Jamie S. Foster

Perturbations to host–microbe interactions, such as environmental stress, can alter and disrupt homeostasis. In this study, we examined the effects of a stressor, simulated microgravity, on beneficial bacteria behaviours when colonising their host. We studied the bacterium Vibrio fischeri, which establishes a mutualistic association in a symbiosis‐specific organ within the bobtail squid, Euprymna scolopes . To elucidate how animal–microbe interactions are affected by the stress of microgravity, squid were inoculated with different bacterial strains exhibiting either a dominant‐ or sharing‐colonisation behaviour in High Aspect Ratio Vessels, which simulate the low‐shear environment of microgravity. The colonisation behaviours of the sharing and dominant strains under modelled microgravity conditions were determined by counting light‐organ homogenate of squids as well as confocal microscopy to assess the partitioning of different strains within the light organ. The results indicated that although the colonisation behaviours of the strains did not change, the population levels of the sharing strains were at lower relative abundance in single‐colonised animals exposed to modelled microgravity compared to unit gravity; in addition, there were shifts in the relative abundance of strains in co‐colonised squids. Together these results suggest that the initiation of beneficial interactions between microbes and animals can be altered by environmental stress, such as simulated microgravity.