Biofilms as complex fluids
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troduction In natural and industrial environments, nearly every surface with a little moisture and nutrients is colonized by bacteria that can live in sessile, interface-associated aggregates referred to as biofilms.1,2 Biofilm formation is marked by the production of an extracellular matrix (ECM), which is composed primarily of polysaccharides3 and proteins.4 This matrix provides a scaffolding structure that holds the community of cells together and provides the biofilm with mechanical integrity. Bacterial biofilms play a crucial role in global ecology,5,6 can be beneficial for water treatment and waste sequestration,7 but are also responsible for many bacteria-related problems, including tooth decay,8,9 implant infections,10,11 hospital-acquired infections,12 and fouling of industrial processes.13 As such, a general framework for understanding biofilm material properties is essential for both the removal of biofilms and the optimization of biofilm properties. Biofilms can also be viewed from the perspective of soft condensed matter physics; this provides a valuable, alternative materials-related understanding of their structure and properties. In this picture, biofilms are composites of colloids embedded in a cross-linked polymer gel. The bacterial cells are analogous to colloidal particles, and in the absence of the ECM, a bacterial colony behaves as a colloidal fluid. The polysaccharide polymers in the ECM are cross-linked by proteins and multivalent cations,
and the ECM is analogous to a cross-linked polymer gel. In addition, bacteria within the biofilm produce surfactants in order to communicate with one another; these surfactants also control interfacial properties (see the Shrout et al. article in this issue).14,15 The production and assembly of cells, polymer, cross-links, and surfactants result in a structure that is heterogeneous and dynamic. In this article, we discuss the physical role of these bacterial biofilm building blocks and review recent progress toward understanding biofilms as composite complex fluid materials. Polymer gels have an equilibrium water content that is determined by their composition,16 and biofilms can control their water content by regulating the composition of their ECM. In this manner, they exert control of their mechanical properties, as the mechanics of most soft materials are set by their water content. We gain insight into the mechanical properties of bacterial biofilms, such as their viscoelasticity, by looking to the mechanical behavior of standard complex fluids such as soft colloids and polymer gels. Recognizing the components of a biofilm as complex fluids facilitates a new understanding of biofilm dynamics. For example, cells can tune the abundance of polymer or surfactant in the extracellular space in response to environmental cues (see Chai et al. and Renner et al. articles in this issue), locally modulating the material properties of the biofilm, and in this
James N. Wilking, Harvard University, Cambridge, MA 02139, USA; [email protected] Thomas E. Angelini, University of Flor
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