Synthase-dependent exopolysaccharide secretion in Gram-negative bacteria

Synthesis of Rhizobial Exopolysaccharides and Their Importance for Symbiosis with Legume Plants

Rhizobia dwell and multiply in the soil and represent a unique group of bacteria able to enter into a symbiotic interaction with plants from the Fabaceae family and fix atmospheric nitrogen inside de novo created plant organs, called nodules. One of the key determinants of the successful interaction between these bacteria and plants are exopolysaccharides, which represent species-specific homo- and heteropolymers of different carbohydrate units frequently decorated by non-carbohydrate substituents. Exopolysaccharides are typically built from repeat units assembled by the Wzx/Wzy-dependent pathway, where individual subunits are synthesized in conjunction with the lipid anchor undecaprenylphosphate (und-PP), due to the activity of glycosyltransferases. Complete oligosaccharide repeat units are transferred to the periplasmic space by the activity of the Wzx flippase, and, while still being anchored in the membrane, they are joined by the polymerase Wzy. Here we have focused on the genetic control over the process of exopolysaccharides (EPS) biosynthesis in rhizobia, with emphasis put on the recent advancements in understanding the mode of action of the key proteins operating in the pathway. A role played by exopolysaccharide in Rhizobium-legume symbiosis, including recent data confirming the signaling function of EPS, is also discussed.

Trigger phosphodiesterases as a novel class of c-di-GMP effector proteins

The bacterial second messenger c-di-GMP controls bacterial biofilm formation, motility, cell cycle progression, development and virulence. It is synthesized by diguanylate cyclases (with GGDEF domains), degraded by specific phosphodiesterases (PDEs, with EAL of HD-GYP domains) and sensed by a wide variety of c-di-GMP-binding effectors that control diverse targets. c-di-GMP-binding effectors can be riboswitches as well as proteins with highly diverse structures and functions. The latter include ‘degenerate’ GGDEF/EAL domain proteins that are enzymatically inactive but still able to bind c-di-GMP. Surprisingly, two enzymatically active ‘trigger PDEs’, the Escherichia coli proteins PdeR and PdeL, have recently been added to this list of c-di-GMP-sensing effectors. Mechanistically, trigger PDEs are multifunctional. They directly and specifically interact with a macromolecular target (e.g. with a transcription factor or directly with a promoter region), whose activity they control by their binding and degradation of c-di-GMP—their PDE activity thus represents the c-di-GMP sensor or effector function. In this process, c-di-GMP serves as a regulatory ligand, but in contrast to classical allosteric control, this ligand is also degraded. The resulting kinetics and circuitry of control are ideally suited for trigger PDEs to serve as key components in regulatory switches.

This article is part of the themed issue ‘The new bacteriology’.

Another Brick in the Wall: a Rhamnan Polysaccharide Trapped inside Peptidoglycan of Lactococcus lactis

Polysaccharides are ubiquitous components of the Gram-positive bacterial cell wall. In Lactococcus lactis, a polysaccharide pellicle (PSP) forms a layer at the cell surface. The PSP structure varies among lactococcal strains; in L. lactis MG1363, the PSP is composed of repeating hexasaccharide phosphate units. Here, we report the presence of an additional neutral polysaccharide in L. lactis MG1363 that is a rhamnan composed of α-l-Rha trisaccharide repeating units. This rhamnan is still present in mutants devoid of the PSP, indicating that its synthesis can occur independently of PSP synthesis. High-resolution magic-angle spinning nuclear magnetic resonance (HR-MAS NMR) analysis of whole bacterial cells identified a PSP at the surface of wild-type cells. In contrast, rhamnan was detected only at the surface of PSP-negative mutant cells, indicating that rhamnan is located underneath the surface-exposed PSP and is trapped inside peptidoglycan. The genetic determinants of rhamnan biosynthesis appear to be within the same genetic locus that encodes the PSP biosynthetic machinery, except the gene tagO encoding the initiating glycosyltransferase. We present a model of rhamnan biosynthesis based on an ABC transporter-dependent pathway. Conditional mutants producing reduced amounts of rhamnan exhibit strong morphological defects and impaired division, indicating that rhamnan is essential for normal growth and division. Finally, a mutation leading to reduced expression of lcpA, encoding a protein of the LytR-CpsA-Psr (LCP) family, was shown to severely affect cell wall structure. In lcpA mutant cells, in contrast to wild-type cells, rhamnan was detected by HR-MAS NMR, suggesting that LcpA participates in the attachment of rhamnan to peptidoglycan.IMPORTANCE In the cell wall of Gram-positive bacteria, the peptidoglycan sacculus is considered the major structural component, maintaining cell shape and integrity. It is decorated with other glycopolymers, including polysaccharides, the roles of which are not fully elucidated. In the ovococcus Lactococcus lactis, a polysaccharide with a different structure between strains forms a layer at the bacterial surface and acts as the receptor for various bacteriophages that typically exhibit a narrow host range. The present report describes the identification of a novel polysaccharide in the L. lactis cell wall, a rhamnan that is trapped inside the peptidoglycan and covalently bound to it. We propose a model of rhamnan synthesis based on an ABC transporter-dependent pathway. Rhamnan appears as a conserved component of the lactococcal cell wall playing an essential role in growth and division, thus highlighting the importance of polysaccharides in the cell wall integrity of Gram-positive ovococci.

Recent advances in studying single bacteria and biofilm mechanics

Bacterial biofilms correspond to surface-associated bacterial communities embedded in hydrogel-like matrix, in which high cell density, reduced diffusion and physico-chemical heterogeneity play a protective role and induce novel behaviors. In this review, we present recent advances on the understanding of how bacterial mechanical properties, from single cell to high-cell density community, determine biofilm tri-dimensional growth and eventual dispersion and we attempt to draw a parallel between these properties and the mechanical properties of other well-studied hydrogels and living systems.

Abundance and co-occurrence of extracellular capsules increase environmental breadth: Implications for the emergence of pathogens

Extracellular capsules constitute the outermost layer of many bacteria, are major virulence factors, and affect antimicrobial therapies. They have been used as epidemiological markers and recently became vaccination targets. Despite the efforts to biochemically serotype capsules in a few model pathogens, little is known of their taxonomic and environmental distribution. We developed, validated, and made available a computational tool, CapsuleFinder, to identify capsules in genomes. The analysis of over 2500 prokaryotic genomes, accessible in a database, revealed that ca. 50% of them—including Archaea—encode a capsule. The Wzx/Wzy-dependent capsular group was by far the most abundant. Surprisingly, a fifth of the genomes encode more than one capsule system—often from different groups—and their non-random co-occurrence suggests the existence of negative and positive epistatic interactions. To understand the role of multiple capsules, we queried more than 6700 metagenomes for the presence of species encoding capsules and showed that their distribution varied between environmental categories and, within the human microbiome, between body locations. Species encoding capsules, and especially those encoding multiple capsules, had larger environmental breadths than the other species. Accordingly, capsules were more frequent in environmental bacteria than in pathogens and, within the latter, they were more frequent among facultative pathogens. Nevertheless, capsules were frequent in clinical samples, and were usually associated with fast-growing bacteria with high infectious doses. Our results suggest that capsules increase the environmental range of bacteria and make them more resilient to environmental perturbations. Capsules might allow opportunistic pathogens to profit from empty ecological niches or environmental perturbations, such as those resulting from antibiotic therapy, to colonize the host. Capsule-associated virulence might thus be a by-product of environmental adaptation. Understanding the role of capsules in natural environments might enlighten their function in pathogenesis.

Extracellular capsules protect bacterial cells from external aggressions such as antibiotics or desiccation, but can also be targeted by vaccines. Since little was known about their frequency across Prokaryotes, we created and made freely available a computational tool, CapsuleFinder, to identify them from genomic data. Surprisingly, its use showed that many bacterial strains, especially those with the largest genomes, encode several capsules. The frequencies of the different combinations of capsule groups depended strongly on the phyla and the groups themselves, suggesting the existence of epistatic interactions between capsules. Bacteria encoding capsule systems were found in many natural environments, and were frequent in the human microbiome. In contrast to their frequent association with virulence, we found many more capsules in non-pathogens or facultative pathogens than among obligatory pathogens. We suggest that capsules increase the environmental breadth of bacteria thereby facilitating host colonization by opportunistic pathogens.

Lack of the PGA exopolysaccharide in Salmonella as an adaptive trait for survival in the host

Many bacteria build biofilm matrices using a conserved exopolysaccharide named PGA or PNAG (poly-β-1,6-N-acetyl-D-glucosamine). Interestingly, while E. coli and other members of the family Enterobacteriaceae encode the pgaABCD operon responsible for PGA synthesis, Salmonella lacks it. The evolutionary force driving this difference remains to be determined. Here, we report that Salmonella lost the pgaABCD operon after the divergence of Salmonella and Citrobacter clades, and previous to the diversification of the currently sequenced Salmonella strains. Reconstitution of the PGA machinery endows Salmonella with the capacity to produce PGA in a cyclic dimeric GMP (c-di-GMP) dependent manner. Outside the host, the PGA polysaccharide does not seem to provide any significant benefit to Salmonella: resistance against chlorine treatment, ultraviolet light irradiation, heavy metal stress and phage infection remained the same as in a strain producing cellulose, the main biofilm exopolysaccharide naturally produced by Salmonella. In contrast, PGA production proved to be deleterious to Salmonella survival inside the host, since it increased susceptibility to bile salts and oxidative stress, and hindered the capacity of S. Enteritidis to survive inside macrophages and to colonize extraintestinal organs, including the gallbladder. Altogether, our observations indicate that PGA is an antivirulence factor whose loss may have been a necessary event during Salmonella speciation to permit survival inside the host.

During bacterial evolution, specific traits that optimize the organism’s fitness are selected. The production of exopolysaccharides is widespread among bacteria in which they play a protective shielding role as main constituents of biofilms. In contrast to closely related siblings, Salmonella has lost the capacity to produce the exopolysaccharide PGA. Our study reveals that Salmonella lost pga genes, and that the driving force for such a loss may have been the detrimental impact that PGA has during Salmonella invasion of internal organs where it augments the susceptibility to bile salts and oxygen radicals, reducing bacterial survival inside macrophages and rendering Salmonella avirulent. These results suggest that gene-loss has played an important role during Salmonella evolution.

Versatile modes of cellular regulation via cyclic dinucleotides

Since the discovery of c-di-GMP almost three decades ago, cyclic dinucleotides (CDNs) have emerged as widely used signaling molecules in most kingdoms of life. The family of second messengers now includes c-di-AMP and distinct versions of mixed cyclic GMP-AMP (cGAMP) compounds. In addition to these nucleotides, a vast number of proteins for the production and turnover of these molecules have been described, as well as effectors that translate the signals into physiological responses. The latter include, but are not limited to, mechanisms for adaptation and survival in prokaryotes, persistence and virulence of bacterial pathogens, and immune responses to viral and bacterial invasion in eukaryotes. In this review, we will focus on recent discoveries and emerging themes that illustrate the ubiquity and versatility of cyclic dinucleotide function at the transcriptional and post-translational levels and, in particular, on insights gained through mechanistic structure-function analyses.

Cohesive Properties of the Caulobacter crescentus Holdfast Adhesin Are Regulated by a Novel c-di-GMP Effector Protein

When encountering surfaces, many bacteria produce adhesins to facilitate their initial attachment and to irreversibly glue themselves to the solid substrate. A central molecule regulating the processes of this motile-sessile transition is the second messenger c-di-GMP, which stimulates the production of a variety of exopolysaccharide adhesins in different bacterial model organisms. In Caulobacter crescentus, c-di-GMP regulates the synthesis of the polar holdfast adhesin during the cell cycle, yet the molecular and cellular details of this control are currently unknown. Here we identify HfsK, a member of a versatile N-acetyltransferase family, as a novel c-di-GMP effector involved in holdfast biogenesis. Cells lacking HfsK form highly malleable holdfast structures with reduced adhesive strength that cannot support surface colonization. We present indirect evidence that HfsK modifies the polysaccharide component of holdfast to buttress its cohesive properties. HfsK is a soluble protein but associates with the cell membrane during most of the cell cycle. Coincident with peak c-di-GMP levels during the C. crescentus cell cycle, HfsK relocalizes to the cytosol in a c-di-GMP-dependent manner. Our results indicate that this c-di-GMP-mediated dynamic positioning controls HfsK activity, leading to its inactivation at high c-di-GMP levels. A short C-terminal extension is essential for the membrane association, c-di-GMP binding, and activity of HfsK. We propose a model in which c-di-GMP binding leads to the dispersal and inactivation of HfsK as part of holdfast biogenesis progression.

Exopolysaccharide (EPS) adhesins are important determinants of bacterial surface colonization and biofilm formation. Biofilms are a major cause of chronic infections and are responsible for biofouling on water-exposed surfaces. To tackle these problems, it is essential to dissect the processes leading to surface colonization at the molecular and cellular levels. Here we describe a novel c-di-GMP effector, HfsK, that contributes to the cohesive properties and stability of the holdfast adhesin in C. crescentus. We demonstrate for the first time that c-di-GMP, in addition to its role in the regulation of the rate of EPS production, also modulates the physicochemical properties of bacterial adhesins. By demonstrating how c-di-GMP coordinates the activity and subcellular localization of HfsK, we provide a novel understanding of the cellular processes involved in adhesin biogenesis control. Homologs of HfsK are found in representatives of different bacterial phyla, suggesting that they play important roles in various EPS synthesis systems.

E. coli Group 1 Capsular Polysaccharide Exportation Nanomachinary as a Plausible Antivirulence Target in the Perspective of Emerging Antimicrobial Resistance

Bacteria evolving resistance against the action of multiple drugs and its ability to disseminate the multidrug resistance trait(s) across various strains of the same bacteria or different bacterial species impose serious threat to public health. Evolution of such multidrug resistance is due to the fact that, most of the antibiotics target bacterial survival mechanisms which exert selective pressure on the bacteria and aids them to escape from the action of antibiotics. Nonetheless, targeting bacterial virulence strategies such as bacterial surface associated polysaccharides biosynthesis and their surface accumulation mechanisms may be an attractive strategy, as they impose less selective pressure on the bacteria. Capsular polysaccharide (CPS) or K-antigen that is located on the bacterial surface armors bacteria from host immune response. Thus, unencapsulating bacteria would be a good strategy for drug design, besides CPS itself being a good vaccine target, by interfering with CPS biosynthesis and surface assembly pathway. Gram-negative Escherichia coli uses Wzy-polymerase dependent (Groups 1 and 4) and ATP dependent (Groups 1 and 3) pathways for CPS production. Considering E. coli as a case in point, this review explains the structure and functional roles of proteins involved in Group 1 Wzy dependent CPS biosynthesis, surface expression and anchorage in relevance to drug and vaccine developments.

The signaling protein MucG negatively affects the production and the molecular mass of alginate in Azotobacter vinelandii

Azotobacter vinelandii is a soil bacterium that produces the polysaccharide alginate. In this work, we identified a miniTn5 mutant, named GG9, which showed increased alginate production of higher molecular mass, and increased expression of the alginate biosynthetic genes algD and alg8 when compared to its parental strain. The miniTn5 was inserted within ORF Avin07920 encoding a hypothetical protein. Avin07910, located immediately downstream and predicted to form an operon with Avin07920, encodes an inner membrane multi-domain signaling protein here named mucG. Insertional inactivation of mucG resulted in a phenotype of increased alginate production of higher molecular mass similar to that of mutant GG9. The MucG protein contains a periplasmic and putative HAMP and PAS domains, which are linked to GGDEF and EAL domains. The last two domains are potentially involved in the synthesis and degradation, respectively, of bis-(3'-5')-cyclic dimeric GMP (c-di-GMP), a secondary messenger that has been reported to be essential for alginate production. Therefore, we hypothesized that the negative effect of MucG on the production of this polymer could be explained by the putative phosphodiesterase activity of the EAL domain. Indeed, we found that alanine replacement mutagenesis of the MucG EAL motif or deletion of the entire EAL domain resulted in increased alginate production of higher molecular mass similar to the GG9 and mucG mutants. To our knowledge, this is the first reported protein that simultaneous affects the production of alginate and its molecular mass.

Glycolipid substrates for ABC transporters required for the assembly of bacterial cell-envelope and cell-surface glycoconjugates

Glycoconjugates, molecules that contain sugar components, are major components of the cell envelopes of bacteria and cover much of their exposed surfaces. These molecules are involved in interactions with the surrounding environment and, in pathogens, play critical roles in the interplay with the host immune system. Despite the remarkable diversity in glycoconjugate structures, most are assembled by glycosyltransferases that act on lipid acceptors at the cytosolic membrane. The resulting glycolipids are then transported to the cell surface in processes that frequently begin with ATP-binding cassette transporters. This review summarizes current understanding of the structure and biosynthesis of glycolipid substrates and the structure and functions of their transporters. This article is part of a Special Issue entitled: Bacterial Lipids edited by Russell E. Bishop.

BcsZ inhibits biofilm phenotypes and promotes virulence by blocking cellulose production in Salmonella enterica serovar Typhimurium

Background

Cellulose, a 1,4 beta-glucan polysaccharide, is produced by a variety of organisms including bacteria. Although the production of cellulose has a high biological, ecological and economical impact, regulatory mechanisms of cellulose biosynthesis are mostly unknown. Family eight cellulases are regularly associated with cellulose biosynthesis operons in bacteria; however, their function is poorly characterized. In this study, we analysed the role of the cellulase BcsZ encoded by the bcsABZC cellulose biosynthesis operon of Salmonella enterica serovar Typhimurium (S. Typhimurium) in biofilm related behavior. We also investigated the involvement of BcsZ in pathogenesis of S. Typhimurium including a murine typhoid fever infection model.

Result

In S. Typhimurium, cellulase BcsZ with a putative periplasmic location negatively regulates cellulose biosynthesis. Moreover, as assessed with a non-polar mutant, BcsZ affects cellulose-associated phenotypes such as the rdar biofilm morphotype, cell clumping, biofilm formation, pellicle formation and flagella-dependent motility. Strikingly, although upregulation of cellulose biosynthesis was not observed on agar plate medium at 37 °C, BcsZ is required for efficient pathogen-host interaction. Key virulence phenotypes of S. Typhimurium such as invasion of epithelial cells and proliferation in macrophages were positively regulated by BcsZ. Further on, a bcsZ mutant was outcompeted by the wild type in organ colonization in the murine typhoid fever infection model. Selected phenotypes were relieved upon deletion of the cellulose synthase BcsA and/or the central biofilm activator CsgD.

Conclusion

Although the protein scaffold has an additional physiological role, our findings indicate that the catalytic activity of BcsZ effectively downregulates CsgD activated cellulose biosynthesis. Repression of cellulose production by BcsZ subsequently enables Salmonella to efficiently colonize the host.

Electronic supplementary material

The online version of this article (doi:10.1186/s12934-016-0576-6) contains supplementary material, which is available to authorized users.

A Look inside the Listeria monocytogenes Biofilms Extracellular Matrix

Listeria monocytogenes is a foodborne pathogen able to persist in food industry and is responsible for a severe illness called listeriosis. The ability of L. monocytogenes to persist in environments is due to its capacity to form biofilms that are a sessile community of microorganisms embedded in a self-produced matrix of extracellular polymeric substances (EPS's). In this review, we summarized recent efforts performed in order to better characterize the polymeric substances that compose the extracellular matrix (ECM) of L. monocytogenes biofilms. EPS extraction and analysis led to the identification of polysaccharides, proteins, extracellular DNA, and other molecules within the listerial ECM. All this knowledge will be useful for increasing food protection, suggesting effective strategies for the minimization of persistence of L. monocytogenes in food industry environments.

Mycoplasma agalactiae Secretion of β-(1→6)-Glucan, a Rare Polysaccharide in Prokaryotes, Is Governed by High-Frequency Phase Variation

Mycoplasmas are minimal, wall-less bacteria but have retained the ability to secrete complex carbohydrate polymers that constitute a glycocalyx. In members of the Mycoplasma mycoides cluster, which are important ruminant pathogens, the glycocalyx includes both cell-attached and cell-free polysaccharides. This report explores the potential secretion of polysaccharides by M. agalactiae, another ruminant pathogen that belongs to a distant phylogenetic group. Comparative genomic analyses showed that M. agalactiae possesses all the genes required for polysaccharide secretion. Notably, a putative synthase gene (gsmA) was identified, by in silico reconstruction of the biosynthetic pathway, that could be involved in both polymerization and export of the carbohydrate polymers. M. agalactiae polysaccharides were then purified in vitro and found to be mainly cell attached, with a linear β-(1→6)-glucopyranose structure [β-(1→6)-glucan]. Secretion of β-(1→6)-glucan was further shown to rely on the presence of a functional gsmA gene, whose expression is subjected to high-frequency phase variation. This event is governed by the spontaneous intraclonal variation in length of a poly(G) tract located in the gsmA coding sequence and was shown to occur in most of the M. agalactiae clinical isolates tested in this study. M. agalactiae susceptibility to serum-killing activity appeared to be dictated by ON/OFF switching of β-(1→6)-glucan secretion, suggesting a role of this phenomenon in survival of the pathogen when it invades the host bloodstream. Finally, β-(1→6)-glucan secretion was not restricted to M. agalactiae but was detected also in M. mycoides subsp. capri PG3(T), another pathogen of small ruminants.

IMPORTANCE

Many if not all bacteria are able to secrete polysaccharides, either attached to the cell surface or exported unbound into the extracellular environment. Both types of polysaccharides can play a role in bacterium-host interactions. Mycoplasmas are no exception despite their poor overall metabolic capacity. We showed here that M. agalactiae secretes a capsular β-(1→6)-glucopyranose thanks to a specific glycosyltransferase with synthase activity. This secretion is governed by high-frequency ON/OFF phase variation that might be crucial in mycoplasma host dissemination, as cell-attached β-(1→6)-glucopyranose increases serum-killing susceptibility. Our results provide functional genetic data about mycoplasmal glycosyltransferases with dual functions, i.e., assembly and export of the sugar polymers across the cell membrane. Furthermore, we demonstrated that nonprotein epitopes can be subjected to surface antigenic variation in mycoplasmas. Finally, the present report contributes to unravel the role of secreted polysaccharides in the virulence and pathogenicity of these peculiar bacteria.

Biofilm Exopolysaccharides of Pathogenic Fungi: Lessons from Bacteria

Exopolysaccharides play an important structural and functional role in the development and maintenance of microbial biofilms. Although the majority of research to date has focused on the exopolysaccharide systems of biofilm-forming bacteria, recent studies have demonstrated that medically relevant fungi such as Candida albicans and Aspergillus fumigatus also form biofilms during infection. These fungal biofilms share many similarities with those of bacteria, including the presence of secreted exopolysaccharides as core components of the extracellular matrix. This review will highlight our current understanding of fungal biofilm exopolysaccharides, as well as the parallels that can be drawn with those of their bacterial counterparts.

Adhesins Involved in Attachment to Abiotic Surfaces by Gram-Negative Bacteria

During the first step of biofilm formation, initial attachment is dictated by physicochemical and electrostatic interactions between the surface and the bacterial envelope. Depending on the nature of these interactions, attachment can be transient or permanent. To achieve irreversible attachment, bacterial cells have developed a series of surface adhesins promoting specific or nonspecific adhesion under various environmental conditions. This article reviews the recent advances in our understanding of the secretion, assembly, and regulation of the bacterial adhesins during biofilm formation, with a particular emphasis on the fimbrial, nonfimbrial, and discrete polysaccharide adhesins in Gram-negative bacteria.

Comparison of O-Antigen Gene Clusters of All O-Serogroups of Escherichia coli and Proposal for Adopting a New Nomenclature for O-Typing

Escherichia coli strains are classified based on O-antigens that are components of the lipopolysaccharide (LPS) in the cell envelope. O-antigens are important virulence factors, targets of both the innate and adaptive immune system, and play a role in host-pathogen interactions. Because they are highly immunogenic and display antigenic specificity unique for each strain, O-antigens are the biomarkers for designating O-types. Immunologically, 185 O-serogroups and 11 OX-groups exist for classification. Conventional serotyping for O-typing entails agglutination reactions between the O-antigen and antisera generated against each O-group. The procedure is labor intensive, not always accurate, and exhibits equivocal results. In this report, we present the sequences of 71 O-antigen gene clusters (O-AGC) and a comparison of all 196 O- and OX-groups. Many of the designated O-types, applied for classification over several decades, exhibited similar nucleotide sequences of the O-AGCs and cross-reacted serologically. Some O-AGCs carried insertion sequences and others had only a few nucleotide differences between them. Thus, based on these findings, it is proposed that several of the E. coli O-groups may be merged. Knowledge of the O-AGC sequences facilitates the development of molecular diagnostic platforms that are rapid, accurate, and reliable that can replace conventional serotyping. Additionally, with the scientific knowledge presented, new frontiers in the discovery of biomarkers, understanding the roles of O-antigens in the innate and adaptive immune system and pathogenesis, the development of glycoconjugate vaccines, and other investigations, can be explored.

Exploiting the commons: cyclic diguanylate regulation of bacterial exopolysaccharide production

Nowadays, there is increasing interest for bacterial polysaccharides in a wide variety of industrial sectors. This is due to their chemical and reological properties, and also the possibility to be obtained by fermentation processes. Biosynthesis of a growing number of exopolysaccharides (EPS) has been reported to be regulated by the ubiquitous second messenger c-di-GMP in a limited number of bacterial species. Since most bacteria are yet unexplored, it is likely that an unsuspected number and variety of EPS structures activated by c-di-GMP await to be uncovered. In the search of new EPS, manipulation of bacterial c-di-GMP metabolism can be combined with high throughput approaches for screening of large collections of bacteria. In addition, c-di-GMP activation of EPS production and promotion of cell aggregation may have direct applications in environmental industries related with biofuel production or wastewater treatments.

Alginate Biosynthesis inAzotobacter vinelandii: Overview of Molecular Mechanisms in Connection with the Oxygen Availability

The Gram-negative bacteriumAzotobacter vinelandiican synthetize the biopolymer alginate that has material properties appropriate for plenty of applications in industry as well as in medicine. In order to settle the foundation for improving alginate production without compromising its quality, a better understanding of the polymer biosynthesis and the mechanism of regulation during fermentation processes is necessary. This knowledge is crucial for the development of novel production strategies. Here, we highlight the key aspects of alginate biosynthesis that can lead to producing an alginate with specific material properties with particular focus on the role of oxygen availability linked with the molecular mechanisms involved in the alginate production.

Systems Biology of Microbial Exopolysaccharides Production

Exopolysaccharides (EPSs) produced by diverse group of microbial systems are rapidly emerging as new and industrially important biomaterials. Due to their unique and complex chemical structures and many interesting physicochemical and rheological properties with novel functionality, the microbial EPSs find wide range of commercial applications in various fields of the economy such as food, feed, packaging, chemical, textile, cosmetics and pharmaceutical industry, agriculture, and medicine. EPSs are mainly associated with high-value applications, and they have received considerable research attention over recent decades with their biocompatibility, biodegradability, and both environmental and human compatibility. However, only a few microbial EPSs have achieved to be used commercially due to their high production costs. The emerging need to overcome economic hurdles and the increasing significance of microbial EPSs in industrial and medical biotechnology call for the elucidation of the interrelations between metabolic pathways and EPS biosynthesis mechanism in order to control and hence enhance its microbial productivity. Moreover, a better understanding of biosynthesis mechanism is a significant issue for improvement of product quality and properties and also for the design of novel strains. Therefore, a systems-based approach constitutes an important step toward understanding the interplay between metabolism and EPS biosynthesis and further enhances its metabolic performance for industrial application. In this review, primarily the microbial EPSs, their biosynthesis mechanism, and important factors for their production will be discussed. After this brief introduction, recent literature on the application of omics technologies and systems biology tools for the improvement of production yields will be critically evaluated. Special focus will be given to EPSs with high market value such as xanthan, levan, pullulan, and dextran.

AmrZ regulates cellulose production in Pseudomonas syringae pv. tomato DC3000

In Pseudomonas syringae pv. tomato DC3000, the second messenger c-di-GMP has been previously shown to stimulate pellicle formation and cellulose biosynthesis. A screen for genes involved in cellulose production under high c-di-GMP intracellular levels led to the identification of insertions in two genes, wssB and wssE, belonging to the Pto DC3000 cellulose biosynthesis operon wssABCDEFGHI. Interestingly, beside cellulose-deficient mutants, colonies with a rougher appearance than the wild type also arouse among the transposants. Those mutants carry insertions in amrZ, a gene encoding a transcriptional regulator in different Pseudomonas. Here, we provide evidence that AmrZ is involved in the regulation of bacterial cellulose production at transcriptional level by binding to the promoter region of the wssABCDEFGHI operon and repressing cellulose biosynthesis genes. Mutation of amrZ promotes wrinkly colony morphology, increased cellulose production and loss of motility in Pto DC3000. AmrZ regulon includes putative c-di-GMP metabolising proteins, like AdcA and MorA, which may also impact those phenotypes. Furthermore, an amrZ but not a cellulose-deficient mutant turned out to be impaired in pathogenesis, indicating that AmrZ is a key regulator of Pto DC3000 virulence probably by controlling bacterial processes other than cellulose production.

Phylum-wide analysis of genes/proteins related to the last steps of assembly and export of extracellular polymeric substances (EPS) in cyanobacteria

Many cyanobacteria produce extracellular polymeric substances (EPS) with particular characteristics (e.g. anionic nature and presence of sulfate) that make them suitable for industrial processes such as bioremediation of heavy metals or thickening, suspending or emulsifying agents. Nevertheless, their biosynthetic pathway(s) are still largely unknown, limiting their utilization. In this work, a phylum-wide analysis of genes/proteins putatively involved in the assembly and export of EPS in cyanobacteria was performed. Our results demonstrated that most strains harbor genes encoding proteins related to the three main pathways: Wzy-, ABC transporter-, and Synthase-dependent, but often not the complete set defining one pathway. Multiple gene copies are mainly correlated to larger genomes, and the strains with reduced genomes (e.g. the clade of marine unicellular Synechococcus and Prochlorococcus), seem to have lost most of the EPS-related genes. Overall, the distribution of the different genes/proteins within the cyanobacteria phylum raises the hypothesis that cyanobacterial EPS production may not strictly follow one of the pathways previously characterized. Moreover, for the proteins involved in EPS polymerization, amino acid patterns were defined and validated constituting a novel and robust tool to identify proteins with similar functions and giving a first insight to which polymer biosynthesis they are related to.

Insights into the structure and function of membrane-integrated processive glycosyltransferases

Complex carbohydrates perform essential functions in life, including energy storage, cell signaling, protein targeting, quality control, as well as supporting cell structure and stability. Extracellular polysaccharides (EPS) represent mainly structural polymers and are found in essentially all kingdoms of life. For example, EPS are important biofilm and capsule components in bacteria, represent major constituents in cell walls of fungi, algae, arthropods and plants, and modulate the extracellular matrix in vertebrates. Different mechanisms evolved by which EPS are synthesized. Here, we review the structures and functions of membrane-integrated processive glycosyltransferases (GTs) implicated in the synthesis and secretion of chitin, alginate, hyaluronan and poly-N-acetylglucosamine (PNAG).

Characterization of the Pseudomonas aeruginosa Glycoside Hydrolase PslG Reveals That Its Levels Are Critical for Psl Polysaccharide Biosynthesis and Biofilm Formation

A key component of colonization, biofilm formation, and protection of the opportunistic human pathogen Pseudomonas aeruginosa is the biosynthesis of the exopolysaccharide Psl. Composed of a pentameric repeating unit of mannose, glucose, and rhamnose, the biosynthesis of Psl is proposed to occur via a Wzx/Wzy-dependent mechanism. Previous genetic studies have shown that the putative glycoside hydrolase PslG is essential for Psl biosynthesis. To understand the function of this protein, the apo-structure of the periplasmic domain of PslG (PslG(31-442)) and its complex with mannose were determined to 2.0 and 1.9 Å resolution, respectively. Despite a domain architecture and positioning of catalytic residues similar to those of other family 39 glycoside hydrolases, PslG(31-442) exhibits a unique 32-Å-long active site groove that is distinct from other structurally characterized family members. PslG formed a complex with two mannose monosaccharides in this groove, consistent with binding data obtained from intrinsic tryptophan fluorescence. PslG was able to catalyze the hydrolysis of surface-associated Psl, and this activity was abolished in a E165Q/E276Q double catalytic variant. Surprisingly, P. aeruginosa variants with these chromosomal mutations as well as a pslG deletion mutant were still capable of forming Psl biofilms. However, overexpression of PslG in a pslG deletion background impaired biofilm formation and resulted in less surface-associated Psl, suggesting that regulation of this enzyme is important during polysaccharide biosynthesis.

Applications of imaging for bacterial systems biology

Imaging has fueled exciting advances in bacterial cell biology, which have led to exquisite understanding of mechanisms of protein localization and cell growth in select cases. Nonetheless, it remains a challenge to connect subcellular dynamics to cellular phenotypes. In this review, I explore synergies between imaging and systems approaches to bacterial physiology. I highlight how single-cell, time-lapse imaging under environmental or chemical perturbations yields insights that complement traditional observations based on population-level growth on long time-scales. Next, I discuss applications of high-throughput fluorescence imaging to dissect genetic pathways and drug targets. Finally, I describe how confocal imaging is illuminating the role of spatial organization in the structure and function of bacterial communities, from biofilms to the intestinal microbiota.

Sph3 Is a Glycoside Hydrolase Required for the Biosynthesis of Galactosaminogalactan in Aspergillus fumigatus

Aspergillus fumigatus is the most virulent species within the Aspergillus genus and causes invasive infections with high mortality rates. The exopolysaccharide galactosaminogalactan (GAG) contributes to the virulence of A. fumigatus. A co-regulated five-gene cluster has been identified and proposed to encode the proteins required for GAG biosynthesis. One of these genes, sph3, is predicted to encode a protein belonging to the spherulin 4 family, a protein family with no known function. Construction of an sph3-deficient mutant demonstrated that the gene is necessary for GAG production. To determine the role of Sph3 in GAG biosynthesis, we determined the structure of Aspergillus clavatus Sph3 to 1.25 Å. The structure revealed a (β/α)8 fold, with similarities to glycoside hydrolase families 18, 27, and 84. Recombinant Sph3 displayed hydrolytic activity against both purified and cell wall-associated GAG. Structural and sequence alignments identified three conserved acidic residues, Asp-166, Glu-167, and Glu-222, that are located within the putative active site groove. In vitro and in vivo mutagenesis analysis demonstrated that all three residues are important for activity. Variants of Asp-166 yielded the greatest decrease in activity suggesting a role in catalysis. This work shows that Sph3 is a glycoside hydrolase essential for GAG production and defines a new glycoside hydrolase family, GH135.

The protein BpsB is a poly-β-1,6-N-acetyl-D-glucosamine deacetylase required for biofilm formation in Bordetella bronchiseptica

Bordetella pertussis and Bordetella bronchiseptica are the causative agents of whooping cough in humans and a variety of respiratory diseases in animals, respectively. Bordetella species produce an exopolysaccharide, known as the Bordetella polysaccharide (Bps), which is encoded by the bpsABCD operon. Bps is required for Bordetella biofilm formation, colonization of the respiratory tract, and confers protection from complement-mediated killing. In this report, we have investigated the role of BpsB in the biosynthesis of Bps and biofilm formation by B. bronchiseptica. BpsB is a two-domain protein that localizes to the periplasm and outer membrane. BpsB displays metal- and length-dependent deacetylation on poly-β-1,6-N-acetyl-d-glucosamine (PNAG) oligomers, supporting previous immunogenic data that suggests Bps is a PNAG polymer. BpsB can use a variety of divalent metal cations for deacetylase activity and showed highest activity in the presence of Ni(2+) and Co(2+). The structure of the BpsB deacetylase domain is similar to the PNAG deacetylases PgaB and IcaB and contains the same circularly permuted family four carbohydrate esterase motifs. Unlike PgaB from Escherichia coli, BpsB is not required for polymer export and has unique structural differences that allow the N-terminal deacetylase domain to be active when purified in isolation from the C-terminal domain. Our enzymatic characterizations highlight the importance of conserved active site residues in PNAG deacetylation and demonstrate that the C-terminal domain is required for maximal deacetylation of longer PNAG oligomers. Furthermore, we show that BpsB is critical for the formation and complex architecture of B. bronchiseptica biofilms.

Challenges and perspectives in combinatorial assembly of novel exopolysaccharide biosynthesis pathways

Because of their rheological properties various microbial polysaccharides are applied as thickeners and viscosifiers both in food and non-food industries. A broad variety of microorganisms secrete structurally diverse exopolysaccharides (EPS) that contribute to their surface attachment, protection against abiotic or biotic stress factors, and nutrient gathering. Theoretically, a massive number of EPS structures are possible through variations in monosaccharide sequences, condensation linkages and non-sugar decorations. Given the already-high diversity of EPS structures, taken together with the principal of combinatorial biosynthetic pathways, microbial polysaccharides are an attractive class of macromolecules with which to generate novel structures via synthetic biology approaches. However, previous manipulations primarily focused on increasing polysaccharide yield, with structural modifications restricted to removal of side chains or non-sugar decorations. This article outlines the biosynthetic pathways of the bacterial heteroexopolysaccharides xanthan and succinoglycan, which are used as thickening and stabilizing agents in food and non-food industries. Challenges and perspectives of combining synthetic biology approaches with directed evolution to overcome obstacles in assembly of novel EPS biosynthesis pathways are discussed.

Identification of Poly-N-acetylglucosamine as a Major Polysaccharide Component of the Bacillus subtilis Biofilm Matrix

Bacillus subtilis is intensively studied as a model organism for the development of bacterial biofilms or pellicles. A key component is currently undefined exopolysaccharides produced from proteins encoded by genes within the eps locus. Within this locus are four genes, epsHIJK, known to be essential for pellicle formation. We show they encode proteins synthesizing the broadly expressed microbial carbohydrate poly-N-acetylglucosamine (PNAG). PNAG was present in both pellicle and planktonic wild-type B. subtilis cells and in strains with deletions in the epsA-G and -L-O genes but not in strains deleted for epsH-K. Cloning of the B. subtilis epsH-K genes into Escherichia coli with in-frame deletions in the PNAG biosynthetic genes pgaA-D, respectively, restored PNAG production in E. coli. Cloning the entire B. subtilis epsHIJK locus into pga-deleted E. coli, Klebsiella pneumoniae, or alginate-negative Pseudomonas aeruginosa restored or conferred PNAG production. Bioinformatic and structural predictions of the EpsHIJK proteins suggest EpsH and EpsJ are glycosyltransferases (GT) with a GT-A fold; EpsI is a GT with a GT-B fold, and EpsK is an α-helical membrane transporter. B. subtilis, E. coli, and pga-deleted E. coli carrying the epsHIJK genes on a plasmid were all susceptible to opsonic killing by antibodies to PNAG. The immunochemical and genetic data identify the genes and proteins used by B. subtilis to produce PNAG as a significant carbohydrate factor essential for pellicle formation.

Bacterial cellulose biosynthesis: diversity of operons, subunits, products, and functions

Recent studies of bacterial cellulose biosynthesis, including structural characterization of a functional cellulose synthase complex, provided the first mechanistic insight into this fascinating process. In most studied bacteria, just two subunits, BcsA and BcsB, are necessary and sufficient for the formation of the polysaccharide chain in vitro. Other subunits - which differ among various taxa - affect the enzymatic activity and product yield in vivo by modulating (i) the expression of the biosynthesis apparatus, (ii) the export of the nascent β-D-glucan polymer to the cell surface, and (iii) the organization of cellulose fibers into a higher-order structure. These auxiliary subunits play key roles in determining the quantity and structure of resulting biofilms, which is particularly important for the interactions of bacteria with higher organisms - leading to rhizosphere colonization and modulating the virulence of cellulose-producing bacterial pathogens inside and outside of host cells. We review the organization of four principal types of cellulose synthase operon found in various bacterial genomes, identify additional bcs genes that encode components of the cellulose biosynthesis and secretion machinery, and propose a unified nomenclature for these genes and subunits. We also discuss the role of cellulose as a key component of biofilms and in the choice between acute infection and persistence in the host.

Alginate-modifying enzymes: biological roles and biotechnological uses

Alginate denotes a group of industrially important 1-4-linked biopolymers composed of the C-5-epimers β-D-mannuronic acid (M) and α-L-guluronic acid (G). The polysaccharide is manufactured from brown algae where it constitutes the main structural cell wall polymer. The physical properties of a given alginate molecule, e.g., gel-strength, water-binding capacity, viscosity and biocompatibility, are determined by polymer length, the relative amount and distribution of G residues and the acetyl content, all of which are controlled by alginate modifying enzymes. Alginate has also been isolated from some bacteria belonging to the genera Pseudomonas and Azotobacter, and bacterially synthesized alginate may be O-acetylated at O-2 and/or O-3. Initially, alginate is synthesized as polymannuronic acid, and some M residues are subsequently epimerized to G residues. In bacteria a mannuronan C-5-epimerase (AlgG) and an alginate acetylase (AlgX) are integral parts of the protein complex necessary for alginate polymerization and export. All alginate-producing bacteria use periplasmic alginate lyases to remove alginate molecules aberrantly released to the periplasm. Alginate lyases are also produced by organisms that utilize alginate as carbon source. Most alginate-producing organisms encode more than one mannuronan C-5 epimerase, each introducing its specific pattern of G residues. Acetylation protects against further epimerization and from most alginate lyases. An enzyme from Pseudomonas syringae with alginate deacetylase activity has been reported. Functional and structural studies reveal that alginate lyases and epimerases have related enzyme mechanisms and catalytic sites. Alginate lyases are now utilized as tools for alginate characterization. Secreted epimerases have been shown to function well in vitro, and have been engineered further in order to obtain enzymes that can provide alginates with new and desired properties for use in medical and pharmaceutical applications.

Bacterial exopolysaccharides: biosynthesis pathways and engineering strategies

Bacteria produce a wide range of exopolysaccharides which are synthesized via different biosynthesis pathways. The genes responsible for synthesis are often clustered within the genome of the respective production organism. A better understanding of the fundamental processes involved in exopolysaccharide biosynthesis and the regulation of these processes is critical toward genetic, metabolic and protein-engineering approaches to produce tailor-made polymers. These designer polymers will exhibit superior material properties targeting medical and industrial applications. Exploiting the natural design space for production of a variety of biopolymer will open up a range of new applications. Here, we summarize the key aspects of microbial exopolysaccharide biosynthesis and highlight the latest engineering approaches toward the production of tailor-made variants with the potential to be used as valuable renewable and high-performance products for medical and industrial applications.

The sweet tooth of bacteria: common themes in bacterial glycoconjugates

Humans have been increasingly recognized as being superorganisms, living in close contact with a microbiota on all their mucosal surfaces. However, most studies on the human microbiota have focused on gaining comprehensive insights into the composition of the microbiota under different health conditions (e.g., enterotypes), while there is also a need for detailed knowledge of the different molecules that mediate interactions with the host. Glycoconjugates are an interesting class of molecules for detailed studies, as they form a strain-specific barcode on the surface of bacteria, mediating specific interactions with the host. Strikingly, most glycoconjugates are synthesized by similar biosynthesis mechanisms. Bacteria can produce their major glycoconjugates by using a sequential or an en bloc mechanism, with both mechanistic options coexisting in many species for different macromolecules. In this review, these common themes are conceptualized and illustrated for all major classes of known bacterial glycoconjugates, with a special focus on the rather recently emergent field of glycosylated proteins. We describe the biosynthesis and importance of glycoconjugates in both pathogenic and beneficial bacteria and in both Gram-positive and -negative organisms. The focus lies on microorganisms important for human physiology. In addition, the potential for a better knowledge of bacterial glycoconjugates in the emerging field of glycoengineering and other perspectives is discussed.

Dimeric c-di-GMP is required for post-translational regulation of alginate production in Pseudomonas aeruginosa

Pseudomonas aeruginosa is an opportunistic human pathogen that secretes the exopolysaccharide alginate during infection of the respiratory tract of individuals afflicted with cystic fibrosis and chronic obstructive pulmonary disease. Among the proteins required for alginate production, Alg44 has been identified as an inner membrane protein whose bis-(3',5')-cyclic dimeric guanosine monophosphate (c-di-GMP) binding activity post-translationally regulates alginate secretion. In this study, we report the 1.8 Å crystal structure of the cytoplasmic region of Alg44 in complex with dimeric self-intercalated c-di-GMP and characterize its dinucleotide-binding site using mutational analysis. The structure shows that the c-di-GMP binding region of Alg44 adopts a PilZ domain fold with a dimerization mode not previously observed for this family of proteins. Calorimetric binding analysis of residues in the c-di-GMP binding site demonstrate that mutation of Arg-17 and Arg-95 alters the binding stoichiometry between c-di-GMP and Alg44 from 2:1 to 1:1. Introduction of these mutant alleles on the P. aeruginosa chromosome show that the residues required for binding of dimeric c-di-GMP in vitro are also required for efficient alginate production in vivo. These results suggest that the dimeric form of c-di-GMP represents the biologically active signaling molecule needed for the secretion of an important virulence factor produced by P. aeruginosa.

Listeria monocytogenes exopolysaccharide: origin, structure, biosynthetic machinery and c-di-GMP-dependent regulation

Elevated levels of the second messenger c-di-GMP activate biosynthesis of an unknown exopolysaccharide (EPS) in the food-borne pathogen Listeria monocytogenes. This EPS strongly protects cells against disinfectants and desiccation, indicating its potential significance for listerial persistence in the environment and for food safety. We analyzed the potential phylogenetic origin of this EPS, determined its complete structure, characterized genes involved in its biosynthesis and hydrolysis and identified diguanylate cyclases activating its synthesis. Phylogenetic analysis of EPS biosynthesis proteins suggests that they have evolved within monoderms. Scanning electron microscopy revealed that L. monocytogenes EPS is cell surface-bound. Secreted carbohydrates represent exclusively cell-wall debris. Based on carbohydrate composition, linkage and NMR analysis, the structure of the purified EPS is identified as a β-1,4-linked N-acetylmannosamine chain decorated with terminal α-1,6-linked galactose. All genes of the pssA-E operon are required for EPS production and so is a separately located pssZ gene. We show that PssZ has an EPS-specific glycosylhydrolase activity. Exogenously added PssZ prevents EPS-mediated cell aggregation and disperses preformed aggregates, whereas an E72Q mutant in the presumed catalytic residue is much less active. The diguanylate cyclases DgcA and DgcB, whose genes are located next to pssZ, are primarily responsible for c-di-GMP-dependent EPS production.

Novel mixed-linkage β-glucan activated by c-di-GMP in Sinorhizobium meliloti

An artificial increase of cyclic diguanylate (c-di-GMP) levels in Sinorhizobium meliloti 8530, a bacterium that does not carry known cellulose synthesis genes, leads to overproduction of a substance that binds the dyes Congo red and calcofluor. Sugar composition and methylation analyses and NMR studies identified this compound as a linear mixed-linkage (1 → 3)(1 → 4)-β-D-glucan (ML β-glucan), not previously described in bacteria but resembling ML β-glucans found in plants and lichens. This unique polymer is hydrolyzed by the specific endoglucanase lichenase, but, unlike lichenan and barley glucan, it generates a disaccharidic → 4)-β-D-Glcp-(1 → 3)-β-D-Glcp-(1 → repeating unit. A two-gene operon bgsBA required for production of this ML β-glucan is conserved among several genera within the order Rhizobiales, where bgsA encodes a glycosyl transferase with domain resemblance and phylogenetic relationship to curdlan synthases and to bacterial cellulose synthases. ML β-glucan synthesis is subjected to both transcriptional and posttranslational regulation. bgsBA transcription is dependent on the exopolysaccharide/quorum sensing ExpR/SinI regulatory system, and posttranslational regulation seems to involve allosteric activation of the ML β-glucan synthase BgsA by c-di-GMP binding to its C-terminal domain. To our knowledge, this is the first report on a linear mixed-linkage (1 → 3)(1 → 4)-β-glucan produced by a bacterium. The S. meliloti ML β-glucan participates in bacterial aggregation and biofilm formation and is required for efficient attachment to the roots of a host plant, resembling the biological role of cellulose in other bacteria.

Biosynthesis and function of extracellular glycans in cyanobacteria

The cell surface of cyanobacteria is covered with glycans that confer versatility and adaptability to a multitude of environmental factors. The complex carbohydrates act as barriers against different types of stress and play a role in intra- as well as inter-species interactions. In this review, we summarize the current knowledge of the chemical composition, biosynthesis and biological function of exo- and lipo-polysaccharides from cyanobacteria and give an overview of sugar-binding lectins characterized from cyanobacteria. We discuss similarities with well-studied enterobacterial systems and highlight the unique features of cyanobacteria. We pay special attention to colony formation and EPS biosynthesis in the bloom-forming cyanobacterium, Microcystis aeruginosa.

A molecular description of cellulose biosynthesis

Cellulose is the most abundant biopolymer on Earth, and certain organisms from bacteria to plants and animals synthesize cellulose as an extracellular polymer for various biological functions. Humans have used cellulose for millennia as a material and an energy source, and the advent of a lignocellulosic fuel industry will elevate it to the primary carbon source for the burgeoning renewable energy sector. Despite the biological and societal importance of cellulose, the molecular mechanism by which it is synthesized is now only beginning to emerge. On the basis of recent advances in structural and molecular biology on bacterial cellulose synthases, we review emerging concepts of how the enzymes polymerize glucose molecules, how the nascent polymer is transported across the plasma membrane, and how bacterial cellulose biosynthesis is regulated during biofilm formation. Additionally, we review evolutionary commonalities and differences between cellulose synthases that modulate the nature of the cellulose product formed.

Exopolysaccharides produced by marine bacteria and their applications as glycosaminoglycan-like molecules

Although polysaccharides are ubiquitous and the most abundant renewable bio-components, their studies, covered by the glycochemistry and glycobiology fields, remain a challenge due to their high molecular diversity and complexity. Polysaccharides are industrially used in food products; human therapeutics fall into a more recent research field and pharmaceutical industry is looking for more and more molecules with enhanced activities. Glycosaminoglycans (GAGs) found in animal tissues play a critical role in cellular physiological and pathological processes as they bind many cellular components. Therefore, they present a great potential for the design and preparation of therapeutic drugs. On the other hand, microorganisms producing exopolysaccharides (EPS) are renewable resources meeting well the actual industrial demand. In particular, the diversity of marine microorganisms is still largely unexplored offering great opportunities to discover high value products such as new molecules and biocatalysts. EPS-producing bacteria from the marine environment will be reviewed with a focus on marine-derived EPS from bacteria isolated from deep-sea hydrothermal vents. Information on chemical and structural features, putative pathways of biosynthesis, novel strategies for chemical and enzymatic modifications and potentialities in the biomedical field will be provided. An integrated approach should be used to increase the basic knowledge on these compounds and their applications; new clean environmentally friendly processes for the production of carbohydrate bioactive compounds should also be proposed for a sustainable industry.

PssP2 is a polysaccharide co-polymerase involved in exopolysaccharide chain-length determination in Rhizobium leguminosarum

Production of extracellular polysaccharides is a complex process engaging proteins localized in different subcellular compartments, yet communicating with each other or even directly interacting in multicomponent complexes. Proteins involved in polymerization and transport of exopolysaccharide (EPS) in Rhizobium leguminosarum are encoded within the chromosomal Pss-I cluster. However, genes implicated in polysaccharide synthesis are common in rhizobia, with several homologues of pss genes identified in other regions of the R. leguminosarum genome. One such region is chromosomally located Pss-II encoding proteins homologous to known components of the Wzx/Wzy-dependent polysaccharide synthesis and transport systems. The pssP2 gene encodes a protein similar to polysaccharide co-polymerases involved in determination of the length of polysaccharide chains in capsule and O-antigen biosynthesis. In this work, a mutant with a disrupted pssP2 gene was constructed and its capabilities to produce EPS and enter into a symbiotic relationship with clover were studied. The pssP2 mutant, while not altered in lipopolysaccharide (LPS), displayed changes in molecular mass distribution profile of EPS. Lack of the full-length PssP2 protein resulted in a reduction of high molecular weight EPS, yet polymerized to a longer length than in the RtTA1 wild type. The mutant strain was also more efficient in symbiotic performance. The functional interrelation between PssP2 and proteins encoded within the Pss-I region was further supported by data from bacterial two-hybrid assays providing evidence for PssP2 interactions with PssT polymerase, as well as glycosyltransferase PssC. A possible role for PssP2 in a complex involved in EPS chain-length determination is discussed.

Cyclic di-GMP-dependent signaling pathways in the pathogenic Firmicute Listeria monocytogenes

We characterized key components and major targets of the c-di-GMP signaling pathways in the foodborne pathogen Listeria monocytogenes, identified a new c-di-GMP-inducible exopolysaccharide responsible for motility inhibition, cell aggregation, and enhanced tolerance to disinfectants and desiccation, and provided first insights into the role of c-di-GMP signaling in listerial virulence. Genome-wide genetic and biochemical analyses of c-di-GMP signaling pathways revealed that L. monocytogenes has three GGDEF domain proteins, DgcA (Lmo1911), DgcB (Lmo1912) and DgcC (Lmo2174), that possess diguanylate cyclase activity, and three EAL domain proteins, PdeB (Lmo0131), PdeC (Lmo1914) and PdeD (Lmo0111), that possess c-di-GMP phosphodiesterase activity. Deletion of all phosphodiesterase genes (ΔpdeB/C/D) or expression of a heterologous diguanylate cyclase stimulated production of a previously unknown exopolysaccharide. The synthesis of this exopolysaccharide was attributed to the pssA-E (lmo0527-0531) gene cluster. The last gene of the cluster encodes the fourth listerial GGDEF domain protein, PssE, that functions as an I-site c-di-GMP receptor essential for exopolysaccharide synthesis. The c-di-GMP-inducible exopolysaccharide causes cell aggregation in minimal medium and impairs bacterial migration in semi-solid agar, however, it does not promote biofilm formation on abiotic surfaces. The exopolysaccharide also greatly enhances bacterial tolerance to commonly used disinfectants as well as desiccation, which may contribute to survival of L. monocytogenes on contaminated food products and in food-processing facilities. The exopolysaccharide and another, as yet unknown c-di-GMP-dependent target, drastically decrease listerial invasiveness in enterocytes in vitro, and lower pathogen load in the liver and gallbladder of mice infected via an oral route, which suggests that elevated c-di-GMP levels play an overall negative role in listerial virulence.

Listeria monocytogenes is ubiquitously present in the environment, highly adaptable and tolerant to various stresses. L. monocytogenes is also a foodborne pathogen associated with the largest foodborne outbreaks in recent US history. Signaling pathways involving the second messenger c-di-GMP play important roles in increased stress survival of proteobacteria and mycobacteria, yet roles of c-di-GMP signaling pathways in L. monocytogenes have remained unexplored. Here, we identified and systematically characterized functions of the proteins involved in c-di-GMP synthesis, degradation and sensing. We show that elevated c-di-GMP levels in L. monocytogenes result in synthesis of a previously unknown exopolysaccharide that promotes cell aggregation, inhibits motility in semi-solid media, and importantly, enhances bacterial tolerance to commonly used disinfectants as well as desiccation. These properties of the exopolysaccharide may increase listerial survival in food processing plants as well as on produce during transportation and storage. Elevated c-di-GMP levels also grossly diminish listerial invasiveness in enterocytes in vitro, and impair bacterial accumulation in selected mouse organs during oral infection.

Modification and periplasmic translocation of the biofilm exopolysaccharide poly-β-1,6-N-acetyl-D-glucosamine

Poly-β-1,6-N-acetyl-D-glucosamine (PNAG) is an exopolysaccharide produced by a wide variety of medically important bacteria. Polyglucosamine subunit B (PgaB) is responsible for the de-N-acetylation of PNAG, a process required for polymer export and biofilm formation. PgaB is located in the periplasm and likely bridges the inner membrane synthesis and outer membrane export machinery. Here, we present structural, functional, and molecular simulation data that suggest PgaB associates with PNAG continuously during periplasmic transport. We show that the association of PgaB's N- and C-terminal domains forms a cleft required for the binding and de-N-acetylation of PNAG. Molecular dynamics (MD) simulations of PgaB show a binding preference for N-acetylglucosamine (GlcNAc) to the N-terminal domain and glucosammonium to the C-terminal domain. Continuous ligand binding density is observed that extends around PgaB from the N-terminal domain active site to an electronegative groove on the C-terminal domain that would allow for a processive mechanism. PgaB's C-terminal domain (PgaB310-672) directly binds PNAG oligomers with dissociation constants of ∼1-3 mM, and the structures of PgaB310-672 in complex with β-1,6-(GlcNAc)6, GlcNAc, and glucosamine reveal a unique binding mode suitable for interaction with de-N-acetylated PNAG (dPNAG). Furthermore, PgaB310-672 contains a β-hairpin loop (βHL) important for binding PNAG that was disordered in previous PgaB42-655 structures and is highly dynamic in the MD simulations. We propose that conformational changes in PgaB310-672 mediated by the βHL on binding of PNAG/dPNAG play an important role in the targeting of the polymer for export and its release.

A different path: revealing the function of staphylococcal proteins in biofilm formation

Staphylococcus aureus and Staphylococcus epidermidis cause dangerous and difficult to treat medical device-related infections through their ability to form biofilms. Extracellular poly-N-acetylglucosamine (PNAG) facilitates biofilm formation and is a vaccination target, yet details of its biosynthesis by the icaADBC gene products is limited. IcaC is the proposed transporter for PNAG export, however a comparison of the Ica proteins to homologous exo-polysaccharide synthases suggests that the common IcaAD protein components both synthesise and transport the PNAG. The limited distribution of icaC to the Staphylococcaceae and its membership of a family of membrane-bound acyltransferases, leads us to suggest that IcaC is responsible for the known O-succinylation of PNAG that occurs in staphylococci, identifying a potentially new therapeutic target specific for these bacteria.

Mechanism of activation of bacterial cellulose synthase by cyclic di-GMP

The bacterial signaling molecule cyclic-di-GMP stimulates the synthesis of bacterial cellulose, frequently found in biofilms. Bacterial cellulose is synthesized and translocated across the inner membrane by a complex of the cellulose synthase BcsA and BcsB subunits. Here we present crystal structures of the cyclic-di-GMP-activated BcsA–B complex. The structures reveal that cyclic-di-GMP releases an auto-inhibited state of the enzyme by breaking a salt bridge which otherwise tethers a conserved gating loop that controls access to and substrate coordination at the active site. Disrupting the salt bridge by mutagenesis generates a constitutively active cellulose synthase. Additionally, the cyclic-di-GMP activated BcsA–B complex contains a nascent cellulose polymer whose terminal glucose unit rests at a novel location above BcsA’s active site where it is positioned for catalysis. Our mechanistic insights are the first examples of how cyclic-di-GMP allosterically modulates enzymatic functions.

Responses to elevated c-di-GMP levels in mutualistic and pathogenic plant-interacting bacteria

Despite a recent burst of research, knowledge on c-di-GMP signaling pathways remains largely fragmentary and molecular mechanisms of regulation and even c-di-GMP targets are yet unknown for most bacteria. Besides genomics or bioinformatics, accompanying alternative approaches are necessary to reveal c-di-GMP regulation in bacteria with complex lifestyles. We have approached this study by artificially altering the c-di-GMP economy of diverse pathogenic and mutualistic plant-interacting bacteria and examining the effects on the interaction with their respective host plants. Phytopathogenic Pseudomonas and symbiotic Rhizobium strains with enhanced levels of intracellular c-di-GMP displayed common free-living responses: reduction of motility, increased production of extracellular polysaccharides and enhanced biofilm formation. Regarding the interaction with the host plants, P. savastanoi pv. savastanoi cells containing high c-di-GMP levels formed larger knots on olive plants which, however, displayed reduced necrosis. In contrast, development of disease symptoms in P. syringae-tomato or P. syringae-bean interactions did not seem significantly affected by high c-di-GMP. On the other hand, increasing c-di-GMP levels in symbiotic R. etli and R. leguminosarum strains favoured the early stages of the interaction since enhanced adhesion to plant roots, but decreased symbiotic efficiency as plant growth and nitrogen contents were reduced. Our results remark the importance of c-di-GMP economy for plant-interacting bacteria and show the usefulness of our approach to reveal particular stages during plant-bacteria associations which are sensitive to changes in c-di-GMP levels.