Progress in Our Understanding of Wzx Flippase for Translocation of Bacterial Membrane Lipid-Linked Oligosaccharide

Rhizobial Secretion of Truncated Exopolysaccharides Severely Impairs the Mesorhizobium-Lotus Symbiosis

The symbiosis between Mesorhizobium japonicum R7A and Lotus japonicus Gifu is an important model system for investigating the role of bacterial exopolysaccharides (EPS) in plant-microbe interactions. Previously, we showed that R7A exoB mutants that are affected at an early stage of EPS synthesis and in lipopolysaccharide (LPS) synthesis induce effective nodules on L. japonicus Gifu after a delay, whereas exoU mutants affected in the biosynthesis of the EPS side chain induce small uninfected nodule primordia and are impaired in infection. The presence of a halo around the exoU mutant when grown on Calcofluor-containing media suggested the mutant secreted a truncated version of R7A EPS. A nonpolar ΔexoA mutant defective in the addition of the first glucose residue to the EPS backbone was also severely impaired symbiotically. Here, we used a suppressor screen to show that the severe symbiotic phenotype of the exoU mutant was due to the secretion of an acetylated pentasaccharide, as both monomers and oligomers, by the same Wzx/Wzy system that transports wild-type exopolysaccharide. We also present evidence that the ΔexoA mutant secretes an oligosaccharide by the same transport system, contributing to its symbiotic phenotype. In contrast, ΔexoYF and polar exoA and exoL mutants have a similar phenotype to exoB mutants, forming effective nodules after a delay. These studies provide substantial evidence that secreted incompatible EPS is perceived by the plant, leading to abrogation of the infection process. [Formula: see text] Copyright © 2024 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Repeat-Unit Elongations To Produce Bacterial Complex Long Polysaccharide Chains, an O-Antigen Perspective

The O-antigen, a long polysaccharide that constitutes the distal part of the outer membrane-anchored lipopolysaccharide, is one of the critical components in the protective outer membrane of Gram-negative bacteria. Most species produce one of the structurally diverse O-antigens, with nearly all the polysaccharide components having complex structures made by the Wzx/Wzy pathway. This pathway produces repeat-units of mostly 3-8 sugars on the cytosolic face of the cytoplasmic membrane that is translocated by Wzx flippase to the periplasmic face and polymerized by Wzy polymerase to give long-chain polysaccharides. The Wzy polymerase is a highly diverse integral membrane protein typically containing 10-14 transmembrane segments. Biochemical evidence confirmed that Wzy polymerase is the sole driver of polymerization, and recent progress also began to demystify its interacting partner, Wzz, shedding some light to speculate how the proteins may operate together during polysaccharide biogenesis. However, our knowledge of how the highly variable Wzy proteins work as part of the O-antigen processing machinery remains poor. Here, we discuss the progress to the current understanding of repeat-unit polymerization and propose an updated model to explain the formation of additional short chain O-antigen polymers found in the lipopolysaccharide of diverse Gram-negative species and their importance in the biosynthetic process.

Extensive Diversity in Escherichia coli Group 3 Capsules Is Driven by Recombination and Plasmid Transfer from Multiple Species

Bacterial capsules provide protection against environmental challenges and host immunity. Historically, Escherichia coli K serotyping scheme, which relies on the hypervariable capsules, has identified around 80 K forms that fall into four distinct groups. Based on recent work by us and others, we predicted that E. coli capsular diversity is grossly underestimated. We exploited group 3 capsule gene clusters, the best genetically defined capsule group in E. coli, to analyze publicly available E. coli sequences for overlooked capsular diversity within the species. We report the discovery of seven novel group 3 clusters that fall into two distinct subgroups (3A and 3B). The majority of the 3B capsule clusters were found on plasmids, contrary to the defining feature of group 3 capsule genes localizing at the serA locus on the E. coli chromosome. Other new group 3 capsule clusters were derived from ancestral sequences through recombination events between shared genes found within the serotype variable central region 2. Intriguingly, flanking regions 1 and 3, known to be conserved areas among capsule clusters, showed considerable intra-subgroup variation in clusters from the 3B subgroup, containing genes of shared ancestry with other Enterobacteriaceae species. Variation of group 3 kps clusters within dominant E. coli lineages, including multidrug-resistant pathogenic lineages, further supports that E. coli capsules are undergoing rigorous change. Given the pivotal role of capsular polysaccharides in phage predation, our findings raise attention to the need of monitoring kps evolutionary dynamics in pathogenic E. coli in supporting phage therapy. IMPORTANCE Capsular polysaccharides protect pathogenic bacteria against environmental challenges, host immunity, and phage predations. The historical Escherichia coli K typing scheme, which relies on the hypervariable capsular polysaccharide, has identified around 80 different K forms that fall into four distinct groups. Taking advantage of the supposedly compact and genetically well-defined group 3 gene clusters, we analyzed published E. coli sequences to identify seven new gene clusters and revealed an unexpected capsular diversity. Genetic analysis revealed that group 3 gene clusters shared closely related serotype-specific region 2 and were diversified through recombination events and plasmid transfer between multiple Enterobacteriaceae species. Overall, capsular polysaccharides in E. coli are undergoing rigorous change. Given the pivotal role capsules play in phage interactions, this work highlighted the need to monitor the evolutionary dynamics of capsules in pathogenic E. coli for effective phage therapy.

SSP: An In Silico Tool for Salmonella Species Serotyping Using the Sequences of O-Antigen Biosynthesis Proteins and H-Antigen Filament Proteins

Over 2500 Salmonella species (alternatively, serovars) encompassing different combinations of O-, H1- and H2-antigens are present in nature and cause millions of deaths worldwide every year. Since conventional serotyping is time-consuming, a user-friendly Salmonellaspecies serotyping (SSP) web tool (https://project.iith.ac.in/SSP/) is developed here to predict the serotypes using Salmonella protein(s) or whole proteome sequences. Prior to SSP implementation, a detailed analysis of protein sequences involved in O-antigen biosynthesis and H-antigen formation is carried out to assess their serotype specificity. Intriguingly, the results indicate that the initializing transferases WbaP, WecA and GNE can efficiently distinguish the O-antigens, which have Gal, GlcNAc and GalNAc as initial sugars respectively. Rigorous analysis shows that Wzx and Wzy are sufficient to distinguish the O-types. Exceptionally, some situations warrant additional proteins. Thus, 150 additional transferases, RfbE for O2, O9 and O9,46 types, Orf17.4 for O3,10 and O1,3,19 types, WecB, WbbE and WbbF for O54 and, Wzm and Wzt for O67 are utilized in serotyping. An in-depth analysis of 302 reference datasets representing 56 H1- and 20 H2-types leads to the identification and utilization of 61 unique sequence patterns of FliC and FljB in H-typing. A test dataset of 2136 whole proteome sequences covering 740 Salmonella serovars, including 13 new species are successfully predicted with 99.72% accuracy. Prior to this, all the O-, H1- and H2-antigens are predicted accurately when tested independently. Indeed, SSP also identifies wrongly annotated Salmonella species; hence, it can easily identify new species that emerge with any combination of O-, H1- and H2-antigens. Thus, SSP can act as a valuable tool in the surveillance of Salmonella species.

The lipid linked oligosaccharide polymerase Wzy and its regulating co-polymerase, Wzz, from enterobacterial common antigen biosynthesis form a complex

The enterobacterial common antigen (ECA) is a carbohydrate polymer that is associated with the cell envelope in the Enterobacteriaceae. ECA contains a repeating trisaccharide which is polymerized by WzyE, a member of the Wzy membrane protein polymerase superfamily. WzyE activity is regulated by a membrane protein polysaccharide co-polymerase, WzzE. Förster resonance energy transfer experiments demonstrate that WzyE and WzzE from Pectobacterium atrosepticum form a complex in vivo, and immunoblotting and cryo-electron microscopy (cryo-EM) analysis confirm a defined stoichiometry of approximately eight WzzE to one WzyE. Low-resolution cryo-EM reconstructions of the complex, aided by an antibody recognizing the C-terminus of WzyE, reveals WzyE sits in the central membrane lumen formed by the octameric arrangement of the transmembrane helices of WzzE. The pairing of Wzy and Wzz is found in polymerization systems for other bacterial polymers, including lipopolysaccharide O-antigens and capsular polysaccharides. The data provide new structural insight into a conserved mechanism for regulating polysaccharide chain length in bacteria.

Multiple immunodominant O-epitopes co-expression in live attenuated Salmonella serovars induce cross-protective immune responses against S. Paratyphi A, S. Typhimurium and S. Enteritidis

Salmonella enterica subsp. enterica (S. enterica) is a significant public health concern and is estimated to cause more than 300,000 deaths annually. Nowadays, the vaccines available for human Salmonellosis prevention are all targeting just one serovar, i.e., S. Typhi, leaving a huge potential risk of Salmonella disease epidemiology change. In this study, we explored the strategy of multiple immunodominant O-epitopes co-expression in S. enterica serovars and evaluated their immunogenicity to induce cross-immune responses and cross-protections against S. Paratyphi A, S. Typhimurium and S. Enteritidis. We found that nucleotide sugar precursors CDP-Abe and CDP-Par (or CDP-Tyv) could be utilized by S. enterica serovars simultaneously, exhibiting O2&O4 (or O4&O9) double immunodominant O-serotypes without obvious growth defects. More importantly, a triple immunodominant O2&O4&O9 O-serotypes could be achieved in S. Typhimurium by improving the substrate pool of CDP-Par, glycosyltransferase WbaV and flippase Wzx via a dual-plasmid overexpressing system. Through immunization in a murine model, we found that double or triple O-serotypes live attenuated vaccine candidates could induce significantly higher heterologous serovar-specific antibodies than their wild-type parent strain. Meanwhile, the bacterial agglutination, serum bactericidal assays and protection efficacy experiments had all shown that these elicited serum antibodies are cross-reactive and cross-protective. Our work highlights the potential of developing a new type of live attenuated Salmonella vaccines against S. Paratyphi A, S. Typhimurium and S. Enteritidis simultaneously.

Dual adhesive unipolar polysaccharides synthesized by overlapping biosynthetic pathways in Agrobacterium tumefaciens

Agrobacterium tumefaciens is a member of the Alphaproteobacteria that pathogenises plants and associates with biotic and abiotic surfaces via a single cellular pole. A. tumefaciens produces the unipolar polysaccharide (UPP) at the site of surface contact. UPP production is normally surface-contact inducible, but elevated levels of the second messenger cyclic diguanylate monophosphate (cdGMP) bypass this requirement. Multiple lines of evidence suggest that the UPP has a central polysaccharide component. Using an A. tumefaciens derivative with elevated cdGMP and mutationally disabled for other dispensable polysaccharides, a series of related genetic screens have identified a large number of genes involved in UPP biosynthesis, most of which are Wzx-Wzy-type polysaccharide biosynthetic components. Extensive analyses of UPP production in these mutants have revealed that the UPP is composed of two genetically, chemically, and spatially discrete forms of polysaccharide, and that each requires a specific Wzy-type polymerase. Other important biosynthetic, processing, and regulatory functions for UPP production are also revealed, some of which are common to both polysaccharides, and a subset of which are specific to each type. Many of the UPP genes identified are conserved among diverse rhizobia, whereas others are more lineage specific.

Structure and Mechanism of the Lipid Flippase MurJ

Biosynthesis of many important polysaccharides (including peptidoglycan, lipopolysaccharide, and N-linked glycans) necessitates the transport of lipid-linked oligosaccharides (LLO) across membranes from their cytosolic site of synthesis to their sites of utilization. Much of our current understanding of LLO transport comes from genetic, biochemical, and structural studies of the multidrug/oligosaccharidyl-lipid/polysaccharide (MOP) superfamily protein MurJ, which flips the peptidoglycan precursor lipid II. MurJ plays a pivotal role in bacterial cell wall synthesis and is an emerging antibiotic target. Here, we review the mechanism of LLO flipping by MurJ, including the structural basis for lipid II flipping and ion coupling. We then discuss inhibition of MurJ by antibacterials, including humimycins and the phage M lysis protein, as well as how studies on MurJ could provide insight into other flippases, both within and beyond the MOP superfamily. Expected final online publication date for the Annual Review of Biochemistry, Volume 91 is June 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Development of an O-polysaccharide based recombinant glycoconjugate vaccine in engineered E. coli against ExPEC O1

Extraintestinal pathogenic Escherichia coli O1 is a frequently identified serotype that causes serious infections and is often refractory to antimicrobial therapy. Glycoconjugate vaccine represents a promising measure to reduce ExPEC infections. Herein, we designed an O1-specific glyco-optimized chassis strain for manufacture of O-polysaccharide (OPS) antigen and OPS-based bioconjugate. Specifically, OPS and OPS-based glycoprotein were synthesized in glyco-optimized chassis strain, when compared to the unmeasurable level of the parent strain. The optimal expression of oligosaccharyltransferase and carrier protein further improved the titer. MS analysis elucidated the correct structure of resulting bioconjugate at routine and unreported glycosylation sequons of carrier protein, with a higher glycosylation efficiency. Finally, purified bioconjugate stimulated mouse to generate specific IgG antibodies and protected them against virulent ExPEC O1 challenge. The plug-and-play glyco-optimized platform is suitable for bioconjugate synthesis, thus providing a potential platform for future medical applications.

Lipopolysaccharide lipid A: A promising molecule for new immunity-based therapies and antibiotics

Lipopolysaccharides (LPS) are the main components of the external leaflet of the Gram-negative outer membrane and consist of three different moieties: lipid A, core oligosaccharide, and O-polysaccharide. The lipid A is a glucosamine disaccharide with different levels of acylation and phosphorylation, beside carrying, in certain cases, additional substituents on the sugar backbone. It is also the main immunostimulatory part of the LPS, as its recognition by the host immune system represents a fundamental event for detection of perilous microorganisms. Moreover, an uncontrolled immune response caused by a large amount of circulating LPS can lead to dramatic outcomes for human health, such as septic shock. The immunostimulant properties of an LPS incredibly vary depending on lipid A chemical structure, and for this reason, natural and synthetic variants of the lipid A are under study to develop new drugs that mimic or antagonise its natural effects. Here, we review past and recent findings on the lipid A as an antibiotic target and immune-therapeutic molecule, with a special attention on the crucial role of the chemical structure and its exploitation for conceiving novel strategies for treatment of several immune-related pathologies.

High-Throughput Mutagenesis and Cross-Complementation Experiments Reveal Substrate Preference and Critical Residues of the Capsule Transporters in Streptococcus pneumoniae

MOP (Multidrug/Oligosaccharidyl-lipid/Polysaccharide) family transporters are found in almost all life forms. They are responsible for transporting lipid-linked precursors across the cell membrane to support the synthesis of various glycoconjugates. While significant progress has been made in elucidating their transport mechanism, how these transporters select their substrates remains unclear. Here, we systematically tested the MOP transporters in the Streptococcus pneumoniae capsule pathway for their ability to translocate noncognate capsule precursors. Sequence similarity cannot predict whether these transporters are interchangeable. We showed that subtle changes in the central aqueous cavity of the transporter are sufficient to accommodate a different cargo. These changes can occur naturally, suggesting a potential mechanism of expanding substrate selectivity. A directed evolution experiment was performed to identify gain-of-function variants that translocate a noncognate cargo. Coupled with a high-throughput mutagenesis and sequencing (Mut-seq) experiment, residues that are functionally important for the capsule transporter were revealed. Lastly, we showed that the expression of a flippase that can transport unfinished precursors resulted in an increased susceptibility to bacitracin and mild cell shape defects, which may be a driving force to maintain transporter specificity. IMPORTANCE All licensed pneumococcal vaccines target the capsular polysaccharide (CPS). This layer is highly variable and is important for virulence in many bacterial pathogens. Most of the CPSs are produced by the Wzx/Wzy mechanism. In this pathway, CPS repeating units are synthesized in the cytoplasm, which must be flipped across the cytoplasmic membrane before polymerization. This step is mediated by the widely conserved MOP (Multidrug/Oligosaccharidyl-lipid/Polysaccharide) family transporters. Here, we systematically evaluated the interchangeability of these transporters and identified the residues important for substrate specificity and function. Understanding how CPS is synthesized will inform glycoengineering, vaccine development, and antimicrobial discovery.

Genomic and molecular evidence reveals novel pathways associated with cell surface polysaccharides in bacteria

Amino acid (acyl carrier protein) ligases (AALs) are a relatively new family of bacterial amino acid adenylating enzymes with unknown function(s). Here, genomic enzymology tools that employ sequence similarity networks and genome context analyses were used to hypothesize the metabolic function(s) of AALs. In over 50% of species, aal and its cognate acyl carrier protein (acp) genes, along with three more genes, formed a highly conserved AAL cassette. AAL cassettes were strongly associated with surface polysaccharide gene clusters in Proteobacteria and Actinobacteria, yet were prevalent among soil and rhizosphere-associated α- and β-Proteobacteria, including symbiotic α- and β-rhizobia and some Mycolata. Based on these associations, AAL cassettes were proposed to encode a noncanonical Acp-dependent polysaccharide modification route. Genomic-inferred predictions were substantiated by published experimental evidence, revealing a role for AAL cassettes in biosynthesis of biofilm-forming exopolysaccharide in pathogenic Burkholderia and expression of aal and acp genes in nitrogen-fixing Rhizobium bacteroids. Aal and acp genes were associated with dltBD-like homologs that modify cell wall teichoic acids with d-alanine, including in Paenibacillus and certain other bacteria. Characterization of pathways that involve AAL and Acp may lead to developing new plant and human disease-controlling agents as well as strains with improved nitrogen fixation capacity.

Bacterial Extracellular Polymers: A Review

Prokaryotic microbial cells especially bacteria are highly emphases for their exopolysaccharides (EPS) production. EPS are the higher molecular weight natural extracellular compounds observe at the surface of the bacterial cells. Nowadays bacterial EPS represent rapidly emerging as new and industrially important biomaterials because it having tremendous physical and chemical properties with novel functionality. Due to its industrial demand as well as research studies the different extraction processes have been discovered to remove the EPS from the microbial biofilm. The novelties of EPS are also based on the microbial habitat conditions such as higher temperature, lower temperature, acidic, alkaliphilic, saline, etc. Based on its chemical structure they can be homopolysaccharide or heteropolysaccharide. EPSs have a wide range of applications in various industries such as food, textile, pharmaceutical, heavy metal recovery, agriculture, etc. So, this review focus on the understanding of the structure, different extraction processes, biosynthesis and genetic engineering of EPS as well as their desirable biotechnological applications.

A rationally designed oral vaccine induces immunoglobulin A in the murine gut that directs the evolution of attenuated Salmonella variants

The ability of gut bacterial pathogens to escape immunity by antigenic variation-particularly via changes to surface-exposed antigens-is a major barrier to immune clearance¹. However, not all variants are equally fit in all environments^2,3. It should therefore be possible to exploit such immune escape mechanisms to direct an evolutionary trade-off. Here, we demonstrate this phenomenon using Salmonella enterica subspecies enterica serovar Typhimurium (S.Tm). A dominant surface antigen of S.Tm is its O-antigen: a long, repetitive glycan that can be rapidly varied by mutations in biosynthetic pathways or by phase variation^4,5. We quantified the selective advantage of O-antigen variants in the presence and absence of O-antigen-specific immunoglobulin A and identified a set of evolutionary trajectories allowing immune escape without an associated fitness cost in naive mice. Through the use of rationally designed oral vaccines, we induced immunoglobulin A responses blocking all of these trajectories. This selected for Salmonella mutants carrying deletions of the O-antigen polymerase gene wzyB. Due to their short O-antigen, these evolved mutants were more susceptible to environmental stressors (detergents or complement) and predation (bacteriophages) and were impaired in gut colonization and virulence in mice. Therefore, a rationally induced cocktail of intestinal antibodies can direct an evolutionary trade-off in S.Tm. This lays the foundations for the exploration of mucosal vaccines capable of setting evolutionary traps as a prophylactic strategy.

Survival strategies of ammonia-oxidizing archaea (AOA) in a full-scale WWTP treating mixed landfill leachate containing copper ions and operating at low-intensity of aeration

Recent studies indicate that ammonia-oxidizing archaea (AOA) may play an important role in nitrogen removal by wastewater treatment plants (WWTPs). However, our knowledge of the mechanisms employed by AOA for growth and survival in full-scale WWTPs is still limited. Here, metagenomic and metatranscriptomic analyses combined with a laboratory cultivation experiment revealed that three active AOAs (WS9, WS192, and WS208) belonging to family Nitrososphaeraceae were active in the deep oxidation ditch (DOD) of a full-scale WWTP treating landfill leachate, which is configured with three continuous aerobic-anoxic (OA) modules with low-intensity aeration (≤ 1.5 mg/L). AOA coexisted with AOB and complete ammonia oxidizers (Comammox), while the ammonia-oxidizing microbial (AOM) community was unexpectedly dominated by the novel AOA strain WS9. The low aeration, long retention time, and relatively high inputs of ammonium and copper might be responsible for the survival of AOA over AOB and Comammox, while the dominance of WS9, specifically may be enhanced by substrate preference and uniquely encoded retention strategies. The urease-negative WS9 is specifically adapted for ammonia acquisition as evidenced by the high expression of an ammonium transporter, whereas two metabolically versatile urease-positive AOA strains (WS192 and WS208) can likely supplement ammonia needs with urea. This study provides important information for the survival and application of the eutrophic Nitrososphaeraceae AOA and advances our understanding of archaea-dominated ammonia oxidation in a full-scale wastewater treatment system.

Enterobacterial Common Antigen: Synthesis and Function of an Enigmatic Molecule

The outer membrane (OM) of Gram-negative bacteria poses a barrier to antibiotic entry due to its high impermeability. Thus, there is an urgent need to study the function and biogenesis of the OM. In Enterobacterales, an order of bacteria with many pathogenic members, one of the components of the OM is enterobacterial common antigen (ECA). We have known of the presence of ECA on the cell surface of Enterobacterales for many years, but its properties have only more recently begun to be unraveled. ECA is a carbohydrate antigen built of repeating units of three amino sugars, the structure of which is conserved throughout Enterobacterales. There are three forms of ECA, two of which (ECAPG and ECALPS) are located on the cell surface, while one (ECACYC) is located in the periplasm. Awareness of the importance of ECA has increased due to studies of its function that show it plays a vital role in bacterial physiology and interaction with the environment. Here, we review the discovery of ECA, the pathways for the biosynthesis of ECA, and the interactions of its various forms. In addition, we consider the role of ECA in the host immune response, as well as its potential roles in host-pathogen interaction. Furthermore, we explore recent work that offers insights into the cellular function of ECA. This review provides a glimpse of the biological significance of this enigmatic molecule.

Assembly of Bacterial Capsular Polysaccharides and Exopolysaccharides

Polysaccharides are dominant features of most bacterial surfaces and are displayed in different formats. Many bacteria produce abundant long-chain capsular polysaccharides, which can maintain a strong association and form a capsule structure enveloping the cell and/or take the form of exopolysaccharides that are mostly secreted into the immediate environment. These polymers afford the producing bacteria protection from a wide range of physical, chemical, and biological stresses, support biofilms, and play critical roles in interactions between bacteria and their immediate environments. Their biological and physical properties also drive a variety of industrial and biomedical applications. Despite the immense variation in capsular polysaccharide and exopolysaccharide structures, patterns are evident in strategies used for their assembly and export. This review describes recent advances in understanding those strategies, based on a wealth of biochemical investigations of select prototypes, supported by complementary insight from expanding structural biology initiatives. This provides a framework to identify and distinguish new systems emanating from genomic studies.

Modulation of bacterial multicellularity via spatio-specific polysaccharide secretion

The development of multicellularity is a key evolutionary transition allowing for differentiation of physiological functions across a cell population that confers survival benefits; among unicellular bacteria, this can lead to complex developmental behaviors and the formation of higher-order community structures. Herein, we demonstrate that in the social δ-proteobacterium Myxococcus xanthus, the secretion of a novel biosurfactant polysaccharide (BPS) is spatially modulated within communities, mediating swarm migration as well as the formation of multicellular swarm biofilms and fruiting bodies. BPS is a type IV pilus (T4P)-inhibited acidic polymer built of randomly acetylated β-linked tetrasaccharide repeats. Both BPS and exopolysaccharide (EPS) are produced by dedicated Wzx/Wzy-dependent polysaccharide-assembly pathways distinct from that responsible for spore-coat assembly. While EPS is preferentially produced at the lower-density swarm periphery, BPS production is favored in the higher-density swarm interior; this is consistent with the former being known to stimulate T4P retraction needed for community expansion and a function for the latter in promoting initial cell dispersal. Together, these data reveal the central role of secreted polysaccharides in the intricate behaviors coordinating bacterial multicellularity.

A study of the social bacterium Myxococcus xanthus reveals that the bacteria preferentially secrete specific polysaccharides within distinct zones of a swarm to facilitate spreading across a surface.

Lipopolysaccharide O-antigens-bacterial glycans made to measure

Lipopolysaccharides are critical components of bacterial outer membranes. The more conserved lipid A part of the lipopolysaccharide molecule is a major element in the permeability barrier imposed by the outer membrane and offers a pathogen-associated molecular pattern recognized by innate immune systems. In contrast, the long-chain O-antigen polysaccharide (O-PS) shows remarkable structural diversity and fulfills a range of functions, depending on bacterial lifestyles. O-PS production is vital for the success of clinically important Gram-negative pathogens. The biological properties and functions of O-PSs are mostly independent of specific structures, but the size distribution of O-PS chains is particularly important in many contexts. Despite the vast O-PS chemical diversity, most are produced in bacterial cells by two assembly strategies, and the different mechanisms employed in these pathways to regulate chain-length distribution are emerging. Here, we review our current understanding of the mechanisms involved in regulating O-PS chain-length distribution and discuss their impact on microbial cell biology.

A Diverse Repertoire of Exopolysaccharide Biosynthesis Gene Clusters in Lactobacillus Revealed by Comparative Analysis in 106 Sequenced Genomes

Production of exopolysaccharides (EPS) is one of the unique features of Lactobacillus genus. EPS not only have many physiological roles such as in stress tolerance, quorum sensing and biofilm formation, but also have numerous applications in the food and pharmaceutical industries. In this study, we identified and compared EPS biosynthesis gene clusters in 106 sequenced Lactobacillus genomes representing 27 species. Of the 146 identified clusters, only 41 showed the typical generic organization of genes as reported earlier. Hierarchical clustering showed highly varied nature of the clusters in terms of the gene composition; nonetheless, habitat-wise grouping was observed for the gene clusters from host-adapted and nomadic strains. Of the core genes required for EPS biosynthesis, epsA, B, C, D and E showed higher conservation, whereas gt, wzx and wzy showed high variability in terms of the number and composition of the protein families. Analysis of the distribution pattern of the protein families indicated a higher proportion of mutually exclusive families in clusters from host-adapted and nomadic strains, whereas those from the free-living group had very few unique families. Taken together, this analysis highlights high variability in the EPS gene clusters amongst Lactobacillus with some of their properties correlated to the habitats.

Function and Biogenesis of Lipopolysaccharides

The cell envelope is the first line of defense between a bacterium and the world-at-large. Often, the initial steps that determine the outcome of chemical warfare, bacteriophage infections, and battles with other bacteria or the immune system greatly depend on the structure and composition of the bacterial cell surface. One of the most studied bacterial surface molecules is the glycolipid known as lipopolysaccharide (LPS), which is produced by most Gram-negative bacteria. Much of the initial attention LPS received in the early 1900s was owed to its ability to stimulate the immune system, for which the glycolipid was commonly known as endotoxin. It was later discovered that LPS also creates a permeability barrier at the cell surface and is a main contributor to the innate resistance that Gram-negative bacteria display against many antimicrobials. Not surprisingly, these important properties of LPS have driven a vast and still prolific body of literature for more than a hundred years. LPS research has also led to pioneering studies in bacterial envelope biogenesis and physiology, mostly using Escherichia coli and Salmonella as model systems. In this review, we will focus on the fundamental knowledge we have gained from studies of the complex structure of the LPS molecule and the biochemical pathways for its synthesis, as well as the transport of LPS across the bacterial envelope and its assembly at the cell surface.

Current Progress in the Structural and Biochemical Characterization of Proteins Involved in the Assembly of Lipopolysaccharide

The lipid component of the outer leaflet of the outer membrane of Gram-negative bacteria is primarily composed of the glycolipid lipopolysaccharide (LPS), which serves to form a protective barrier against hydrophobic toxins and many antibiotics. LPS is comprised of three regions: the lipid A membrane anchor, the nonrepeating core oligosaccharide, and the repeating O-antigen polysaccharide. The lipid A portion is also referred to as endotoxin as its overstimulation of the toll-like receptor 4 during systemic infection precipitates potentially fatal septic shock. Because of the importance of LPS for the viability and virulence of human pathogens, understanding how LPS is synthesized and transported to the outer leaflet of the outer membrane is important for developing novel antibiotics to combat resistant Gram-negative strains. The following review describes the current state of our understanding of the proteins responsible for the synthesis and transport of LPS with an emphasis on the contribution of protein structures to our understanding of their functions. Because the lipid A portion of LPS is relatively well conserved, a detailed description of the biosynthetic enzymes in the Raetz pathway of lipid A synthesis is provided. Conversely, less well-conserved biosynthetic enzymes later in LPS synthesis are described primarily to demonstrate conserved principles of LPS synthesis. Finally, the conserved LPS transport systems are described in detail.

FtsW activity and lipid II synthesis are required for recruitment of MurJ to midcell during cell division in Escherichia coli

Peptidoglycan (PG) is the unique cell shape-determining component of the bacterial envelope, and is a key target for antibiotics. PG synthesis requires the transmembrane movement of the precursor lipid II, and MurJ has been shown to provide this activity in Escherichia coli. However, how MurJ functions in vivo has not been reported. Here we show that MurJ localizes both in the lateral membrane and at midcell, and is recruited to midcell simultaneously with late-localizing divisome proteins and proteins MraY and MurG. MurJ septal localization is dependent on the presence of a complete and active divisome, lipid II synthesis and PBP3/FtsW activities. Inactivation of MurJ, either directly by mutation or through binding with MTSES, did not affect the midcell localization of MurJ. Our study visualizes MurJ localization in vivo and reveals a possible mechanism of MurJ recruitment during cell division.

Loss of specificity variants of WzxC suggest that substrate recognition is coupled with transporter opening in MOP-family flippases

Bacteria produce a variety of surface-exposed polysaccharides important for cell integrity, biofilm formation and evasion of the host immune response. Synthesis of these polymers often involves the assembly of monomer oligosaccharide units on the lipid carrier undecaprenyl-phosphate at the inner face of the cytoplasmic membrane. For many polymers, including cell wall peptidoglycan, the lipid-linked precursors must be transported across the membrane by flippases to facilitate polymerization at the membrane surface. Flippase activity for this class of polysaccharides is most often attributed to MOP (Multidrug/Oligosaccharidyl-lipid/Polysaccharide) family proteins. Little is known about how this ubiquitous class of transporters identifies and translocates its cognate precursor over the many different types of lipid-linked oligosaccharides produced by a given bacterial cell. To investigate the specificity determinants of MOP proteins, we selected for variants of the WzxC flippase involved in Escherichia coli capsule (colanic acid) synthesis that can substitute for the essential MurJ MOP-family protein and promote transport of cell wall peptidoglycan precursors. Variants with substitutions predicted to destabilize the inward-open conformation of WzxC lost substrate specificity and supported both capsule and peptidoglycan synthesis. Our results thus suggest that specific substrate recognition by a MOP transporter normally destabilizes the inward-open state, promoting transition to the outward-open conformation and concomitant substrate translocation. Furthermore, the ability of WzxC variants to suppress MurJ inactivation provides strong support for the designation of MurJ as the flippase for peptidoglycan precursors, the identity of which has been controversial.

Wzx flippases exhibiting complex O-unit preferences require a new model for Wzx-substrate interactions

The Wzx flippase is a critical component of the O‐antigen biosynthesis pathway, being responsible for the translocation of oligosaccharide O units across the inner membrane in Gram‐negative bacteria. Recent studies have shown that Wzx has a strong preference for its cognate O unit, but the types of O‐unit structural variance that a given Wzx can accommodate are poorly understood. In this study, we identified two Yersinia pseudotuberculosis Wzx that can distinguish between different terminal dideoxyhexose sugars on a common O‐unit main‐chain, despite both being able to translocate several other structurally‐divergent O units. We also identified other Y. pseudotuberculosis Wzx that can translocate a structurally divergent foreign O unit with high efficiency, and thus exhibit an apparently relaxed substrate preference. It now appears that Wzx substrate preference is more complex than previously suggested, and that not all O‐unit residues are equally important determinants of translocation efficiency. We propose a new “Structure‐Specific Triggering” model in which Wzx translocation proceeds at a low level for a wide variety of substrates, with high‐frequency translocation only being triggered by Wzx interacting with one or more preferred O‐unit structural elements found on its cognate O unit(s).

Recent insights in microbial exopolysaccharide biosynthesis and engineering strategies

The distinct biosynthesis pathways for microbial exopolysaccharide production provide different engineering strategies to tailor the chemical structures of the final polymers. This review focuses on the latest insights in the various pathways and identifies bottlenecks as well as promising targets for tailoring microbial polysaccharide production. The main engineering strategies includes the combinatorial assembly of glycosyltransferases and engineering of the Wzx and Wzy proteins for flipping of repeating units as well as polymerization. In the case of synthase based polysaccharides, the use of epimerases or engineering approaches of the synthase itself as well as overexpression of c-di-GMP levels is identified as one of the most promising strategies. For sucrase-based biosynthesis, the in vitro production by engineered sucrase enzymes or adjusted production conditions is shown as a very promising method.