Link between chemotactic response to Ni2+ and its adsorption onto the Escherichia coli cell surface

Bacterial chemotaxis is of medical, biological, and geological significance. Despite its importance, current chemotaxis measurements fail to account for the speciation of the chemical effector and the protonation state of the bacterial surface. We hypothesize that adsorption of Ni2+ onto the surface of Escherichia coli can influence its effective concentration and therefore influence its ability to induce a repellent response. By measuring repellent response at different pH values, the influence of Ni2+ adsorption on chemotaxis was assessed. In addition, we tested the effect of different Ni2+ chelating agents. Our data indicate that adsorption reactions influence the chemotactic response to Ni2+. We use potentiometric titration and Ni2+ adsorption experiments to develop and constrain a thermodynamic model capable of quantifying the concentration of Ni2+ at the bacteria/solution interface. Results from this model predict that the concentration of adsorbed Ni2+ is linearly proportional to the magnitude of the chemotactic response in E. coli. If adsorption is linked to chemotaxis in other cases, then chemotactic responses in realistic settings depend on a number of environmental factors such as pH, competing binding agents (e.g., aqueous organic acids, natural organic matter, mineral surfaces, etc.), and ionic strength. Our modeling approach quantifies adsorbed species on bacterial surfaces and may be used to predict the responses of different species to a variety of chemoeffectors. Our data suggest that specified changes in environmental conditions can be used to tune chemotactic responses in natural biological and geological settings.