Anokhi Kholwadwala 

Background: Compared to Earth, those living aboard the ISS are exposed to reduced microbial diversity and higher concentrations of synthetic chemicals¹. Astronauts exposed to this environment experience increased gut permeability wherein substances from the gut leak into the bloodstream causing issues like inflammation². The presence of microgravity is another major factor affecting human health in space. Studies performed in simulated microgravity environments on Earth have demonstrated an increase in bacterial taxa commonly associated with a sedentary lifestyle, a decrease in beneficial bacteria such as those producing short chain fatty acids, and an increase in inflammatory biomarkers³.

Methods: The initial search terms used for this literature review in Google Scholar and PubMed were “space” AND “microbiome.”

Results: A significant impact of these microbiome changes comes at their intersection with immune suppression. In one study on Earth, researchers seeded U937 cells into a Rotary Cell Culture System to simulate microgravity⁴. The cells were infected with Escherichia coli and compared against a control group of cells similarly infected with E. coli, but cultured outside of the Rotary Cell Culture System⁴. The researchers found that simulated microgravity suppresses the innate immune system through inhibition of the Erk MAPK and p38 pathways, resulting in decreased production of TNF and IL-6⁴. As a follow up to that experiment, the researchers also found that introducing a probiotic to mice in a simulated microgravity environment decreased both the microbiome alterations and the degree of immune suppression experienced by the mice⁴. Another study taking periodic saliva samples from nine astronauts on the ISS found that Varicella zoster virus was detected in the saliva of four of the nine crew members, with reactivation occurring in space and peaking by day 90 of their mission⁵.

Conclusions : Given the documented effects of the space environment on the microbiome and immune suppression, researchers propose a personalized prebiotic, postbiotic and symbiotic routine to be followed before, during and after each astronaut’s time in space [6]. They suggest that doing so would reduce the microbiome alterations and immune disturbances experienced by astronauts. By tailoring each plan to existing astronaut health issues and continuing this supplementation back on Earth, we may be able to reduce any long-term effects of microbiome alteration [6]. As we embark on longer duration missions in space, we need to plan for the long term health of our astronauts, especially through purposeful modulation of crew microbiomes.

Works cited

1. Salido RA, Zhao HN, McDonald D, et al. The International Space Station has a unique and extreme microbial and chemical environment driven by use patterns. Cell. 2025;188(7):2022-2041.e23. doi:10.1016/j.cell.2025.01.039
2. Akinsuyi OS, Xhumari J, Ojeda A, Roesch LFW. Gut permeability among astronauts during space missions. Life Sci Space Res. 2024;41:171-180. doi:10.1016/j.lssr.2024.03.003
3. Ramos-Nascimento A, Grenga L, Haange SB, et al. Human gut microbiome and metabolite dynamics under simulated microgravity. Gut Microbes. 2023;15(2):2259033. doi:10.1080/19490976.2023.2259033
4. Wang J, Han C, Lu Z, et al. Simulated microgravity suppresses MAPK pathway-mediated innate immune response to bacterial infection and induces gut microbiota dysbiosis. FASEB J. 2020;34(10):12345-12356. doi:10.1096/fj.202001428R
5. Voorhies AA, Ott CM, Mehta S, et al. Study of the impact of long-duration space missions at the International Space Station on the astronaut microbiome. Sci Rep. 2019;9:9911. doi:10.1038/s41598-019-46303-8
6. Arora S, Puri S, Bhambri N. A designer diet layout for astronauts using a microbiome mediated approach. FEMS Microbiol Lett. 2022;369(1):fnac049. doi:10.1093/femsle/fnac049