Adaptation to temperature stress by Vibrio fischeri facilitates this microbe's symbiosis with the Hawaiian bobtail squid (Euprymna scolopes)

For microorganisms cycling between free-living and host-associated stages, where reproduction occurs in both of these lifestyles, an interesting inquiry is whether adaptation to stress during the free-living stage can impact microbial fitness in the host. To address this topic, the mutualism between the Hawaiian bobtail squid (Euprymna scolopes) and the marine bioluminescent bacterium Vibrio fischeri was utilized. Using microbial experimental evolution, V. fischeri was selected to low (8⁰C), high (34⁰C), and fluctuating temperature stress (8⁰C/34⁰C) for 2,000 generations. The temperatures 8⁰C and 34⁰C were the lower and upper growth limits, respectively. V. fischeri was also selected to benign temperatures (21⁰C and 28⁰C) for 2,000 generations, which served as controls. V. fischeri demonstrated significant adaptation to low, high, and fluctuating temperature stress. V. fischeri did not display significant adaptation to the benign temperatures. Adaptation to stressful temperatures facilitated V. fischeri’s ability to colonize the squid host relative to the ancestral lines. Bioluminescence levels also increased. Evolution to benign temperatures did not manifest these results. In summary, microbial adaptation to stress during the free-living stage can promote coevolution between hosts and microorganisms.