The Effect of Lipopolysaccharides on the Electrostatic Properties of Gram-Negative General Porins from Enterobacteriaceae

We investigated, by using all-atom molecular dynamics simulations, the effect of the outer membrane of Gram-negative bacteria, composed in the outer leaflet by polar/charged lipopolysaccharides (LPS), on the electrostatic properties of general porins from the Enterobacteriaceae family. General porins constitute the main path for the facilitated diffusion of polar antibiotics through the outer membrane. As model system we selected OmpK36 from Klebsiella pneumoniae, the ortholog of OmpC from Escherichia coli. This species presents high variability of amino acid composition of porins, with the effect to increase its resistance to the penetration of antibiotics. The various properties we analyzed seem to indicate that LPS acts as an independent layer without affecting the internal electrostatic properties of OmpK36. The only apparent effect on the microsecond time scale we sampled is the appearance of calcium ions, when present at moderate concentration in solution, inside the pore. However, we noticed increased fluctuations of the polarization density and only minor changes on its average value.

Comparative effects of different bacterial lipopolysaccharides on modulation of immune levels to improve survival of the black tiger shrimp

To prevent loss from disease, immunostimulants have been used as dietary supplements to improve immunity and survival of shrimps. Among the various types of immunostimulants, there is increasing evidence that a diet enriched with bacterial lipopolysaccharide can reduce the mortality rate of shrimp under exposure to pathogens. Here, the immunostimulatory effects of bacterial lipopolysaccharide (LPS) from various bacterial sources were explored. Bacterial LPS was extracted from a shrimp pathogen, Vibrio harveyi and its effects were compared with the commercially available LPS from the non-shrimp pathogen, Escherichia coli. Our results revealed that the LPS from V. harveyi was different in molecular size but contained similar functional groups to that from E. coli. To understand their molecular mechanisms, bacterial LPS from the two sources were applied as a supplementary diet and fed to juvenile shrimp for 4-week feeding period before tissue samples were collected for transcriptomic analysis by next generation sequencing. Gene expression profiling revealed that major immune-related genes such as pattern recognition proteins (PRPs), proteinases and proteinase inhibitors, prophenoloxidase systems (proPO system), antimicrobial peptides (AMPs), signaling transduction pathways, heat shock proteins (HSPs), oxidative stress responses, and other immune-related molecules such as mucins and peritrophins were modulated in the groups of shrimp fed with bacterial LPS from both sources, but at different levels. The results suggest that bacterial LPS could modulate shrimp immune system, and different LPS sources led to different activation of immune pathways. Additionally, metabolic-related genes were affected by LPS, suggesting that energy was required for immune stimulation. In the V. harveyi pathogen challenge trial, all shrimp groups fed with diets containing LPS from both bacterial sources showed better survival than the control group without LPS. When comparing groups fed with LPS supplemented diets, the higher concentration of LPS (8 μg/body weight) from E. coli resulted in a better survival rate than a lower concentration (4 μg/body weight). Conversely, shrimp fed with a diet containing LPS from V. harveyi showed a lower survival rate when a higher dose of LPS (8 μg/body weight) was administered than the group fed with a lower concentration of LPS (4 μg/body weight). This could be due to overstimulation of shrimp immune responses, especially by LPS derived from shrimp pathogens, resulting in a reverse effect. These results confirm that immunity in shrimp upon administration of bacterial LPS depends on the origin and dose of the LPS administered.

Identification and prioritization of potential therapeutic molecules against LpxA from Acinetobacter baumannii - A computational study

A. baumannii is a ubiquitously found gram-negative, multi-drug resistant bacterial species from the ESKAPE family of pathogens known to be the causative agent for hospital-acquired infections such as pneumonia, meningitis, endocarditis, septicaemia and urinary tract infections. A. baumannii is implicated as a contributor to bloodstream infections in approximately 2% of all worldwide infections. Hence, exploring novel therapeutic agents against the bacterium is essential. LpxA or UDP-N-acetylglucosamine acetyltransferase is an essential enzyme important in Lipid A biosynthesis which catalyses the reversible transfer of an acetyl group on the glucosamine 3-OH of the UDP-GlcNAc which is a crucial step in the biosynthesis of the protective Lipopolysaccharides (LPS) layer of the bacteria which upon disruption can lead to the elimination of the bacterium which delineates LpxA as an appreciable drug target from A. baumannii. The present study performs high throughput virtual screening of LpxA against the enamine-HTSC-large-molecule library and performs toxicity and ADME screening to identify the three promising lead molecules subjected to molecular dynamics simulations. Global and essential dynamics analysis of LpxA and its complexes along with FEL and MM/PBSA based binding free energy delineate Z367461724 and Z219244584 as potential inhibitors against LpxA from A. baumannii.

Engineered bacterial membrane vesicles are promising carriers for vaccine design and tumor immunotherapy

Bacterial membrane vesicles (BMVs) have emerged as novel and promising platforms for the development of vaccines and immunotherapeutic strategies against infectious and noninfectious diseases. The rich microbe-associated molecular patterns (MAMPs) and nanoscale membrane vesicle structure of BMVs make them highly immunogenic. In addition, BMVs can be endowed with more functions via genetic and chemical modifications. This article reviews the immunological characteristics and effects of BMVs, techniques for BMV production and modification, and the applications of BMVs as vaccines or vaccine carriers. In summary, given their versatile characteristics and immunomodulatory properties, BMVs can be used for clinical vaccine or immunotherapy applications.

Induced fusion and aggregation of bacterial outer membrane vesicles: Experimental and theoretical analysis

Recombinantly engineered bacterial outer membrane vesicles (OMVs) are promising vaccine delivery vehicles. The diversity of exogenous antigens delivered by OMVs can be enhanced by induced fusion of OMV populations. To date there are no reports of induced fusion of bacterial OMVs. Here we measure the pH and salt-induced aggregation and fusion of OMVs and analyze the processes against the Derjaguin-Landau-Verwey-Overbeek (DLVO) colloidal stability model. Vesicle aggregation and fusion kinetics were investigated for OMVs isolated from native E. coli (Nissle 1917) and lipopolysaccharide (LPS) modified E. coli (ClearColi) strains to evaluate the effect of lipid type on vesicle aggregation and fusion. Electrolytes and low pHs induced OMV aggregation for both native and modified LPS constructs, approaching a calculated fusion efficiency of ~25% (i.e. ~1/4 of collision events lead to fusion). However, high fusion efficiency was achieved for Nissle OMVs solely with decreased pH as opposed to a combination of low pH and increased divalent counterion concentration for ClearColi OMVs. The lipid composition of the OMVs from Nissle negatively impacted fusion in the presence of electrolytes, causing higher deviations from DLVO-predicted critical coagulation concentrations with monovalent counterions. The outcome of the work is a defined set of conditions under which investigators can induce OMVs to fuse and make various combinations of vesicle compositions.

Phage Display Detection of Mimotopes that Are Shared Epitopes of Clinically and Epidemiologically Relevant Enterobacteria

Background: Escherichia coli and Salmonella are etiologic agents of intestinal infections. A previous study showed the presence of shared epitopes between lipopolysaccharides (LPSs) of E. coli O157 and Salmonella. Aim: Using phage display, the aim of this study is to identify mimotopes of shared epitopes in different enterobacterial LPSs. Methods: We use anti-LPS IgG from E. coli O157 and Salmonella to select peptide mimotopes of the M13 phage. The amino acid sequence of the mimotopes is used to synthesize peptides, which are in turn used to immunize rabbits. The antibody response of the resulting sera against the LPSs and synthetic peptides (SPs) is analyzed by ELISA and by Western blot assays, indicating that LPS sites are recognized by the same antibody. In a complementary test, the reactions of human serum samples obtained from the general population against the SPs and LPSs are also analyzed. Results: From the last biopanning phase, sixty phagotopes are selected. The analysis of the peptide mimotope amino acid sequences shows that in 4 of them the S/N/A/PF motif is a common sequence. Antibodies from the sera of immunized rabbits with SP287/3, SP459/1, SP308/3, and SP073/14 react against both their own peptide and the different LPSs. The Western blot test shows a sera reaction against both the lateral chains and the cores of the LPSs. The analysis of the human sera shows a response against the SPs and LPSs. Conclusion: The designed synthetic peptides are mimotopes of LPS epitopes of Salmonella and E. coli that possess immunogenic capacity. These mimotopes could be considered for use in the design of vaccines against both enterobacteria.

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.

Phages for Phage Therapy: Isolation, Characterization, and Host Range Breadth

For a bacteriophage to be useful for phage therapy it must be both isolated from the environment and shown to have certain characteristics beyond just killing strains of the target bacterial pathogen. These include desirable characteristics such as a relatively broad host range and a lack of other characteristics such as carrying toxin genes and the ability to form a lysogen. While phages are commonly isolated first and subsequently characterized, it is possible to alter isolation procedures to bias the isolation toward phages with desirable characteristics. Some of these variations are regularly used by some groups while others have only been shown in a few publications. In this review I will describe (1) isolation procedures and variations that are designed to isolate phages with broader host ranges, (2) characterization procedures used to show that a phage may have utility in phage therapy, including some of the limits of such characterization, and (3) results of a survey and discussion with phage researchers in industry and academia on the practice of characterization of phages.

Abundant production of exopolysaccharide by EAEC strains enhances the formation of bacterial biofilms in contaminated sprouts

Enteroaggregative E. coli (EAEC) is associated with food-borne outbreaks of diarrhea and growth faltering among children in developing countries. A Shiga toxin-producing EAEC strain of serotype O104:H4 strain caused one of the largest outbreaks of a food-borne infection in Europe in 2011. The outbreak was traced to contaminated fenugreek sprouts, yet the mechanisms whereby such persistent contamination of sprouts could have occurred are not clear. We found that under ambient conditions of temperature and in minimal media, pathogenic Shiga toxin-producing EAEC O104:H4 227-11 and non-Shiga toxin-producing 042 strains both produce high levels of exopolysaccharide structures (EPS) that are released to the external milieu. The exopolysaccharide was identified as colanic acid (CA). Unexpectedly, Shiga-toxin producing EAEC strain 227-11 produced 3-6-fold higher levels of CA than the 042 strain, suggesting differential regulation of the CA in the two strains. The presence of CA was accompanied by the formation of large biofilm structures on the surface of sprouts. The wcaF-wza chromosomal locus was required for the synthesis of CA in EAEC 042. Deletion in the glycosyltransferase wcaE gene abolished the production of CA in 042, and resulted in diminished adherence to sprouts when co-cultured at ambient temperature. In conclusion, this work suggests that copious production of CA may contribute to persistence of EAEC in the environment and suggests a potential explanation for the large Shiga toxin-producing EAEC outbreak in 2011.

Genetics and evolution of Yersinia pseudotuberculosis O-specific polysaccharides: a novel pattern of O-antigen diversity

O-antigen polysaccharide is a major immunogenic feature of the lipopolysaccharide of Gram-negative bacteria, and most species produce a large variety of forms that differ substantially from one another. There are 18 known O-antigen forms in the Yersinia pseudotuberculosis complex, which are typical in being composed of multiple copies of a short oligosaccharide called an O unit. The O-antigen gene clusters are located between the hemH and gsk genes, and are atypical as 15 of them are closely related, each having one of five downstream gene modules for alternative main-chain synthesis, and one of seven upstream modules for alternative side-branch sugar synthesis. As a result, many of the genes are in more than one gene cluster. The gene order in each module is such that, in general, the earlier a gene product functions in O-unit synthesis, the closer the gene is to the 5΄ end for side-branch modules or the 3΄ end for main-chain modules. We propose a model whereby natural selection could generate the observed pattern in gene order, a pattern that has also been observed in other species.

Outer membrane vesicles derived from Salmonella Typhimurium mutants with truncated LPS induce cross-protective immune responses against infection of Salmonella enterica serovars in the mouse model

Salmonella enterica cause diarrheal and systemic diseases and are of considerable concern worldwide. Vaccines that are cross-protective against multiple serovars could provide effective control of Salmonella-mediated diseases. Bacteria-derived outer membrane vesicles (OMVs) are highly immunogenic and are capable of eliciting protective immune responses. Alterations in lipopolysaccharide (LPS) length can result in outer membrane remodeling and composition of outer membrane proteins (OMPs) changing. In this study, we investigated the impact of truncated LPS on both the production and immunogenicity of Salmonella OMVs, including the ability of OMVs to elicit cross-protection against challenge by heterologous Salmonella strains. We found that mutations in waaJ and rfbP enhanced vesiculation, while mutations in waaC, waaF and waaG inhibited this process. Animal experiments indicated that OMVs from waaC, rfaH and rfbP mutants induced stronger serum immune responses compared to OMVs from the parent strain, while all elicited protective responses against the wild-type S. Typhimurium challenge. Furthermore, intranasal or intraperitoneal immunization with OMVs derived from the waaC and rfbP mutants elicited significantly higher cross-reactive IgG responses and provided enhanced cross-protection against S. Choleraesuis and S. Enteritidis challenge than the wild-type OMVs. These results indicate that truncated-LPS OMVs are capable of conferring cross protection against multiple serotypes of Salmonella infection.

Immunogenicity and Cross-Protective Efficacy Induced by Outer Membrane Proteins from Salmonella Typhimurium Mutants with Truncated LPS in Mice

Lipopolysaccharide (LPS) is a major virulence factor present in the outer membrane of Salmonella enterica serovar Typhimurium (S. Typhimurium). Outer membrane proteins (OMPs) from Salmonella show high immunogenicity and provide protection against Salmonella infection, and truncated LPS alters the outer membrane composition of the cell wall. In our previous study, we demonstrated that Salmonella mutants carrying truncated LPS failed to induce strong immune responses and cross-reaction to other enteric bacteria, due to their high attenuation and low colonization in the host. Therefore, we plan to investigate whether outer membrane proteins from Salmonella mutants with truncated LPS resulting from a series of nonpolar mutations, including ∆waaC12, ∆waaF15, ∆waaG42, ∆rfaH49, ∆waaI43, ∆waaJ44, ∆waaL46, ∆wbaP45 and ∆wzy-48, affect immunogenicity and provide protection against diverse Salmonella challenge. In this study, the immunogenicity and cross-protection efficiency of purified OMPs from all mutants were investigated to explore a potential OMP vaccine to protect against homologous or heterologous serotype Salmonella challenge. The results demonstrated that OMPs from three Salmonella mutants (∆waaC12, ∆waaJ44 and ∆waaL46) induced higher immune responses and provided good protection against homologous S. Typhimurium. The OMPs from these three mutants were also selected to determine the cross-protective efficacy against homologous and heterologous serotype Salmonella. Our results indicated that the mutant ∆waaC12 can elicit higher cross-reactivity and can provide good protection against S. Choleraesuis and S. Enteritidis infection and that the cross-reactivity may be ascribed to an antigen of approximately 18.4–30 kDa.

Cryptococcus neoformans capsular polysaccharides form branched and complex filamentous networks viewed by high-resolution microscopy

Cryptococcus neoformans is a fungal pathogen that causes life-threatening infections in immunocompromised individuals. Its main virulence factor is an extracellular polysaccharide capsule whose structure, assembly and dynamics remain poorly understood. In this study, we apply improved protocols for sample preparation and recently-developed scanning microscopy techniques to visualize the ultrastructure of the C. neoformans capsule at high-resolution (up to 1 nm) and improved structural preservation. Although most capsule structures in nature consist of linear polymers, we show here that the C. neoformans capsule is a 'microgel-like' structure composed of branched polysaccharides. Moreover, we imaged the capsule-to-cell wall link, which is formed by thin fibers that branch out of thicker capsule filaments, and have one end firmly embedded in the cell wall structure. Together, our findings provide compelling ultrastructural evidence for a branched and complex capsule conformation, which may have important implications for the biological activity of the capsule as a virulence factor.

The sweet tooth of bacteria: common themes in bacterial glycoconjugates

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

Biosynthesis and function of extracellular glycans in cyanobacteria

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

Modulation of Caenorhabditis elegans immune response and modification of Shigella endotoxin upon interaction

To analyze the pathogenesis at both physiological and molecular level using the model organism, Caenorhabditis elegans at different developmental stages in response to Shigella spp. and its pathogen associated molecular patterns such as lipopolysaccharide. The solid plate and liquid culture-based infection assays revealed that Shigella spp. infects C. elegans and had an impact on the brood size and pharyngeal pumping rate. LPS of Shigella spp. was toxic to C. elegans. qPCR analysis revealed that host innate immune genes have been modulated upon Shigella spp. infections and its LPS challenges. Non-destructive analysis was performed to kinetically assess the alterations in LPS during interaction of Shigella spp. with C. elegans. The modulation of innate immune genes attributed the surrendering of host immune system to Shigella spp. by favoring the infection. LPS appeared to have a major role in Shigella-mediated pathogenesis and Shigella employs a tactic behavior of modifying its LPS content to escape from the recognition of host immune system.

Synthesis and immunogenicity evaluation of Salmonella enterica serovar Paratyphi A O-specific polysaccharide conjugated to diphtheria toxoid

Salmonella enterica serovar Paratyphi A (S. Paratyphi A) is a human restricted pathogen that can cause systemic infection (paratyphoid fever) with recently increased incidence particularly in developing countries. Currently there is no licensed vaccine for prevention of infection from S. Paratyphi A. In this study the O-specific polysaccharide (OSP) of S. Paratyphi A was conjugated to diphtheria toxoid (DT) with and without adipic acid dihydrazide (ADH) as a linker. Binding of the OSP to a carrier protein was intended to convert a T-cell independent OSP response to a T-cell dependent response inducing higher levels of anti-OSP antibodies and immunological memory. These conjugates (OSP-AH-DT and OSP-DT) were evaluated for their immunogenicity in mice. The S. Paratyphi A OSP-DT conjugate induced a poor anti-OSP response less than that observed with LPS while the OSP-AH-DT conjugate induced a significantly higher antibody titer compared with LPS alone. The study also demonstrated diphtheria toxoid as a potential carrier protein for conjugate vaccine candidates using S. Paratyphi A OSP.

Francisella tularensis Schu S4 lipopolysaccharide core sugar and O-antigen mutants are attenuated in a mouse model of tularemia

The virulence factors mediating Francisella pathogenesis are being investigated, with an emphasis on understanding how the organism evades innate immunity mechanisms. Francisella tularensis produces a lipopolysaccharide (LPS) that is essentially inert and a polysaccharide capsule that helps the organism to evade detection by components of innate immunity. Using an F. tularensis Schu S4 mutant library, we identified strains that are disrupted for capsule and O-antigen production. These serum-sensitive strains lack both capsule production and O-antigen laddering. Analysis of the predicted protein sequences for the disrupted genes (FTT1236 and FTT1238c) revealed similarity to those for waa (rfa) biosynthetic genes in other bacteria. Mass spectrometry further revealed that these proteins are involved in LPS core sugar biosynthesis and the ligation of O antigen to the LPS core sugars. The 50% lethal dose (LD50) values of these strains are increased 100- to 1,000-fold for mice. Histopathology revealed that the immune response to the F. tularensis mutant strains was significantly different from that observed with wild-type-infected mice. The lung tissue from mutant-infected mice had widespread necrotic debris, but the spleens lacked necrosis and displayed neutrophilia. In contrast, the lungs of wild-type-infected mice had nominal necrosis, but the spleens had widespread necrosis. These data indicate that murine death caused by wild-type strains occurs by a mechanism different from that by which the mutant strains kill mice. Mice immunized with these mutant strains displayed >10-fold protective effects against virulent type A F. tularensis challenge.

Structural analysis of O-polysaccharide chains extracted from different Salmonella Typhimurium strains

Salmonella Typhimurium is the major cause of invasive nontyphoidal Salmonella disease in Africa, with high mortality among children and HIV-infected individuals. Currently, no vaccine is available for use in humans. Antibodies directed against the O-polysaccharide of the lipopolysaccharide molecule of Salmonella mediate bacterial killing and are protective, and conjugation of the O-polysaccharide to a carrier protein represents a possible strategy for vaccine development. Here we have purified the O-polysaccharide from six different strains of S. Typhimurium and fully characterized them using analytical methods including HPLC-SEC, HPAEC-PAD, GC, GC-MS, 1D and 2D NMR spectroscopy. All the O-polysaccharide samples showed a similar bimodal molecular mass distribution, but differed with respect to the amount and position of O-acetylation and glucosylation. For some strains, O-acetyl groups were found not only on C-2 of abequose (factor 5 specificity), but also on C-2 and C-3 of rhamnose; glucose was found to be linked 1→4 or 1→6 to galactose in different amounts according to the strain of origin. This structural variability could have an impact on the immunogenicity of corresponding glycoconjugate vaccines and different strains need to be evaluated in order to identify the appropriate source of O-polysaccharide to use for the development of a candidate conjugate vaccine with broad coverage against S. Typhimurium.

Effect of deletion of genes involved in lipopolysaccharide core and O-antigen synthesis on virulence and immunogenicity of Salmonella enterica serovar typhimurium

Lipopolysaccharide (LPS) is a major virulence factor of Salmonella enterica serovar Typhimurium and is composed of lipid A, core oligosaccharide (C-OS), and O-antigen polysaccharide (O-PS). While the functions of the gene products involved in synthesis of core and O-antigen have been elucidated, the effect of removing O-antigen and core sugars on the virulence and immunogenicity of Salmonella enterica serovar Typhimurium has not been systematically studied. We introduced nonpolar, defined deletion mutations in waaG (rfaG), waaI (rfaI), rfaH, waaJ (rfaJ), wbaP (rfbP), waaL (rfaL), or wzy (rfc) into wild-type S. Typhimurium. The LPS structure was confirmed, and a number of in vitro and in vivo properties of each mutant were analyzed. All mutants were significantly attenuated compared to the wild-type parent when administered orally to BALB/c mice and were less invasive in host tissues. Strains with ΔwaaG and ΔwaaI mutations, in particular, were deficient in colonization of Peyer's patches and liver. This deficiency could be partially overcome in the ΔwaaI mutant when it was administered intranasally. In the context of an attenuated vaccine strain delivering the pneumococcal antigen PspA, all of the mutations tested resulted in reduced immune responses against PspA and Salmonella antigens. Our results indicate that nonreversible truncation of the outer core is not a viable option for developing a live oral Salmonella vaccine, while a wzy mutant that retains one O-antigen unit is adequate for stimulating the optimal protective immunity to homologous or heterologous antigens by oral, intranasal, or intraperitoneal routes of administration.

Salmonella-induced mucosal lectin RegIIIβ kills competing gut microbiota

Intestinal inflammation induces alterations of the gut microbiota and promotes overgrowth of the enteric pathogen Salmonella enterica by largely unknown mechanisms. Here, we identified a host factor involved in this process. Specifically, the C-type lectin RegIIIβ is strongly upregulated during mucosal infection and released into the gut lumen. In vitro, RegIIIβ kills diverse commensal gut bacteria but not Salmonella enterica subspecies I serovar Typhimurium (S. Typhimurium). Protection of the pathogen was attributable to its specific cell envelope structure. Co-infection experiments with an avirulent S. Typhimurium mutant and a RegIIIβ-sensitive commensal E. coli strain demonstrated that feeding of RegIIIβ was sufficient for suppressing commensals in the absence of all other changes inflicted by mucosal disease. These data suggest that RegIIIβ production by the host can promote S. Typhimurium infection by eliminating inhibitory gut microbiota.

Evidence that WapB is a 1,2-glucosyltransferase of Pseudomonas aeruginosa involved in Lipopolysaccharide outer core biosynthesis

Pseudomonas aeruginosa is an important opportunistic pathogen infecting debilitated individuals. One of the major virulence factors expressed by P. aeruginosa is lipopolysaccharide (LPS), which is composed of lipid A, core oligosaccharide (OS), and O-antigen polysaccharide. The core OS is divided into inner and outer regions. Although the structure of the outer core OS has been elucidated, the functions and mechanisms of the glycosyltransferases involved in core OS biogenesis are currently unknown. Here, we show that a previously uncharacterized gene, pa1014, is involved in outer core biosynthesis, and we propose to rename this gene wapB. We constructed a chromosomal mutant, wapB::Gm, in a PAO1 (O5 serotype) strain background. Characterization of the LPS from the mutant by Western immunoblotting showed a lack of reactivity to PAO1 outer core-specific monoclonal antibody (MAb) 5c-101. The chemical structure of the core OS of the wapB mutant was elucidated using nuclear magnetic resonance spectroscopy and mass spectrometry techniques and revealed that the core OS of the wapB mutant lacked the terminal β-1,2-linked-d-glucose residue. Complementation of the mutant with wapB in trans restored the core structure to one that is identical to that of the wild type. Eleven of the 20 P. aeruginosa International Antigenic Typing Scheme (IATS) serotypes produce LPSs that lack the terminal d-glucose residue (Glc(IV)). Interestingly, expressing wapB in each of these 11 serotypes modifies each of their outer core OS structures, which became reactive to MAb 5c-101 in Western immunoblotting, suggesting the presence of a terminal d-glucose in these core OS structures. Our results strongly suggested that wapB encodes a 1,2-glucosyltransferase.

Diversity of non-stoichiometric substitutions on the lipopolysaccharide of E. coli C demonstrated by electrospray ionization single quadrupole mass spectrometry

The lipopolysaccharide (LPS) of enterobacteria frequently contains various numbers of charged non-stoichiometric substituents such as phosphate (P) and ethanolamine (EtN) groups and a third residue of 3-deoxy-D-manno-2-octulosonic acid (KDO) on the R-core polysaccharide backbone. These substituents can modify the biological activities of LPS including varying the stability of the outer membrane, tolerance to cationic antibiotics, pathogenicity, and sensitivity to enterobacteria bacteriophages. These diverse substituents can be clearly detected in degraded samples of LPS from E. coli C using electrospray ionization single quadrupole mass spectrometry (ESI-Q-MS) from a 0.1 mg/mL solution in a 50:50 mixture of methanol and 10 mM ammonium acetate (pH 6.8). The O-deacylated derivative showed multiple peaks of [M-3H](3-) ions which corresponded to species having up to eight phosphates, two ethanolamines, and an additional KDO on the backbone of Hex(5) Hep(3) KDO(2) GlcN(2) C14:0(3-OH)(2). The major components of the O,N-deacylated derivative were the species associated with four and five phosphates on Hex(5) Hep(3) KDO(2) GlcN(2). The polysaccharide portion of LPS also revealed species which corresponded to Hex(5) Hep(3) KDO associated with two to four phosphates and an ethanolamine. The present method was proved to be useful to investigate the structural diversity of enterobacterial LPS.

Regulated delayed expression of rfaH in an attenuated Salmonella enterica serovar typhimurium vaccine enhances immunogenicity of outer membrane proteins and a heterologous antigen

RfaH is a transcriptional antiterminator that reduces the polarity of long operons encoding secreted and surface-associated cell components of Salmonella enterica serovar Typhimurium, including O antigen and lipopolysaccharide core sugars. A DeltarfaH mutant strain is attenuated in mice (50% lethal dose [LD(50)], >10(8) CFU). To examine the potential for using rfaH in conjunction with other attenuating mutations, we designed a series of strains in which we replaced the native rfaH promoter with the tightly regulated arabinose-dependent araC P(BAD) promoter so that rfaH expression was dependent on exogenously supplied arabinose provided during in vitro growth. Following colonization of host lymphoid tissues, where arabinose was not available, the P(BAD) promoter was no longer active and rfaH was not expressed. In the absence of RfaH, O antigen and core sugars were not synthesized. We constructed three mutant strains that expressed different levels of RfaH by altering the ribosome-binding sequence and start codon. One mutation, DeltaP(rfaH178), was introduced into the attenuated vaccine strain chi9241 (DeltapabA DeltapabB DeltaasdA) expressing the pneumococcal surface protein PspA from an Asd(+) balanced-lethal plasmid. Mice immunized with this strain and boosted 4 weeks later induced higher levels of serum immunoglobulin G specific for PspA and for outer membrane proteins from other enteric bacteria than either an isogenic DeltarfaH derivative or the isogenic RfaH(+) parent. Eight weeks after primary oral immunization, mice were challenged with 200 LD(50) of virulent Streptococcus pneumoniae WU2. Immunization with DeltaP(rfaH178) mutant strains led to increased levels of protection compared to that of the parent chi9241 and of a DeltarfaH derivative of chi9241.

The capsule of the fungal pathogen Cryptococcus neoformans

The capsule of the fungal pathogen Cryptococcus neoformans has been studied extensively in recent decades and a large body of information is now available to the scientific community. Well-known aspects of the capsule include its structure, antigenic properties and its function as a virulence factor. The capsule is composed primarily of two polysaccharides, glucuronoxylomannan (GXM) and galactoxylomannan (GalXM), in addition to a smaller proportion of mannoproteins (MPs). Most of the studies on the composition of the capsule have focused on GXM, which comprises more than 90% of the capsule's polysaccharide mass. It is GalXM, however, that is of particular scientific interest because of its immunological properties. The molecular structure of these polysaccharides is very complex and has not yet been fully elucidated. Both GXM and GalXM are high molecular mass polymers with the mass of GXM equaling roughly 10 times that of GalXM. Recent findings suggest, however, that the actual molecular weight might be different to what it has traditionally been thought to be. In addition to their structural roles in the polysaccharide capsule, these molecules have been associated with many deleterious effects on the immune response. Capsular components are therefore considered key virulence determinants in C. neoformans, which has motivated their use in vaccines and made them targets for monoclonal antibody treatments. In this review, we will provide an update on the current knowledge of the C. neoformans capsule, covering aspects related to its structure, synthesis and particularly, its role as a virulence factor.

Vesicular Trans-Cell Wall Transport in Fungi: A Mechanism for the Delivery of Virulence-Associated Macromolecules?

Fungal cells are encaged in rigid, complex cell walls. Until recently, there was remarkably little information regarding the trans-fungal cell wall transfer of intracellular macromolecules to the extracellular space. Recently, several studies have begun to elucidate the mechanisms that fungal cells utilize to secrete a wide variety of macromolecules through the cell wall. The combined use of transmission electron microscopy, serology, biochemistry, proteomics and lipidomics have revealed that the fungal pathogens Cryptococcus neoformans, Histoplasma capsulatum, Candida albicans, Candida parapsilosis and Sporothrix schenckii, as well as the model yeast Saccharomyces cerevisiae, each produces extracellular vesicles that carry lipids, proteins, polysaccharides and pigment-like structures of unquestionable biological significance. Compositional analysis of the C. neoformans and H. capsulatum extracellular vesicles suggests that they may function as 'virulence bags', with the potential to modulate the host-pathogen interaction in favor of the fungus. The cellular origin of the extracellular vesicles remains unknown, but morphological and biochemical features indicate that they are similar to the well-described mammalian exosomes.

Identification of a Wzy polymerase required for group IV capsular polysaccharide and lipopolysaccharide biosynthesis in Vibrio vulnificus

The estuarine bacterium Vibrio vulnificus is a human and animal pathogen. The expression of capsular polysaccharide (CPS) is essential for virulence. We used a new mini-Tn10 delivery vector, pNKTXI-SceI, to generate a mutant library and identify genes essential for CPS biosynthesis. Twenty-one acapsular mutants were isolated, and the disrupted gene in one mutant, coding for a polysaccharide polymerase (wzy), is described here. A wecA gene initiating glycosyltransferase was among the genes identified in the region flanking the wzy gene. This, together with the known structure of the CPS, supports a group IV capsule designation for the locus; however, its overall organization mirrored that of group I capsules. This new arrangement may be linked to our finding that the CPS region appears to have been recently acquired by horizontal transfer. Alcian Blue staining and immunoblotting with antisera against the wild-type strain indicated that the wzy::Tn10 mutant failed to produce CPS and was attenuated relative to the wild type in a septicemic mouse model. Interestingly, immunoblotting revealed that the mutant was also defective in lipopolysaccharide (LPS) production. However, the core-plus-one O-antigen pattern typical of wzy mutations was apparent. CPS production, LPS production, and virulence were restored following complementation with the wild-type wzy gene. Hence, Wzy participates in both CPS and LPS biosynthesis and is required for virulence in strain 27562. To our knowledge, this is the first functional demonstration of a Wzy polysaccharide polymerase in V. vulnificus and is the first to show a link between LPS and CPS biosynthesis.

Promethazine renders Escherichia coli susceptible to penicillin G: real-time measurement of bacterial susceptibility by fluoro-luminometry

Gram-negative bacteria are resistant to many hydrophobic antibiotics (such as penicillin G) owing to the highly hydrophilic saccharide part of lipopolysaccharide in the cell membrane, whilst most hydrophilic antibiotics (such as ampicillin) are more freely diffused into cells through aqueous porins. In this study, we investigated the possibility of altering the membrane permeability of Escherichia coli with tricyclic cationic compounds, such as the non-antibiotic drug promethazine. We also established the activity of these compounds as modifiers of antibiotic resistance in bacteria by a fluoro-luminometric approach. According to the results, promethazine has no bacteriostatic effect on E. coli at concentrations <64 microg/mL. However, promethazine at these concentrations in combination with penicillin G produced a significant synergistic activity against E. coli. Specifically, a constant promethazine concentration of 32 microg/mL in combination with penicillin G concentrations of 16-128 microg/mL suppressed the growth and viability of bacteria and converted penicillin G-resistant cells to being susceptible to this antibiotic with a minimum inhibitory concentration of 128 microg/mL. In contrast to penicillin G, the efficacy of ampicillin was apparently not increased in the presence of promethazine, suggesting that promethazine directly affects the membrane permeability of bacteria or alternatively inhibits the function of efflux pumps. In conclusion, we conclude that exposing E. coli cells to a given antibiotic in combination with promethazine can increase the susceptibility of bacteria; this effect is reliably assessed on a real-time basis using kinetic fluoro-luminometric measurements.

Molecular basis for substrate selectivity and specificity by an LPS biosynthetic enzyme

Diversity in the polysaccharide component of lipopolysaccharide (LPS) contributes to the persistence and pathogenesis of Gram-negative bacteria. The Nudix hydrolase GDP-mannose mannosyl hydrolase (Gmm) contributes to this diversity by regulating the concentration of mannose in LPS biosynthetic pathways. Here, we present seven high-resolution crystal structures of Gmm from the enteropathogenic E. coli strain O128: the structure of the apo enzyme, the cocrystal structure of Gmm bound to the product Mg2+-GDP, two cocrystal structures of precatalytic and turnover complexes of Gmm-Ca2+-GDP-alpha-d-mannose, and three cocrystal structures of an inactive mutant (His-124 --> Leu) Gmm bound to substrates GDP-alpha-d-mannose, GDP-alpha-d-glucose, and GDP-beta-l-fucose. These crystal structures help explain the molecular basis for substrate specificity and promiscuity and provide a structural framework for reconciling previously determined kinetic parameters. Unexpectedly, these structures reveal concerted changes in the enzyme structure that result in the formation of a catalytically competent active site only in the presence of the substrate/product. These structural views of the enzyme may provide a rationale for the design of inhibitors that target the biosynthesis of LPS by pathogenic bacteria.

Modification of Lipopolysaccharide with Colanic Acid (M-antigen) Repeats in Escherichia coli

Colanic acid (CA) or M-antigen is an exopolysaccharide produced by many enterobacteria, including the majority of Escherichia coli strains. Unlike other capsular polysaccharides, which have a close association with the bacterial surface, CA forms a loosely associated saccharide mesh that coats the bacteria, often within biofilms. Herein we show that a highly mucoid strain of E. coli K-12 ligates CA repeats to a significant proportion of lipopolysaccharide (LPS) core acceptor molecules, forming the novel LPS glycoform we call MLPS.MLPS biosynthesis is dependent upon (i) CA induction, (ii) LPS core biosynthesis, and (iii) the O-antigen ligase WaaL. Compositional analysis, mass spectrometry, and nuclear magnetic resonance spectroscopy of a purified MLPS sample confirmed the presence of a CA repeat unit identical in carbohydrate sequence, but differing at multiple positions in anomeric configuration and linkage, from published structures of extracellular CA. The attachment point was identified as O-7 of the L-glycero-D-manno-heptose of the outer LPS core, the same position used for O-antigen ligation. When O-antigen biosynthesis was restored in the K-12 background and grown under conditions meeting the above specifications, only MLPS was observed, suggesting E. coli can reversibly change its proximal covalently linked cell surface polysaccharide coat from O-antigen to CA in response to certain environmental stimuli. The identification of MLPS has implications for potential underlying mechanisms coordinating the synthesis of various surface polysaccharides.

The Pseudomonas aeruginosa lipid A deacylase: selection for expression and loss within the cystic fibrosis airway

Lipopolysaccharide (LPS) is the major surface component of gram-negative bacteria, and a component of LPS, lipid A, is recognized by the innate immune system through the Toll-like receptor 4/MD-2 complex. Pseudomonas aeruginosa, an environmental gram-negative bacterium that opportunistically infects the respiratory tracts of patients with cystic fibrosis (CF), can synthesize various structures of lipid A. Lipid A from P. aeruginosa strains isolated from infants with CF has a specific structure that includes the removal of the 3 position 3-OH C10 fatty acid. Here we demonstrate increased expression of the P. aeruginosa lipid A 3-O-deacylase (PagL) in isolates from CF infants compared to that in environmental isolates. PagL activity was increased in environmental isolates by growth in medium limited for magnesium and decreased by growth at low temperature in laboratory-adapted strains of P. aeruginosa. P. aeruginosa PagL was shown to be an outer membrane protein by isopycnic density gradient centrifugation. Heterologous expression of P. aeruginosa pagL in Salmonella enterica serovar Typhimurium and Escherichia coli resulted in removal of the 3-OH C14 fatty acid from lipid A, indicating that P. aeruginosa PagL recognizes either 3-OH C10 or 3-OH C14. Finally, deacylated lipid A species were not observed in some clinical P. aeruginosa isolates from patients with severe pulmonary disease, suggesting that loss of PagL function can occur during long-term adaptation to the CF airway.

The core lipopolysaccharide of Escherichia coli is a ligand for the dendritic-cell-specific intercellular adhesion molecule nonintegrin CD209 receptor

The dendritic-cell-specific intercellular adhesion molecule nonintegrin (DC-SIGN) CD209 is a receptor for Escherichia coli K-12 that promotes bacterial adherence and phagocytosis. However, the ligand of E. coli for DC-SIGN has not yet been identified. In this study, we found that DC-SIGN did not mediate the phagocytosis of several pathogenic strains of E. coli, including enteropathogenic E. coli, enterohemorrhagic E. coli, enterotoxigenic E. coli, and uropathogenic E. coli, in dendritic cells or HeLa cells expressing human DC-SIGN antigen. However, we showed that an outer core lipopolysaccharide (LPS) (rough) mutant, unlike an inner core LPS (deep rough) mutant or O-antigen-expressing recombinant of E. coli K-12 was phagocytosed. These results demonstrate that the host cells expressing DC-SIGN can phagocytose E. coli in part by interacting with the complete core region of the LPS molecule. These results provide a mechanism for how O antigen acts as an antiphagocytic factor.