Evidence that the algI/algJ gene cassette, required for O acetylation of Pseudomonas aeruginosa alginate, evolved by lateral gene transfer

The mechanism of peptidoglycan O-acetylation in Gram-negative bacteria typifies bacterial MBOAT-SGNH acyltransferases

Bacterial cell envelope polymers are commonly modified with acyl groups that provide fitness advantages. Many polymer acylation pathways involve pairs of membrane-bound O-acyltransferase (MBOAT) and SGNH family proteins. As an example, the MBOAT protein PatA and the SGNH protein PatB are required in Gram-negative bacteria for peptidoglycan O-acetylation. The mechanism for how MBOAT-SGNH transferases move acyl groups from acyl-CoA donors made in the cytoplasm to extracellular polymers is unclear. Using the peptidoglycan O-acetyltransferase proteins PatAB, we explore the mechanism of MBOAT-SGNH pairs. We find that the MBOAT protein PatA catalyzes auto-acetylation of an invariant Tyr residue in its conserved C-terminal hexapeptide motif. We also show that PatB can use a synthetic hexapeptide containing an acetylated tyrosine to donate an acetyl group to a peptidoglycan mimetic. Finally, we report the structure of PatB, finding that it has structural features that shape its activity as an O-acetyltransferase and distinguish it from other SGNH esterases and hydrolases. Taken together, our results support a model for peptidoglycan acylation in which a tyrosine-containing peptide at the MBOAT's C-terminus shuttles an acyl group from the MBOAT active site to the SGNH active site, where it is transferred to peptidoglycan. This model likely applies to other systems containing MBOAT-SGNH pairs, such as those that O-acetylate alginate, cellulose, and secondary cell wall polysaccharides.

Acyltransferases that Modify Cell Surface Polymers Across the Membrane

Cell surface oligosaccharides and related polymers are commonly decorated with acyl esters that alter their structural properties and influence their interactions with other molecules. In many cases, these esters are added to polymers that are already positioned on the extracytoplasmic side of a membrane, presenting cells with a chemical challenge because the high-energy acyl donors used for these modifications are made in the cytoplasm. How activated acyl groups are passed from the cytoplasm to extra-cytoplasmic polymers has been a longstanding question. Recent mechanistic work has shown that many bacterial acyl transfer pathways operate by shuttling acyl groups through two covalent intermediates to their final destination on an extracellular polymer. Key to these and other pathways are cross-membrane acyltransferases─enzymes that catalyze transfer of acyl groups from a donor on one side of the membrane to a recipient on the other side. Here we review what has been learned recently about how cross-membrane acyltransferases in polymer acylation pathways function, highlighting the chemical and biosynthetic logic used by two key protein families, membrane-bound O-acyltransferases (MBOATs) and acyltransferase-3 (AT3) proteins. We also point out outstanding questions and avenues for further exploration.

Alginate Formulation for Wound Healing Applications

Significance: Alginate, sourced from seaweed, holds significant importance in industrial and biomedical domains due to its versatile properties. Its chemical composition, primarily comprising β-D-mannuronic acid and α-L-guluronic acid, governs its physical and biological attributes. This polysaccharide, extracted from brown algae and bacteria, offers diverse compositions impacting key factors such as molecular weight, flexibility, solubility, and stability. Recent Advances: Commercial extraction methods yield soluble sodium alginate essential for various biomedical applications. Extraction processes involve chemical treatments converting insoluble alginic acid salts into soluble forms. While biosynthesis pathways in bacteria and algae share similarities, differences in enzyme utilization and product characteristics are noted. Critical Issues: Despite its widespread applicability, challenges persist regarding alginate's stability, biodegradability, and bioactivity. Further understanding of its interactions in complex biological environments and the optimization of extraction and synthesis processes are imperative. Additionally, concerns regarding immune responses to alginate-based implants necessitate thorough investigation. Future Directions: Future research endeavors aim to enhance alginate's stability and bioactivity, facilitating its broader utilization in regenerative medicine and therapeutic interventions. Novel approaches focusing on tailored hydrogel formations, advanced drug delivery systems, and optimized cellular encapsulation techniques hold promise. Continued exploration of alginate's potential in tissue engineering and wound healing, alongside efforts to address critical issues, will drive advancements in biomedical applications.

New insights into pathogenic performances during peroxydisulfate composting: sources, pathways, and influencing factors

Livestock manure treatment technology and composting and its products have been widely used in agricultural soil. However, little was known about the variations in the fate of pathogens and the factors affecting their pathogenic ability during this process, which posed threats to ecological safety and public health globally. This study used a metagenomic approach to profile the behaviors of pathogens during peroxydisulfate composting. Results showed that Pseudomonas aeruginosa, Klebsiella pneumoniae, Escherichia coli, Burkholderia pseudomallei, and Mycobacterium tuberculosis were the main secretors of virulence factors (VFs) in composting system; their abundance and the virulence factor-related genes they carried were better downregulated under the role of peroxydisulfate. In addition, peroxydisulfate composting ensured the lower moisture, weakening the swimming mobility behavior of pathogens through suppressing the abundance of genes associated with flagellar formation, and impaired the communication between pathogens by regulating quorum sensing (QS)- and quorum quenching (QQ)-related genes. Moreover, reduced abundance of resistomes restricted pathogens disseminating infection. In summary, this study provided useful strategies in managing pathogen pathogenic ability during composting based on pathogenic source (pathogens), pathway (VFs), influencing factors (QS/QQ, physicochemical habitats), and resistomes.

The mechanism of peptidoglycan O-acetylation in Gram-negative bacteria typifies bacterial MBOAT-SGNH acyltransferases

Bacterial cell envelope polymers are commonly modified with acyl groups that provide fitness advantages. Many polymer acylation pathways involve pairs of membrane-bound O-acyltransferase (MBOAT) and SGNH family proteins. As an example, the MBOAT protein PatA and the SGNH protein PatB are required in Gram-negative bacteria for peptidoglycan O-acetylation. The mechanism for how MBOAT-SGNH transferases move acyl groups from acyl-CoA donors made in the cytoplasm to extracellular polymers is unclear. Using the peptidoglycan O-acetyltransferase proteins PatAB, we explore the mechanism of MBOAT-SGNH pairs. We find that the MBOAT protein PatA catalyzes auto-acetylation of an invariant Tyr residue in its conserved C-terminal hexapeptide motif. We also show that PatB can use a synthetic hexapeptide containing an acetylated tyrosine to donate an acetyl group to a peptidoglycan mimetic. Finally, we report the structure of PatB, finding that it has structural features that shape its activity as an O-acetyltransferase and distinguish it from other SGNH esterases and hydrolases. Taken together, our results support a model for peptidoglycan acylation in which a tyrosine-containing peptide at the MBOAT's C-terminus shuttles an acyl group from the MBOAT active site to the SGNH active site, where it is transferred to peptidoglycan. This model likely applies to other systems containing MBOAT-SGNH pairs, such as those that O-acetylate alginate, cellulose, and secondary cell wall polysaccharides. The use of an acyl-tyrosine intermediate for MBOAT-SGNH acyl transfer is also shared with AT3-SGNH proteins, a second major group of acyltransferases that modify cell envelope polymers.

Pseudomonas aeruginosa AlgF is a protein-protein interaction mediator required for acetylation of the alginate exopolysaccharide

Enzymatic modifications of bacterial exopolysaccharides enhance immune evasion and persistence during infection. In the Gram-negative opportunistic pathogen Pseudomonas aeruginosa, acetylation of alginate reduces opsonic killing by phagocytes and improves reactive oxygen species scavenging. Although it is well known that alginate acetylation in P. aeruginosa requires AlgI, AlgJ, AlgF, and AlgX, how these proteins coordinate polymer modification at a molecular level remains unclear. Here, we describe the structural characterization of AlgF and its protein interaction network. We characterize direct interactions between AlgF and both AlgJ and AlgX in vitro and demonstrate an association between AlgF and AlgX, as well as AlgJ and AlgI, in P. aeruginosa. We determine that AlgF does not exhibit acetylesterase activity and is unable to bind to polymannuronate in vitro. Therefore, we propose that AlgF functions to mediate protein-protein interactions between alginate acetylation enzymes, forming the periplasmic AlgJFXK (AlgJ-AlgF-AlgX-AlgK) acetylation and export complex required for robust biofilm formation.

The SGNH hydrolase family: a template for carbohydrate diversity

The substitution and de-substitution of carbohydrate materials are important steps in the biosynthesis and/or breakdown of a wide variety of biologically important polymers. The SGNH hydrolase superfamily is a group of related and well-studied proteins with a highly conserved catalytic fold and mechanism composed of 16 member families. SGNH hydrolases can be found in vertebrates, plants, fungi, bacteria, and archaea, and play a variety of important biological roles related to biomass conversion, pathogenesis, and cell signaling. The SGNH hydrolase superfamily is chiefly composed of a diverse range of carbohydrate-modifying enzymes, including but not limited to the carbohydrate esterase families 2, 3, 6, 12 and 17 under the carbohydrate-active enzyme classification system and database (CAZy.org). In this review, we summarize the structural and functional features that delineate these subfamilies of SGNH hydrolases, and which generate the wide variety of substrate preferences and enzymatic activities observed of these proteins to date.

Analysis of the alginate O-acetylation machinery in Pseudomonas aeruginosa

O-acetylation of alginate produced by the opportunistic human pathogen Pseudomonas aeruginosa significantly contributes to its pathogenesis. Three proteins, AlgI, AlgJ and AlgF have been implicated to form a complex and act together with AlgX for O-acetylation of alginate. AlgI was proposed to transfer the acetyl group across the cytoplasmic membrane, while periplasmic AlgJ was hypothesised to transfer the acetyl group to AlgX that acetylates alginate. To elucidate the proposed O-acetylation multiprotein complex, isogenic knockout mutants of algI, algJ and algF genes were generated in the constitutively alginate overproducing P. aeruginosa PDO300 to enable mutual stability studies. All knockout mutants were O-acetylation negative and complementation with the respective genes in cis or trans restored O-acetylation of alginate. Interestingly, only the AlgF deletion impaired alginate production suggesting a link to the alginate polymerisation/secretion multiprotein complex. Mutual stability experiments indicated that AlgI and AlgF interact independent of AlgJ as well as impact on stability of the alginate polymerisation/secretion multiprotein complex. Deletion of AlgJ did not destabilise AlgX and vice versa. When the alginate polymerase, Alg8, was absent, then AlgI and AlgF stability was strongly impaired supporting a link of the O-acetylation machinery with alginate polymerisation. Pull-down experiments suggested that AlgI interacts with AlgJ, while AlgF interacts with AlgJ and AlgI. Overall, these results suggested that AlgI-AlgJ-AlgF form a multiprotein complex linked via Alg8 to the envelope-spanning alginate polymerisation/secretion multiprotein complex to mediate O-acetylation of nascent alginate. Here, we provide the first insight on how the O-acetylation machinery is associated with alginate production.

Recent Advances in Pharmaceutical Potential of Brown Algal Polysaccharides and their Derivatives

Marine plants, animals and microorganisms display steady growth in the ocean and are abundant carbohydrate resources. Specifically, natural polysaccharides obtained from brown algae have been drawing increasing attention owing to their great potential in pharmaceutical applications. This review describes the structural and biological features of brown algal polysaccharides, including alginates, fucoidans, and laminarins, and it highlights recently developed approaches used to obtain the oligo- and polysaccharides with defined structures. Functional modification of these polysaccharides promotes their advanced applications in biomedical materials for controlled release and targeted drug delivery, etc. Moreover, brown algal polysaccharides and their derivatives possess numerous biological activities with anticancer, anticoagulant, wound healing, and antiviral properties. In addition, we also discuss carbohydrate- based substrates from brown algae, which are currently in clinical and preclinical studies, as well as the marine drugs that are already on the market. The present review summarizes the recent development in carbohydratebased products from brown algae, with promising findings that could rapidly facilitate the future discovery of novel marine drugs.

Chemical synthesis of guanosine diphosphate mannuronic acid (GDP-ManA) and its C-4-O-methyl and C-4-deoxy congeners

Described is the first synthesis of guanosine diphosphate mannuronic acid (GDP-ManA), the sugar donor used by algae and bacteria for the production of alginate, an anionic polysaccharide composed of β-d-mannuronic acid (ManA) and α-l-guluronic acid (GulA). Understanding the biosynthesis of these polyanionic polysaccharides on the molecular level, opens up avenues to use and modulate the biosynthesis machinery for biotechnological and therapeutic applications. The synthesis reported here delivers multi-milligram amounts of the GDP-ManA donor that can be used to study the polymerase (Alg8 in Pseudomonas aeruginosa) that generates the poly-ManA chain. Also reported is the assembly of two close analogues of GDP-ManA: the first bears a C-4-O-methyl group, while the second has been deoxygenated at this position. Both molecules may be used as "chain stoppers" in future enzymatic ManA polymerisation reactions. The crucial pyrophosphate linkage of the GDP-mannuronic acids has been constructed by the phosphorylation of the appropriate ManA-1-phosphates with a guanosine phosphoramidite.

Presence of Bacterial Virulence Gene Homologues in the dibenzo-p-dioxins degrading bacterium Sphingomonas wittichii

Sphingomonas wittichii, a close relative of the human pathogen Sphingomonas paucimobilis, is a microorganism of great interest to the bioremediation community for its ability of biodegradation to a large number of toxic polychlorinated dioxins. In the present study we investigated the presence of different virulence factors and genes in S. wittichii. We utilized phylogenetic, comparative genomics and bioinformatics analysis to investigate the potentiality of S. wittichii as a potential virulent pathogen. The 16SrDNA phylogenetic tree showed that the closest bacterial taxon to S. wittichii is Brucella followed by Helicobacter, Campylobacter, Pseudomonas then Legionella. Despite their close phylogenetic relationship, S. wittichii did not share any virulence factors with Helicobacter or Campylobacter. On the contrary, in spite of the phylogenetic divergence between S. wittichii and Pseudomonas spp., they shared many major virulence factors, such as, adherence, antiphagocytosis, Iron uptake, proteases and quorum sensing. S. wittichii contains several major virulence factors resembling Pseudomonas sp., Legionella sp., Brucella sp. and Bordetella sp. virulence factors. Similarity of virulence factors did not match phylogenetic relationships. These findings suggest horizontal gene transfer of virulence factors rather than sharing a common pathogenic ancestor. S. wittichii is a potential virulent bacterium. Another possibility is that reductive evolution process attenuated S. wittichii pathogenic capabilities. Thus plenty of care must be taken when using this bacterium in soil remediation purposes.

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.

Enzymatic modifications of exopolysaccharides enhance bacterial persistence

Biofilms are surface-attached communities of bacterial cells embedded in a self-produced matrix that are found ubiquitously in nature. The biofilm matrix is composed of various extracellular polymeric substances, which confer advantages to the encapsulated bacteria by protecting them from eradication. The matrix composition varies between species and is dependent on the environmental niche that the bacteria inhabit. Exopolysaccharides (EPS) play a variety of important roles in biofilm formation in numerous bacterial species. The ability of bacteria to thrive in a broad range of environmental settings is reflected in part by the structural diversity of the EPS produced both within individual bacterial strains as well as by different species. This variability is achieved through polymerization of distinct sugar moieties into homo- or hetero-polymers, as well as post-polymerization modification of the polysaccharide. Specific enzymes that are unique to the production of each polymer can transfer or remove non-carbohydrate moieties, or in other cases, epimerize the sugar units. These modifications alter the physicochemical properties of the polymer, which in turn can affect bacterial pathogenicity, virulence, and environmental adaptability. Herein, we review the diversity of modifications that the EPS alginate, the Pel polysaccharide, Vibrio polysaccharide, cepacian, glycosaminoglycans, and poly-N-acetyl-glucosamine undergo during biosynthesis. These are EPS produced by human pathogenic bacteria for which studies have begun to unravel the effect modifications have on their physicochemical and biological properties. The biological advantages these polymer modifications confer to the bacteria that produce them will be discussed. The expanding list of identified modifications will allow future efforts to focus on linking these modifications to specific biosynthetic genes and biofilm phenotypes.

In silico detection of virulence gene homologues in the human pathogen sphingomonas spp

There is an ongoing debate about the clinical significance of Sphingomonas paucimobilis as a virulent bacterial pathogen. In the present study, we investigated the presence of different virulence factors and genes in Sphingomonas bacteria. We utilized phylogenetic, comparative genomics and bioinformatics analysis to investigate the potentiality of Sphingomonas bacteria as virulent pathogenic bacteria. The 16S ribosomal RNA gene (16S rDNA) phylogenetic tree showed that the closest bacterial taxon to Sphingomonas is Brucella with a bootstrap value of 87 followed by Helicobacter, Campylobacter, Pseudomonas, and then Legionella. Sphingomonas shared no virulence factors with Helicobacter or Campylobacter, despite their close phylogenic relationship. In spite of the phylogenetic divergence between Sphingomonas and Pseudomonas, they shared many major virulence factors, such as adherence, antiphagocytosis, iron uptake, proteases, and quorum sensing. In conclusion, Sphingomonas spp. contains several major virulence factors resembling Pseudomonas sp., Legionella sp., Brucella sp., and Bordetella sp. virulence factors. Similarity of virulence factors did not match phylogenetic relationships. These findings suggest horizontal gene transfer of virulence factors rather than sharing a common pathogenic ancestor. Sphingomonas spp. is potential virulent bacterial pathogen.

Cytotoxicity of TiO2 nanoparticles towards freshwater sediment microorganisms at low exposure concentrations

There is a persistent need to assess the effects of TiO2 nanoparticles on the aquatic ecosystem owing to their increasing usage in consumer products and risk of environmental release. The current study is focused on TiO2 nanoparticle-induced acute toxicity at sub-ppm level (≤1ppm) on the three different freshwater sediment bacterial isolates and their consortium under two different irradiation (visible light and dark) conditions. The consortium of the bacterial isolates was found to be less affected by the exposure to the nanoparticles compared to the individual cells. The oxidative stress contributed considerably towards the cytotoxicity under both light and dark conditions. A statistically significant increase in membrane permeability was noted under the dark conditions as compared to the light conditions. The optical and fluorescence microscopic images showed aggregation and chain formation of the bacterial cells, when exposed to the nanoparticles. The electron microscopic (SEM, TEM) observations suggested considerable damage of cells and bio-uptake of nanoparticles. The exopolysaccrides (EPS) production and biofilm formation were noted to increase in the presence of the nanoparticles, and expression of the key genes involved in biofilm formation was studied by RT-PCR.

Mechanism of action of peptidoglycan O-acetyltransferase B involves a Ser-His-Asp catalytic triad

The O-acetylation of the essential cell wall polymer peptidoglycan is essential in many bacteria for their integrity and survival, and it is catalyzed by peptidoglycan O-acetlytransferase B (PatB). Using PatB from Neisseria gonorrhoeae as the model, we have shown previously that the enzyme has specificity for polymeric muropeptides that possess tri- and tetrapeptide stems and that rates of reaction increase with increasing degrees of polymerization. Here, we present the catalytic mechanism of action of PatB, the first to be described for an O-acetyltransferase of any bacterial exopolysaccharide. The influence of pH on PatB activity was investigated, and pKa values of 6.4-6.45 and 6.25-6.35 for the enzyme-substrate complex (kcat vs pH) and the free enzyme (kcat·KM(-1) vs pH), respectively, were determined for the respective cosubstrates. The enzyme is partially inactivated by sulfonyl fluorides but not by EDTA, suggesting the participation of a serine residue in its catalytic mechanism. Alignment of the known and hypothetical PatB amino acid sequences identified Ser133, Asp302, and His305 as three invariant amino acid residues that could potentially serve as a catalytic triad. Replacement of Asp302 with Ala resulted in an enzyme with less than 20% residual activity, whereas activity was barely detectable with (His305 → Ala)PatB and (Ser133 → Ala)PatB was totally inactive. The reaction intermediate of the transferase reaction involving acetyl- and propionyl-acyl donors was trapped on both the wild-type and (Asp302 → Ala) enzymes and LC-MS/MS analysis of tryptic peptides identified Ser133 as the catalytic nucleophile. A transacetylase mechanism is proposed based on the mechanism of action of serine esterases.

P. aeruginosa SGNH hydrolase-like proteins AlgJ and AlgX have similar topology but separate and distinct roles in alginate acetylation

The O-acetylation of polysaccharides is a common modification used by pathogenic organisms to protect against external forces. Pseudomonas aeruginosa secretes the anionic, O-acetylated exopolysaccharide alginate during chronic infection in the lungs of cystic fibrosis patients to form the major constituent of a protective biofilm matrix. Four proteins have been implicated in the O-acetylation of alginate, AlgIJF and AlgX. To probe the biological function of AlgJ, we determined its structure to 1.83 Å resolution. AlgJ is a SGNH hydrolase-like protein, which while structurally similar to the N-terminal domain of AlgX exhibits a distinctly different electrostatic surface potential. Consistent with other SGNH hydrolases, we identified a conserved catalytic triad composed of D190, H192 and S288 and demonstrated that AlgJ exhibits acetylesterase activity in vitro. Residues in the AlgJ signature motifs were found to form an extensive network of interactions that are critical for O-acetylation of alginate in vivo. Using two different electrospray ionization mass spectrometry (ESI-MS) assays we compared the abilities of AlgJ and AlgX to bind and acetylate alginate. Binding studies using defined length polymannuronic acid revealed that AlgJ exhibits either weak or no detectable polymer binding while AlgX binds polymannuronic acid specifically in a length-dependent manner. Additionally, AlgX was capable of utilizing the surrogate acetyl-donor 4-nitrophenyl acetate to catalyze the O-acetylation of polymannuronic acid. Our results, combined with previously published in vivo data, suggest that the annotated O-acetyltransferases AlgJ and AlgX have separate and distinct roles in O-acetylation. Our refined model for alginate acetylation places AlgX as the terminal acetlytransferase and provides a rationale for the variability in the number of proteins required for polysaccharide O-acetylation.

Bacteria utilize many defense strategies to protect themselves against external forces. One mechanism used by the bacterium Pseudomonas aeruginosa is the production of the long sugar polymer alginate. The bacteria use this polymer to form a biofilm – a barrier to protect against antibiotics and the host immune response. During its biosynthesis alginate undergoes a chemical modification whereby acetate is added to the polymer. Acetylation of alginate is important as this modification makes the bacterial biofilm less susceptible to recognition and clearance by the host immune system. In this paper we present the atomic structure of AlgJ; one of four proteins required for O-acetylation of the polymer. AlgJ is structurally similar to AlgX, which we have shown previously is also required for alginate acetylation. To understand why both enzymes are required for O-acetylation we functionally characterized the proteins and found that although AlgJ exhibits acetylesterase activity – catalyzing the removal of acetyl groups from a surrogate substrate – it does not bind to short mannuornic acid polymers. In contrast, AlgX bound alginate in a length-dependent manner and was capable of transfering acetate from a surrogate substrate onto alginate. This has allowed us to not only understand how acetate is added to alginate, but increases our understanding of how acetate is added to other bacterial sugar polymers.

Biomolecular Mechanisms of Pseudomonas aeruginosa and Escherichia coli Biofilm Formation

Pseudomonas aeruginosa and Escherichia coli are the most prevalent Gram-negative biofilm forming medical device associated pathogens, particularly with respect to catheter associated urinary tract infections. In a similar manner to Gram-positive bacteria, Gram-negative biofilm formation is fundamentally determined by a series of steps outlined more fully in this review, namely adhesion, cellular aggregation, and the production of an extracellular polymeric matrix. More specifically this review will explore the biosynthesis and role of pili and flagella in Gram-negative adhesion and accumulation on surfaces in Pseudomonas aeruginosa and Escherichia coli. The process of biofilm maturation is compared and contrasted in both species, namely the production of the exopolysaccharides via the polysaccharide synthesis locus (Psl), pellicle Formation (Pel) and alginic acid synthesis in Pseudomonas aeruginosa, and UDP-4-amino-4-deoxy-l-arabinose and colonic acid synthesis in Escherichia coli. An emphasis is placed on the importance of the LuxR homologue sdiA; the luxS/autoinducer-II; an autoinducer-III/epinephrine/norepinephrine and indole mediated Quorum sensing systems in enabling Gram-negative bacteria to adapt to their environments. The majority of Gram-negative biofilms consist of polysaccharides of a simple sugar structure (either homo- or heteropolysaccharides) that provide an optimum environment for the survival and maturation of bacteria, allowing them to display increased resistance to antibiotics and predation.

Microbial alginate production, modification and its applications

Alginate is an important polysaccharide used widely in the food, textile, printing and pharmaceutical industries for its viscosifying, and gelling properties. All commercially produced alginates are isolated from farmed brown seaweeds. These algal alginates suffer from heterogeneity in composition and material properties. Here, we will discuss alginates produced by bacteria; the molecular mechanisms involved in their biosynthesis; and the potential to utilize these bacterially produced or modified alginates for high-value applications where defined material properties are required.

Structural and functional characterization of Pseudomonas aeruginosa AlgX: role of AlgX in alginate acetylation

The exopolysaccharide alginate, produced by mucoid Pseudomonas aeruginosa in the lungs of cystic fibrosis patients, undergoes two different chemical modifications as it is synthesized that alter the properties of the polymer and hence the biofilm. One modification, acetylation, causes the cells in the biofilm to adhere better to lung epithelium, form microcolonies, and resist the effects of the host immune system and/or antibiotics. Alginate biosynthesis requires 12 proteins encoded by the algD operon, including AlgX, and although this protein is essential for polymer production, its exact role is unknown. In this study, we present the X-ray crystal structure of AlgX at 2.15 Å resolution. The structure reveals that AlgX is a two-domain protein, with an N-terminal domain with structural homology to members of the SGNH hydrolase superfamily and a C-terminal carbohydrate-binding module. A number of residues in the carbohydrate-binding module form a substrate recognition "pinch point" that we propose aids in alginate binding and orientation. Although the topology of the N-terminal domain deviates from canonical SGNH hydrolases, the residues that constitute the Ser-His-Asp catalytic triad characteristic of this family are structurally conserved. In vivo studies reveal that site-specific mutation of these residues results in non-acetylated alginate. This catalytic triad is also required for acetylesterase activity in vitro. Our data suggest that not only does AlgX protect the polymer as it passages through the periplasm but that it also plays a role in alginate acetylation. Our results provide the first structural insight for a wide group of closely related bacterial polysaccharide acetyltransferases.

Synthase-dependent exopolysaccharide secretion in Gram-negative bacteria

The biosynthesis and export of bacterial cell-surface polysaccharides is known to occur through several distinct mechanisms. Recent advances in the biochemistry and structural biology of several proteins in synthase-dependent polysaccharide secretion systems have identified key conserved components of this pathway in Gram-negative bacteria. These components include an inner-membrane-embedded polysaccharide synthase, a periplasmic tetratricopeptide repeat (TPR)-containing scaffold protein, and an outer-membrane β-barrel porin. There is also increasing evidence that many synthase-dependent systems are post-translationally regulated by the bacterial second messenger bis-(3'-5')-cyclic dimeric guanosine monophosphate (c-di-GMP). Here, we compare these core proteins in the context of the alginate, cellulose, and poly-β-D-N-acetylglucosamine (PNAG) secretion systems.

Biosynthesis of the Pseudomonas aeruginosa Extracellular Polysaccharides, Alginate, Pel, and Psl

Pseudomonas aeruginosa thrives in many aqueous environments and is an opportunistic pathogen that can cause both acute and chronic infections. Environmental conditions and host defenses cause differing stresses on the bacteria, and to survive in vastly different environments, P. aeruginosa must be able to adapt to its surroundings. One strategy for bacterial adaptation is to self-encapsulate with matrix material, primarily composed of secreted extracellular polysaccharides. P. aeruginosa has the genetic capacity to produce at least three secreted polysaccharides; alginate, Psl, and Pel. These polysaccharides differ in chemical structure and in their biosynthetic mechanisms. Since alginate is often associated with chronic pulmonary infections, its biosynthetic pathway is the best characterized. However, alginate is only produced by a subset of P. aeruginosa strains. Most environmental and other clinical isolates secrete either Pel or Psl. Little information is available on the biosynthesis of these polysaccharides. Here, we review the literature on the alginate biosynthetic pathway, with emphasis on recent findings describing the structure of alginate biosynthetic proteins. This information combined with the characterization of the domain architecture of proteins encoded on the Psl and Pel operons allowed us to make predictive models for the biosynthesis of these two polysaccharides. The results indicate that alginate and Pel share certain features, including some biosynthetic proteins with structurally or functionally similar properties. In contrast, Psl biosynthesis resembles the EPS/CPS capsular biosynthesis pathway of Escherichia coli, where the Psl pentameric subunits are assembled in association with an isoprenoid lipid carrier. These models and the environmental cues that cause the cells to produce predominantly one polysaccharide over the others are subjects of current investigation.

Genome sequence of poultry pathogen Riemerella anatipestifer strain RA-YM

Riemerella anatipestifer is a Gram-negative, rod-shaped bacterium associated with epizootic infections in poultry. R. anatipestifer strain RA-YM, belonging to the serotype 1 prevalent in China, is a clinically isolated strain with high-level virulence. Here, we report the first genome sequence of this species.

O-Acetylation of peptidoglycan is required for proper cell separation and S-layer anchoring in Bacillus anthracis

O-Acetylation of the MurNAc moiety of peptidoglycan is typically associated with bacterial resistance to lysozyme, a muramidase that serves as a central component of innate immunity. Here, we report that the peptidoglycan of Bacillus anthracis, the etiological agent of anthrax, is O-acetylated and that, unusually, this modification is produced by two unrelated families of O-acetyltransferases. Also, in contrast to other bacteria, O-acetylation of B. anthracis peptidoglycan is combined with N-deacetylation to confer resistance of cells to lysozyme. Activity of the Pat O-acetyltransferases is required for the separation of the daughter cells following bacterial division and for anchoring of one of the major S-layer proteins. Our results indicate that peptidoglycan O-acetylation modulates endogenous muramidase activity affecting the cell-surface properties and morphology of this important pathogen.

ClpXP proteases positively regulate alginate overexpression and mucoid conversion in Pseudomonas aeruginosa

Overproduction of the exopolysaccharide alginate and conversion to a mucoid phenotype in Pseudomonas aeruginosa are markers for the onset of chronic lung infection in cystic fibrosis (CF). Alginate production is regulated by the extracytoplasmic function (ECF) sigma factor AlgU/T and the cognate anti-sigma factor MucA. Many clinical mucoid isolates carry loss-of-function mutations in mucA. These mutations, including the most common mucA22 allele, cause C-terminal truncations in MucA, indicating that an inability to regulate AlgU activity by MucA is associated with conversion to the mucoid phenotype. Here we report that a mutation in a stable mucoid strain derived from the parental strain PAO1, designated PAO581, that does not contain the mucA22 allele, was due to a single-base deletion in mucA (DeltaT180), generating another type of C-terminal truncation. A global mariner transposon screen in PAO581 for non-mucoid isolates led to the identification of three regulators of alginate production, clpP (PA1801), clpX (PA1802), and a clpP paralogue (PA3326, designated clpP2). The PAO581 null mutants of clpP, clpX and clpP2 showed decreased AlgU transcriptional activity and an accumulation of haemagglutinin (HA)-tagged N-terminal MucA protein with an apparent molecular mass of 15 kDa. The clpP and clpX mutants of a CF mucoid isolate revert to the non-mucoid phenotype. The ClpXP and ClpP2 proteins appear to be part of a proteolytic network that degrades the cytoplasmic portion of truncated MucA proteins to release the sequestered AlgU, which drives alginate biosynthesis.

Alginate and chitosan functionalization for micronutrient encapsulation

A new method for encapsulation of micronutrients was successfully developed. The encapsulation matrix consisted of two polymers (alginate and chitosan), which were functionalized by acylation with palmitoyl chloride. The structural modifications of polymers were confirmed by Fourier transform infrared (FTIR) spectroscopy. Beads were formed by ionic gelation, and their mechanical and physical characteristics (puncture strength and deformation, viscoelasticity, water vapor permeability, and rate of gel swelling) were evaluated using beads or films made of bead-forming solutions. Functionalization increased elasticity and water impermeability of polymer films. Stability of selected encapsulated micronutrients (ferrous fumarate, ascorbic acid, and beta-carotene) was also evaluated under two levels of temperature (23 and 45 degrees C) and relative humidity (56 and 100%) for 6 months. Encapsulation strongly increased the stability of micronutrients. No difference was observed in the encapsulated micronutrients' stability between nonfunctionalized and functionalized beads. Finally, a release study in gastrointestinal media was conducted. Results showed that beads were not susceptible to enzymatic and acidic attacks during stomach transit. This research demonstrates the potential of a new encapsulation method to protect bioactive molecules from temperature, moisture, and acidic conditions.

Genetics of bacterial alginate: alginate genes distribution, organization and biosynthesis in bacteria

Bacterial alginate genes are chromosomal and fairly widespread among rRNA homology group I Pseudomonads and Azotobacter. In both genera, the genetic pathway of alginate biosynthesis is mostly similar and the identified genes are identically organized into biosynthetic, regulatory and genetic switching clusters. In spite of these similarities,still there are transcriptional and functional variations between P. aeruginosa and A. vinelandii. In P. aeruginosa all biosynthetic genes except algC transcribe in polycistronic manner under the control of algD promoter while in A. vinelandii, these are organized into many transcriptional units. Of these, algA and algC are transcribed each from two different and algD from three different promoters. Unlike P. aeruginosa, the promoters of these transcriptional units except one of algC and algD are algT-independent. Both bacterial species carry homologous algG gene for Ca(2+)-independent epimerization. But besides algG, A. vinelandii also has algE1-7 genes which encode C-5-epimerases involved in the complex steps of Ca(2+)-dependent epimerization. A hierarchy of alginate genes expression under sigma(22)(algT) control exists in P. aeruginosa where algT is required for transcription of the response regulators algB and algR, which in turn are necessary for expression of algD and its downstream biosynthetic genes. Although algTmucABCD genes cluster play similar regulatory roles in both P. aeruginosa and A. vinelandii but unlike, transcription of A. vinelandii, algR is independent of sigma(22). These differences could be due to the fact that in A. vinelandii alginate plays a role as an integrated part in desiccation-resistant cyst which is not found in P. aeruginosa.

Bacterial alginates: from biosynthesis to applications

Alginate is a polysaccharide belonging to the family of linear (unbranched), non-repeating copolymers, consisting of variable amounts of beta-D-mannuronic acid and its C5-epimer alpha- L-guluronic acid linked via beta-1,4-glycosidic bonds. Like DNA, alginate is a negatively charged polymer, imparting material properties ranging from viscous solutions to gel-like structures in the presence of divalent cations. Bacterial alginates are synthesized by only two bacterial genera, Pseudomonas and Azotobacter, and have been extensively studied over the last 40 years. While primarily synthesized in form of polymannuronic acid, alginate undergoes chemical modifications comprising acetylation and epimerization, which occurs during periplasmic transfer and before final export through the outer membrane. Alginate with its unique material properties and characteristics has been increasingly considered as biomaterial for medical applications. The genetic modification of alginate producing microorganisms could enable biotechnological production of new alginates with unique, tailor-made properties, suitable for medical and industrial applications.

Mutations affecting peptidoglycan acetylation in Neisseria gonorrhoeae and Neisseria meningitidis

Neisseria gonorrhoeae acetylates its cell wall peptidoglycan (PG) at the C-6 position on N-acetylmuramic acid. To understand the effects of PG acetylation on PG metabolism and release of PG fragments, we have made mutations in the genes responsible for PG acetylation. An insertion mutation in a putative PG acetylase gene (designated pacA) resulted in loss of PG acetylation as detected by a high-performance liquid chromatography-based assay. Sequence analysis of a naturally occurring non-acetylating strain revealed the presence of a 26-bp deletion in pacA. Introduction of the deletion mutation into wild-type gonococci resulted in lack of acetylation, and the phenotype was complemented by the addition of a wild-type copy of pacA at a distant location on the chromosome. Mutations were also introduced into three genes downstream of pacA. The gene directly downstream of pacA was required for acetylation and was designated pacB, whereas the next two genes were not required. Sequences highly similar to pacA and pacB were also found in N. meningitidis and N. lactamica strains, and an insertion in the meningococcal pacA eliminated PG acetylation. Phenotypic analyses of an N. gonorrhoeae pacA mutant did not show any decrease in lysozyme resistance or serum resistance, and the release of PG fragments during growth was unchanged. However, purified PG from the wild-type strain was significantly more resistant to the action of human lysozyme than was PG purified from the pacA mutant. Interestingly, the pacA mutant was more sensitive to EDTA, a compound known to trigger autolysis.