Cellulose and Cellulose Secretion in Bacterial Biofilms

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. Here we provide an overview of the intricate molecular mechanisms of BC biogenesis across multiple types of Bcs secretion systems, with a focus on Gram-negative bacteria secreting crystalline or chemically modified biofilm cellulose.