High Levels of Cyclic Diguanylate Interfere with Beneficial Bacterial Colonization

Effect of siaD on Ag-8 to improve resistance to crown gall in grapes and related mechanisms

Crown gall caused by Agrobacterium vitis (A. vitis) is one of the crucial issues restricting the to grape industry. In this study, Agrobacterium tumefaciens (Ag-8) was separated from the soil that could prevent the occurrence of grape crown gall. By the mutagenesis of Ag-8 transposon, the siaD gene deletion strain (ΔsiaD) showed significantly lower efficacy in grape and tomato plants for controlling grape crown gall, but the relevant mechanism was not clear. The biofilm formation and motility of ΔsiaD were significantly decreased, and the colonization ability of ΔsiaD in tomato roots was significantly reduced. RNA-seq analysis showed that the expression of nemR significantly reduced in the ΔsiaD and that the expression of nemR showed a high correlation with biofilm and motility. Further studies showed that the nemR gene deletion strain of Ag-8 (ΔnemR) showed significantly reduced motility, biofilm formation and control of grape crown gall compared to Ag-8, and the nemR gene complementary strain of Ag-8 (ΔnemR-comp) recovered to Ag-8 wild-type levels. The inoculation experiments of preventive, curative or simultaneous treatment further showed that the preferential inoculation with Ag-8 reduced the incidence of grape crown gall on tomato plants, and studies showed that the mutation of siaD affected the site competition between Ag-8 and A. vitis, and that the mutation of nemR was consistent with the previous results. This study provides a new strategy for the prevention and control of grape crown gall, which is of great significance to the grape industry to increase production and income.

Functional analysis of cyclic diguanylate-modulating proteins in Vibrio fischeri

As bacterial symbionts transition from a motile free-living state to a sessile biofilm state, they must coordinate behavior changes suitable to each lifestyle. Cyclic diguanylate (c-di-GMP) is an intracellular signaling molecule that can regulate this transition, and it is synthesized by diguanylate cyclase (DGC) enzymes and degraded by phosphodiesterase (PDE) enzymes. Generally, c-di-GMP inhibits motility and promotes biofilm formation. While c-di-GMP and the enzymes that contribute to its metabolism have been well-studied in pathogens, considerably less focus has been placed on c-di-GMP regulation in beneficial symbionts. Vibrio fischeri is the sole beneficial symbiont of the Hawaiian bobtail squid ( Euprymna scolopes ) light organ, and the bacterium requires both motility and biofilm formation to efficiently colonize. C-di-GMP regulates swimming motility and cellulose exopolysaccharide production in V. fischeri . The genome encodes 50 DGCs and PDEs, and while a few of these proteins have been characterized, the majority have not undergone comprehensive characterization. In this study, we use protein overexpression to systematically characterize the functional potential of all 50 V. fischeri proteins. All 28 predicted DGCs and 14 predicted PDEs displayed at least one phenotype consistent with their predicted function, and a majority of each displayed multiple phenotypes. Finally, active site mutant analysis of proteins with the potential for both DGC and PDE activities revealed potential activities for these proteins. This work presents a systems-level functional analysis of a family of signaling proteins in a tractable animal symbiont and will inform future efforts to characterize the roles of individual proteins during lifestyle transitions.

IMPORTANCE

C-di-GMP is a critical second messenger that mediates bacterial behaviors, and V. fischeri colonization of its Hawaiian bobtail squid host presents a tractable model in which to interrogate the role of c-di-GMP during animal colonization. This work provides systems-level characterization of the 50 proteins predicted to modulate c-di-GMP levels. By combining multiple assays, we generated a rich understanding of which proteins have the capacity to influence c-di-GMP levels and behaviors. Our functional approach yielded insights into how proteins with domains to both synthesize and degrade c-di-GMP may impact bacterial behaviors. Finally, we integrated published data to provide a broader picture of each of the 50 proteins analyzed. This study will inform future work to define specific pathways by which c-di-GMP regulates symbiotic behaviors and transitions.

Negative feedback of cyclic di-GMP levels optimizes switching between sessile and motile lifestyles in Vibrio cholerae

The signaling molecule cyclic di-GMP (cdG) controls the switch between bacterial motility and biofilm production, and fluctuations in cellular levels of cdG have been implicated in Vibrio cholerae pathogenesis. Intracellular concentrations of cdG are controlled by the interplay of diguanylate cyclase (DGC) enzymes, which synthesize cdG to promote biofilms, and phosphodiesterase (PDE) enzymes, which hydrolyse cdG to drive motility. To track the complete regulatory logic of how V. cholerae responds to changing cdG levels, we followed a timecourse of overexpression of either the V. harveyi diguanylate cyclase QrgB or a variant of QrgB lacking catalytic activity (QrgB*). We find that QrgB increases cdG levels relative to QrgB* for 30 minutes after overexpression, but the effect of QrgB on cdG levels plateaus at 30 minutes, indicating tight adaptive control of cdG levels. In contrast, loss of VpsR, a master regulator activating biofilm formation upon binding to cdG, leads to higher baseline levels of cdG and continuously increasing cdG through 60 minutes after QrgB induction, revealing the existence of a negative feedback loop on cdG levels operating through VpsR. Through a combination of RNA polymerase ChIP-seq, RNA-seq, and genetic approaches, we show that transcription of a gene encoding a PDE, cdgC, is activated by VpsR at high cdG concentrations, mediating this negative feedback on cdG levels. We further identify a transcript encoded within, and antisense to, the cdgC open reading frame which we name sRNA negative regulator of CdgC (SnrC). RNA polymerase ChIP-seq and RNA-seq demonstrate SnrC to be expressed specifically under conditions of high cdG in the absence of VpsR. Ectopic SnrC expression increases cdG levels in a manner dependent on CdgC, demonstrating that its effect on cdG levels is likely through interference with CdgC production. Further, although cells lacking cdgC exhibit enhanced biofilm formation, these mutants are outcompeted by wild type V. cholerae in colonization assays that reward a combination of attachment, dispersal, and motility behaviors. These results underscore the importance of negative feedback regulation of cdG to maintain appropriate homeostatic levels for efficient transitioning between biofilm formation and motility, both of which are necessary over the course of the V. cholerae infection cycle.

Comparative Genomic and Functional Analysis of c-di-GMP Metabolism and Regulatory Proteins in Bacillus velezensis LQ-3

Bacillus velezensis is a promising candidate for biocontrol applications. A common second messenger molecule, bis-(3,5)-cyclic-dimeric-guanosine monophosphate (c-di-GMP), has the ability to regulate a range of physiological functions that impact the effectiveness of biocontrol. However, the status of the c-di-GMP signaling pathway in biocontrol strain LQ-3 remains unknown. Strain LQ-3, which was isolated from wheat rhizosphere soil, has shown effective control of wheat sharp eyespot and has been identified as B. velezensis through whole-genome sequencing analyses. In this study, we investigated the intracellular c-di-GMP signaling pathway of LQ-3 and further performed a comparative genomic analysis of LQ-3 and 29 other B. velezensis strains. The results revealed the presence of four proteins containing the GGDEF domain, which is the conserved domain for c-di-GMP synthesis enzymes. Additionally, two proteins were identified with the EAL domain, which represents the conserved domain for c-di-GMP degradation enzymes. Furthermore, one protein was found to possess a PilZ domain, indicative of the conserved domain for c-di-GMP receptors in LQ-3. These proteins are called DgcK, DgcP, YybT, YdaK, PdeH, YkuI, and DgrA, respectively; they are distributed in a similar manner across the strains and belong to the signal transduction family. We selected five genes from the aforementioned seven genes for further study, excluding YybT and DgrA. They all play a role in regulating the motility, biofilm formation, and colonization of LQ-3. This study reveals the c-di-GMP signaling pathway associated with biocontrol features in B. velezensis LQ-3, providing guidance for the prevention and control of wheat sharp eyespot by LQ-3.

A cyclic di-GMP-binding adaptor protein interacts with a N5-glutamine methyltransferase to regulate the pathogenesis in Xanthomonas citri subsp. citri

The second messenger cyclic diguanylate monophosphate (c-di-GMP) regulates a wide range of bacterial behaviours through diverse mechanisms and binding receptors. Single-domain PilZ proteins, the most widespread and abundant known c-di-GMP receptors in bacteria, act as trans-acting adaptor proteins that enable c-di-GMP to control signalling pathways with high specificity. This study identifies a single-domain PilZ protein, XAC3402 (renamed N5MapZ), from the phytopathogen Xanthomonas citri subsp. citri (Xcc), which modulates Xcc virulence by directly interacting with the methyltransferase HemK. Through yeast two-hybrid, co-immunoprecipitation and immunofluorescent staining, we demonstrated that N5MapZ and HemK interact directly under both in vitro and in vivo conditions, with the strength of the protein-protein interaction decreasing at high c-di-GMP concentrations. This finding distinguishes N5MapZ from other characterized single-domain PilZ proteins, as it was previously known that c-di-GMP enhances the interaction between those single-domain PilZs and their protein partners. This observation is further supported by the fact that the c-di-GMP binding-defective mutant N5MapZR10A can interact with HemK to inhibit the methylation of the class 1 translation termination release factor PrfA. Additionally, we found that HemK plays an important role in Xcc pathogenesis, as the deletion of hemK leads to extensive phenotypic changes, including reduced virulence in citrus plants, decreased motility, production of extracellular enzymes and stress tolerance. Gene expression analysis has revealed that c-di-GMP and the HemK-mediated pathway regulate the expression of multiple virulence effector proteins, uncovering a novel regulatory mechanism through which c-di-GMP regulates Xcc virulence by mediating PrfA methylation via the single-domain PilZ adaptor protein N5MapZ.

A Label-Free Approach for Relative Spatial Quantitation of c-di-GMP in Microbial Biofilms

Microbial biofilms represent an important lifestyle for bacteria and are dynamic three-dimensional structures. Cyclic dimeric guanosine monophosphate (c-di-GMP) is a ubiquitous signaling molecule that is known to be tightly regulated with biofilm processes. While measurements of global levels of c-di-GMP have proven valuable toward understanding the genetic control of c-di-GMP production, there is a need for tools to observe the local changes of c-di-GMP production in biofilm processes. We have developed a label-free method for the direct detection of c-di-GMP in microbial colony biofilms using matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI). We applied this method to the enteric pathogen Vibrio cholerae, the marine symbiont V. fischeri, and the opportunistic pathogen Pseudomonas aeruginosa PA14 and detected spatial and temporal changes in c-di-GMP signal that accompanied genetic alterations in factors that synthesize and degrade the compound. We further demonstrated how this method can be simultaneously applied to detect additional metabolites of interest from a single sample.

Lighting the way: how the Vibrio fischeri model microbe reveals the complexity of Earth's "simplest" life forms

Vibrio (Aliivibrio) fischeri's initial rise to fame derived from its alluring production of blue-green light. Subsequent studies to probe the mechanisms underlying this bioluminescence helped the field discover the phenomenon now known as quorum sensing. Orthologs of quorum-sensing regulators (i.e., LuxR and LuxI) originally identified in V. fischeri were subsequently uncovered in a plethora of bacterial species, and analogous pathways were found in yet others. Over the past three decades, the study of this microbe has greatly expanded to probe the unique role of V. fischeri as the exclusive symbiont of the light organ of the Hawaiian bobtail squid, Euprymna scolopes. Buoyed by this optically amenable host and by persistent and insightful researchers who have applied novel and cross-disciplinary approaches, V. fischeri has developed into a robust model for microbe-host associations. It has contributed to our understanding of how bacteria experience and respond to specific, often fluxing environmental conditions and the mechanisms by which bacteria impact the development of their host. It has also deepened our understanding of numerous microbial processes such as motility and chemotaxis, biofilm formation and dispersal, and bacterial competition, and of the relevance of specific bacterial genes in the context of colonizing an animal host. Parallels in these processes between this symbiont and bacteria studied as pathogens are readily apparent, demonstrating functional conservation across diverse associations and permitting a reinterpretation of "pathogenesis." Collectively, these advances built a foundation for microbiome studies and have positioned V. fischeri to continue to expand the frontiers of our understanding of the microbial world inside animals.

The global regulation of c-di-GMP and cAMP in bacteria

Nucleotide second messengers are highly versatile signaling molecules that regulate a variety of key biological processes in bacteria. The best-studied examples are cyclic AMP (cAMP) and bis-(3'-5')-cyclic dimeric guanosine monophosphate (c-di-GMP), which both act as global regulators. Global regulatory frameworks of c-di-GMP and cAMP in bacteria show several parallels but also significant variances. In this review, we illustrate the global regulatory models of the two nucleotide second messengers, compare the different regulatory frameworks between c-di-GMP and cAMP, and discuss the mechanisms and physiological significance of cross-regulation between c-di-GMP and cAMP. c-di-GMP responds to numerous signals dependent on a great number of metabolic enzymes, and it regulates various signal transduction pathways through its huge number of effectors with varying activities. In contrast, due to the limited quantity, the cAMP metabolic enzymes and its major effector are regulated at different levels by diverse signals. cAMP performs its global regulatory function primarily by controlling the transcription of a large number of genes via cAMP receptor protein (CRP) in most bacteria. This review can help us understand how bacteria use the two typical nucleotide second messengers to effectively coordinate and integrate various physiological processes, providing theoretical guidelines for future research.

Vibrio fischeri: a model for host-associated biofilm formation

Multicellular communities of adherent bacteria known as biofilms are often detrimental in the context of a human host, making it important to study their formation and dispersal, especially in animal models. One such model is the symbiosis between the squid Euprymna scolopes and the bacterium Vibrio fischeri. Juvenile squid hatch aposymbiotically and selectively acquire their symbiont from natural seawater containing diverse environmental microbes. Successful pairing is facilitated by ciliary movements that direct bacteria to quiet zones on the surface of the squid's symbiotic light organ where V. fischeri forms a small aggregate or biofilm. Subsequently, the bacteria disperse from that aggregate to enter the organ, ultimately reaching and colonizing deep crypt spaces. Although transient, aggregate formation is critical for optimal colonization and is tightly controlled. In vitro studies have identified a variety of polysaccharides and proteins that comprise the extracellular matrix. Some of the most well-characterized matrix factors include the symbiosis polysaccharide (SYP), cellulose polysaccharide, and LapV adhesin. In this review, we discuss these components, their regulation, and other less understood V. fischeri biofilm contributors. We also highlight what is currently known about dispersal from these aggregates and host cues that may promote it. Finally, we briefly describe discoveries gleaned from the study of other V. fischeri isolates. By unraveling the complexities involved in V. fischeri's control over matrix components, we may begin to understand how the host environment triggers transient biofilm formation and dispersal to promote this unique symbiotic relationship.

Transcriptional pathways across colony biofilm models in the symbiont Vibrio fischeri

Beneficial microbial symbionts that are horizontally acquired by their animal hosts undergo a lifestyle transition from free-living in the environment to associating with host tissues. In the model symbiosis between the Hawaiian bobtail squid and its microbial symbiont Vibrio fischeri, one mechanism used to make this transition during host colonization is the formation of biofilm-like aggregates in host mucosa. Previous work identified factors that are sufficient to induce V. fischeri biofilm formation, yet much remains unknown regarding the breadth of target genes induced by these factors. Here, we probed two widely used in vitro models of biofilm formation to identify novel regulatory pathways in the squid symbiont V. fischeri ES114. We discovered a shared set of 232 genes that demonstrated similar patterns in expression in both models. These genes comprise multiple exopolysaccharide loci that are upregulated and flagellar motility genes that are downregulated, with a consistent decrease in measured swimming motility. Furthermore, we identified genes regulated downstream of the key sensor kinase RscS that are induced independent of the response regulator SypG. Our data suggest that transcriptional regulator VpsR plays a strong role in expression of at least a subset of these genes. Overall, this study adds to our understanding of the genes involved in V. fischeri biofilm regulation while revealing new regulatory pathways branching from previously characterized signaling networks.IMPORTANCEThe V. fischeri-squid system provides an opportunity to study biofilm development both in the animal host and in culture-based biofilm models that capture key aspects of in vivo signaling. In this work, we report the results of the transcriptomic profiling of two V. fischeri biofilm models followed by phenotypic validation and examination of novel signaling pathway architecture. Remarkable consistency between the models provides a strong basis for future studies using either approach or both. A subset of the factors identified by the approaches were validated in the work, and the body of transcriptomic data provides a number of leads for future studies in culture and during animal colonization.

A label-free approach for relative spatial quantitation of c-di-GMP in microbial biofilms

Microbial biofilms represent an important lifestyle for bacteria and are dynamic three dimensional structures. Cyclic dimeric guanosine monophosphate (c-di-GMP) is a ubiquitous signaling molecule that is known to be tightly regulated with biofilm processes. While measurements of global levels of c-di-GMP have proven valuable towards understanding the genetic control of c-di-GMP production, there is a need for tools to observe the local changes of c-di-GMP production in biofilm processes. We have developed a label-free method for the direct detection of c-di-GMP in microbial colony biofilms using matrix-assisted laser desorption ionization mass spectrometry imaging (MALDI-MSI). We applied this method to the enteric pathogen Vibrio cholerae, the marine symbiont V. fischeri, and the opportunistic pathogen Pseudomonas aeruginosa PA14 and detected spatial and temporal changes in c-di-GMP signal that accompanied genetic alterations in factors that synthesize and degrade the compound. We further demonstrated how this method can be simultaneously applied to detect additional metabolites of interest in a single experiment.

Transcriptional pathways across colony biofilm models in the symbiont Vibrio fischeri

Beneficial microbial symbionts that are horizontally acquired by their animal hosts undergo a lifestyle transition from free-living in the environment to associated with host tissues. In the model symbiosis between the Hawaiian bobtail squid and its microbial symbiont Vibrio fischeri, one mechanism used to make this transition during host colonization is the formation of biofilm-like aggregates in host mucosa. Previous work identified factors that are sufficient to induce V. fischeri biofilm formation, yet much remains unknown regarding the breadth of target genes induced by these factors. Here, we probed two widely-used in vitro models of biofilm formation to identify novel regulatory pathways in the squid symbiont V. fischeri ES114. We discovered a shared set of 232 genes that demonstrated similar patterns in expression in both models. These genes comprise multiple exopolysaccharide loci that are upregulated and flagellar motility genes that are downregulated, with a consistent decrease in measured swimming motility. Furthermore, we identified genes regulated downstream of the key sensor kinase RscS that are induced independent of the response regulator SypG. Our data suggest that putative response regulator VpsR plays a strong role in expression of at least a subset of these genes. Overall, this study adds to our understanding of the genes involved in V. fischeri biofilm regulation, while revealing new regulatory pathways branching from previously characterized signaling networks.

Exopolysaccharides Producing Bacteria: A Review

Bacterial exopolysaccharides (EPS) are essential natural biopolymers used in different areas including biomedicine, food, cosmetic, petroleum, and pharmaceuticals and also in environmental remediation. The interest in them is primarily due to their unique structure and properties such as biocompatibility, biodegradability, higher purity, hydrophilic nature, anti-inflammatory, antioxidant, anti-cancer, antibacterial, and immune-modulating and prebiotic activities. The present review summarizes the current research progress on bacterial EPSs including their properties, biological functions, and promising applications in the various fields of science, industry, medicine, and technology, as well as characteristics and the isolation sources of EPSs-producing bacterial strains. This review provides an overview of the latest advances in the study of such important industrial exopolysaccharides as xanthan, bacterial cellulose, and levan. Finally, current study limitations and future directions are discussed.

Cyclic di-GMP regulates bacterial colonization and further biocontrol efficacy of Bacillus velezensis against apple ring rot disease via its potential receptor YdaK

Bacillus species are among the most investigated beneficial bacteria and widely used in agricultural systems as biological control agents. Its biocontrol efficacy is controlled by diverse regulators. Cyclic diguanylate (c-di-GMP) is a nearly universal second messenger in bacteria and modulates various important physiological processes, including motility, biofilm formation, antifungal antibiotic production and host colonization. However, the impact of c-di-GMP on biocontrol efficacy of beneficial bacteria is unknown. Bacillus velezensis PG12 is an effective biocontrol strain against apple ring rot disease caused by Botryosphaeria dothidea. In this study, the contribution of c-di-GMP to biocontrol efficacy of B. velezensis PG12 was investigated. Deletion of single gene encoding diguanylate cyclase or phosphodiesterase did not affect its biocontrol efficacy against apple ring rot. However, artificial modulation of c-di-GMP level in the cells leads to a significant change of biocontrol efficacy, suggesting that c-di-GMP positively regulates biocontrol efficacy of B. velezensis PG12 against apple ring rot disease. More evidences indicate that c-di-GMP does not affect the antagonistic activity of B. velezensis PG12 against B. dothidea in vitro and in vivo, but positively regulates biofilm formation of B. velezensis PG12 and its colonization on apple fruits. Importantly, deletion of ydaK could rescue the inhibition of biofilm formation, bacterial colonization and biocontrol efficacy caused by low c-di-GMP level, indicating that YdaK is the potential c-di-GMP receptor to regulate biofilm formation, colonization and effective biological control. However, YdaK did not affect the antagonistic activity of B. velezensis PG12 against B. dothidea. Based on these findings, we propose that c-di-GMP regulates biofilm formation, subsequently the bacterial colonization on apple fruits and thus biocontrol efficacy of B. velezensis through its receptor YdaK. This is the first report showing that c-di-GMP plays a role in biocontrol efficacy of beneficial bacteria.

The enhanced expression of genes encoding diguanylate cyclases under cold stress contributes to the adhesion and biofilm formation of Shewanella putrefaciens WS13

Shewanella putrefaciens is a special spoilage bacterium of seafood during cold storage, which is easy to form biofilm and bring serious hazard to the seafood quality. Life cycle of biofilm starts after bacterial adhesion, which is essential for the formation and development of biofilm. As a ubiquitous second messenger in bacteria, c-di-GMP regulates the conversion between bacterial planktonic state and biofilm state. In this study, the adhesion and biofilm formation of S. putrefaciens WS13 under 4°C were compared to those under 30°C. Atom force microscope and scanning electron microscope were used to study the bacterial adhesion. Biofilm was analyzed by Fourier transform infrared spectroscopy, Bradford assay and phenol-sulfuric acid method. High-performance liquid chromatographic-tandem mass spectrometric and quantitative real-time PCR were applied to study c-di-GMP level and genes encoding diguanylate cyclases in cells, respectively. Results showed that the swarming mobility of S. putrefaciens WS13 was weaker under 4°C, however, the adhesive force under 4°C was 4–5 times higher than that under 30°C. Biofilm biomass, extracellular polysaccharides and extracellular proteins were 2.5 times, 3 times, and 1.6 times more than those under 30°C, respectively, but biofilm composition formed under both temperatures were similar. c-di-GMP level in S. putrefaciens WS13 under 30°C was no more than half of that in the corresponding growth stage under 4°C. Quantitative real-time PCR analysis also showed that the expression of genes encoding diguanylate cyclases were significantly enhanced under 4°C than that under 30°C. S. putrefaciens WS13 adapted to the cold stress by enhancing the expression of genes encoding diguanylate cyclases to promote bacterial adhesion and biofilm formation. This study provides a theoretical foundation for the research on the cold adaptation mechanism of specific spoilage bacteria of seafood based on c-di-GMP, and also provides a new idea to control seafood quality from the perspective of microbial molecular biology.

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

Simple Summary

Bacterial exopolysaccharides (EPS) are secreted biopolymers with often critical roles in bacterial physiology and ecology. In addition to their biological role, there is increasing interest for EPS in various industrial sectors. β-glucans are among the most important ones including cellulose as the most abundant organic polymer on earth, but also newcomers, such as the bacterial Mixed Linkage β-Glucan (MLG), displaying a unique repeating unit suggestive of biotechnological potential. In this work we describe Rhizobium etli as the first bacterium reported to be able to produce these two linear β-glucans cellulose and MLG. Rhizobium etli is an agronomic relevant rhizobacteria able to perform Biological Nitrogen Fixation (BNF) in a symbiotic association with common bean plants. The production and regulation of cellulose and MLG by Rhizobium etli CFN42 is discussed and their impact on its free-living and symbiotic lifestyles evaluated.

Abstract

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 β-glucans are polysaccharides formed by D-glucose units linked by β-glycosidic bonds, which include curdlan, cellulose, and the new described Mixed Linkage β-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 β-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 β-glucans separately contribute to the fitness of interaction between R. etli and its host. Overproduction of these β-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.