Biofilms as Complex Fluids
James N. Wilking , Thomas E. Angelini , Agnese Seminara , Michael P. Brenner , and David A. Weitz, MRS bulletin, volume 36, 385-392 (2011)
Author: Sofia Magkiriadou, Fall 2011
The authors discuss how biofilms, i.e. bacterial colonies embedded in an extra-cellulal matrix and usually growing on surfaces, can be studied and often understood as colloidal materials where the bacteria resemble colloidal particles and the extra-cellular matrix a cross-linked polymer matrix. This matrix provides the bacterial colony with mechanical stability and, as will be discussed, its mechanical properties depend directly on the chemical behaviour of the bacteria. Biofilms can be found on almost any surface, ranging from nature, where they play a vital role in the regulation of the environment, to hospitals, where they can be a source of infections, while the growth of some bacterial species in this form is even being pursued as a means to purify polluted areas. Thus, the study of biofilms is very important.
The bacteria forming a biofilm may have a spherical or rod-like shape. The extra-cellular matrix is usually composed of polysaccharides cross-linked by proteins and multivalent cations, and it additionally contains surfactants excreted by the bacteria for communication. With this consistency, the matrix is analogous to a polymer gel. Contrary to a simple gel, however, this system is very dynamic as its properties change depending on the bacterial behaviour. The whole system can range in thickness from a few microns to a few millimeters.
Interestingly, while the bacteria that comprise biofilms lack mobility organs and are structurally very rigid, the biofilms as a whole are very responsive to mechanical perturbations from the environment. This has made it difficult to understand them comprehensively as they keep changing; most of our knowledge comes from however model systems which have been studied in the laboratory.
Control of water content
In a simple textbook system of a polymer solution separated by pure water by a semi-permeable membrane, the polymer exerts osmotic pressure. If this polymer is cross-linked, it will swell up to the point where this pressure is balanced by the polymer mesh's elastic shear modulus Ge. Thus, for a given concentration of polymer and cross-linking proteins, there is an equilibrium water concentration; by controlling the relative production of the constituents of their extra-cellular matrix, the bacteria are thus able to control the water content of their environment, which is directly related to the mechanical properties of the biofilm.
Viscoelastic materials exhibit a time-dependent response to mechanical perturbations. Therefore, two quantities are important for the characterisation of viscoelasticity: the elastic shear modulus Ge, which is a measure of the magnitude of the force required to deform a material by a certain amount, and the relaxation time te, which is a measure of the characteristic time over which this deformation occurs. These quantities depend directly on the water content. In colloidal suspensions with low volume fraction the relaxation times are really short; at intermediate volume fractions the system becomes highly elastic; at high volume fraction the viscoelastic properties approach that of the material of the individual particles. Biofilms with viscoelastic properties that match these expectations have been observed. However, many biofilms have a very low bacterial volume fraction, in which case their viscoelastic properties are determined by those of the extra-cellular matrix that surrounds them. In this case, it is useful to think about the factors that determine these properties for a polymer gel: the ambient temperature KbT and the mesh size ksi (Ge ~ KT/ksi^3). However, these properties of a biofilm are strongly inter-dependent on their environment; the same bacteria, when prospering on an acqueous surface where water is abundant, will develop films with very different mechanical properties than when on a surface where they need to do work to obtain water.
Biofilms are mechanically very sensitive, and care must be taken in order to observe them non-destructively. There exist well-developed experimental soft matter techniques which are appropriate for such measurements: passive microrheology, microbead force spectroscopy, and microfluidics are a few examples.
BIOFILM DYNAMICS Surfactant heterogeneity and cooperative motility
Bacteria can be very mobile. The mechanisms of their mobility can either be found in each bacterial body, in the form of flagella, pilli, other organs, or arise as collective behavior through mechanisms with or without such organs. This latter case that is not yet clearly understood, although it seems clear that amphiphilic molecules play a central role. In particular, inhomogeneities in surfactant concentration are known to lead to gradients in interfacial tension which can, in turn, give rise to spreading forces. Such behavior has been observed in some biofilms and analogies between their spreading patterns and those of a droplet, which is known to be driven by surfactant inhomogeneities, hint to the importance of this collective motion mechanism.
Material heterogeneity and nutrient depletion gradients
Given the large size of biofilms relative to the bacteria that live in them, the distribution of nutrients is often not homogeneous, not only due to genetic variation between the cells, but also due to uneven diffusion of available elements (for instance if a certain nutrient is available peripherally, only the outermost bacteria will avail themselves of it). Such inhomogeneities play an important role in the mechanism of biofilm formation as they can act as limiting factors in the determination of the size and shape of the film. Moreover, local concentration fluctuations of nutrients affect the metabolic rates and behavior of the proximate bacteria, which can result in local variation of the mechanical properties of the biofilm since, as discussed, those are heavily dependent upon the chemicals excreted by the cells.