Environmental regulation of exopolysaccharide production in Sinorhizobium meliloti

Small molecule control of virulence gene expression in Francisella tularensis

In Francisella tularensis, the SspA protein family members MglA and SspA form a complex that associates with RNA polymerase (RNAP) to positively control the expression of virulence genes critical for the intramacrophage growth and survival of the organism. Although the association of the MglA-SspA complex with RNAP is evidently central to its role in controlling gene expression, the molecular details of how MglA and SspA exert their effects are not known. Here we show that in the live vaccine strain of F. tularensis (LVS), the MglA-SspA complex works in concert with a putative DNA-binding protein we have called PigR, together with the alarmone guanosine tetraphosphate (ppGpp), to regulate the expression of target genes. In particular, we present evidence that MglA, SspA, PigR and ppGpp regulate expression of the same set of genes, and show that mglA, sspA, pigR and ppGpp null mutants exhibit similar intramacrophage growth defects and are strongly attenuated for virulence in mice. We show further that PigR interacts directly with the MglA-SspA complex, suggesting that the central role of the MglA and SspA proteins in the control of virulence gene expression is to serve as a target for a transcription activator. Finally, we present evidence that ppGpp exerts its effects by promoting the interaction between PigR and the RNAP-associated MglA-SspA complex. Through its responsiveness to ppGpp, the contact between PigR and the MglA-SspA complex allows the integration of nutritional cues into the regulatory network governing virulence gene expression.

Guanosine tetraphosphate (ppGpp) is a small molecule that is produced by many different bacteria in response to nutrient limitation. Although ppGpp has been shown to play an important role in controlling the expression of virulence genes in several pathogenic bacteria, few studies have addressed how this occurs. Here we show that in the intracellular pathogen F. tularensis, ppGpp plays a critical role in controlling the expression of genes required for intracellular replication and virulence, and we uncover the molecular basis for its effect. In particular, we show that ppGpp works in concert with three other essential regulators of virulence gene expression in F. tularensis—a putative DNA-binding protein that we have called PigR and the SspA protein family members MglA and SspA. Our study provides evidence that ppGpp functions to promote the interaction between PigR and a component of F. tularensis RNA polymerase (RNAP) comprising the MglA and SspA proteins. By influencing the interaction between PigR and the RNAP-associated MglA-SspA complex, ppGpp serves to tie the nutritional status of the cell to the expression of genes that are essential for survival in the host.

An integrated view of biofilm formation in rhizobia

Biofilms are bacterial communities enclosed within an extracellular matrix of polysaccharides produced by the bacteria, which adhere to a living or an inert macrosurface. In nature, biofilms constitute a protected growth modality allowing bacteria to survive in hostile environments. Studies of environmental isolates have revealed a highly ordered, three-dimensional organization of the extracellular matrix, which has important implications for biofilm physiology. The zone of soil immediately surrounding a plant root where complex biological and ecological processes occur, termed rhizosphere, forms an environment that fulfills the requirements for biofilm formation, including sufficient moisture and supply of nutrients, which are provided by the plant. Biofilm formation on plants appears to be associated with symbiotic and pathogenic responses, but it is unclear how plants regulate the association. Biofilms function as structures resistant against stress factors such as desiccation, UV radiation, predation, and antibiosis, which help create protective niches for rhizobia. However, the role of biofilms in rhizobial-legume symbiosis remains to be clarified. Here, the mechanisms involved in bacterial biofilm formation and attachment on plant roots, and the relation of these mechanisms to rhizobial function and survival are reviewed.

Null mutations in Sinorhizobium meliloti exoS and chvI demonstrate the importance of this two-component regulatory system for symbiosis

Exopolysaccharides, either succinoglycan or galactoglucan, are essential for the establishment of the symbiosis between Sinorhizobium meliloti and Medicago sativa (alfalfa). The ExoS/ChvI two-component regulatory system is known as a regulator of succinoglycan production but the genes that are directly regulated by ChvI have not been determined. Difficulty isolating exoS and chvI null mutants has prompted the suggestion that these genes are essential for S. meliloti viability. We have successfully isolated exoS and chvI null mutants using a merodiploid-facilitated strategy. We present evidence that the S. meliloti ExoS/ChvI two-component regulatory system is essential for symbiosis with alfalfa. Phenotypic analyses of exoS and chvI null mutant strains demonstrate that ExoS/ChvI controls both succinoglycan and galactoglucan production and is required for growth on over 21 different carbon sources. These new findings suggest that the ExoS/ChvI regulatory targets might not be the exo genes that are specific for succinoglycan biosynthesis but rather genes that have common influence on both succinoglycan and galactoglucan production. Other studied alpha-proteobacteria ExoS/ChvI orthologues are required for the bacteria to invade or persist in host cells and thus we present more evidence that this two-component regulatory system is essential for alpha-proteobacterial host interaction.

The low-molecular-weight fraction of exopolysaccharide II from Sinorhizobium meliloti is a crucial determinant of biofilm formation

Sinorhizobium meliloti is a soil bacterium that elicits the formation of root organs called nodules on its host plant, Medicago sativa. Inside these structures, the bacteria are able to convert atmospheric nitrogen into ammonia, which is then used by the plant as a nitrogen source. The synthesis by S. meliloti of at least one exopolysaccharide, succinoglycan or EPS II, is essential for a successful symbiosis. While exopolysaccharide-deficient mutants induce the formation of nodules, they fail to invade them, and as a result, no nitrogen fixation occurs. Interestingly, the low-molecular-weight fractions of these exopolysaccharides are the symbiotically active forms, and it has been suggested that they act as signals to the host plant to initiate infection thread formation. In this work, we explored the role of these rhizobial exopolysaccharides in biofilm formation and their importance in the symbiotic relationship with the host. We showed that the ExpR/Sin quorum-sensing system controls biofilm formation in S. meliloti through the production of EPS II, which provides the matrix for the development of structured and highly organized biofilms. Moreover, the presence of the low-molecular-weight fraction of EPS II is vital for biofilm formation, both in vitro and in vivo. This is the first report where the symbiotically active fraction of EPS II is shown to be a critical factor for biofilm formation and root colonization. Thus, the ability of S. meliloti to properly attach to root surfaces and form biofilms conferred by the synthesis of exopolysaccharides may embody the main function of these symbiotically essential molecules.

The high-affinity phosphate transporter Pst in Proteus mirabilis HI4320 and its importance in biofilm formation

Proteus mirabilis causes urinary tract infections (UTIs) in individuals requiring long-term indwelling catheterization. The pathogenesis of this uropathogen is mediated by a number of virulence factors and the formation of crystalline biofilms. In addition, micro-organisms have evolved complex systems for the acquisition of nutrients, including the phosphate-specific transport system, which has been shown to be important in biofilm formation and pathogenesis. A functional Pst system is important during UTIs caused by P. mirabilis HI4320, since transposon mutants in the PstS periplasmic binding protein and the PstA permease protein were attenuated in the CBA mouse model of UTI. These mutants displayed a defect in biofilm formation when grown in human urine. This study focuses on a comparison of the proteomes during biofilm and planktonic growth in phosphate-rich medium and human urine, and microscopic investigations of biofilms formed by the pst mutants. Our data suggest that (i) the Deltapst mutants, and particularly the DeltapstS mutant, are defective in biofilm formation, and (ii) the proteomes of these mutants differ significantly from that of the wild-type. Therefore, since the Pst system of P. mirabilis HI4320 negatively regulates biofilm formation, this system is important for the pathogenesis of these organisms during complicated UTIs.

Competitive and cooperative effects in quorum-sensing-regulated galactoglucan biosynthesis in Sinorhizobium meliloti

The symbiotic nitrogen-fixing bacterium Sinorhizobium meliloti possesses the Sin quorum-sensing system based on N-acyl homoserine lactones (AHLs) as signal molecules. The Sin system consists of SinI, the AHL synthase, and SinR, the LuxR-type regulator. This system regulates the expression of a multitude of S. meliloti genes through ExpR, another LuxR-type regulator. Analysis of the activity of the sinI promoter showed that the expression of sinI is dependent on sinR and enhanced by a combination of expR and Sin AHLs. The characterization of the ExpR binding site upstream of sinI and the identification of binding sites upstream of the galactoglucan biosynthesis genes wgaA (expA1) and wgeA (expE1) allowed the definition of a consensus sequence for these binding sites. Based on this consensus, two additional ExpR binding sites in the promoter regions of exoI and exsH, two genes related to the production of succinoglycan, were found. The specific binding of ExpR to the wgaA and wgeA promoters was enhanced in the presence of oxo-C(14)-HL. Positive regulation of the galactoglucan biosynthesis genes by ExpR was shown to be dependent on WggR (ExpG) and influenced by MucR, both of which are previously characterized regulators of these genes. Based on these results, a reworked model of the Sin-ExpR quorum-sensing regulation scheme of galactoglucan production in S. meliloti is suggested.

Fine-tuning of galactoglucan biosynthesis in Sinorhizobium meliloti by differential WggR (ExpG)-, PhoB-, and MucR-dependent regulation of two promoters

Depending on the phosphate concentration encountered in the environment Sinorhizobium meliloti 2011 synthesizes two different exopolysaccharides (EPS). Galactoglucan (EPS II) is produced under phosphate starvation but also in the presence of extra copies of the transcriptional regulator WggR (ExpG) or as a consequence of a mutation in mucR. The galactoglucan biosynthesis gene cluster contains the operons wga (expA), wge (expE), wgd (expD), and wggR (expG). Two promoters, differentially controlled by WggR, PhoB, and MucR, were identified upstream of each of these operons. The proximal promoters of the wga, wge, and wgd transcription units were constitutively active when separated from the upstream regulatory sequences. Promoter activity studies and the positions of predicted PhoB and WggR binding sites suggested that the proximal promoters are cooperatively induced by PhoB and WggR. MucR was shown to strongly inhibit the distal promoters and bound to the DNA in the vicinity of the distal transcription start sites. An additional inhibitory effect on the distal promoter of the structural galactoglucan biosynthesis genes was identified as a new feature of WggR in a mucR mutant. A regulatory model of the fine-tuning of galactoglucan production is proposed.

Influence of phosphate and ammonia on the growth, exopolysaccharide production and symbiosis of Rhizobium leguminosarum bv. trifolii TA1 with clover (Trifolium pratense)

The Rhizobium-legume interaction is sensitive to a number of environmental factors, among which phosphate (Pi) and ammonium availability are the most important. We investigated the effect of Pi and ammonia concentration on exopolysaccharide production and symbiosis Trifolium pratense with Rhizobium leguminosarum bv. trifolii TA1 (RtTA1). The optimal Pi concentration in the bacterial growth medium for RtTA1 growth and exopolysaccharide production was in the range from 0.9 mM to 8.1 mM. Independently of Pi concentration, ammonium (NH4Cl) concentration above 8.1 mM in the culture medium significantly decreased EPS production, indicating a regulatory role of this nutrient on the EPS production in the RtTA1 strain. Pi availability has a beneficial effect on both partners of symbiosis. Pi concentration in the plant medium in the range from 1.7 mM to 5 mM was optimal for nodule formation, nodule occupancy and nitrogen fixation ability. Despite of T. pratense cv. Bryza tolerance on high Pi concentration, 20 mM Pi occurs to be nearly phytotoxic, which negatively affects almost all symbiotic parameters. Large amounts of starch were accumulated in the nodules formed by clover grown on medium containing high Pi concentration.

Escherichia coli transcription factor YncC (McbR) regulates colanic acid and biofilm formation by repressing expression of periplasmic protein YbiM (McbA)

Quorum-sensing signal autoinducer 2 (AI-2) stimulates Escherichia coli biofilm formation through the motility regulator MqsR that induces expression of the putative transcription factor encoded by yncC. Here, we show that YncC increases biofilm formation by repressing overproduction of the exopolysaccharide identified as colanic acid (corroborated by decreasing mucoidy and increased sensitivity to bacteriophage P1 infection). Differential gene expression and gel shift assays demonstrated that YncC is a repressor of the predicted periplasmic protein-encoding gene, ybiM, which was corroborated by the isogenic yncC ybiM double mutation that repressed the yncC phenotypes (biofilm formation, colanic acid overproduction, mucoidy and bacteriophage resistance). Through nickel-enrichment DNA microarrays and additional gel shift assays, we found that the putative transcription factor B3023 (directly upstream of mqsR) binds the yncC promoter. Overexpressing MqsR, AI-2 import regulators LsrR/LsrK and AI-2 exporter TqsA induced yncC transcription, whereas the AI-2 synthase LuxS and B3023 repressed yncC. MqsR has a toxic effect on E. coli bacterial growth, which is partially reduced by the b3023 mutation. Therefore, AI-2 quorum-sensing control of biofilm formation is mediated through regulator MqsR that induces expression of the transcription factor YncC. YncC inhibits the expression of periplasmic YbiM, which prevents overproduction of colanic acid (excess colanic acid causes mucoidy) and prevents YbiM from inhibiting biofilm formation.

The phosphate regulon and bacterial virulence: a regulatory network connecting phosphate homeostasis and pathogenesis

Bacterial pathogens regulate virulence factor gene expression coordinately in response to environmental stimuli, including nutrient starvation. The phosphate (Pho) regulon plays a key role in phosphate homeostasis. It is controlled by the PhoR/PhoB two-component regulatory system. PhoR is an integral membrane signaling histidine kinase that, through an interaction with the ABC-type phosphate-specific transport (Pst) system and a protein called PhoU, somehow senses environmental inorganic phosphate (P(i)) levels. Under conditions of P(i) limitation (or in the absence of a Pst component or PhoU), PhoR activates its partner response regulator PhoB by phosphorylation, which, in turn, up- or down-regulates target genes. Single-cell profiling of PhoB activation has shown recently that Pho regulon gene expression exhibits a stochastic, "all-or-none" behavior. Recent studies have also shown that the Pho regulon plays a role in the virulence of several bacteria. Here, we present a comprehensive overview of the role of the Pho regulon in bacterial virulence. The Pho regulon is clearly not a simple regulatory circuit for controlling phosphate homeostasis; it is part of a complex network important for both bacterial virulence and stress response.

Regulation of motility by the ExpR/Sin quorum-sensing system in Sinorhizobium meliloti

A successful symbiotic relationship between Sinorhizobium meliloti and its host Medicago sativa (alfalfa) depends on several signaling mechanisms, such as the biosynthesis of exopolysaccharides (EPS) by S. meliloti. Previous work in our laboratory has shown that a quorum-sensing mechanism controls the production of the symbiotically active EPS II. Recent microarray analysis of the whole-genome expression profile of S. meliloti reveals that the ExpR/Sin quorum-sensing system regulates additional physiological processes that include low-molecular-weight succinoglycan production, nitrogen utilization, metal transport, motility, and chemotaxis. Nearly half of the flagellar genes and their dependence on quorum sensing are prominently displayed in our microarray analyses. We extend those observations in this work and confirm the findings by real-time PCR expression analysis of selected genes, including the flaF, flbT, flaC, cheY1, and flgB genes, involved in motility and chemotaxis. These genes code for regulators of flagellum synthesis, the chemotactic response, or parts of the flagellar apparatus. Gene expression analyses and visualization of flagella by electron microscopy performed at different points in the growth phase support our proposed model in which quorum sensing downregulates motility in S. meliloti. We demonstrate that the ExpR/Sin quorum-sensing system controls motility gene expression through the VisN/VisR/Rem relay. We also show that the ExoS-dependent two-component system suppresses motility gene expression through VisN and Rem in parallel to quorum sensing. This study contributes to our understanding of the mechanisms that govern motility in S. meliloti.

Polyphosphate kinase 1 is a pathogenesis determinant in Campylobacter jejuni

Campylobacter jejuni is the leading cause of bacterial gastroenteritis in the developed world. Despite its prevalence, relatively little is known about C. jejuni's precise pathogenesis mechanisms, particularly in comparison to other well-studied enteric organisms such as Escherichia coli and Salmonella spp. Altered expression of phosphate genes in a C. jejuni stringent response mutant, together with known correlations between the stringent response, polyphosphate (poly-P), and virulence in other bacteria, led us to investigate the role of poly-P in C. jejuni stress survival and pathogenesis. All sequenced C. jejuni strains harbor a conserved putative polyphosphate kinase 1 predicted to be principally responsible for poly-P synthesis. We generated a targeted ppk1 deletion mutant (Deltappk1) in C. jejuni strain 81-176 and found that Deltappk1, as well as the DeltaspoT stringent response mutant, exhibited low levels of poly-P at all growth stages. In contrast, wild-type C. jejuni poly-P levels increased significantly as the bacteria transitioned from log to stationary phase. Phenotypic analyses revealed that the Deltappk1 mutant was defective for survival during osmotic shock and low-nutrient stress. However, certain phenotypes associated with ppk1 deletion in other bacteria (i.e., motility and oxidative stress) were unaffected in the C. jejuni Deltappk1 mutant, which also displayed an unexpected increase in biofilm formation. The C. jejuni Deltappk1 mutant was also defective for the virulence-associated phenotype of intraepithelial cell survival in a tissue culture infection model and exhibited a striking, dose-dependent chick colonization defect. These results indicate that poly-P utilization and accumulation contribute significantly to C. jejuni pathogenesis and affect its ability to adapt to specific stresses and stringencies. Furthermore, our study demonstrates that poly-P likely plays both similar and unique roles in C. jejuni compared to its roles in other bacteria and that poly-P metabolism is linked to stringent response mechanisms in C. jejuni.

The ExpR/Sin quorum-sensing system controls succinoglycan production in Sinorhizobium meliloti

Sinorhizobium meliloti is a gram-negative soil bacterium capable of forming a symbiotic nitrogen-fixing relationship with its plant host, Medicago sativa. Various bacterially produced factors are essential for successful nodulation. For example, at least one of two exopolysaccharides produced by S. meliloti (succinoglycan or EPS II) is required for nodule invasion. Both of these polymers are produced in high- and low-molecular-weight (HMW and LMW, respectively) fractions; however, only the LMW forms of either succinoglycan or EPS II are active in nodule invasion. The production of LMW succinoglycan can be generated by direct synthesis or through the depolymerization of HMW products by the action of two specific endoglycanases, ExsH and ExoK. Here, we show that the ExpR/Sin quorum-sensing system in S. meliloti is involved in the regulation of genes responsible for succinoglycan biosynthesis as well as in the production of LMW succinoglycan. Therefore, quorum sensing, which has been shown to regulate the production of EPS II, also plays an important role in succinoglycan biosynthesis.

Roles of poly-3-hydroxybutyrate (PHB) and glycogen in symbiosis of Sinorhizobium meliloti with Medicago sp

Poly-3-hydroxybutyrate (PHB) and glycogen are major carbon storage compounds in Sinorhizobium meliloti. The roles of PHB and glycogen in rhizobia-legume symbiosis are not fully understood. Glycogen synthase mutations were constructed by in-frame deletion (glgA1) or insertion (glgA2). These mutations were combined with a phbC mutation to make all combinations of double and triple mutants. PHB was not detectable in any of the mutants containing the phbC mutation; glycogen was not detectable in any of the mutants containing the glgA1 mutation. PHB levels were significantly lower in the glgA1 mutant, while glycogen levels were increased in the phbC mutant. Exopolysaccharide (EPS) was not detected in any of the phbC mutants, while the glgA1 and glgA2 mutants produced levels of EPS similar to the wild-type. Symbiotic properties of these strains were investigated on Medicago truncatula and Medicago sativa. The results indicated that the strains unable to synthesize PHB, or glycogen, were still able to form nodules and fix nitrogen. However, phbC mutations caused greater nodule formation delay on M. truncatula than on M. sativa. Time-course studies showed that (1) the ability to synthesize PHB is important for N(2) fixation in M. truncatula nodules and younger M. sativa nodules, and (2) the blocking of glycogen synthesis resulted in lower levels of N(2) fixation on M. truncatula and older nodules on M. sativa. These data have important implications for understanding how PHB and glycogen function in the interactions of S. meliloti with Medicago spp.

Effects of nutritional and environmental conditions on Sinorhizobium meliloti biofilm formation

Rhizobia are non-spore-forming soil bacteria that fix atmospheric nitrogen into ammonia in a symbiosis with legume roots. However, in the absence of a legume host, rhizobia manage to survive and hence must have evolved strategies to adapt to diverse environmental conditions. The capacity to respond to variations in nutrient availability enables the persistence of rhizobial species in soil, and consequently improves their ability to colonize and to survive in the host plant. Rhizobia, like many other soil bacteria, persist in nature most likely in sessile communities known as biofilms, which are most often composed of multiple microbial species. We have been employing in vitro assays to study environmental parameters that might influence biofilm formation in the Medicago symbiont Sinorhizobium meliloti. These parameters include carbon source, amount of nitrate, phosphate, calcium and magnesium as well as the effects of osmolarity and pH. The microtiter plate assay facilitates the detection of subtle differences in rhizobial biofilms in response to these parameters, thereby providing insight into how environmental stress or nutritional status influences rhizobial survival. Nutrients such as sucrose, phosphate and calcium enhance biofilm formation as their concentrations increase, whereas extreme temperatures and pH negatively affect biofilm formation.

The stringent response is required for Helicobacter pylori survival of stationary phase, exposure to acid, and aerobic shock

The gastric pathogen Helicobacter pylori must adapt to fluctuating conditions in the harsh environment of the human stomach with the use of a minimal number of transcriptional regulators. We investigated whether H. pylori utilizes the stringent response, involving signaling through the alarmone (p)ppGpp, as a survival strategy during environmental stresses. We show that the H. pylori homologue of the bifunctional (p)ppGpp synthetase and hydrolase SpoT is responsible for all cellular (p)ppGpp production in response to starvation conditions. Furthermore, the H. pylori spoT gene complements the growth defect of Escherichia coli mutants lacking (p)ppGpp. An H. pylori spoT deletion mutant is impaired for stationary-phase survival and undergoes a premature transformation to a coccoid morphology. In addition, the spoT deletion mutant is unable to survive specific environmental stresses, including aerobic shock and acid exposure, which are likely to be encountered by this bacterium during infection and transmission.

Isolation of salt-sensitive mutants of Sinorhizobium meliloti strain Rm1021

The determinants necessary for adaptation to high NaCl concentrations and competition for nodule occupancy in Sinorhizobium meliloti were investigated genetically. Mutations in fabG as well as smc02909 (transmembrane transglycosylase), trigger factor (tig) and smc00717 (probably ftsE) gave rise to strains that were unable to tolerate high salt and were uncompetitive for nodule occupancy relative to the wild-type. Moreover exoF1, exoA and pgm determinants were determined to be necessary for strain Rm1021 to survive high NaCl and/or MgCl(2) concentrations. The introduction of an expR(+) allele was capable of suppressing the Mg(2+) sensitivity associated with the exoF1, but not the exoA, mutation in a manner independent of exopolysaccharide II (EPS II)-associated mucoidy. The results also show that the EPS II-associated mucoid phenotype was affected by either Mg(2+)or K(+), but not by Li(+), Ca(2+), or high osmolarity.

Helicobacter pylori initiates the stringent response upon nutrient and pH downshift

Helicobacter pylori was previously reported to lack a stringent response. In contrast, we show that after nutrient downshift, H. pylori produced abundant ppGpp and less total RNA. pH downshift also caused (p)ppGpp accumulation. Our observations indicate that nutrient deprivation and acid shock activate the stringent response in H. pylori.

Rhizobial exopolysaccharides: genetic control and symbiotic functions

Specific complex interactions between soil bacteria belonging to Rhizobium, Sinorhizobium, Mesorhizobium, Phylorhizobium, Bradyrhizobium and Azorhizobium commonly known as rhizobia, and their host leguminous plants result in development of root nodules. Nodules are new organs that consist mainly of plant cells infected with bacteroids that provide the host plant with fixed nitrogen. Proper nodule development requires the synthesis and perception of signal molecules such as lipochitooligosaccharides, called Nod factors that are important for induction of nodule development. Bacterial surface polysaccharides are also crucial for establishment of successful symbiosis with legumes. Sugar polymers of rhizobia are composed of a number of different polysaccharides, such as lipopolysaccharides (LPS), capsular polysaccharides (CPS or K-antigens), neutral β-1, 2-glucans and acidic extracellular polysaccharides (EPS). Despite extensive research, the molecular function of the surface polysaccharides in symbiosis remains unclear.

This review focuses on exopolysaccharides that are especially important for the invasion that leads to formation of indetermined (with persistent meristem) type of nodules on legumes such as clover, vetch, peas or alfalfa. The significance of EPS synthesis in symbiotic interactions of Rhizobium leguminosarum with clover is especially noticed. Accumulating data suggest that exopolysaccharides may be involved in invasion and nodule development, bacterial release from infection threads, bacteroid development, suppression of plant defense response and protection against plant antimicrobial compounds. Rhizobial exopolysaccharides are species-specific heteropolysaccharide polymers composed of common sugars that are substituted with non-carbohydrate residues. Synthesis of repeating units of exopolysaccharide, their modification, polymerization and export to the cell surface is controlled by clusters of genes, named exo/exs, exp or pss that are localized on rhizobial megaplasmids or chromosome. The function of these genes was identified by isolation and characterization of several mutants disabled in exopolysaccharide synthesis. The effect of exopolysaccharide deficiency on nodule development has been extensively studied. Production of exopolysaccharides is influenced by a complex network of environmental factors such as phosphate, nitrogen or sulphur. There is a strong suggestion that production of a variety of symbiotically active polysaccharides may allow rhizobial strains to adapt to changing environmental conditions and interact efficiently with legumes.

L-Canavanine made by Medicago sativa interferes with quorum sensing in Sinorhizobium meliloti

Sinorhizobium meliloti is a gram-negative soil bacterium, capable of establishing a nitrogen-fixing symbiosis with its legume host, alfalfa (Medicago sativa). Quorum sensing plays a crucial role in this symbiosis, where it influences the nodulation process and the synthesis of the symbiotically important exopolysaccharide II (EPS II). S. meliloti has three quorum-sensing systems (Sin, Tra, and Mel) that use N-acyl homoserine lactones as their quorum-sensing signal molecule. Increasing evidence indicates that certain eukaryotic hosts involved in symbiotic or pathogenic relationships with gram-negative bacteria produce quorum-sensing-interfering (QSI) compounds that can cross-communicate with the bacterial quorum-sensing system. Our studies of alfalfa seed exudates suggested the presence of multiple signal molecules capable of interfering with quorum-sensing-regulated gene expression in different bacterial strains. In this work, we choose one of these QSI molecules (SWI) for further characterization. SWI inhibited violacein production, a phenotype that is regulated by quorum sensing in Chromobacterium violaceum. In addition, this signal molecule also inhibits the expression of the S. meliloti exp genes, responsible for the production of EPS II, a quorum-sensing-regulated phenotype. We identified this molecule as l-canavanine, an arginine analog, produced in large quantities by alfalfa and other legumes.

The role of nutrient presence on the adhesion kinetics of Burkholderia cepacia G4g and ENV435g

The adhesion kinetics of Burkholderia cepacia G4g and ENV435g have been investigated in a radial stagnation point flow (RSPF) system under well-controlled hydrodynamics and solution chemistry. The sensitivity of adhesion behavior to nutrient condition was also examined. Supplementary cell characterization techniques were conducted to evaluate the viability, hydrophobicity, electrophoretic mobility, size, and charge density of cells grown in both nutrient rich Luria broth (LB) and nutrient poor basal salts medium (BSM). Comparable adhesion kinetics were observed for the wild-type (G4g) and mutant (ENV435g) grown in the same medium; however, the attachment efficiency increased with the level of nutrient presence for both cell types by approximately 60%. Nutrient condition altered deposition due to its impact on the surface charge characteristics and size of the cells. Adhesion behavior was consistent with expectations based on classical Derjaguin-Landau-Verwey-Overbeek (DLVO) theory for colloidal interactions, as the adhesion efficiency increased with ionic strength. However, the results also suggest the involvement of non-DLVO type interactions that influence cell adhesion. Systematic experimentation with B. cepacia in the RSPF system demonstrated that the ENV435g mutant is not "adhesion deficient"; rather, adhesion for both the G4g and ENV435g was a function of the nutrient condition and resulting cell surface chemistry.

The Campylobacter jejuni stringent response controls specific stress survival and virulence-associated phenotypes

Campylobacter jejuni is a highly prevalent food-borne pathogen that causes diarrhoeal disease in humans. A natural zoonotic, it must overcome significant stresses both in vivo and during transmission despite the absence of several traditional stress response genes. Although relatively little is understood about its mechanisms of pathogenesis, its ability to interact with and invade human intestinal epithelial cells closely correlates with virulence. A C. jejuni microarray-based screen revealed that several known virulence genes and several uncharacterized genes, including spoT, were rapidly upregulated during infection of human epithelial cells. spoT and its homologue relA have been shown in other bacteria to regulate the stringent response, an important stress response that to date had not been demonstrated for C. jejuni or any other epsilon-proteobacteria. We have found that C. jejuni mounts a stringent response that is regulated by spoT. Detailed analyses of a C. jejuni delta spoT mutant revealed that the stringent response is required for several specific stress, transmission and antibiotic resistance-related phenotypes. These include stationary phase survival, growth and survival under low CO2/high O2 conditions, and rifampicin resistance. A secondary suppressor strain that specifically rescues the low CO2 growth defect of the delta spoT mutant was also isolated. The stringent response additionally proved to be required for the virulence-related phenotypes of adherence, invasion, and intracellular survival in two human epithelial cell culture models of infection; spoT is the first C. jejuni gene shown to participate in longer term survival in epithelial cells. Microarray analyses comparing wild-type to the delta spoT mutant also revealed a strong correlation between gene expression profiles and phenotype differences observed. Together, these data demonstrate a critical role for the C. jejuni stringent response in multiple aspects of C. jejuni biology and pathogenesis and, further, may lend novel insight into unexplored features of the stringent response in other prokaryotic organisms.

Detection of and response to signals involved in host-microbe interactions by plant-associated bacteria

Diverse interactions between hosts and microbes are initiated by the detection of host-released chemical signals. Detection of these signals leads to altered patterns of gene expression that culminate in specific and adaptive changes in bacterial physiology that are required for these associations. This concept was first demonstrated for the members of the family Rhizobiaceae and was later found to apply to many other plant-associated bacteria as well as to microbes that colonize human and animal hosts. The family Rhizobiaceae includes various genera of rhizobia as well as species of Agrobacterium. Rhizobia are symbionts of legumes, which fix nitrogen within root nodules, while Agrobacterium tumefaciens is a pathogen that causes crown gall tumors on a wide variety of plants. The plant-released signals that are recognized by these bacteria are low-molecular-weight, diffusible molecules and are detected by the bacteria through specific receptor proteins. Similar phenomena are observed with other plant pathogens, including Pseudomonas syringae, Ralstonia solanacearum, and Erwinia spp., although here the signals and signal receptors are not as well defined. In some cases, nutritional conditions such as iron limitation or the lack of nitrogen sources seem to provide a significant cue. While much has been learned about the process of host detection over the past 20 years, our knowledge is far from being complete. The complex nature of the plant-microbe interactions makes it extremely challenging to gain a comprehensive picture of host detection in natural environments, and thus many signals and signal recognition systems remain to be described.

The symbiotic defect in a Sinorhizobium meliloti lipopolysaccharide mutant can be overcome by expression of other surface polysaccharides

In this work we have examined the extent of functional complementation in symbiosis among different Sinorhizobium meliloti surface polysaccharides including lipopolysaccharide (LPS). We show that a symbiotic deficiency associated with an LPS defect can be reversed by appropriate expression of other surface polysaccharides such as galactoglucan (EPSII) and a particular form of capsular polysaccharide (KdoPS). It is noteworthy that, while succinoglycan EPSI and LPS cannot functionally substitute for each other, they can both be replaced by the same common set of polysaccharides (i.e., EPSII/KdoPS). The complex pattern of functional complementation in symbiosis among S. meliloti surface polysaccharides was shown to be different in Medicago truncatula compared to that previously reported for M. sativa.

Four promoters subject to regulation by ExoR and PhoB direct transcription of the Sinorhizobium melilotiexoYFQ operon involved in the biosynthesis of succinoglycan

Succinoglycan (EPS I), the main acidic exopolysaccharide of Sinorhizobium meliloti, is required for the initiation and elongation of infection threads during nodulation of the host plant alfalfa. The gene products of the exoYFQ operon are involved in the first step of succinoglycan biosynthesis as well as in the polymerisation of subunits to the high-molecular-mass form of this exopolysaccharide. One promoter region that directs transcription of exoX and two promoter regions that drive transcription of exoY were mapped in the exoX-exoY intergenic region. The distal exoY promoter region containing three putative -10 promoter elements was active under standard growth conditions and was subject to ExoR-dependent regulation. Although this promoter region was stimulated in a phoB mutant, no PHO box-like sequences were found, suggesting an indirect regulatory effect of PhoB. The proximal promoter contains a PHO box-like sequence in the putative -35 region and was affected by low and high phosphate concentrations dependent on PhoB. In the case of deleted upstream regions, this promoter was also controlled by ExoR. An additional promoter displaying activity in exoR, mucR and phoB mutants under standard conditions was identified upstream of exoF. The putative -35 promoter element of this promoter is covered by a second PHO box-like sequence.

Environmental modulation of the pssTNOP gene expression in Rhizobium leguminosarum bv. trifolii

Exopolysaccharide production by Rhizobium leguminosarum bv. trifolii is required for successful establishment of nitrogen-fixing symbiosis with clover (Trifolium pratense L.). Using plasmid-borne transcriptional fusions of promoters of pss genes with promoterless lacZ the effect of root exudate, phosphate, and ammonia on expression of pssT, pssN, pssO, and pssP genes in wild-type strain RtTA1 background was determined. A stimulating effect of these environmental factors on pssO and pssP gene expression was observed. The putative pssO gene promoter was determined to be a strong promoter within which the divergent nod-box element was identified. The pssO promoter was slightly inducible in a flavonoid-dependent manner in wild-type R. leguminosarum bv. trifolii strains RtTA1 and ANU843 and very weakly active in a mutant of strain ANU843 that lacks the regulatory nodD gene. The expression of pssO and pssP genes in planta was investigated using plasmid-borne pssO-gusA and pssP-gusA fusions under different phosphate availability to clover. The level of pssO-gusA fusion expression was shown to be dependent on phosphate concentration in the plant growth medium.

Phosphorus limitation enhances biofilm formation of the plant pathogen Agrobacterium tumefaciens through the PhoR-PhoB regulatory system

The plant pathogen Agrobacterium tumefaciens forms architecturally complex biofilms on inert surfaces. Adherence of A. tumefaciens C58 was significantly enhanced under phosphate limitation compared to phosphate-replete conditions, despite slower overall growth under low-phosphate conditions. Replacement of Pi with sn-glycerol-3-phosphate and 2-aminoethylphosphonate yielded similar results. The increase in surface interactions under phosphate limitation was observed in both static culture and continuous-culture flow cells. Statistical analysis of confocal micrographs obtained from the flow cell biofilms revealed that phosphate limitation increased both the overall attached biomass and the surface coverage, whereas the maximum thickness of the biofilm was not affected. Functions encoded on the two large plasmids of A. tumefaciens C58, pTiC58 and pAtC58, were not required for the observed phosphate effect. The phosphate concentration at which increased attachment was observed triggered the phosphate limitation response, controlled in many bacteria by the two-component regulatory system PhoR-PhoB. The A. tumefaciens phoB and phoR orthologues could only be disrupted in the presence of plasmid-borne copies of the genes, suggesting that this regulatory system might be essential. Expression of the A. tumefaciens phoB gene from a tightly regulated inducible promoter, however, correlated with the amount of biofilm under both phosphate-limiting and nonlimiting conditions, demonstrating that components of the Pho regulon influence A. tumefaciens surface interactions.

Global transcriptional analysis of the phosphate starvation response in Sinorhizobium meliloti strains 1021 and 2011

The global response to phosphate starvation was analysed at the transcriptional level in two closely related strains of Sinorhizobium meliloti, Rm1021 and Rm2011. The Pho regulon is known to be induced by PhoB under conditions of phosphate limitation. Ninety-eight genes were found to be significantly induced (more than three-fold) in a phoB -dependent manner in phosphate-stressed cells, and phoB -independent repression of 86 genes was observed. Possible roles of these genes in the phosphate stress response are discussed. Twenty new putative PHO box sequences were identified in regions upstream of 17 of the transcriptional units that showed phoB -dependent, or partially phoB -dependent, regulation, indicating direct regulation of these genes by PhoB. Despite the overall similarity between the phosphate stress responses in Rm1021 and Rm2011, lower induction rates were found for a set of phoB -dependent genes in Rm1021. Moreover, Rm1021 exhibited moderate constitutive activation of 12 phosphate starvation-inducible, phoB -dependent genes when cells were grown in a complex medium. A 1-bp deletion was observed in the pstC ORF in Rm1021, which results in truncation of the protein product. This mutation is probably responsible for the expression of phosphate starvation-inducible genes in Rm1021 in the absence of phosphate stress.

Cell surface characteristics of two halotolerant strains of Sinorhizobium meliloti

The halotolerant Sinorhizobium meliloti strain Rmd201 and its variant Rmd201 a were examined for their cell surface properties. The variant strain formed rough colonies and was found to be more hydrophobic. Growth of the variant strain was not affected appreciably when NaCl concentration of the medium was increased from 2 mM to 700 mM. Exopolysaccharide (EPS) and the lipopolysaccharide (LPS) content of the variant strain was found to be 7 and 14 times less, respectively, than the parent strain. However, enhanced synthesis of high molecular weight LPS bands were observed in SDS-PAGE analysis in the variant strain when the NaCl concentration was raised from 2 mM to 700 mM. Ribose and glucosamine were present in the variant LPS only. Mannose appeared as a major LPS constituent of the variant when grown in high salt containing medium. All these cell surface characteristics indicated that there were significant differences between the halotolerant strains of S. meliloti. The changes in the cell surface of the variant strain indicated the possible mutation in the gene(s) of cell surface polysaccharide biosynthesis.

Surface polysaccharide involvement in establishing the rhizobium-legume symbiosis

When the rhizosphere is nitrogen-starved, legumes and rhizobia (soil bacteria) enter into a symbiosis that enables the fixation of atmospheric dinitrogen. This implies a complex chemical dialogue between partners and drastic changes on both plant roots and bacteria. Several recent works pointed out the importance of rhizobial surface polysaccharides in the establishing of the highly specific symbiosis between symbionts. Exopolysaccharides appear to be essential for the early infection process. Lipopolysaccharides exhibit specific roles in the later stages of the nodulation processes such as the penetration of the infection thread into the cortical cells or the setting up of the nitrogen-fixing phenotype. More generally, even if active at different steps of the establishing of the symbiosis, all the polysaccharide classes seem to be involved in complex processes of plant defense inhibition that allow plant root invasion. Their chemistry is important for structural recognition as well as for physico-chemical properties.

Quorum sensing controls exopolysaccharide production in Sinorhizobium meliloti

Sinorhizobium meliloti is a soil bacterium capable of invading and establishing a symbiotic relationship with alfalfa plants. This invasion process requires the synthesis, by S. meliloti, of at least one of the two symbiotically important exopolysaccharides, succinoglycan and EPS II. We have previously shown that the sinRI locus of S. meliloti encodes a quorum-sensing system that plays a role in the symbiotic process. Here we show that the sinRI locus exerts one level of control through regulation of EPS II synthesis. Disruption of the autoinducer synthase gene, sinI, abolished EPS II production as well as the expression of several genes in the exp operon that are responsible for EPS II synthesis. This phenotype was complemented by the addition of acyl homoserine lactone (AHL) extracts from the wild-type strain but not from a sinI mutant, indicating that the sinRI-specified AHLs are required for exp gene expression. This was further confirmed by the observation that synthetic palmitoleyl homoserine lactone (C(16:1)-HL), one of the previously identified sinRI-specified AHLs, specifically restored exp gene expression. Most importantly, the absence of symbiotically active EPS II in a sinI mutant was confirmed in plant nodulation assays, emphasizing the role of quorum sensing in symbiosis.

A LuxR homolog controls production of symbiotically active extracellular polysaccharide II by Sinorhizobium meliloti

Production of complex extracellular polysaccharides (EPSs) by the nitrogen-fixing soil bacterium Sinorhizobium meliloti is required for efficient invasion of root nodules on the host plant alfalfa. Any one of three S. meliloti polysaccharides, succinoglycan, EPS II, or K antigen, can mediate infection thread initiation and extension (root nodule invasion) on alfalfa. Of these three polysaccharides, the only symbiotically active polysaccharide produced by S. meliloti wild-type strain Rm1021 is succinoglycan. The expR101 mutation is required to turn on production of symbiotically active forms of EPS II in strain Rm1021. In this study, we have determined the nature of the expR101 mutation in S. meliloti. The expR101 mutation, a spontaneous dominant mutation, results from precise, reading frame-restoring excision of an insertion sequence from the coding region of expR, a gene whose predicted protein product is highly homologous to the Rhizobium leguminosarum bv. viciae RhiR protein and a number of other homologs of Vibrio fischeri LuxR that function as receptors for N-acylhomoserine lactones (AHLs) in quorum-sensing regulation of gene expression. S. meliloti ExpR activates transcription of genes involved in EPS II production in a density-dependent fashion, and it does so at much lower cell densities than many quorum-sensing systems. High-pressure liquid chromatographic fractionation of S. meliloti culture filtrate extracts revealed at least three peaks with AHL activity, one of which activated ExpR-dependent expression of the expE operon.

The Sinorhizobium meliloti stringent response affects multiple aspects of symbiosis

Sinorhizobium meliloti and host legumes enter into a nitrogen-fixing, symbiotic relationship triggered by an exchange of signals between bacteria and plant. S. meliloti produces Nod factor, which elicits the formation of nodules on plant roots, and succinoglycan, an exopolysaccharide that allows for bacterial invasion and colonization of the host. The biosynthesis of these molecules is well defined, but the specific regulation of these compounds is not completely understood. Bacteria control complex regulatory networks by the production of ppGpp, the effector molecule of the stringent response, which induces physiological change in response to adverse growth conditions and can also control bacterial development and virulence. Through detailed analysis of an S. meliloti mutant incapable of producing ppGpp, we show that the stringent response is required for nodule formation and regulates the production of succinoglycan. Although it remains unknown whether these phenotypes are connected, we have isolated suppressor strains that restore both defects and potentially identify key downstream regulatory genes. These results indicate that the S. meliloti stringent response has roles in both succinoglycan production and nodule formation and, more importantly, that control of bacterial physiology in response to the plant and surrounding environment is critical to the establishment of a successful symbiosis.

MucR and mucS activate exp genes transcription and galactoglucan production in Sinorhizobium meliloti EFB1

When grown under standard conditions, Sinorhizobium meliloti EFB1 simultaneously produces two acidic exopolysaccharides, succinoglycan and galactoglucan, yielding very mucoid colonies. In this strain, MucR is essential for galactoglucan synthesis. A mutation in the mucS gene resulted in less mucoid colonies than in the wild-type EFB1. This mucS- strain was complemented to the wild-type phenotype by the cloned mucS gene, indicating that mucS is necessary for a wild-type level of galactoglucan production. Reverse transcription-polymerase chain reaction analysis of exp genes, which encode the pathway for galactoglucan production, in EFB1 and in the mutants affected in mucS, mucR, and both genes simultaneously, showed that MucS is a transcriptional activator of the exp genes but does not affect its own transcription. Furthermore, MucR is necessary for mucS transcriptional activation. As introduction of a cloned mucS gene in a mucR- strain yielded colonies less mucoid than the wild type, MucR could also activate exp genes transcription through other pathways. Deletion analysis of the expE promoter showed a region important for transcription and MucS activation. This region, containing a palindrome, is present in the putative expA, expC, expD, and expE promoters but not in the mucS promoter, suggesting that it is the target for MucS. A mucR-mucS- mutant, which does not produce galactoglucan, was impaired in competitive nodulation of alfalfa in soil microcosms, indicating another possible role for this exopolysaccharide in symbiosis.

Alfalfa root nodule invasion efficiency is dependent on Sinorhizobium meliloti polysaccharides

The soil bacterium Sinorhizobium meliloti is capable of entering into a nitrogen-fixing symbiosis with Medicago sativa (alfalfa). Particular low-molecular-weight forms of certain polysaccharides produced by S. meliloti are crucial for establishing this symbiosis. Alfalfa nodule invasion by S. meliloti can be mediated by any one of three symbiotically important polysaccharides: succinoglycan, EPS II, or K antigen (also referred to as KPS). Using green fluorescent protein-labeled S. meliloti cells, we have shown that there are significant differences in the details and efficiencies of nodule invasion mediated by these polysaccharides. Succinoglycan is highly efficient in mediating both infection thread initiation and extension. However, EPS II is significantly less efficient than succinoglycan at mediating both invasion steps, and K antigen is significantly less efficient than succinoglycan at mediating infection thread extension. In the case of EPS II-mediated symbioses, the reduction in invasion efficiency results in stunted host plant growth relative to plants inoculated with succinoglycan or K-antigen-producing strains. Additionally, EPS II- and K-antigen-mediated infection threads are 8 to 10 times more likely to have aberrant morphologies than those mediated by succinoglycan. These data have important implications for understanding how S. meliloti polysaccharides are functioning in the plant-bacterium interaction, and models are discussed.