The Role of Two Linear β-Glucans Activated by c-di-GMP in Rhizobium etli CFN42

Bacterial exopolysaccharides (EPS) have been implicated in a variety of functions that assist in bacterial survival, colonization, and host-microbe interactions. Among them, bacterial linear beta-glucans are polysaccharides formed by D-glucose units linked by beta-glycosidic bonds, which include curdlan, cellulose, and the new described Mixed Linkage beta-Glucan (MLG). Bis-(3',5')-cyclic dimeric guanosine monophosphate (c-di-GMP) is a universal bacterial second messenger that usually promote EPS production. Here, we report Rhizobium etli as the first bacterium capable of producing cellulose and MLG. Significant amounts of these two beta-glucans are not produced under free-living laboratory conditions, but their production is triggered upon elevation of intracellular c-di-GMP levels, both contributing to Congo red (CR+) and Calcofluor (CF+) phenotypes. Cellulose turned out to be more relevant for free-living phenotypes promoting flocculation and biofilm formation under high c-di-GMP conditions. None of these two EPS are essential for attachment to roots of Phaseolus vulgaris, neither for nodulation nor for symbiotic nitrogen fixation. However, both beta-glucans separately contribute to the fitness of interaction between R. etli and its host. Overproduction of these beta-glucans, particularly cellulose, appears detrimental for symbiosis. This indicates that their activation by c-di-GMP must be strictly regulated in time and space and should be controlled by different, yet unknown, regulatory pathways.

Automated summary

Bacterial exopolysaccharides play important roles in bacterial survival, colonization, and host-microbe interactions. Rhizobium etli is the first bacterium capable of producing cellulose and mixed linkage beta-glucan. The production of these polysaccharides is regulated by c-di-GMP levels and contributes to the interaction between the bacterium and its host.