The idea of prolonged human space flight to another planet or an asteroid may have been the fodder of Jules Verne's imagination or Ray Bradbury's dreams, but today such ventures are the source of serious discussion at NASA. What potential frailties of the human experience may become manifest during an 18-month round trip mission to Mars? At one point, crew members will be over 200 million miles from Earth with a 20 minute delay in one-way communication. The complex interactions between humans and microbes in and on our bodies (i.e., our microbiome) evolved on Earth in the presence of gravity. During a mission to Mars or to a far-off asteroid, exposure to microgravity will have myriad effects on astronauts, as well as their microbiomes. In addition, more intensive radiation exposure and stress experienced during prolonged isolation will have additional, potentially detrimental effects.
Dysregulation of the human immune response occurs in microgravity, which may increase vulnerability to infection. At the same time, bacteria in microgravity have greater resistance to antibiotics, they may become more virulent, and the growth kinetics of bacteria is enhanced. What about the confines of a spacecraft? Past missions demonstrate that living and working quarters can become heavily contaminated with microorganisms on high-touch surfaces and floating in the air. Microgravity also profoundly alters the aerobiology of microbes expelled in a cough or sneeze. Thus, during a mission to Mars or to a far-off asteroid, astronauts may have a somewhat immunosuppressed physiologic status while microbes become empowered, and all of this occurs in constrained living and working quarters cohabitated for 18 months.
Preflight countermeasures directed at the crew are the cornerstone to preventing infection during prolonged space travel. These interventions involve a robust vaccination programs, screening for and treatment of latent infections such as tuberculosis that may reactivate in the presence of an impaired immune response. Basic infection control education is similarly important. Engineering design issues are also important. Considerations include antibiofouling potable water systems and self-disinfecting or growth-inhibiting environmental surfaces. Power and space constraints must be addressed if interventions such as HEPA filtration of air are to be utilized. The choices of germicidal agents for environmental cleaning and hand hygiene are limited by toxicity from off-gassing that may enter the air-handling or potable water systems. As such, disinfectants that are effective and safe in this unique environment are needed.
There remain many controversial issues such as use of probiotics to adjust for alterations in an astronaut's microbiome resulting from microgravity and determining the extent to which food should be gamma-irradiated prior to being brought on board. As tempting as it may seem to have astronauts ingest sterile food for 18 months as a means to mitigate risk of foodborne infection, the impending effect on the intestinal microbiome must be considered. Storage of antibiotics and other medications must be done to shield the contents from radiation exposure that may reduce drug stability during prolonged space travel. Easy to use and interpret, low-energy requiring diagnostic testing equipment must be developed to assess an astronaut who may develop a febrile illness. Quarantine of material brought back to Earth will be necessary. These and many other issues will need to be addressed to minimize risk and increase the likelihood of a successful journey to Mars. No doubt, there will be bountiful civilian applications from the research and planning for such a challenging endeavor, and in the end, scientific studies during the mission will give us a greater understanding about the effects of gravity on the evolution of humans, the microbes we keep and balance between them.