Vi polysaccharide and conjugated vaccines afford similar early, IgM or IgG-independent control of infection but boosting with conjugated Vi vaccines sustains the efficacy of immune responses

Introduction

Vaccination with Vi capsular polysaccharide (Vi-PS) or protein-Vi typhoid conjugate vaccine (TCV) can protect adults against Salmonella Typhi infections. TCVs offer better protection than Vi-PS in infants and may offer better protection in adults. Potential reasons for why TCV may be superior in adults are not fully understood.

Methods and results

Here, we immunized wild-type (WT) mice and mice deficient in IgG or IgM with Vi-PS or TCVs (Vi conjugated to tetanus toxoid or CRM197) for up to seven months, with and without subsequent challenge with Vi-expressing Salmonella Typhimurium. Unexpectedly, IgM or IgG alone were similarly able to reduce bacterial burdens in tissues, and this was observed in response to conjugated or unconjugated Vi vaccines and was independent of antibody being of high affinity. Only in the longer-term after immunization (>5 months) were differences observed in tissue bacterial burdens of mice immunized with Vi-PS or TCV. These differences related to the maintenance of antibody responses at higher levels in mice boosted with TCV, with the rate of fall in IgG titres induced to Vi-PS being greater than for TCV.

Discussion

Therefore, Vi-specific IgM or IgG are independently capable of protecting from infection and any superior protection from vaccination with TCV in adults may relate to responses being able to persist better rather than from differences in the antibody isotypes induced. These findings suggest that enhancing our understanding of how responses to vaccines are maintained may inform on how to maximize protection afforded by conjugate vaccines against encapsulated pathogens such as S. Typhi.

High-performance porphyrin-like graphene quantum dots for immuno-sensing of Salmonella typhi

The extraordinary optical properties of porphyrins have inspired their applications in various fields. Herein, we introduce iron porphyrin bio-mimicked graphene quantum dots (Fe-N-GQDs) as a novel paramagnetic and fluorescent label. The Fe-N-GQD was prepared by the mechanochemical mixing of Fe, N, and C sources followed by pyrolysis at high-temperature and next, the solvothermal treatment was performed. The Fe-N sites in graphene matrix, the structural alterations during the solvothermal treatment, the optical properties, and paramagnetic behaviour were studied using FTIR, Raman and X-ray spectroscopies, and Vibrating sample magnetometer. The structural studies revealed that under solvothermal condition, Fe-N doped graphene sheets cut into ultra-small Fe-N-GQDs containing well-dispersed particles with an average diameter of about 2.5 nm. As a result of Fe-N doping, the photoluminescence quantum yield was enhanced to 86% and strong paramagnetic behaviour was observed. Due to the rich oxygen-containing groups at Fe-N-GQDs surface, it has proper sites for bio-conjugation. The bioconjugated Fe-N-GQDs serve as donors in a prominent fluorescence resonance energy transfer system, while graphene oxide acts as an acceptor. The proposed immunosensor was successfully applied for the detection of Salmonella Typhi Vi antigen in real human serum in the concentration range from 1 pg/mL to 1 μg/mL with the detection limit of 1 pg/mL.

Adsorption of Vi Capsular Antigen of Salmonella Typhi in Chitosan-Poly (Methacrylic Acid) Nanoparticles

The development of a nanoparticulate system for the carrier antigen is now an important tool in the vaccination process, since a smaller number of doses is necessary for effective immunization. Thus, in this work a nanoparticulate system using polymers of chitosan and poly (methacrylic acid) (CS–PMAA) to adsorb the Vi antigen of Salmonella Typhi was developed. CS–PMAA nanoparticles with different proportions of chitosan and poly (methacrylic acid) were obtained and reached sizes from 123.9 ± 2.48 to 234.9 ± 2.66 nm, and spherical shapes were seen in transmission microscopy. At pH 7.2, the nanoparticles had a cationic surface charge that contributed to the adsorption of the Vi antigen. Qualitative analyses of the isolated Vi antigen were performed using Fourier-transform infrared spectroscopy, which indicated the presence of all the characteristic bands of the capsular polysaccharide, and nuclear magnetic resonance, which showed signals for the five hydrogens and the N-acetyl and O-acetyl groups which are characteristic of the Vi antigen structure. In the adsorption kinetics study, the Vi capsular antigen, contained in a phosphate buffer solution of pH 7.2, experienced 55% adsorption on the 1–1% (CS–PMAA) nanoparticles. The adsorption kinetics results showed the ability of the nanoparticulate system to adsorb the Vi antigen.

Small alarmones (p)ppGpp regulate virulence associated traits and pathogenesis of Salmonella enterica serovar Typhi

How Salmonella enterica serovar Typhi (S. Typhi), an important human pathogen, survives the stressful microenvironments inside the gastrointestinal tract and within macrophages remains poorly understood. We report here that S. Typhi has a bonafide stringent response (SR) system, which is mediated by (p)ppGpp and regulates multiple virulence-associated traits and the pathogenicity of the S. Typhi Ty2 strain. In an iron overload mouse model of S. Typhi infection, the (p)ppGpp⁰ (Ty2ΔRelAΔSpoT) strain showed minimal systemic spread and no mortality, as opposed to 100% death of the mice challenged with the isogenic wild-type strain. Ty2ΔRelAΔSpoT had markedly elongated morphology with incomplete septa formation and demonstrated severely attenuated motility and chemotaxis due to the loss of flagella. Absence of the Vi-polysaccharide capsule rendered the mutant strain highly susceptible to complement-mediated lysis. The phenotypes of Ty2ΔRelAΔSpoT was contributed by transcriptional repression of several genes, including fliC, tviA, and ftsZ, as found by reverse transcriptase quantitative polymerase chain reaction and gene complementation studies. Finally, Ty2ΔRelAΔSpoT had markedly reduced invasion into intestinal epithelial cells and significantly attenuated survival within macrophages. To the best of our knowledge, this was the first study that addressed SR in S. Typhi and showed that (p)ppGpp was essential for optimal pathogenic fitness of the organism.

The invasome of Salmonella Dublin as revealed by whole genome sequencing

Background

Salmonella enterica serovar Dublin is a zoonotic infection that can be transmitted from cattle to humans through consumption of contaminated milk and milk products. Outbreaks of human infections by S. Dublin have been reported in several countries including high-income countries. A high proportion of S. Dublin cases in humans are associated with invasive disease and systemic illness. The genetic basis of virulence in S. Dublin is not well characterized.

Methods

Whole genome sequencing was applied to a set of clinical invasive and non-invasive S. Dublin isolates from different countries in order to characterize the putative genetic determinants involved in the virulence and invasiveness of S. Dublin in humans.

Results

We identified several virulence factors that form the bacterial invasome and may contribute to increasing bacterial virulence and pathogenicity including mainly Gifsy-2 prophage, two different type 6 secretion systems (T6SSs) harbored by Salmonella pathogenicity islands; SPI-6 and SPI-19 respectively and virulence genes; ggt and PagN. Although Vi antigen and the virulence plasmid have been reported previously to contribute to the virulence of S. Dublin we did not detect them in all invasive isolates indicating that they are not the main virulence determinants in S. Dublin.

Conclusion

Several virulence factors within the genome of S. Dublin might contribute to the ability of S. Dublin to invade humans’ blood but there were no genomic markers that differentiate invasive from non-invasive isolates suggesting that host immune response play a crucial role in the clinical outcome of S. Dublin infection.

Electronic supplementary material

The online version of this article (doi:10.1186/s12879-017-2628-x) contains supplementary material, which is available to authorized users.

Unique lipid anchor attaches Vi antigen capsule to the surface of Salmonella enterica serovar Typhi

Polysaccharide capsules are surface structures that are critical for the virulence of many Gram-negative pathogenic bacteria. Salmonella enterica serovar Typhi is the etiological agent of typhoid fever. It produces a capsular polysaccharide known as "Vi antigen," which is composed of nonstoichiometrically O-acetylated α-1,4-linked N-acetylgalactosaminuronic acid residues. This glycan is a component of currently available vaccines. The genetic locus for Vi antigen production is also present in soil bacteria belonging to the genus Achromobacter Vi antigen assembly follows a widespread general strategy with a characteristic glycan export step involving an ATP-binding cassette transporter. However, Vi antigen producers lack the enzymes that build the conserved terminal glycolipid characterizing other capsules using this method. Achromobacter species possess a Vi antigen-specific depolymerase enzyme missing in S enterica Typhi, and we exploited this enzyme to isolate acylated Vi antigen termini. Mass spectrometry analysis revealed a reducing terminal N-acetylhexosamine residue modified with two β-hydroxyl acyl chains. This terminal structure resembles one half of lipid A, the hydrophobic portion of bacterial lipopolysaccharides. The VexE protein encoded in the Vi antigen biosynthesis locus shares similarity with LpxL, an acyltransferase from lipid A biosynthesis. In the absence of VexE, Vi antigen is produced, but its physical properties are altered, its export is impaired, and a Vi capsule structure is not assembled on the cell surface. The structure of the lipidated terminus dictates a unique assembly mechanism and has potential implications in pathogenesis and vaccine production.