Establishing a Role for Bacterial Cellulose in Environmental Interactions: Lessons Learned from Diverse Biofilm-Producing Proteobacteria

Abstract

Bacterial cellulose (BC) serves as a molecular glue to facilitate intra- and inter-domain interactions in nature. Biosynthesis of BC-containing biofilms occurs in a variety of Proteobacteria that inhabit diverse ecological niches. The enzymatic and regulatory systems responsible for the polymerization, exportation, and regulation of BC are equally as diverse. Though the magnitude and environmental consequences of BC production are species-specific, the common role of BC-containing biofilms is to establish close contact with a preferred host to facilitate efficient host-bacteria interactions. Universally, BC aids in attachment, adherence, and subsequent colonization of a substrate. Bi-directional interactions influence host physiology, bacterial physiology, and regulation of BC biosynthesis, primarily through modulation of intracellular bis-(3'→5')-cyclic diguanylate (c-di-GMP) levels. Depending on the circumstance, BC producers exhibit a pathogenic or symbiotic relationship with plant, animal, or fungal hosts. Rhizobiaceae species colonize plant roots, Pseudomonadaceae inhabit the phyllosphere, Acetobacteriaceae associate with sugar-loving insects and inhabit the carposphere, Enterobacteriaceae use fresh produce as vehicles to infect animal hosts, and Vibrionaceae, particularly Aliivibrio fischeri, colonize the light organ of squid. This review will highlight the diversity of the biosynthesis and regulation of BC in nature by discussing various examples of Proteobacteria that use BC-containing biofilms to facilitate host-bacteria interactions. Through discussion of current data we will establish new directions for the elucidation of BC biosynthesis, its regulation and its ecophysiological roles.

Keywords: Komagataeibacter (Gluconacetobacter) xylinus; animal–bacteria interactions; bacterial cellulose; biofilms; c-di-GMP; ecophysiology; fungal–bacteria interactions; plant–bacteria interactions.

FIGURE 1

Chemical structures of the key compounds discussed in this review. Cellobiose (A), a proposed monomeric unit of cellulose; c-di-GMP (B), the key secondary messenger involved in biofilm production. Plant cell wall components (PCWCs): hemicelluloses xylan (C), xyloglucan (D), and glucomannan (E); pectin (F); and lignin monomers p-coumaryl alcohol (G), coniferyl alcohol (H), and sinapyl alcohol (I). Plant-derived compounds (PDCs): ascorbic acid (J), indole-3-acetic acid (K), abscisic acid (L), zeatin (M), gibberellic acid (N), ethylene (O), and indole (P).

FIGURE 2

The number of publications regarding BC and Komagataeibacter xylinus. BC publications are shown by gray bars, while those studying BC production by K. xylinus are shown by black bars. The 1987 discovery that c-di-GMP activates BcsA and controls BC production marks a turning point for research in the bacterial cellulose (BC) field (^∗).

FIGURE 3

The turnover of c-di-GMP is controlled by environmental conditions. Extracellular cues that signal conditions suitable for colonization activate GGDEF domain-containing DGC enzymes that synthesize c-di-GMP. High levels of c-di-GMP binds the PilZ domain within the BcsA glycosyltransferase and triggers the production of a BC-containing biofilm. In contrast, extracellular signals associated with an unsuitable growth environment activate EAL/HD-GYP domain-containing PDEs that degrade c-di-GMP and produce the linear dinucleotide, pGpG. The resulting low c-di-GMP and high pGpG levels activate motility and virulence mechanisms that allow the bacterium to move to a more optimal environment for colonization. Negative feedback occurs between c-di-GMP and DGCs, as well as between GTP and PDEs.

FIGURE 4

Colonization of legume roots by Rhizobium spp. Plant-derived flavonoids (Flv) and Rhizobium Nod factors positively regulate each other (A). Bacterial cellulose (BC) and surface polysaccharides (SPs) are produced by Rhizobium to assist in attachment and subsequent biofilm formation on plant roots (B). See text for details. Green triangles indicate that a characteristic would be promoted, while red triangles indicate that a characteristic would be reduced.

FIGURE 5

Bacterial cellulose- and unipolar polysaccharide (UPP)-mediated root colonization in Agrobacterium tumefaciens is regulated by c-di-GMP, which is controlled by various DGCs and PDEs. The positive regulators of flagellar-mediated motility, VisN and VisR, inhibit the activity of DGCs and are responsible for the inverse regulation of motility and biofilm formation. Green triangles indicate that a characteristic would be promoted.

FIGURE 6

A schematic illustrating the effect that the PCWCs lignin, pectin and hemicellulose have on the structure of BC when K. xylinus colonizes fruit. Green triangles indicate that a characteristic would be promoted, while red triangles indicate that a characteristic would be reduced. All PCWCs associate with the BC enhancing anchoring to and colonization of the fruit by bacteria.

FIGURE 7

Bi-directional transfer of phytohormones influences bacterial physiology and fruit development during K. xylinus fruit colonization. The fruit on the left represents an unripe fruit that contains high levels of indole-3-acetic acid (IAA), zeatin (Z), and gibberellic acid (GA₃). The fruit on the right represents a ripe fruit that contains high levels of abscisic acid (ABA) and ascorbic acid (AsA). See text for details. Green triangles indicate that a characteristic would be promoted, while red triangles indicate that a characteristic would be reduced.

FIGURE 8

Regulation of BC biosynthesis in Enterobacteriaceae. BC biosynthesis is regulated by c-di-GMP through the action of many diguanylate cyclases (DGCs). Accumulation of c-di-GMP inhibits the multifunctional phosphodiesterase (PDE) YdaM, which breaks down c-di-GMP, and inhibits both the DGC YciR and transcription factor (TF) MlrA through direct interaction. YciR produces c-di-GMP but also stimulates MlrA activity through direct interaction. High c-di-GMP levels allow MlrA to dramatically increase RpoS induced CsgD expression. RpoS and CsgD are key regulators for biofilm regulation. CsgD-induced expression of AdrA is the primary pathway for BC biosynthesis, however DGCs that are either dependent or independent of CsgD for activation, such as the DGC YedQ have been identified. It is likely that other c-di-GMP pools, such as the YegE/YciR formed c-di-GMP, may also contribute to BcsA activation. Green triangles indicate that a characteristic would be promoted, while red triangles indicate that a characteristic would be reduced.

FIGURE 9

Sugar-loving insects, such as the fruit fly Drosophila melanogaster, acquires and deposits acetic acid bacteria (AAB) onto fruit in nature. AAB residing in the gut of the insect get deposited on the fruit and encourage ripening, perpetuating this positive feedback loop. This process facilitates the fruit-fruit transmission of AAB that inhabit the carposphere, such as K. xylinus.

FIGURE 10

Regulatory mechanisms responsible for production of BC and the symbiosis polysaccharide (SYP) when Aliivibrio fischeri colonizes the light organ of squid. Pathways in red are involved with BC biosynthesis, pathways in blue are involved with SYP biosynthesis, and pathways in green are responsible for bioluminescence. Green triangles indicate that a characteristic would be promoted. The dashed arrow shows a proposed pathway.