Chemotaxis Mediated by Non-Adaptive Dynamics

Chemotaxis is the ability of bacteria to locate high attractant sources in the environment. The extensive knowledge gained from the pathway in the model organism Escherichia coli shows that adaptation to stimuli is a hallmark underlying chemotaxis. Studies on certain mutant strains and other bacterial species, however, indicate that some form of chemotaxis could also be achieved without adaptation. It is not clear how efficient such chemotaxis is, how it could be mediated, and how widespread it is among different bacterial species. In order to explore alternative pathway structures and dynamics that can underlie chemotaxis, we employ an evolutionary approach. This approach starts with a population of bacteria that move in a virtual environment based on the dynamics of simple pathways they harbour. As mutations lead to changes in pathway structure and dynamics, bacteria with better ability to localize with high attractant sources gain a selective advantage. We find that chemotaxis via non-adaptive dynamics evolves consistently under different model assumptions and environments. These dynamics directly couple tumbling probability of the cell to increasing attractant levels. Further analyses of evolved pathway structures show that this alternative behaviour can be mediated with as few as two components. The non-adaptive mechanisms mediating chemotaxis provide an explanation for experimental observations made in mutant strains of E. coli and in wild type Rhodobacter sphaeroides and that could not be explained with existing knowledge. These mechanisms could allow a straightforward link between cell metabolism and chemotaxis. Furthermore, they could have acted as the origin of the conventional chemotaxis involving adaptation. This is the preliminary version of a paper that was published in PLoS Computational Biology. The original publication is available at http://www.ploscompbiol.org/home.action