Biofilms as complex fluids
Entry by Emily Redston, AP 225, Fall 2011
Work in progress
Biofilms as Complex Fluids by J. N. Wilking, T. E. Angelini, A. Seminara, M. P. Brenner, and D. A. Weitz. MRS Bulletin, 26, 385-391 (2011)
Bacterial biofilms can be found on nearly every surface as long as there is moisture and nutrients. They can have a positive impact in areas such as water treatment and waste sequestration, but they also play a devastating role in many bacteria-related problems like tooth decay and hospital-acquired infections. A better understanding of the structure, mechanics, and dynamics of biofilms is necessary for both their removal and for the optimization of their properties.
Viewing biofilms as a complex fluid is a good starting point for analyzing their structure and properties. A bioflim can be seen as a composite of colloids (bacterial cells) embedded in a cross-linked polymer gel (extracellular matrix -- ECM).
Bacterial cells are rigid and have well-defined shapes like spheres or rods. Since the bacteria within a biofilm are mostly sessile and cannot generate forces outside of the cell, they control the structural and mechanical properties of the biofilm by regulating the composition of the ECM. The ECM is primarily composed of polysaccharides cross-linked by proteins and multivalent cations. This matrix is the scaffold that holds the bacteria together; it gives the biofilm its mechanical integrety.
Unfortunately, it is difficult to get a full picture of the biofilm material properties due to the highly variably nature of the ECM. The ECM is often composed of multiple species, so it is poorly understood.
By exploiting the relation between biofilms and soft matter (specifically the polymer-like behavior of the ECM), one can understand a great deal about how the bacteria is able to control the water content in the film. To maximize its entropy, an entangled polymer placed in contact with a reservoir of liquid will swell.