Host-microbe interactions in microgravity: assessment and implications

Abstract

Spaceflight imposes several unique stresses on biological life that together can have a profound impact on the homeostasis between eukaryotes and their associated microbes. One such stressor, microgravity, has been shown to alter host-microbe interactions at the genetic and physiological levels. Recent sequencing of the microbiomes associated with plants and animals have shown that these interactions are essential for maintaining host health through the regulation of several metabolic and immune responses. Disruptions to various environmental parameters or community characteristics may impact the resiliency of the microbiome, thus potentially driving host-microbe associations towards disease. In this review, we discuss our current understanding of host-microbe interactions in microgravity and assess the impact of this unique environmental stress on the normal physiological and genetic responses of both pathogenic and mutualistic associations. As humans move beyond our biosphere and undergo longer duration space flights, it will be essential to more fully understand microbial fitness in microgravity conditions in order to maintain a healthy homeostasis between humans, plants and their respective microbiomes.

Figure 1

Overview of the complexity of human microbiome. Circle size reflects the approximate relative abundance of the various microbes known to associate with humans. Relative gene abundance is derived from the human microbiome sequencing project [19,22]. Question marks reflect uncertainty or potential underestimation of gene abundance.

Figure 2

Impact of microgravity on the developmental time line of mutualistic symbiosis between the squid Euprymna scolopes and bacterial partner Vibrio fischeri. (A) Juvenile squid just after hatching. Blue rectangle marks the location within the host mantle cavity of light organ, the site of symbiosis. Bar, 0.25 mm (B) Light organ at hatching showing the elongated surface epithelium that forms appendage like structures on either side of the light organ. Bar, 75 μm. (C) One half of light organ depicting the movement of hemocytes (green) moving into the blood sinus contained within the surface epithelium upon exposure to bacteria. Bar, 30 μm. (D) Light organ exposed to bacterial lipopolysaccharide showing pronounced cell death staining pattern along the superficial epithelium. Bar, 30 µm. (E) Image of one half of light organ depicting the loss of the superficial epithelial appendage structures 96 h after colonization with V. fischeri. Bar, 30 µm. (F) Exposure to microgravity alters the developmental time line of the symbiosis under normal gravity and modeled microgravity conditions using a RWV bioreactor. Events listed in black do not change under microgravity conditions. Those events in red are delayed during modeled microgravity, where as those events in green are accelerated.