The adaptability to ever-changing environment is one of the fundamental characteristics that differentiates living matters from non-living ones. The combination of passive adaptation by Darwinian evolution and active adaptation by sensing and decision-making shapes the mechanisms of adaptability. In this work, we outline that the biological adaptation shares a very similar structure with the stochastic and information-thermodynamics. Fitness, the macroscopic quantity to characterize evolutionary success, plays the similar role as the free energy in this structure. The fluctuation relation of fitness and information derived from the structure can be regarded as an extension of the concept of the evolutionary stable strategy.

Vibrio cholerae, an etiological agent of cholera, is attracted by various amino acids and taurine, a component of bile. A chemoreceptor homolog (Mlp37) was found to mediate taxis to both taurine and amino acids. Direct binding of taurine and amino acids to the periplasmic fragment of Mlp37 was observed by isothermal titration calorimetry. Ligand-occupied structures of the Mlp37 fragment revealed that the ligand-binding pocket has an opening, large enough to accommodate a larger side chain, and contains several common residues that can interact with taurine and different amino acids without substantial displacements. This structural information allowed us to visualize ligand binding to Mlp37 with fluorescently labeled serine.