Microbiome and immunity: A critical link for long-duration space exploration missions

Abstract

The human microbiome is the collection of trillions of microorganisms that live in and on the human body. It is mostly composed of bacteria, viruses, archaea, and unicellular eukaryotes that have coevolved with us over millions of years. The microbiome is essential to human health, being involved in the processing and absorption of complex nutrients, the synthesis of vitamins and antioxidants, the maturation and modulation of the immune system and the prevention of infections by pathogenic organisms. Therefore, qualitative or quantitative shifts in the gut microbiota, commonly known as dysbiosis, as a consequence of host pathobiology, alterations of diet, medications, and other environmental triggers, can perturb critical inter-microbe as well as host-microbe relationships to initiate pathophysiological processes leading to human disease. To maintain health, the immune system plays an essential role in maintaining a delicate balance between eliminating invading pathogens and keeping the homeostatic relationship with beneficial resident microorganisms of the gastrointestinal tract. At the same time, resident bacteria have a significant immunomodulatory activity that profoundly shape mammalian immunity. During a mission to space, astronauts are exposed to a number of stressors (microgravity, sleep deprivation, radiation, dietary changes, etc.) that are likely to affect the composition and dynamic of the astronauts’ microbiome. Several studies on specific culturable commensal and opportunistic pathogenic bacteria performed under real or simulated microgravity conditions suggest that space travel can affect the composition and function of the microbiome, including bacterial virulence, antibiotic resistance, and growth. This chapter will focus on what is currently known about the interaction between the human intestinal microbiota and the immune response and how conditions found in space might alter this interaction and pose a risk to astronauts’ health.