Bacterial biofilms : Mechanisms of activation and diversification of the bacterial cellulose synthases

Cellulose — or the unbranched chains of β-1,4-linked D-glucose moieties — represents the major component of the plant cell wall and thus the most abundant biopolymer on Earth. In addition, cellulose is secreted by many and diverse viral, prokaryotic, and eukaryotic organisms, where it most commonly confers resistance to environmental stress and/or contributes to the establishment of multicellularity. Bacteria, in particular, have co-opted secreted cellulosic polymers as extracellular biofilm components, which - together with extracellular DNA and proteinaceous fimbriae, such as adhesins, amyloid curli, flagellar filaments and/or type IV pili - lead to the development of a complex, spatially, and functionally differentiated matrix for macrocolony homeostasis. Bacterial cellulose (BC) is polymerized from a pre-energized UDP-glucose substrate and extruded through the (inner) cell membrane by processive multidomain synthases, which assemble into complex multicomponent Bcs secretion systems and are most often under the control of the intracellular second messenger c di-GMP. The secreted cellulosic chains can be decorated with additional chemical groups or can pack with various degrees of crystallinity depending on dedicated enzymatic complexes and/or cytoskeletal scaffolds. Here I review our recent progress in the understanding of synthase-dependent EPS biogenesis with focus on common and idiosyncratic molecular mechanisms across diverse cellulose secretion systems.