Rheological properties of viscoelastic biofilm extracellular polymeric substances and comparison to the behavior of calcium alginate gels

In this publication we present a detailed study of viscoelastic biofilms of Pseudomonas aeruginosa. Sample solutions were extracted from biofilm layers grown on Pseudomonas isolation agar. This aqueous solutions of extracellular polymeric substances exhibit weak elastic effects caused by entanglements and a small number of permanent junction points formed by calcium ions. The cross-linking mechanisms are confirmed by the Cox–Merz rule and dynamic frequency sweep tests, which result in an average lifetime of junction points of the order of 17 ms. The experimental data reveal 3.4×1017 elastically effective chains per liter of solution and no significant temperature effects in the regime between 2 and 24 °C. This result coincides pretty well with the concentration of dissolved polymer chains (2.9×1017 molecules\l). Upon addition of calcium ions, one observes the formation of stable supermolecular networks with permanent junction points. These cross-linking points did not show thermal fluctuations in time zones between 10 ms and several hours. The entanglement density of these gels is of the same order as observed in the non-cross -linked sol state (entrapped entanglements). In spite of the different molecular composition alginate gels show the same type of cross-linking mechanism as gels of extracellular polymeric substances.