ABSTRACT
Cellulose is a widespread component of bacterial biofilms, where its properties of exceptional water retention, high tensile strength and stiffness prevents dehydration and mechanical disruption of the biofilm. Bacteria in the Gluconacetobacter genus secrete crystalline cellulose, with a structure very similar to that found in plant cell walls. How this higher-order structure is produced is poorly understood. We used cryo-electron tomography and focused ion beam milling of native bacterial biofilms to image cellulose-synthesizing G. hansenii and G. xylinus bacteria in a frozen-hydrated, near-native state. We confirm previous results suggesting that cellulose crystallization occurs serially following its secretion along one side of the cell, leading to a cellulose ribbon that can reach several microns in length and combine with ribbons from other cells to form a robust biofilm matrix. We were able to take direct measurements in a near-native state of the cellulose sheets. Our results also reveal a novel cytoskeletal structure, that we name the cortical belt, adjacent to the inner membrane and underlying the sites where cellulose is seen emerging from the cell. We find that this structure is not present in other cellulose-synthesizing bacterial species, Agrobacterium tumefaciens and Escherichia coli 1094, which do not produce organized cellulose ribbons. We therefore propose that the cortical belt holds the cellulose synthase complexes in a line, to form higher-order cellulose structures such as sheets and ribbons.
Importance This work’s relevance for the microbiology community is two-fold: It delivers for the first time high-resolution near-native snapshots of the Gluconacetobacter spp. (previously Komagataibacter spp.) in the process of cellulose ribbon synthesis, in their native biofilm environment. It puts forward a non-characterized cytoskeleton element associated with the side of the cell where the cellulose synthesis occurs. This represents a step forward in the understanding of the cell-guided process of crystalline cellulose synthesis, particularly studied in the Gluconacetobacter genus and still not fully understood. Additionally, our successful attempt to cryo-FIB mill through biofilms to image the cells in their native environment will drive the community to use this tool for the morphological characterization of other studied biofilms.
- Copyright © 2020 Nicolas et al.
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