Molecular aspects of bacterial nanocellulose biosynthesis

Abstract

Bacterial nanocellulose (BNC) produced by aerobic bacteria is a biopolymer with sophisticated technical properties. Although the potential for economically relevant applications is huge, the cost of BNC still limits its application to a few biomedical devices and the edible product Nata de Coco, made available by traditional fermentation methods in Asian countries. Thus, a wider economic relevance of BNC is still dependent on breakthrough developments on the production technology. On the other hand, the development of modified strains able to overproduce BNC with new properties - e.g. porosity, density of fibres crosslinking, mechanical properties, etc. - will certainly allow to overcome investment and cost production issues and enlarge the scope of BNC applications. This review discusses current knowledge about the molecular basis of BNC biosynthesis, its regulations and, finally, presents a perspective on the genetic modification of BNC producers made possible by the new tools available for genetic engineering.

Figure 1

Pathways for the biosynthesis of BNC by K. xylinus and assembly of cellulose molecules into nanofibrils: (1) Glucokinase‐ATP, (2) Phosphoglucomutase, (3) Glucose‐6‐phosphate dehydrogenase, (4) 6‐phosphogluconate dehydrogenase, (5) Phosphorribulose isomerase, (6) Phosphorribulose epimeraase, (7) Transaketolase, (8) Transaldolase, (9) Phosphoglucoisomerase, (10) Fructokinase, (11) Fructokinase ATP, (12) Aldolase, (13) Triosephosphate isomerase, (14) Glyceraldehyde 3‐phosphate dehydrogenase, (15) Phosphoglycerate mutase, (16) Enolase, (17) Pyruvate kinase (18) Pyruvate biphosphate kinase, (19) Pyruvate dehydrogenase, (20) Alcohol dehydrogenase and (21) Aldehyde dehydrogenase.

Figure 2

A. Organization of cellulose synthase operon and its flanking regions in Komagataeibacter xylinus E25 Accession no. CP004360 (Ia, locus tags H845_449 → H845_455).B. A cartoon showing the domain organisation of cellulose synthase operon and its flanking regions in Komagataeibacter xylinus E25. Domains were identified by a combined use of Blast (Altschul et al., 1997); HMMER/Pfam (Bateman et al., 1999); and SMART (Schultz, 2000).

Figure 3

Crystal structure of the BcsA–BcsB complex (Morgan et al., 2013).

Figure 4

The influence of environmental factors on the biological functions of the bacterial cell (McDougald et al., 2012).

Figure 5

A. Comparison of the domain organization of the PAS domain‐containing proteins in DGC2 and PDEA1 from K. xylinus.B. The connection among oxygen level, cellular c‐di‐GMP concentration and cellulose yield in K. xylinus based on Qi et al. (2009).

Figure 6

Gating loop positions in the absence and presence of c‐di‐GMP (Morgan et al., 2014).

Figure 7

Stable strain of K. hansenii ATCC 53582 and unstable strain K. xylinus E25 generating Cel− and Cel+ forms.A. Stationary culture – thin homogenous BNC membrane formed on the SH medium surface.B. Microscopic pictures of colonies formed by the K. xylinus E25 Cel− and Cel+ forms and K. hansenii ATCC53582.C. Agitated culture – cellulose biosynthesis in the form of small beads (K. hanseii ATCC 53582 and K. xylinus E25 Cel+) or lack of BNC biosynthesis (K. xylinus E25 Cel−).