A combinatorial DNA assembly approach to biosynthesis of N-linked glycans in E. coli.

Glycoengineering of recombinant glycans and glycoconjugates is a rapidly evolving field. However, the production and exploitation of glycans has lagged behind that of proteins and nucleic acids. Biosynthetic glycoconjugate production requires the coordinated cooperation of three key components within a bacterial cell: a substrate protein, a coupling oligosaccharyltransferase, and a glycan biosynthesis locus. While the acceptor protein and oligosaccharyltransferase are the products of single genes, the glycan is a product of a multigene metabolic pathway. Typically, the glycan biosynthesis locus is cloned and transferred en bloc from the native organism to a suitable Escherichia coli strain. However, gene expression within these pathways has been optimized by natural selection in the native host and is unlikely to be optimal for heterologous production in an unrelated organism. In recent years, synthetic biology has addressed the challenges in heterologous expression of multigene systems by deconstructing these pathways and rebuilding them from the bottom up. The use of DNA assembly methods allows the convenient assembly of such pathways by combining defined parts with the requisite coding sequences in a single step. In this study, we apply combinatorial assembly to the heterologous biosynthesis of the Campylobacter jejuni  N-glycosylation (pgl) pathway in E. coli. We engineered reconstructed biosynthesis clusters that faithfully reproduced the C. jejuni heptasaccharide glycan. Furthermore, following a single round of combinatorial assembly and screening, we identified pathway clones that outperform glycan and glycoconjugate production of the native unmodified pgl cluster. This platform offers a flexible method for optimal engineering of glycan structures in E. coli.

Clostridioides difficile para-Cresol Production Is Induced by the Precursor para-Hydroxyphenylacetate.

Clostridioides difficile is an etiological agent for antibiotic-associated diarrheal disease. C. difficile produces a phenolic compound, para-cresol, which selectively targets gammaproteobacteria in the gut, facilitating dysbiosis. C. difficile decarboxylates para-hydroxyphenylacetate (p-HPA) to produce p-cresol by the action of the HpdBCA decarboxylase encoded by the hpdBCA operon. Here, we investigate regulation of the hpdBCA operon and directly compare three independent reporter systems; SNAP-tag, glucuronidase gusA, and alkaline phosphatase phoZ reporters to detect basal and inducible expression. We show that expression of hpdBCA is upregulated in response to elevated p-HPA. In silico analysis identified three putative promoters upstream of hpdBCA operon-P1, P2, and Pσ54; only the P1 promoter was responsible for both basal and p-HPA-inducible expression of hpdBCA We demonstrated that turnover of tyrosine, a precursor for p-HPA, is insufficient to induce expression of the hpdBCA operon above basal levels because it is inefficiently converted to p-HPA in minimal media. We show that induction of the hpdBCA operon in response to p-HPA occurs in a dose-dependent manner. We also identified an inverted palindromic repeat (AAAAAG-N13-CTTTTT) upstream of the hpdBCA start codon (ATG) that is essential for inducing transcription of the hpdBCA operon in response to p-HPA, which drives the production of p-cresol. This provides insights into the regulatory control of p-cresol production, which affords a competitive advantage for C. difficile over other intestinal bacteria, promoting dysbiosis.IMPORTANCEClostridioides difficile infection results from antibiotic-associated dysbiosis. para-Cresol, a phenolic compound produced by C. difficile, selectively targets gammaproteobacteria in the gut, facilitating dysbiosis. Here, we demonstrate that expression of the hpdBCA operon, encoding the HpdBCA decarboxylase which converts p-HPA to p-cresol, is upregulated in response to elevated exogenous p-HPA, with induction occurring between >0.1 and ≤0.25 mg/ml. We determined a single promoter and an inverted palindromic repeat responsible for basal and p-HPA-inducible hpdBCA expression. We identified turnover of tyrosine, a p-HPA precursor, does not induce hpdBCA expression above basal level, indicating that exogenous p-HPA was required for p-cresol production. Identifying regulatory controls of p-cresol production will provide novel therapeutic targets to prevent p-cresol production, reducing C. difficile's competitive advantage.

The Streptococcos suis sortases SrtB and SrtF are essential for disease in pigs.

The porcine pathogen Streptococcus suis colonizes the upper respiratory tracts of pigs, potentially causing septicaemia, meningitis and death, thus placing a severe burden on the agricultural industry worldwide. It is also a zoonotic pathogen that is known to cause systemic infections and meningitis in humans. Understanding how S. suis colonizes and interacts with its hosts is relevant for future strategies of drug and vaccine development. As with other Gram-positive bacteria, S. suis utilizes enzymes known as sortases to attach specific proteins bearing cell wall sorting signals to its surface, where they can play a role in host-pathogen interactions. The surface proteins of bacteria are often important in adhesion to and invasion of host cells. In this study, markerless in-frame deletion mutants of the housekeeping sortase srtA and the two pilus-associated sortases, srtB and srtF, were generated and their importance in S. suis infections was investigated. We found that all three of these sortases are essential to disease in pigs, concluding that their cognate-sorted proteins may also be useful in protecting pigs against infection.

Role of Glycosyltransferases Modifying Type B Flagellin of Emerging Hypervirulent Clostridium difficile Lineages and Their Impact on Motility and Biofilm Formation.

Clostridium difficile is the principal cause of nosocomial infectious diarrhea worldwide. The pathogen modifies its flagellin with either a type A or type B O-linked glycosylation system, which has a contributory role in pathogenesis. We study the functional role of glycosyltransferases modifying type B flagellin in the 023 and 027 hypervirulent C. difficile lineages by mutagenesis of five putative glycosyltransferases and biosynthetic genes. We reveal their roles in the biosynthesis of the flagellin glycan chain and demonstrate that flagellar post-translational modification affects motility and adhesion-related bacterial properties of these strains. We show that the glycosyltransferases 1 and 2 (GT1 and GT2) are responsible for the sequential addition of a GlcNAc and two rhamnoses, respectively, and that GT3 is associated with the incorporation of a novel sulfonated peptidyl-amido sugar moiety whose structure is reported in our accompanying paper (Bouché, L., Panico, M., Hitchen, P., Binet, D., Sastre, F., Faulds-Pain, A., Valiente, E., Vinogradov, E., Aubry, A., Fulton, K., Twine, S., Logan, S. M., Wren, B. W., Dell, A., and Morris, H. R. (2016) J. Biol. Chem. 291, 25439-25449). GT2 is also responsible for methylation of the rhamnoses. Whereas type B modification is not required for flagellar assembly, some mutations that result in truncation or abolition of the glycan reduce bacterial motility and promote autoaggregation and biofilm formation. The complete lack of flagellin modification also significantly reduces adhesion of C. difficile to Caco-2 intestinal epithelial cells but does not affect activation of human TLR5. Our study advances our understanding of the genes involved in flagellar glycosylation and their biological roles in emerging hypervirulent C. difficile strains.

The Type B Flagellin of Hypervirulent Clostridium difficile Is Modified with Novel Sulfonated Peptidylamido-glycans.

Glycosylation of flagellins is a well recognized property of many bacterial species. In this study, we describe the structural characterization of novel flagellar glycans from a number of hypervirulent strains of C. difficile We used mass spectrometry (nano-LC-MS and MS/MS analysis) to identify a number of putative glycopeptides that carried a variety of glycoform substitutions, each of which was linked through an initial N-acetylhexosamine residue to Ser or Thr. Detailed analysis of a LLDGSSTEIR glycopeptide released by tryptic digestion, which carried two variant structures, revealed that the glycopeptide contained, in addition to carbohydrate moieties, a novel structural entity. A variety of electrospray-MS strategies using Q-TOF technology were used to define this entity, including positive and negative ion collisionally activated decomposition MS/MS, which produced unique fragmentation patterns, and high resolution accurate mass measurement to allow derivation of atomic compositions, leading to the suggestion of a taurine-containing peptidylamido-glycan structure. Finally, NMR analysis of flagellin glycopeptides provided complementary information. The glycan portion of the modification was assigned as α-Fuc3N-(1→3)-α-Rha-(1→2)-α-Rha3OMe-(1→3)-ß-GlcNAc-(1→)Ser, and the novel capping moiety was shown to be comprised of taurine, alanine, and glycine. This is the first report of a novel O-linked sulfonated peptidylamido-glycan moiety decorating a flagellin protein.

The post-translational modification of the Clostridium difficile flagellin affects motility, cell surface properties and virulence.

Clostridium difficile is a prominent nosocomial pathogen, proliferating and causing enteric disease in individuals with a compromised gut microflora. We characterized the post-translational modification of flagellin in C. difficile 630. The structure of the modification was solved by nuclear magnetic resonance and shown to contain an N-acetylglucosamine substituted with a phosphorylated N-methyl-l-threonine. A reverse genetics approach investigated the function of the putative four-gene modification locus. All mutants were found to have truncated glycan structures by LC-MS/MS, taking into account bioinformatic analysis, we propose that the open reading frame CD0241 encodes a kinase involved in the transfer of the phosphate to the threonine, the CD0242 protein catalyses the addition of the phosphothreonine to the N-acetylglucosamine moiety and CD0243 transfers the methyl group to the threonine. Some mutations affected motility and caused cells to aggregate to each other and abiotic surfaces. Altering the structure of the flagellin modification impacted on colonization and disease recurrence in a murine model of infection, showing that alterations in the surface architecture of C. difficile vegetative cells can play a significant role in disease. We show that motility is not a requirement for colonization, but that colonization was compromised when the glycan structure was incomplete.

The agr locus regulates virulence and colonization genes in Clostridium difficile 027.

The transcriptional regulator AgrA, a member of the LytTR family of proteins, plays a key role in controlling gene expression in some Gram-positive pathogens, including Staphylococcus aureus and Enterococcus faecalis. AgrA is encoded by the agrACDB global regulatory locus, and orthologues are found within the genome of most Clostridium difficile isolates, including the epidemic lineage 027/BI/NAP1. Comparative RNA sequencing of the wild type and otherwise isogenic agrA null mutant derivatives of C. difficile R20291 revealed a network of approximately 75 differentially regulated transcripts at late exponential growth phase, including many genes associated with flagellar assembly and function, such as the major structural subunit, FliC. Other differentially regulated genes include several involved in bis-(3'-5')-cyclic dimeric GMP (c-di-GMP) synthesis and toxin A expression. C. difficile 027 R20291 agrA mutant derivatives were poorly flagellated and exhibited reduced levels of colonization and relapses in the murine infection model. Thus, the agr locus likely plays a contributory role in the fitness and virulence potential of C. difficile strains in the 027/BI/NAP1 lineage.

Improved bacterial mutagenesis by high-frequency allele exchange, demonstrated in Clostridium difficile and Streptococcus suis.

Here we show that the frequency of mutant isolation by two-step allele exchange can be improved by increasing the length of homologous DNA and the opportunity for recombination, obviating the need for counterselection markers. These principles are demonstrated in Clostridium difficile and Streptococcus suis but are likely to be generally applicable.

Flagellin redundancy in Caulobacter crescentus and its implications for flagellar filament assembly.

Bacterial flagella play key roles in surface attachment and host-bacterial interactions as well as driving motility. Here, we have investigated the ability of Caulobacter crescentus to assemble its flagellar filament from six flagellins: FljJ, FljK, FljL, FljM, FljN, and FljO. Flagellin gene deletion combinations exhibited a range of phenotypes from no motility or impaired motility to full motility. Characterization of the mutant collection showed the following: (i) that there is no strict requirement for any one of the six flagellins to assemble a filament; (ii) that there is a correlation between slower swimming speeds and shorter filament lengths in ΔfljK ΔfljM mutants; (iii) that the flagellins FljM to FljO are less stable than FljJ to FljL; and (iv) that the flagellins FljK, FljL, FljM, FljN, and FljO alone are able to assemble a filament.

Characterisation of Clostridium difficile biofilm formation, a role for Spo0A.

Clostridium difficile is a Gram-positive anaerobic, spore-forming bacillus that is the leading cause of nosocomial diarrhoea worldwide. We demonstrate that C. difficile aggregates and forms biofilms in vitro on abiotic surfaces. These polymicrobial aggregates are attached to each other and to an abiotic surface by an extracellular polymeric substance (EPS). The EPS matrix provides the scaffold bonding together vegetative cells and spores, as well as forming a protective barrier for vegetative cells against oxygen stress. The master regulator of sporulation, Spo0A, may play a key role in biofilm formation, as genetic inactivation of spo0A in strain R20291 exhibits decreased biofilm formation. Our findings highlight an important attribute of C. difficile pathogenesis, which may have significant implications for infection, treatment and relapse.

Structural insight into the Clostridium difficile ethanolamine utilisation microcompartment.

Bacterial microcompartments form a protective proteinaceous barrier around metabolic enzymes that process unstable or toxic chemical intermediates. The genome of the virulent, multidrug-resistant Clostridium difficile 630 strain contains an operon, eut, encoding a bacterial microcompartment with genes for the breakdown of ethanolamine and its utilisation as a source of reduced nitrogen and carbon. The C. difficile eut operon displays regulatory genetic elements and protein encoding regions in common with homologous loci found in the genomes of other bacteria, including the enteric pathogens Salmonella enterica and Enterococcus faecalis. The crystal structures of two microcompartment shell proteins, CD1908 and CD1918, and an uncharacterised protein with potential enzymatic activity, CD1925, were determined by X-ray crystallography. CD1908 and CD1918 display the same protein fold, though the order of secondary structure elements is permuted in CD1908 and this protein displays an N-terminal ß-strand extension. These proteins form hexamers with molecules related by crystallographic and non-crystallographic symmetry. The structure of CD1925 has a cupin ß-barrel fold and a putative active site that is distinct from the metal-ion dependent catalytic cupins. Thin-section transmission electron microscopy of Escherichia coli over-expressing eut proteins indicates that CD1918 is capable of self-association into arrays, suggesting an organisational role for CD1918 in the formation of this microcompartment. The work presented provides the basis for further study of the architecture and function of the C. difficile eut microcompartment, its role in metabolism and the wider consequences of intestinal colonisation and virulence in this pathogen.