Polymer-induced phase separation in suspensions of bacteria

¹  SUPA and School of Physics & Astronomy, The University of Edinburgh, James Clerk Maxwell Building, Kings Buildings - Mayfield Road, Edinburgh EH9 3JZ, UK, EU
²  Institute for Cell Biology, School of Biological Sciences, The University of Edinburgh, Darwin Building, Kings Buildings - Mayfield Road, Edinburgh EH9 3JR, UK, EU
³  Institut für Physik, Johannes Gutenberg Universität - 55099 Mainz, Germany, EU

Corresponding author: w.poon@ed.ac.uk

Received: 8 January 2010
Accepted: 10 March 2010

Abstract

We study phase separation in suspensions of two unrelated species of rod-like bacteria, Escherichia coli and Sinorhizobium meliloti, induced by the addition of two different anionic polyelectrolytes, sodium polystyrene sulfonate or succinoglycan, the former being synthetic and the latter of natural origin. Comparison with the known behaviour of synthetic colloid-polymer mixtures and with simulations show that “depletion” (or, equivalently, “macromolecular crowding”) is the dominant mechanism: exclusion of the non-adsorbing polymer from the region between two neighbouring bacteria creates an unbalanced osmotic force pushing them together. The implications of our results for understanding phenomena such as biofilm formation are discussed.