Updated project metadata. In this study we employed a proteomics method to determine how bacteria respond at the protein level to a transition from a low- to a high-viscosity liquid environment. We applied this method to explore how the symbiotic marine bacterium Vibrio fischeri responds to changes in environmental viscosity associated with the transition from free-living to host-associated states. For this project cultures of ES401 were initially grown in low-viscosity liquid media and then transferred to either fresh low- or high-viscosity liquid media, which was generated by supplementing liquid media with 5% polyvinylpyrrolidone (w/vol). Samples were then incubated for one, 12, or 24 hours, and then collected by centrifugation and cell pellets frozen at -80°C. For each treatment four biological replicates were generated and analyzed.