Quorum Sensing and Cyclic di-GMP Exert Control Over Motility of Vibrio fischeri KB2B1

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.

OpaR Exerts a Dynamic Control over c-di-GMP Homeostasis and cpsA Expression in Vibrio parahaemolyticus through Its Regulation of ScrC and the Trigger Phosphodiesterase TpdA

The second messenger cyclic dimeric GMP (c-di-GMP) plays a central role in controlling decision-making processes that are vitally important for the environmental survival of the human pathogen Vibrio parahaemolyticus. The mechanisms by which c-di-GMP levels and biofilm formation are dynamically controlled in V. parahaemolyticus are poorly understood. Here, we report the involvement of OpaR in controlling c-di-GMP metabolism and its effects on the expression of the trigger phosphodiesterase (PDE) TpdA and the biofilm-matrix related gene cpsA. Our results revealed that OpaR negatively modulates the expression of tpdA by maintaining a baseline level of c-di-GMP. The OpaR-regulated PDEs ScrC, ScrG, and VP0117 enable the upregulation of tpdA, to different degrees, in the absence of OpaR. We also found that TpdA plays the dominant role in c-di-GMP degradation under planktonic conditions compared to the other OpaR-regulated PDEs. In cells growing on solid medium, we observed that the role of the dominant c-di-GMP degrader alternates between ScrC and TpdA. We also report contrasting effects of the absence of OpaR on cpsA expression in cells growing on solid media compared to cells forming biofilms over glass. These results suggest that OpaR can act as a double-edged sword to control cpsA expression and perhaps biofilm development in response to poorly understood environmental factors. Finally, using an in-silico analysis, we indicate outlets of the OpaR regulatory module that can impact decision making during the motile-to-sessile transition in V. parahaemolyticus. IMPORTANCE The second messenger c-di-GMP is extensively used by bacterial cells to control crucial social adaptations such as biofilm formation. Here, we explore the role of the quorum-sensing regulator OpaR, from the human pathogen V. parahaemolyticus, on the dynamic control of c-di-GMP signaling and biofilm-matrix production. We found that OpaR is crucial to c-di-GMP homeostasis in cells growing on Lysogeny Broth agar and that the OpaR-regulated PDEs TpdA and ScrC alternate in the dominant role over time. Furthermore, OpaR plays contrasting roles in controlling the expression of the biofilm-related gene cpsA on different surfaces and growth conditions. This dual role has not been reported for orthologues of OpaR, such as HapR from Vibrio cholerae. It is important to investigate the origins and consequences of the differences in c-di-GMP signaling between closely and distantly related pathogens to better understand pathogenic bacterial behavior and its evolution.

Motility Control as a Possible Link Between Quorum Sensing to Surface Attachment in Vibrio Species

In this chapter, we discuss motility control as a possible link between quorum sensing (QS) to surface attachment in Vibrio species. QS regulates a variety of behaviors that are important for the life cycle of many bacterial species, including virulence factor production, biofilm formation, or metabolic homeostasis. Therefore, without QS, many species of bacteria cannot survive in their natural environments. Here, we summarize several QS systems in different Vibrio species and discuss some of emerging features that suggest QS is intimately connected to motility control. Finally, we speculate the connection between motility and QS is critical for Vibrio species to detect solid surfaces for surface attachment.

High Levels of Cyclic Diguanylate Interfere with Beneficial Bacterial Colonization

During colonization of the Hawaiian bobtail squid (Euprymna scolopes), Vibrio fischeri bacteria undergo a lifestyle transition from a planktonic motile state in the environment to a biofilm state in host mucus. Cyclic diguanylate (c-di-GMP) is a cytoplasmic signaling molecule that is important for regulating motility-biofilm transitions in many bacterial species. V. fischeri encodes 50 proteins predicted to synthesize and/or degrade c-di-GMP, but a role for c-di-GMP regulation during host colonization has not been investigated. We examined strains exhibiting either low or high levels of c-di-GMP during squid colonization and found that while a low-c-di-GMP strain had no colonization defect, a high c-di-GMP strain was severely impaired. Expression of a heterologous c-di-GMP phosphodiesterase restored colonization, demonstrating that the effect is due to high c-di-GMP levels. In the constitutive high-c-di-GMP state, colonizing V. fischeri exhibited reduced motility, altered biofilm aggregate morphology, and a regulatory interaction where transcription of one polysaccharide locus is inhibited by the presence of the other polysaccharide. Our results highlight the importance of proper c-di-GMP regulation during beneficial animal colonization, illustrate multiple pathways regulated by c-di-GMP in the host, and uncover an interplay of multiple exopolysaccharide systems in host-associated aggregates.

Mutational Analysis of Vibrio fischeri c-di-GMP-Modulating Genes Reveals Complex Regulation of Motility

The symbiont Vibrio fischeri uses motility to colonize its host. In numerous bacterial species, motility is negatively controlled by cyclic-di-GMP (c-di-GMP), which is produced by diguanylate cyclases (DGCs) with GGDEF domains and degraded by phosphodiesterases with either EAL or HD-GYP domains. To begin to decode the functions of the 50 Vibrio fischeri genes with GGDEF, EAL, and/or HD-GYP domains, we deleted each gene and assessed each mutant's migration through tryptone broth salt (TBS) soft agar medium containing or lacking magnesium (Mg) and calcium (Ca), which are known to influence V. fischeri motility. We identified 6, 13, and 16 mutants with altered migration in TBS-Mg, TBS, and TBS-Ca soft agar, respectively, a result that underscores the importance of medium conditions in assessing gene function. A biosensor-based assay revealed that Mg and Ca affected c-di-GMP levels negatively and positively, respectively; the severe decrease in c-di-GMP caused by Mg addition correlates with its strong positive impact on bacterial migration. A mutant defective for VF_0494, a homolog of V. cholerae rocS, exhibited a severe defect in migration across all conditions. Motility of a VF_1603 VF_2480 double mutant was also severely defective and could be restored by expression of "c-di-GMP-blind" alleles of master flagellar regulator flrA. Together, this work sheds light on the genes and conditions that influence c-di-GMP-mediated control over motility in V. fischeri and provides a foundation for (i) assessing roles of putative c-di-GMP-binding proteins, (ii) evaluating other c-di-GMP-dependent phenotypes in V. fischeri, (iii) uncovering potential redundancy, and (iv) deciphering signal transduction mechanisms. IMPORTANCE Critical bacterial processes, including motility, are influenced by c-di-GMP, which is controlled by environment-responsive synthetic and degradative enzymes. Because bacteria such as Vibrio fischeri use motility to colonize their hosts, understanding the roles of c-di-GMP-modulating enzymes in controlling motility has the potential to inform on microbe-host interactions. We leveraged recent advances in genetic manipulation to generate 50 mutants defective for putative c-di-GMP synthetic and degradative enzymes. We then assessed the consequences on motility, manipulating levels of magnesium and calcium, which inversely influenced motility and levels of c-di-GMP. Distinct subsets of the 50 genes were required under the different conditions. Our data thus provide needed insight into the functions of these enzymes and environmental factors that influence them.

sRNA chaperone Hfq controls bioluminescence and other phenotypes through Qrr1-dependent and -independent mechanisms in Vibrio fischeri

Colonization of the squid Euprymna scolopes by the bacterium Vibrio fischeri depends on bacterial biofilm formation, motility, and bioluminescence. Previous work has demonstrated an inhibitory role for the small RNA (sRNA) Qrr1 in quorum-induced bioluminescence of V. fischeri, but the contribution of the corresponding sRNA chaperone, Hfq, was not examined. We thus hypothesized that V. fischeri Hfq similarly functions to inhibit bacterial bioluminescence as well as regulate other key steps of symbiosis, including bacterial biofilm formation and motility. Surprisingly, deletion of hfq increased luminescence of V. fischeri beyond what was observed for the loss of qrr1 sRNA. Epistasis experiments revealed that, while Hfq contributes to the Qrr1-dependent regulation of light production, it also functions independently of Qrr1 and its downstream target, LitR. This Hfq-dependent, Qrr1-independent regulation of bioluminescence is also independent of the major repressor of light production in V. fischeri, ArcA. We further determined that Hfq is required for full motility of V. fischeri in a mechanism that partially depends on the Qrr1/LitR regulators. Finally, Hfq also appears to function in the control of biofilm formation: loss of Hfq delayed the timing and diminished the extent of wrinkled colony development, but did not eliminate the production of SYP-polysaccharide-dependent cohesive colonies. Furthermore, loss of Hfq enhanced production of cellulose and resulted in increased Congo red binding. Together, these findings point to Hfq as an important regulator of multiple phenotypes relevant to symbiosis between V. fischeri and its squid host.

Para-Aminobenzoic Acid, Calcium, and c-di-GMP Induce Formation of Cohesive, Syp-Polysaccharide-Dependent Biofilms in Vibrio fischeri

The marine bacterium Vibrio fischeri efficiently colonizes its symbiotic squid host, Euprymna scolopes, by producing a transient biofilm dependent on the symbiosis polysaccharide (SYP). In vitro, however, wild-type strain ES114 fails to form SYP-dependent biofilms. Instead, genetically engineered strains, such as those lacking the negative regulator BinK, have been developed to study this phenomenon. Historically, V. fischeri has been grown using LBS, a complex medium containing tryptone and yeast extract; supplementation with calcium is required to induce biofilm formation by a binK mutant. Here, through our discovery that yeast extract inhibits biofilm formation, we uncover signals and underlying mechanisms that control V. fischeri biofilm formation. In contrast to its inability to form a biofilm on unsupplemented LBS, a binK mutant formed cohesive, SYP-dependent colony biofilms on tTBS, modified LBS that lacks yeast extract. Moreover, wild-type strain ES114 became proficient to form cohesive, SYP-dependent biofilms when grown in tTBS supplemented with both calcium and the vitamin para-aminobenzoic acid (pABA); neither molecule alone was sufficient, indicating that this phenotype relies on coordinating two cues. pABA/calcium supplementation also inhibited bacterial motility. Consistent with these phenotypes, cells grown in tTBS with pABA/calcium were enriched in transcripts for biofilm-related genes and predicted diguanylate cyclases, which produce the second messenger cyclic-di-GMP (c-di-GMP). They also exhibited elevated levels of c-di-GMP, which was required for the observed phenotypes, as phosphodiesterase overproduction abrogated biofilm formation and partially rescued motility. This work thus provides insight into conditions, signals, and processes that promote biofilm formation by V. fischeri.

Getting the Message Out: the Many Modes of Host-Symbiont Communication during Early-Stage Establishment of the Squid-Vibrio Partnership

Symbiosis, by its basic nature, depends on partner interactions that are mediated by cues and signals. This kind of critical reciprocal communication shapes the trajectory of host-microbe associations from their onset through their maturation and is typically mediated by both biochemical and biomechanical influences. Symbiotic partnerships often involve communities composed of dozens to hundreds of microbial species, for which resolving the precise nature of these partner interactions is highly challenging. Naturally occurring binary associations, such as those between certain legumes, nematodes, fishes, and squids, and their specific bacterial partner species offer the opportunity to examine interactions with high resolution and at the scale at which the interactions occur. The goals of this review are to provide the conceptual framework for evolutionarily conserved drivers of host-symbiont communication in animal associations and to offer a window into some mechanisms of this phenomenon as discovered through the study of the squid-vibrio model. The discussion focuses upon the early events that lead to persistence of the symbiotic partnership. The biophysical and biochemical determinants of the initial hours of dialogue between partners and how the symbiosis is shaped by the environment that is created by their reciprocal interactions are key topics that have been difficult to approach in more complex systems. Through our research on the squid-vibrio system, we provide insight into the intricate temporal and spatial complexity that underlies the molecular and cellular events mediating successful microbial colonization of the host animal.