Polymer-induced phase separation in Escherichia coli suspensions

We studied aggregation and phase separation in suspensions of de-flagellated Escherichia coli (AB1157) in phosphate buffer induced by the anionic polyelectrolyte sodium polystyrene sulfonate. We also performed Monte Carlo simulations of this system based on the Asakura-Oosawa model of colloid-polymer mixtures. The results of these simulations, as well as comparison with previous work on synthetic colloid-polymer mixtures, demonstrate that the role of the polymer is to cause a depletion attraction between the E. coli cells. The implication of these results for understanding the role of (predominantly anionic) extracellular polymeric substances (EPS) secreted by bacteria in various natural phenomena such as biofilm formation is discussed.