Bacterial biofilms are integrated, multi-species communities of cells that adhere to almost any surface and are fundamental to the ecology and biology of bacteria. Not only do biofilms contribute to human health and disease, they also play important roles in the context of energy and the environment. The formation of biofilms requires interactions between bacteria and the surfaces they colonize, and both microbe and surface can impact the structure, function, and composition of these communities. Bacteria in biofilms exhibit surprisingly sophisticated social behavior, both cooperative and competitive, made possible by their cell biology. However, they are also hierarchically organized systems governed by complex physical and chemical interactions. Because of this, the study of bacterial biofilms has recently attracted the attention of materials scientists, physicists, chemists, and nanotechnology experts who import not only new tools, but also new concepts and perspectives. This issue reviews recent progress in multidisciplinary studies of biofilms.

This article reviews the physical and chemical constraints of environments on biofilm formation. We provide a perspective on how materials science and engineering can address fundamental questions and unmet technological challenges in this area of microbiology, such as biofilm prevention. Specifically, we discuss three factors that impact the development and organization of bacterial communities. (1) Physical properties of surfaces regulate cell attachment and physiology and affect early stages of biofilm formation. (2) Chemical properties influence the adhesion of cells to surfaces and their development into biofilms and communities. (3) Chemical communication between cells attenuates growth and influences the organization of communities. Mechanisms of spatial and temporal confinement control the dimensions of communities and the diffusion path length for chemical communication between biofilms, which, in turn, influences biofilm phenotypes. Armed with a detailed understanding of biofilm formation, researchers are applying the tools and techniques of materials science and engineering to revolutionize the study and control of bacterial communities growing at interfaces.

Despite modern advancements in sterilization and aseptic procedures, bacterial infections remain a major and significant impediment to the long-term success of medical implants and devices, including catheters, artificial prosthetics, and subcutaneous sensors. It has been estimated that upward of 60% of nosocomial, or hospital-acquired, infections are associated with implants, with an estimated one million cases per year in the United States alone. Current treatment regimens primarily rely on the systemic administration of antibiotics or local administration through irrigation of the surgical site. However, after decades of prophylactic antibiotic use, high infection rates continue to persist, particularly with the emergence of drug-resistant strains such as methicillin-resistant Staphylococcus aureus (MRSA). Consequently, recent research efforts have focused on the use of non-antibiotic-based coatings to inhibit bacteria colonization and subsequent biofilm formation on implant surfaces. In particular, advancements in surface treatment strategies and ongoing development of new antimicrobial agents have led to encouraging progress in the design of better coatings. Here, we present and discuss representative examples of novel surface engineering approaches to address device-associated infections, focusing in particular on coatings that can be easily administered onto implant material surfaces.

Many bacteria grow attached to a surface as biofilms. Several factors dictate biofilm formation, including responses by the colonizing bacteria to their environment. Here we review how bacteria use cell-cell signaling (also called quorum sensing) and motility during biofilm formation. Specifically, we describe quorum sensing and surface motility exhibited by the bacterium Pseudomonas aeruginosa, a ubiquitous environmental organism that acts as an opportunistic human pathogen in immunocompromised individuals. P. aeruginosa uses acyl-homoserine lactone signals during quorum sensing to synchronize gene expression important to the production of polysaccharides, rhamnolipid, and other virulence factors. Surface motility affects the assembly and architecture of biofilms, and some aspects of motility are also influenced by quorum sensing. While some genes and their function are specific to P. aeruginosa, many aspects of biofilm development can be used as a model system to understand how bacteria differentially colonize surfaces.

Bacteria often live in the form of surface-associated communities of cells termed biofilms. Within biofilms, there is a division of labor in which genetically identical cells differentiate to serve distinct functions. This cellular differentiation results from a response to extracellular signals that occur due to changes in the local environment of a cell or in response to signaling molecules that the cells themselves produce. In this review, we discuss differentiation in biofilms, focusing on the molecular mechanisms that regulate differentiation in the bacterium Bacillus subtilis. In this organism, there is a subpopulation of cells within a biofilm that produces a signal, while a different subpopulation of cells responds to it. Studying what signals cells use to communicate with each other within a biofilm will allow for better design of strategies to prevent and disrupt biofilms.

Bacteria living in surface-attached biofilm communities must maintain electrochemical gradients to support basic cellular functions, including chemo-osmotic transport and adenosine triphosphate synthesis. Central to this is the maintenance of electron flow to terminal electron acceptors. These acceptors can be soluble inorganic and organic molecules, such as oxygen, nitrate, sulfate, dimethyl sulfoxide, or fumarate, or solid metal oxides, such as Fe(III) and Mn(IV) oxides. When electrons are transferred to a solid substrate, they may be (1) carried directly to the acceptor via outer membrane cytochromes, (2) carried by electron shuttle molecules, (3) transferred along conductive protein nanowires, or (4) conducted through other extracellular matrices. No matter what the electron acceptor is, in the laboratory, bacterial biofilms are frequently studied while growing on inert surfaces, incapable of electron transfer. However, in natural environments, as well as many industrial and biotechnology settings, biofilms grow on electrically active surfaces. In this review, we propose that the study of bacterial biofilms on redox-active surfaces is important both for the development of industrial processes, such as microbial fuel cells and wastewater treatment systems, as well as for our understanding of how these communities of microbes affect global nutrient cycling, other geobiological processes, and even human disease.

Bacterial biofilms are interface-associated colonies of bacteria embedded in an extracellular matrix that is composed primarily of polymers and proteins. They can be viewed in the context of soft matter physics: the rigid bacteria are analogous to colloids, and the extracellular matrix is a cross-linked polymer gel. This perspective is beneficial for understanding the structure, mechanics, and dynamics of the biofilm. Bacteria regulate the water content of the biofilm by controlling the composition of the extracellular matrix, and thereby controlling the mechanical properties. The mechanics of well-defined soft materials can provide insight into the mechanics of biofilms and, in particular, the viscoelasticity. Furthermore, spatial heterogeneities in gene expression create heterogeneities in polymer and surfactant production. The resulting concentration gradients generate forces within the biofilm that are relevant for biofilm spreading and survival.