Scientists have conducted a wide variety of experiments onboard space shuttles and at the International Space Station. Recent work has shown that bone density in rodents and humans decreases when organisms spend time in space, and this could be related to changes in the gut microbiome. Rodents that were onboard the international Space Station for a month or more had gut microbiomes that were more diverse compared to rodents that had not been to space. The work has been published in Cell Reports.
"This is just another vivid example showing the dynamic interactions between the microbiome and mammalian hosts. The gut microbiome is constantly monitoring and reacting, and that's also the case when you're exposed to microgravity," said senior study author Wenyuan Shi, a microbiologist and chief executive officer at the Forsyth Institute. A causal link between mircobiome changes and the the loss of bone in microgravity has not yet been established, however.
Our bones don't stay the same throughout our lives; they continue to remodel themselves even when we are fully grown; old bone tissue can be continuously replaced with new bone tissue. The microbes in the gut microbiome can also generate molecules that have an impact on our health and well-being. Some of the molecules that are produced during bacterial metabolism interact with the cells that are crucial for bone remodeling.
The environment can have a significant influence on the gut microbiome. The foods we eat, drugs we consume, and environments we're in can all have an effect. Cosmic radiation and microgravity also seem to impact both bacterial and human cells, noted first study author and microbiologist Joseph K. Bedree of the Forsyth Institute. Microgravity exposure can have an impact on many aspects of physiology including the immune system, musculoskeletal system, circadian rhythm, or stress, or example. Any imbalances in those systems could also disrupt the microbial communities in an organism too, added Bedree.
In this study, the researchers analyzed how prolonged microgravity exposure affected the microbiome, and how that may be related to bone density. Twenty rodents were sent to the International Space Station, ten of which came back to Earth alive after 4.5 weeks, and the investigators assessed changes in the rodent microbiomes after their return. The other ten 'extraterrestrial' rodents spent those extra 4.5 weeks in orbit, for a total of nine weeks, before returning to Earth. A group of twenty rodents also stayed on Earth in identical housing during this time, but without the cosmic radiation or microgravity, as a group of "ground control" rodents. The gut microbiomes of all of these rodents were monitored at various time points: before launch, after they returned to Earth, and when the study was complete. Changes in serum metabolites were also analyzed for the space rodents that spent nine weeks in microgravity.
"This is the first time in NASA history that a rodent has been returned to Earth alive," Shi noted. "This meant we were able to gather information about the change in space, and then monitor their microbiome's recovery when they returned. The good news is that even though the microbiome changes in space, these alterations don't appear to persist upon returning to Earth."
Space rodents had more diverse groups of bacteria in their gut microbiomes compared to ground control rodents, and those that spent time in microgravity carried two species of bacteria that were far more abundant: Lactobacillus and Dorea. The levels of these bacteria were also higher in rodents that spent more time in space.The researchers suggested that these two bacteria may be generating metabolites that are associated with exposure to microgravity.
"When someone's in microgravity and experiencing bone loss, it would make sense that their body would try to compensate and that the biological systems within would be doing that as well, but we need to do more mechanistic studies to truly validate these hypotheses," said Bedree.
The study authors noted that coprophagy, a normal rodent behavior in which they consume their own feces, might have affected the microbiome alterations; space rodents could not perform this behavior while rodents that came back to Earth after 4.5 were able to, and this probably influenced the recovery of their microbiome compositions.
Although this research has shown us how space travel changes the microbiome, more work will be necessary to understand how this is related to changes in bone density.