You will be presented with the abstract, introduction, and discussion section from a biomedical research article. Your task is to create a summary that a layperson can understand, capturing the essence of the research article. Keep essential scientific terms, but ensure that the language remains clear and the concepts are explained in an uncomplicated manner.

Title: Mechanical instability and interfacial energy drive biofilm morphogenesis}
Article:
Abstract:
Surface-attached bacterial communities called biofilms display a diversity of morphologies. Although structural and regulatory components required for biofilm formation are known, it is not understood how these essential constituents promote biofilm surface morphology. Here, using Vibrio cholerae as our model system, we combine mechanical measurements, theory and simulation, quantitative image analyses, surface energy characterizations, and mutagenesis to show that mechanical instabilities, including wrinkling and delamination, underlie the morphogenesis program of growing biofilms. We also identify interfacial energy as a key driving force for mechanomorphogenesis because it dictates the generation of new and the annihilation of existing interfaces. Finally, we discover feedback between mechanomorphogenesis and biofilm expansion, which shapes the overall biofilm contour. The morphogenesis principles that we discover in bacterial biofilms, which rely on mechanical instabilities and interfacial energies, should be generally applicable to morphogenesis processes in tissues in higher organisms.
Introduction:
Many of the stunning morphologies that distinguish living entities do not arise exclusively from gene expression programs, but rather from overarching contributions from mechanical forces( Heisenberg and Bellaïche, 2013; Thompson, 1992; Yamada and Cukierman, 2007). Such morphomechanical processes include the formation of ripple-shaped leaves( Liang and Mahadevan, 2009), tendrils and flowers( Gerbode et al., 2012; Liang and Mahadevan, 2011), as well as the dorsal closure and apical constriction-mediated epithelial folding processes that take place during Drosophila embryonic development( He et al., 2014; Solon et al., 2009). One key feature is common to many of these morphogenic transformations: two or more layers of biomaterials are attached to one another but each grows at a different rate( Wang and Zhao, 2015). Inevitably, such growth mismatches generate mechanical stresses, and corresponding shape instabilities, which depend on the mechanical and other material properties of the biological constituents, as well as their geometries. Some examples include villi formation during the development of the human gut and formation of gyri and sulci during cerebrum development( Shyer et al., 2013; Budday et al., 2015; Tallinen et al., 2016). Though ancient in their evolutionary origin, bacterial cells can also display intricate developmental patterns, particularly when they exist in the community lifestyle known as biofilms( Hobley et al., 2015; Humphries et al., 2017; Persat et al., 2015). Biofilms are surface-associated bacterial communities that are embedded in a polymer matrix( O'Toole et al., 2000; Thongsomboon et al., 2018) and are a predominant growth mode for bacteria in nature( Hall-Stoodley et al., 2004; Humphries et al., 2017). Biofilms can be beneficial, for example in waste-water treatment( Nerenberg, 2016), but they also cause significant problems in health and industry( Costerton et al., 1999; Drescher et al., 2013) because they are resistant to physical perturbations and to antibiotics( Kovach et al., 2017; Meylan et al., 2018). Biofilms on surfaces undergo morphogenic transformations, beginning as smooth colonies and, over time, developing complex morphological features( Beyhan and Yildiz, 2007). Genes specifying matrix components that enable polysaccharide production, cell-surface adhesion, and cell–cell adhesion are required for the morphological transition( Hobley et al., 2015). However, the underlying mechanisms that dictate how these biofilm matrix components direct overall morphology are not well-understood. One model focuses on the differential spatial regulation of genes encoding matrix components as the key driver of biofilm morphogenesis( Okegbe et al., 2014). Another model suggests that localized cell death serves as an outlet for mechanical stresses and thus determines biofilm morphology( Asally et al., 2012). Most recently, theory has been put forward to suggest the possibility that global mechanical instabilities are involved in the development of biofilm morphology( Zhang et al., 2016; Zhang et al., 2017). Here, by combining quantitative imaging, biomaterial characterization, mutant analyses, and mechanical theory, we show that the mismatch between the growing biofilm layer and the non-growing substrate causes mechanical instabilities that enable the biofilm to transition from a flat to a wrinkled film, and subsequently to a partially detached film containing delaminated blisters. The sequential instabilities that the film undergoes, coupled with the generation and annihilation of interfaces, drive the evolution of biofilm topography. Our results demonstrate that bacterial biofilms provide a uniquely tractable system for the quantitative investigation of mechanomorphogenesis.
Discussion:
We show here that mechanical instabilities, including wrinkling and delamination, underlie biofilm morphogenesis. Moreover, differences in interfacial energies drive mechanomorphogenesis by dictating the creation or annihilation of new or existing interfaces. Finally, feedback between mechanomorphogenesis and biofilm expansion shapes the overall biofilm contour. Collectively, our findings concerning the connections between a biofilm’s surface morphology and its mechanical and material properties suggest that new genes and/or new compounds that alter biofilm morphology by altering mechanics could be discovered and investigated to address biofilm-related problems. Morphological patterns can certainly involve gene regulation programs. Nonetheless, we expect our mechanical instability findings in V. cholerae biofilms to apply to other systems— from bacteria to humans— because they reveal links between the specific material properties of the biological components and morphological transitions. Regarding bacterial systems, we have already commented on how localized cell death underpins pattern formation at the core of Bacillus subtilis biofilms( Asally et al., 2012). In fact, in light of our mechanomorphogenesis model, localized cell death can be viewed as a source of surface defects that functions to trigger delaminations, similar to the defined surface imperfections that drive delaminations shown in Figure 3E. Another example concerns biofilms of Pseudomonas aeruginosa, an opportunistic pathogen( Costerton et al., 1999). WT P. aeruginosa develops biofilms with a labyrinthine inner pattern surrounded by flat rims( Madsen et al., 2015). By contrast, P. aeruginosa mutants that are incapable of phenazine production( Δphz) form biofilm topography similar to those that we examined here for V. cholerae with disordered cores surrounded by radial features( Dietrich et al., 2013). We suggest that the mechanical principles uncovered here could also drive the morphological transitions in P. aeruginosa biofilms. The WT P. aeruginosa biofilm pattern occurs because cells at the biofilm center display upregulated matrix production( Madsen et al., 2015), whereas cells located at the periphery are downregulated for matrix production. In the case of the Δphz mutant, all of the cells overproduce extracellular polysaccharides( Madsen et al., 2015), so we speculate that the Δphz P. aeruginosa mutant forms peripheral radial wrinkles and subsequently delaminations because of the same mechanical instability described here in V. cholerae. These examples illustrate how gene regulation and spatially differentiated cell physiology can be coupled to mechanical instability to promote biofilm surface morphologies. Recent theoretical work on bacterial biofilms has considered mechanical instabilities. Zhang et al.( 2016) used simulations to suggest that anisotropic growth coupled with wrinkling instability could generate the surface topography observed in bacterial biofilms, and most recently they considered the possibility of delamination( Zhang et al., 2017). Wang and Zhao( 2015) introduced competition between adhesive and elastic energies and computed a phase diagram of the different modes of instability for a film–substrate system. These inspiring theories will be made more valuable by the inclusion of measured biophysical parameters and additional observations generated through experiments. For example, the thin intermediate residual layer that we discovered here is not accurately considered in biofilm simulations, but is required to explain the wrinkling instability in biofilms( Figure 2D). In addition, interfacial energies play a predominant role in driving the morphologies of biological materials that possess soft layers, whereas their roles are minor in classical mechanical systems( Qi et al., 2011). To date, contributions from interfacial energies have been suggested in contexts such as cell sorting in tissues( Brodland, 2002; Foty and Steinberg, 2005), but we are not aware of any work incorporating interfacial energies into mechanical instability models for morphogenesis. Future theoretical analyses can now incorporate measured parameters to understand the rich hierarchical dynamics and the history dependence of mechanomorphogenesis, taking into account biofilm viscoelasticity, interfacial energies, and the consequences of sliding and friction between the biofilm and the substrate( Beroz et al., 2018; Peterson et al., 2015). Though more sophisticated, eukaryotic organisms often employ similar mechanical instability principles to generate fascinating morphologies. Thus, our findings for biofilms are potentially generalizable and relevant for studies of development in higher organisms( Kim et al., 2015). A close analogy is presented by cerebellum development. The cerebellum possesses a thin, soft layer of Purkinje cells that is sandwiched between the rapidly growing external granular layer and the slow-growing internal granular layer( Lejeune et al., 2016b). Through wrinkling instabilities, the cerebellum develops finely spaced parallel grooves called folia. This hard-soft-hard geometry and the associated wrinkling instabilities directly mirror the configuration that we discovered in V. cholerae biofilms. Hence, our work suggests that exploiting mechanical principles to drive key morphogenic events is ancient: it occurs in bacteria, and evolution, as is often the case, has reused prokaryotic processes and principles in eukaryotes. In summary, biofilms represent an intriguing and highly tractable model system to investigate the general role of mechanical forces in morphogenesis, and they provide a convenient system for morpho-engineering.
Engineers have long studied how mechanical instabilities cause patterns to form in inanimate materials , and recently more attention has been given to how such forces affect biological systems . For example , stresses can build up within a tissue if one layer grows faster than an adjacent layer . The tissue can release this stress by wrinkling , folding or creasing . Though ancient and single-celled , bacteria can also develop spectacular patterns when they exist in the lifestyle known as a biofilm: a community of cells adhered to a surface . But do mechanical instabilities drive the patterns seen in biofilms ? To investigate , Yan , Fei , Mao et al . grew biofilms of the bacterium called Vibrio cholerae – which causes the disease cholera – on solid , non-growing ‘substrates’ . This work revealed that as the biofilms grow , their expansion is constrained by the substrate , and this situation generates mechanical stresses . To release the stresses , the biofilm initially folds to form wrinkles . Later , as the biofilm expands further , small parts of it detach from the substrate to form blisters . The same forces that keep water droplets spherical ( known as interfacial forces ) dictate how the blisters evolve , interact , and eventually shape the expanding biofilm . Using these principles , Yan et al . could engineer the biofilm into desired shapes . Collectively , the results presented by Yan et al . connect the shape of the biofilm surface with its material properties , in particular its stiffness . Understanding this relationship could help researchers to develop new ways to remove harmful biofilms , such as those that cause disease or that damage underwater structures . The stiffness of biofilms is already known to affect how well bacteria can resist antibiotics . Future studies could look for new genes or compounds that change the material properties of a biofilm , thereby altering the biofilm surface .