Project, directed by Zuzka Kurtz and Geoffrey Hug.
Clara Honigberg ‘21 is a Biology major from Washington, DC. She can’t wait for her senior year at Mount Holyoke. Aside from her passion for microbiology, she is president of the Ultimate Frisbee Team and can often be found playing fetch with her black cat, Ginny.
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