Structure and function of the Salmonella Typhi chimaeric A(2)B(5) typhoid toxin

Investigating the binding affinity, molecular dynamics, and ADMET properties of 2,3-dihydrobenzofuran derivatives as an inhibitor of fungi, bacteria, and virus protein

Background

2,3-Dihydrobenzofurans (DHB) have proposed as advantages structures, and used as chemical entresol to design small compound libraries. The present study illustrates to explore 2,3-dihydrobenzofurans(DHB) in comparison to selected some derivatives drugs by using molecular docking and molecular dynamics, as well as ADMET studies. The online database “Molinspiration online server” was used to detect the physicochemical pharmacokinetics and drug likeness score of DHB drugs. For estimation of molecular docking, six pathogens, such as Aspergillus niger (PDB id: 1kum), Candida albicans (3dra), Escherichia coli (6og7), Salmonella typhi (4k6l), Influenza (1ru7), and Hepatitis C (4tyd), were chosen due to close biological studies.

Results

From Molinspiration online server has showed that DHB did not violate the “Lipinski five rule” as drugs, leading compound for molecular docking exhibited the potential interaction to the active residue. The binding affinity of DHB2 (−7.00 kcal/mol) against 3dra was higher than DHB8 (−6.40 kcal/mol) and DHB (5.70 kcal/mol) for compounds. The results of molecular docking show that the compounds mentioned in this study are not equally effective against pathogens, such as fungi, viruses, and bacteria. However, DHB2, DHB3, and DHB 8 compounds can work against almost given pathogens which results are derived from auto dock vina in terms of binding affinity around 6.00 kcal/mol, and Fire Dock has values from about 38.0 to 42.0 kcal/mol. To explore the dynamic nature of the interaction, 50 ns molecular dynamics (MD) simulation was performed on the selected protein-DHB complexes. Thus, DHB 8 has greater potential to interact for further for fungi.

Conclusion

Finding from this study can play an effective role as a drug in any biological system. This study as well recommends to researchers to synthesize these DHBs for evaluation of its biological activity.

Graphical abstract

Mechanisms of typhoid toxin neutralization by antibodies targeting glycan receptor binding and nuclease subunits

Nearly all clinical isolates of Salmonella Typhi, the cause of typhoid fever, are antibiotic resistant. All S. Typhi isolates secrete an A2B5 exotoxin called typhoid toxin to benefit the pathogen during infection. Here, we demonstrate that antibiotic-resistant S. Typhi secretes typhoid toxin continuously during infection regardless of antibiotic treatment. We characterize typhoid toxin antibodies targeting glycan-receptor-binding PltB or nuclease CdtB, which neutralize typhoid toxin in vitro and in vivo, as demonstrated by using typhoid toxin secreted by antibiotic-resistant S. Typhi during human cell infection and lethal dose typhoid toxin challenge to mice. TyTx11 generated in this study neutralizes typhoid toxin effectively, comparable to TyTx4 that binds to all PltB subunits available per holotoxin. Cryoelectron microscopy explains that the binding of TyTx11 to CdtB makes this subunit inactive through CdtB catalytic-site conformational change. The identified toxin-neutralizing epitopes are conserved across all S. Typhi clinical isolates, offering critical insights into typhoid toxin-neutralizing strategies.

ADP-ribosylation systems in bacteria and viruses

ADP-ribosylation is an ancient posttranslational modification present in all kingdoms of life. The system likely originated in bacteria where it functions in inter- and intra-species conflict, stress response and pathogenicity. It was repeatedly adopted via lateral transfer by eukaryotes, including humans, where it has a pivotal role in epigenetics, DNA-damage repair, apoptosis, and other crucial pathways including the immune response to pathogenic bacteria and viruses. In other words, the same ammunition used by pathogens is adapted by eukaryotes to fight back. While we know quite a lot about the eukaryotic system, expanding rather patchy knowledge on bacterial and viral ADP-ribosylation would give us not only a better understanding of the system as a whole but a fighting advantage in this constant arms race. By writing this review we hope to put into focus the available information and give a perspective on how this system works and can be exploited in the search for therapeutic targets in the future. The relevance of the subject is especially highlighted by the current situation of being amid the world pandemic caused by a virus harbouring and dependent on a representative of such a system.

Influence of the microenvironment on modulation of the host response by typhoid toxin

Bacterial genotoxins cause DNA damage in eukaryotic cells, resulting in activation of the DNA damage response (DDR) in vitro. These toxins are produced by Gram-negative bacteria, enriched in the microbiota of inflammatory bowel disease (IBD) and colorectal cancer (CRC) patients. However, their role in infection remains poorly characterized. We address the role of typhoid toxin in modulation of the host-microbial interaction in health and disease. Infection with a genotoxigenic Salmonella protects mice from intestinal inflammation. We show that the presence of an active genotoxin promotes DNA fragmentation and senescence in vivo, which is uncoupled from an inflammatory response and unexpectedly associated with induction of an anti-inflammatory environment. The anti-inflammatory response is lost when infection occurs in mice with acute colitis. These data highlight a complex context-dependent crosstalk between bacterial-genotoxin-induced DDR and the host immune response, underlining an unexpected role for bacterial genotoxins.

Enteric Fever Diagnosis: Current Challenges and Future Directions

Enteric fever is a life-threatening systemic febrile disease caused by Salmonella enterica serovars Typhi and Paratyphi (S. Typhi and S. Paratyphi). Unfortunately, the burden of the disease remains high primarily due to the global spread of various drug-resistant Salmonella strains despite continuous advancement in the field. An accurate diagnosis is critical for effective control of the disease. However, enteric fever diagnosis based on clinical presentations is challenging due to overlapping symptoms with other febrile illnesses that are also prevalent in endemic areas. Current laboratory tests display suboptimal sensitivity and specificity, and no diagnostic methods are available for identifying asymptomatic carriers. Several research programs have employed systemic approaches to identify more specific biomarkers for early detection and asymptomatic carrier detection. This review discusses the pros and cons of currently available diagnostic tests for enteric fever, the advancement of research toward improved diagnostic tests, and the challenges of discovering new ideal biomarkers and tests.

Nuclear trafficking of bacterial effector proteins

Bacterial pathogens can subvert host responses by producing effector proteins that directly target the nucleus of eukaryotic cells in animals and plants. Nuclear-targeting proteins are categorised as either: "nucleomodulins," which have epigenetic-modulating activities; or "cyclomodulins," which specifically interfere with the host cell cycle. Bacteria can deliver these effector proteins to eukaryotic cells via a range of strategies. Despite an increasing number of reports describing the effects of bacterial effector proteins on nuclear processes in host cells, the intracellular pathways used by these proteins to traffic to the nucleus have yet to be fully elucidated. This review will describe current knowledge about how nucleomodulins and cyclomodulins enter eukaryotic cells, exploit endocytic pathways and translocate to the nucleus. We will also discuss the secretion of nuclear-targeting proteins or their release in bacterial membrane vesicles and the trafficking pathways employed by each of these forms. Besides their importance for bacterial pathogenesis, some nuclear-targeting proteins have been implicated in the development of chronic diseases and even cancer. A greater understanding of nuclear-targeting proteins and their actions will provide new insights into the pathogenesis of infectious diseases, as well as contribute to advances in the development of novel therapies against bacterial infections and possibly cancer.

Structural and Cellular Insights into the l,d-Transpeptidase YcbB as a Therapeutic Target in Citrobacter rodentium, Salmonella Typhimurium, and Salmonella Typhi Infections

The bacterial cell wall plays a key role in viability and is an important drug target. The cell wall is made of elongated polymers that are cross-linked to one another to form a load-bearing mesh. An alternative cell wall cross-linking mechanism used by the l,d-transpeptidase YcbB has been implicated in the stress-regulated roles of β-lactam resistance, outer membrane defect rescue, and typhoid toxin release. The role for this stress-linked cross-linking in the context of a host infection was unclear. Here, we resolve the crystallographic structures of both Salmonella Typhi YcbB and Citrobacter rodentium YcbB acylated with ertapenem that delineate the conserved structural characteristics of YcbB. In parallel, we show that the general involvement of YcbB in peptidoglycan reinforcement under conditions of bacterial outer envelope stress does not play a significant role in acute infections of mice by C. rodentium and S Typhimurium. Cumulatively, in this work we provide a foundation for the development of novel YcbB-specific antibacterial therapeutics to assist in treatment of increasingly drug-resistant S Typhi infections.

The Majority of Typhoid Toxin-Positive Salmonella Serovars Encode ArtB, an Alternate Binding Subunit

While previous reports had suggested that the typhoid toxin (TT) could potentially use ArtB as an alternate binding subunit, this was thought to play a minor role in the evolution and biology of the toxin. In this study, we establish that both TT genes and artB are widespread among Salmonella enterica subsp. enterica, suggesting that TT likely plays a broader role in Salmonella virulence that extends beyond its proposed role in typhoid fever.

Salmonella enterica encodes a wide array of virulence factors. One novel virulence factor, an A₂B₅ toxin known as the typhoid toxin (TT), was recently identified among a variety of S. enterica serovars. While past studies have shown that some serovars encode both the TT (active subunits CdtB and PltA and binding subunit PltB) and a second binding subunit (ArtB), these serovars were thought to be the exception. Here, we show that genes encoding the TT are detected in more than 100 serovars representing distinct phylogenetic lineages of S. enterica subsp. enterica, although clade B and section Typhi are significantly more likely to encode TT genes than serovars from other clades. Furthermore, we show that 81% of these TT-positive serovars also encode artB, suggesting that the cooccurrence of both toxin binding subunits is considerably more common than previously thought. A combination of in silico modeling, bacterial two-hybrid system screening, and tandem affinity purification (TAP) of toxin subunits suggests that ArtB and PltB interact in vitro, at least under some growth conditions. While different growth conditions yielded slightly higher transcript abundances of artB and pltB, both genes had their highest relative transcript abundances when Salmonella was grown under low-Mg²⁺ conditions, suggesting that ArtB and PltB may compete for inclusion in the TT. Together, our results suggest that ArtB likely plays an important and previously underappreciated role in the biology of the TT produced by typhoidal and nontyphoidal Salmonella.

IMPORTANCE While previous reports had suggested that the typhoid toxin (TT) could potentially use ArtB as an alternate binding subunit, this was thought to play a minor role in the evolution and biology of the toxin. In this study, we establish that both TT genes and artB are widespread among Salmonella enterica subsp. enterica, suggesting that TT likely plays a broader role in Salmonella virulence that extends beyond its proposed role in typhoid fever. Furthermore, our data suggest the selective maintenance of both toxin binding subunits, which may compete for inclusion in the holotoxin. Last, our data support the importance of characterizing diverse nontyphoidal Salmonella (NTS) serovars, as the presence of classically defined typhoidal virulence factors among NTS serovars continues to challenge the typhoid-nontyphoid Salmonella paradigm.

A holin/peptidoglycan hydrolase-dependent protein secretion system

Gram-negative bacteria have evolved numerous pathways to secrete proteins across their complex cell envelopes. Here, we describe a protein secretion system that uses a holin membrane protein in tandem with a cell wall-editing enzyme to mediate the secretion of substrate proteins from the periplasm to the cell exterior. The identity of the cell wall-editing enzymes involved was found to vary across biological systems. For instance, the chitinase secretion pathway of Serratia marcescens uses an endopeptidase to facilitate secretion, whereas the secretion of Typhoid toxin in Salmonella enterica serovar Typhi relies on a muramidase. Various families of holins are also predicted to be involved. Genomic analysis indicates that this pathway is conserved and implicated in the secretion of hydrolytic enzymes and toxins for a range of bacteria. The pairing of holins from different families with various types of peptidoglycan hydrolases suggests that this secretion pathway evolved multiple times. We suggest that the complementary bodies of evidence presented is sufficient to propose that the pathway be named the Type 10 Secretion System (TXSS).

Genotoxic Effect of Salmonella Paratyphi A Infection on Human Primary Gallbladder Cells

Bacterial infections are increasingly being recognized as risk factors for the development of adenocarcinomas. The strong epidemiological evidence linking Helicobacter pylori infection to stomach cancer has paved the way to the demonstration that bacterial infections cause DNA damage in the host cells, initiating transformation. In this regard, the role of bacterial genotoxins has become more relevant. Salmonella enterica serovars Typhi and Paratyphi A have been clinically associated with gallbladder cancer. By harnessing the stem cell potential of cells from healthy human gallbladder explant, we regenerated and propagated the epithelium of this organ in vitro and used these cultures to model S. Paratyphi A infection. This study demonstrates the importance of the typhoid toxin, encoded only by these specific serovars, in causing genomic instability in healthy gallbladder cells, posing intoxicated cells at risk of malignant transformation.

Carcinoma of the gallbladder (GBC) is the most frequent tumor of the biliary tract. Despite epidemiological studies showing a correlation between chronic infection with Salmonella enterica Typhi/Paratyphi A and GBC, the underlying molecular mechanisms of this fatal connection are still uncertain. The murine serovar Salmonella Typhimurium has been shown to promote transformation of genetically predisposed cells by driving mitogenic signaling. However, insights from this strain remain limited as it lacks the typhoid toxin produced by the human serovars Typhi and Paratyphi A. In particular, the CdtB subunit of the typhoid toxin directly induces DNA breaks in host cells, likely promoting transformation. To assess the underlying principles of transformation, we used gallbladder organoids as an infection model for Salmonella Paratyphi A. In this model, bacteria can invade epithelial cells, and we observed host cell DNA damage. The induction of DNA double-strand breaks after infection depended on the typhoid toxin CdtB subunit and extended to neighboring, non-infected cells. By cultivating the organoid derived cells into polarized monolayers in air-liquid interphase, we could extend the duration of the infection, and we observed an initial arrest of the cell cycle that does not depend on the typhoid toxin. Non-infected intoxicated cells instead continued to proliferate despite the DNA damage. Our study highlights the importance of the typhoid toxin in causing genomic instability and corroborates the epidemiological link between Salmonella infection and GBC.

Generation and Characterization of Typhoid Toxin-Neutralizing Human Monoclonal Antibodies

Typhoid toxin is a virulence factor of Salmonella enterica serovar Typhi, the causative agent of typhoid fever, and is thought to be responsible for the symptoms of severe disease. This toxin has a unique A2B5 architecture with two active subunits, the ADP ribosyl transferase PltA and the DNase CdtB, linked to a pentameric B subunit, which is alternatively made of PltB or PltC. Here, we describe the generation and characterization of typhoid toxin-neutralizing human monoclonal antibodies by immunizing genetically engineered mice that have a full set of human immunoglobulin variable region genes. We identified several monoclonal antibodies with strong in vitro and in vivo toxin-neutralizing activity and different mechanisms of toxin neutralization. These antibodies could serve as the basis for the development of novel therapeutic strategies against typhoid fever.

Multimeric Epitope-Scaffold HIV Vaccines Target V1V2 and Differentially Tune Polyfunctional Antibody Responses

The V1V2 region of the HIV-1 envelope is the target of several broadly neutralizing antibodies (bNAbs). Antibodies to V1V2 elicited in the RV144 clinical trial correlated with a reduced risk of HIV infection, but these antibodies were without broad neutralizing activity. Antibodies targeting V1V2 also correlated with a reduced viral load in immunized macaques challenged with simian immunodeficiency virus (SIV) or simian/human immunodeficiency virus (SHIV). To focus immune responses on V1V2, we engrafted the native, glycosylated V1V2 domain onto five different multimeric scaffold proteins and conducted comparative immunogenicity studies in macaques. Vaccinated macaques developed high titers of plasma and mucosal antibodies that targeted structurally distinct V1V2 epitopes. Plasma antibodies displayed limited neutralizing activity but were functionally active for ADCC and phagocytosis, which was detectable 1-2 years after immunizations ended. This study demonstrates that multivalent, glycosylated V1V2-scaffold protein immunogens focus the antibody response on V1V2 and are differentially effective at inducing polyfunctional antibodies with characteristics associated with protection.

Functional Study of Haemophilus ducreyi Cytolethal Distending Toxin Subunit B

The Cytolethal Distending Toxin (CDT) is produced by many Gram-negative pathogenic bacteria responsible for major foodborne diseases worldwide. CDT induces DNA damage and cell cycle arrest in host-cells, eventually leading to senescence or apoptosis. According to structural and sequence comparison, the catalytic subunit CdtB is suggested to possess both nuclease and phosphatase activities, carried by a single catalytic site. However, the impact of each activity on cell-host toxicity is yet to be characterized. Here, we analyze the consequences of cell exposure to different CDT mutated on key CdtB residues, focusing on cell viability, cell cycle defects, and DNA damage induction. A first class of mutant, devoid of any activity, targets putative catalytic (H160A), metal binding (D273R), and DNA binding residues (R117A-R144A-N201A). The second class of mutants (A163R, F156-T158, and the newly identified G114T), which gathers mutations on residues potentially involved in lipid substrate binding, has only partially lost its toxic effects. However, their defects are alleviated when CdtB is artificially introduced inside cells, except for the F156-T158 double mutant that is defective in nuclear addressing. Therefore, our data reveal that CDT toxicity is mainly correlated to CdtB nuclease activity, whereas phosphatase activity may probably be involved in CdtB intracellular trafficking.

Senescence and Host-Pathogen Interactions

Damage to our genomes triggers cellular senescence characterised by stable cell cycle arrest and a pro-inflammatory secretome that prevents the unrestricted growth of cells with pathological potential. In this way, senescence can be considered a powerful innate defence against cancer and viral infection. However, damage accumulated during ageing increases the number of senescent cells and this contributes to the chronic inflammation and deregulation of the immune function, which increases susceptibility to infectious disease in ageing organisms. Bacterial and viral pathogens are masters of exploiting weak points to establish infection and cause devastating diseases. This review considers the emerging importance of senescence in the host-pathogen interaction: we discuss the pathogen exploitation of ageing cells and senescence as a novel hijack target of bacterial pathogens that deploys senescence-inducing toxins to promote infection. The persistent induction of senescence by pathogens, mediated directly through virulence determinants or indirectly through inflammation and chronic infection, also contributes to age-related pathologies such as cancer. This review highlights the dichotomous role of senescence in infection: an innate defence that is exploited by pathogens to cause disease.

Identification of A Putative T6SS Immunity Islet in Salmonella Typhi

Typhoid fever is a major global health problem and is the result of systemic infections caused by the human-adapted bacterial pathogen Salmonella enterica serovar Typhi (S. Typhi). The pathology underlying S. Typhi infections significantly differ from infections caused by broad host range serovars of the same species, which are a common cause of gastroenteritis. Accordingly, identifying S. Typhi genetic factors that impart functionality absent from broad host range serovars offers insights into its unique biology. Here, we used an in-silico approach to explore the function of an uncharacterized 14-gene S. Typhi genomic islet. Our results indicated that this islet was specific to the S. enterica species, where it was encoded by the Typhi and Paratyphi A serovars, but was generally absent from non-typhoidal serovars. Evidence was gathered using comparative genomics and sequence analysis tools, and indicated that this islet was comprised of Type VI secretion system (T6SS) and contact-dependent growth inhibition (CDI) genes, the majority of which appeared to encode orphan immunity proteins that protected against the activities of effectors and toxins absent from the S. Typhi genome. We herein propose that this islet represents an immune system that protects S. Typhi against competing bacteria within the human gut.

Salmonella Typhoid Toxin PltB Subunit and Its Non-typhoidal Salmonella Ortholog Confer Differential Host Adaptation and Virulence

Typhoidal and non-typhoidal Salmonelleae (NTS) cause typhoid fever and gastroenteritis, respectively, in humans. Salmonella typhoid toxin contributes to typhoid disease progression and chronic infection, but little is known about the role of its NTS ortholog. We found that typhoid toxin and its NTS ortholog induce different clinical presentations. The PltB subunit of each toxin exhibits different glycan-binding preferences that correlate with glycan expression profiles of host cells targeted by each bacterium at the primary infection or intoxication sites. Through co-crystal structures of PltB subunits bound to specific glycan receptor moieties, we show that they induce markedly different glycan-binding preferences and virulence outcomes. Furthermore, immunization with the NTS S. Javiana or its toxin offers cross-reactive protection against lethal-dose typhoid toxin challenge. Cumulatively, these results offer insights into the evolution of host adaptations in Salmonella AB toxins, their cell and tissue tropisms, and the design for improved typhoid vaccines and therapeutics.

The role of 9-O-acetylated glycan receptor moieties in the typhoid toxin binding and intoxication

Typhoid toxin is an A2B5 toxin secreted from Salmonella Typhi-infected cells during human infection and is suggested to contribute to typhoid disease progression and the establishment of chronic infection. To deliver the enzymatic 'A' subunits of the toxin to the site of action in host cells, the receptor-binding 'B' subunit PltB binds to the trisaccharide glycan receptor moieties terminated in N-acetylneuraminic acid (Neu5Ac) that is α2-3 or α2-6 linked to the underlying disaccharide, galactose (Gal) and N-acetylglucosamine (GlcNAc). Neu5Ac is present in both unmodified and modified forms, with 9-O-acetylated Neu5Ac being the most common modification in humans. Here we show that host cells associated with typhoid toxin-mediated clinical signs express both unmodified and 9-O-acetylated glycan receptor moieties. We found that PltB binds to 9-O-acetylated α2-3 glycan receptor moieties with a markedly increased affinity, while the binding affinity to 9-O-acetylated α2-6 glycans is only slightly higher, as compared to the affinities of PltB to the unmodified counterparts, respectively. We also present X-ray co-crystal structures of PltB bound to related glycan moieties, which supports the different effects of 9-O-acetylated α2-3 and α2-6 glycan receptor moieties on the toxin binding. Lastly, we demonstrate that the cells exclusively expressing unmodified glycan receptor moieties are less susceptible to typhoid toxin than the cells expressing 9-O-acetylated counterparts, although typhoid toxin intoxicates both cells. These results reveal a fine-tuning mechanism of a bacterial toxin that exploits specific chemical modifications of its glycan receptor moieties for virulence and provide useful insights into the development of therapeutics against typhoid fever.

Bacterial Genotoxin-Induced DNA Damage and Modulation of the Host Immune Microenvironment

: Bacterial genotoxins (BTGX) induce DNA damage, which results in senescence or apoptosis of the target cells if not properly repaired. Three BTGXs have been identified: the cytolethal distending toxin (CDT) family produced by several Gram-negative bacteria, the typhoid toxin produced by several Salmonella enterica serovars, and colibactin, a peptide-polyketide, produced mainly by the phylogenetic group B2 Escherichia coli. The cellular responses induced by BTGXs resemble those of well-characterized carcinogenic agents, and several lines of evidence indicate that bacteria carrying genotoxin genes can contribute to tumor development under specific circumstances. Given their unusual mode of action, it is still enigmatic why these effectors have been acquired by microbes and what is their role in the context of the biology of the producing bacterium, since it is unlikely that their primary purpose is to induce/promote cancer in the mammalian host. In this review, we will discuss the possibility that the DNA damage induced by BTGX modulates the host immune response, acting as immunomodulator, leading to the establishment of a suitable niche for the producing bacterium. We will further highlight open questions that remain to be solved regarding the biology of this unusual family of bacterial toxins.

Prophylactic potential of cytolethal distending toxin B (CdtB) subunit of typhoid toxin against Typhoid fever

Typhoid fever caused by Salmonella enterica serovar Typhi (S.Typhi) continues to be a major problem, especially in developing countries. Due to the rapid emergence of multi-drug-resistant (MDR) strains, which limits the efficacy of conventional antibiotics as well as problems associated with the existing vaccines, efforts are being made to develop effective prophylactic agents. CdtB subunit of typhoid toxin was selected for assessing its vaccine potential due to its high conservation throughout the Typhi strains. In-vitro assessment of DNase activity of cloned and purified CdtB protein showed a significant decrease in the band intensity of DNA. The measure of metabolic activity and morphological alterations assessed using different cell lines in the presence of CdtB protein showed no significant signs of toxicity. These observations were further strengthened by cell cycle analysis, assessed by flow cytometry. Keeping these observations in mind, the immunoprotective potential of CdtB was assessed using S.Typhi induced mouse peritonitis model. A significant titer of IgG antibodies (>128000) against CdtB protein was recorded in the immunized mice by enzyme-linked immunosorbent assay (ELISA), which was also validated by immunoblotting. Active immunization with the protein protected 75% mice against a lethal dose of S.Typhi Ty2. The data indicated a significant (up to 5 log) reduction in the bacterial load in the spleen and liver of immunized-infected mice compared to control (unimmunized-infected) mice which might have resulted in the modulation of histoarchitecture of spleen and liver and the levels of cytokines (IL-6, TNF-α and IL-10) production; thereby indicating the effectiveness of the subunit. The observations deduced from the study give the proof of concept of immunogenic potential of protein. However, further studies involving the immunoreactivity of CdtB with the statistically significant number of sera samples obtained from the human patients would be helpful in establishing the relevance of CdtB protein in humans and for making the strategies to develop it as an effective vaccine candidate.

DNA-Damaging Effectors: New Players in the Effector Arena

In animal cells, nuclear DNA is the target of genotoxins produced by bacterial pathogens that cause genomic mutations eventually leading to apoptosis, senescence, and carcinogenic development. In response to the insult, the DNA damage response (DDR) is activated to ensure lesion repair. Accumulation of DNA breaks is also detected in plants during microbial infection. In this opinion article we propose that phytopathogens can produce DNA-damaging effectors. The recent identification of a functional genotoxin in devastating eukaryotic plant pathogens, such as oomycetes, supports the concept that DNA-damaging effectors may contribute to pathogenicity. Additionally, this raises the question of how plants can perceive these damages and whether this perception can be connected to the plant immune system.

Differential functional patterns of memory CD4+ and CD8+ T-cells from volunteers immunized with Ty21a typhoid vaccine observed using a recombinant Escherichia coli system expressing S. Typhi proteins

It is widely accepted that CD4⁺ and CD8⁺ T-cells play a significant role in protection against Salmonella enterica serovar Typhi (S. Typhi), the causative agent of the typhoid fever. However, the antigen specificity of these T-cells remains largely unknown. Previously, we demonstrated the feasibility of using a recombinant Escherichia coli (E. coli) expression system to uncover the antigen specificity of CD4⁺ and CD8⁺ T cells. Here, we expanded these studies to include the evaluation of 12 additional S. Typhi proteins: 4 outer membrane proteins (OmpH, OmpL, OmpR, OmpX), 3 Vi-polysaccharide biosynthesis proteins (TviA, TviB, TviE), 3 cold shock proteins (CspA, CspB, CspC), and 2 conserved hypothetical proteins (Chp 1 and Chp2), all selected based on the bioinformatic analyses of the content of putative T-cell epitopes. CD4⁺ and CD8⁺ T cells from 15 adult volunteers, obtained before and 42 days after immunization with oral live attenuated Ty21a vaccine, were assessed for their functionality (i.e., production of cytokines and cytotoxic expression markers in response to stimulation with selected antigens) as measured by flow cytometry. Although volunteers differed on their T-cell antigen specificity, we observed T-cell immune responses against all S. Typhi proteins evaluated. These responses included 9 proteins, OmpH, OmpR, TviA, TviE, CspA, CspB, CspC, Chp 1 and Chp 2, which have not been previously reported to elicit T-cell responses. Interestingly, we also observed that, regardless of the protein, the functional patterns of the memory T-cells were different between CD4⁺ and CD8⁺ T cells. In sum, these studies demonstrated the feasibility of using bioinformatic analysis and the E. coli expressing system described here to uncover novel immunogenic T-cell proteins that could serve as potential targets for the production of protein-based vaccines.

Cytolethal Distending Toxin Subunit B: A Review of Structure-Function Relationship

The Cytolethal Distending Toxin (CDT) is a bacterial virulence factor produced by several Gram-negative pathogenic bacteria. These bacteria, found in distinct niches, cause diverse infectious diseases and produce CDTs differing in sequence and structure. CDTs have been involved in the pathogenicity of the associated bacteria by promoting persistent infection. At the host-cell level, CDTs cause cell distension, cell cycle block and DNA damage, eventually leading to cell death. All these effects are attributable to the catalytic CdtB subunit, but its exact mode of action is only beginning to be unraveled. Sequence and 3D structure analyses revealed similarities with better characterized proteins, such as nucleases or phosphatases, and it has been hypothesized that CdtB exerts a biochemical activity close to those enzymes. Here, we review the relationships that have been established between CdtB structure and function, particularly by mutation experiments on predicted key residues in different experimental systems. We discuss the relevance of these approaches and underline the importance of further study in the molecular mechanisms of CDT toxicity, particularly in the context of different pathological conditions.

Typhoid toxin exhausts the RPA response to DNA replication stress driving senescence and Salmonella infection

Salmonella Typhi activates the host DNA damage response through the typhoid toxin, facilitating typhoid symptoms and chronic infections. Here we reveal a non-canonical DNA damage response, which we call RING (response induced by a genotoxin), characterized by accumulation of phosphorylated histone H2AX (γH2AX) at the nuclear periphery. RING is the result of persistent DNA damage mediated by toxin nuclease activity and is characterized by hyperphosphorylation of RPA, a sensor of single-stranded DNA (ssDNA) and DNA replication stress. The toxin overloads the RPA pathway with ssDNA substrate, causing RPA exhaustion and senescence. Senescence is also induced by canonical γΗ2ΑΧ foci revealing distinct mechanisms. Senescence is transmitted to non-intoxicated bystander cells by an unidentified senescence-associated secreted factor that enhances Salmonella infections. Thus, our work uncovers a mechanism by which genotoxic Salmonella exhausts the RPA response by inducing ssDNA formation, driving host cell senescence and facilitating infection.

Glycointeractions in bacterial pathogenesis

Many important interactions between bacterial pathogens and their hosts are highly specific binding events that involve host or pathogen carbohydrate structures (glycans). Glycan interactions can mediate adhesion, invasion and immune evasion and can act as receptors for toxins. Several bacterial pathogens can also enzymatically alter host glycans to reveal binding targets, degrade the host cell glycans or alter the function of host glycoproteins. In recent years, high-throughput screening technologies, such as lectin, glycan and mucin microarrays, have transformed the field by identifying new bacterial-host glycointeractions, which are crucial for colonization, persistence and disease. In this Review, we discuss interactions involving both host and bacterial glycans that have a role in bacterial pathogenesis. We also highlight recent technological advances that have illuminated the glycoscience of microbial pathogenesis.

Infection with genotoxin-producing Salmonella enterica synergises with loss of the tumour suppressor APC in promoting genomic instability via the PI3K pathway in colonic epithelial cells

Several commensal and pathogenic Gram‐negative bacteria produce DNA‐damaging toxins that are considered bona fide carcinogenic agents. The microbiota of colorectal cancer (CRC) patients is enriched in genotoxin‐producing bacteria, but their role in the pathogenesis of CRC is poorly understood. The adenomatous polyposis coli (APC) gene is mutated in familial adenomatous polyposis and in the majority of sporadic CRCs. We investigated whether the loss of APC alters the response of colonic epithelial cells to infection by Salmonella enterica, the only genotoxin‐producing bacterium associated with cancer in humans. Using 2D and organotypic 3D cultures, we found that APC deficiency was associated with sustained activation of the DNA damage response, reduced capacity to repair different types of damage, including DNA breaks and oxidative damage, and failure to induce cell cycle arrest. The reduced DNA repair capacity and inability to activate adequate checkpoint responses was associated with increased genomic instability in APC‐deficient cells exposed to the genotoxic bacterium. Inhibition of the checkpoint response was dependent on activation of the phosphatidylinositol 3‐kinase pathway. These findings highlight the synergistic effect of the loss of APC and infection with genotoxin‐producing bacteria in promoting a microenvironment conducive to malignant transformation.

Alternate subunit assembly diversifies the function of a bacterial toxin

Bacterial toxins with an AB₅ architecture consist of an active (A) subunit inserted into a ring-like platform comprised of five delivery (B) subunits. Salmonella Typhi, the cause of typhoid fever, produces an unusual A₂B₅ toxin known as typhoid toxin. Here, we report that upon infection of human cells, S. Typhi produces two forms of typhoid toxin that have distinct delivery components but share common active subunits. The two typhoid toxins exhibit different trafficking properties, elicit different effects when administered to laboratory animals, and are expressed using different regulatory mechanisms and in response to distinct metabolic cues. Collectively, these results indicate that the evolution of two typhoid toxin variants has conferred functional versatility to this virulence factor. More broadly, this study reveals a new paradigm in toxin biology and suggests that the evolutionary expansion of AB₅ toxins was likely fueled by the plasticity inherent to their structural design coupled to the functional versatility afforded by the combination of homologous toxin components.

Salmonella Typhi produces the typhoid toxin. Here, Fowler et al. show that S. Typhi produces two forms of typhoid toxin that are differentially regulated and display different trafficking properties and different effects when administered to laboratory animals.

The ADP-Ribosylating Toxins of Salmonella

A number of pathogenic bacteria utilize toxins to mediate disease in a susceptible host. The foodborne pathogen Salmonella is one of the most important and well-studied bacterial pathogens. Recently, whole genome sequence characterizations revealed the presence of multiple novel ADP-ribosylating toxins encoded by a variety of Salmonella serovars. In this review, we discuss both the classical (SpvB) and novel (typhoid toxin, ArtAB, and SboC/SeoC) ADP-ribosylating toxins of Salmonella, including the structure and function of these toxins and our current understanding of their contributions to virulence.

Investigation of the role of typhoid toxin in acute typhoid fever in a human challenge model

Salmonella Typhi is a human host-restricted pathogen that is responsible for typhoid fever in approximately 10.9 million people annually¹. The typhoid toxin is postulated to have a central role in disease pathogenesis, the establishment of chronic infection and human host restriction^2–6. However, its precise role in typhoid disease in humans is not fully defined. We studied the role of typhoid toxin in acute infection using a randomized, double-blind S. Typhi human challenge model⁷. Forty healthy volunteers were randomized (1:1) to oral challenge with 10⁴ colony-forming units of wild-type or an isogenic typhoid toxin deletion mutant (TN) of S. Typhi. We observed no significant difference in the rate of typhoid infection (fever ≥38 °C for ≥12 h and/or S. Typhi bacteremia) between participants challenged with wild-type or TN S. Typhi (15 out of 21 (71%) versus 15 out of 19 (79%); P = 0.58). The duration of bacteremia was significantly longer in participants challenged with the TN strain compared with wild-type (47.6 hours (28.9–97.0) versus 30.3(3.6–49.4); P ≤ 0.001). The clinical syndrome was otherwise indistinguishable between wild-type and TN groups. These data suggest that the typhoid toxin is not required for infection and the development of early typhoid fever symptoms within the context of a human challenge model. Further clinical data are required to assess the role of typhoid toxin in severe disease or the establishment of bacterial carriage.

Typhoid toxin is not essential for the pathogenesis of typhoid fever in healthy humans challenged with Salmonella Typhi.

Embracing Diversity: Differences in Virulence Mechanisms, Disease Severity, and Host Adaptations Contribute to the Success of Nontyphoidal Salmonella as a Foodborne Pathogen

Not all Salmonella enterica serovars cause the same disease. S. enterica represents an incredibly diverse species comprising >2,600 unique serovars. While some S. enterica serovars are host-restricted, others infect a wide range of hosts. The diseases that nontyphoidal Salmonella (NTS) serovars cause vary considerably, with some serovars being significantly more likely to cause invasive disease in humans than others. Furthermore, while genomic analyses have advanced our understanding of the genetic diversity of these serovars, they have not been able to fully account for the observed clinical differences. One overarching challenge is that much of what is known about Salmonella's general biology and virulence strategies is concluded from studies examining a select few serovars, especially serovar Typhimurium. As targeted control strategies have been implemented to control select serovars, an increasing number of foodborne outbreaks involving serovars that are less frequently associated with human clinical illness are being detected. Harnessing what is known about the diversity of NTS serovars represents an important factor in achieving the ultimate goal of reducing salmonellosis-associated morbidity and mortality worldwide. In this review we summarize the current understanding of the differences and similarities among NTS serovars, highlighting the virulence mechanisms, genetic differences, and sources that characterize S. enterica diversity and contribute to its success as a foodborne pathogen.

Serodiagnostic evaluation of recombinant CdtB of S. Typhi as a potential candidate for acute typhoid

Typhoid fever caused by human restricted Salmonella typhi presents a considerable health burden on developing South-Asian nations like India. The suboptimal sensitivity and specificity associated with culture-based isolation of etiological agent and the extensively used surface antigen-based serological assays often lead to misdiagnosis and inappropriate antimicrobial treatment. The increasing reports of the emergence of resistant strains and undefined disease burden signify the critical need for an inexpensive, reliable, easy-to-use, and highly sensitive diagnostic test for typhoid fever. Utilizing S. typhi-specific and immunogenic antigens in sero-diagnostic assays could lead to precise diagnosis of acute typhoid and prompt treatment. In this study, we report cloning, expression, and purification of recombinant Cytolethal distending toxin subunit B (CdtB) of S. typhi, which is reported to be highly specific, immunogenic, and expressed only upon S. typhi infection. We further evaluated the purified recombinant CdtB for its diagnostic potential in an IgM-based indirect ELISA format using 33 human samples. Twenty-one serum samples from blood culture confirmed cases (n = 21) of typhoid and 12 samples from healthy controls (n = 12) were tested. The assay showed sensitivity of 100% and specificity of 83.3% respectively with positive and negative predictive values of 91.3 and 100% respectively. Efficient detection of specific IgM antibodies indicates that CdtB could be highly valuable in sero-diagnosis of acute typhoid and rapid screening of clinical samples.

The S. Typhi effector StoD is an E3/E4 ubiquitin ligase which binds K48- and K63-linked diubiquitin

Salmonella Typhi is estimated to cause 100,000–200,000 deaths annually, yet its infection strategy remains elusive. This article reports of the first Typhi-specific effector, which has an E3/E4 ubiquitin ligase activity and can uniquely bind K48- and K63-linked diubiquitin.

Salmonella enterica (e.g., serovars Typhi and Typhimurium) relies on translocation of effectors via type III secretion systems (T3SS). Specialization of typhoidal serovars is thought to be mediated via pseudogenesis. Here, we show that the Salmonella Typhi STY1076/t1865 protein, named StoD, a homologue of the enteropathogenic Escherichia coli/enterohemorrhagic E. coli/Citrobacter rodentium NleG, is a T3SS effector. The StoD C terminus (StoD-C) is a U-box E3 ubiquitin ligase, capable of autoubiquitination in the presence of multiple E2s. The crystal structure of the StoD N terminus (StoD-N) at 2.5 Å resolution revealed a ubiquitin-like fold. In HeLa cells expressing StoD, ubiquitin is redistributed into puncta that colocalize with StoD. Binding assays showed that StoD-N and StoD-C bind the same exposed surface of the β-sheet of ubiquitin, suggesting that StoD could simultaneously interact with two ubiquitin molecules. Consistently, StoD interacted with both K63- (K_D = 5.6 ± 1 μM) and K48-linked diubiquitin (K_D = 15 ± 4 μM). Accordingly, we report the first S. Typhi–specific T3SS effector. We suggest that StoD recognizes and ubiquitinates pre-ubiquitinated targets, thus subverting intracellular signaling by functioning as an E4 enzyme.

Unique features in the intracellular transport of typhoid toxin revealed by a genome-wide screen

Typhoid toxin is a virulence factor for Salmonella Typhi and Paratyphi, the cause of typhoid fever in humans. This toxin has a unique architecture in that its pentameric B subunit, made of PltB, is linked to two enzymatic A subunits, the ADP ribosyl transferase PltA and the deoxyribonuclease CdtB. Typhoid toxin is uniquely adapted to humans, recognizing surface glycoprotein sialoglycans terminated in acetyl neuraminic acid, which are preferentially expressed by human cells. The transport pathway to its cellular targets followed by typhoid toxin after receptor binding is currently unknown. Through a genome-wide CRISPR/Cas9-mediated screen we have characterized the mechanisms by which typhoid toxin is transported within human cells. We found that typhoid toxin hijacks specific elements of the retrograde transport and endoplasmic reticulum-associated degradation machineries to reach its subcellular destination within target cells. Our study reveals unique and common features in the transport mechanisms of bacterial toxins that could serve as the bases for the development of novel anti-toxin therapeutic strategies.

Cell transfection of purified cytolethal distending toxin B subunits allows comparing their nuclease activity while plasmid degradation assay does not

The Cytolethal Distending Toxin (CDT) is produced by many pathogenic bacteria. CDT is known to induce genomic DNA damage to host eukaryotic cells through its catalytic subunit, CdtB. CdtB is structurally homologous to DNase I and has a nuclease activity, dependent on several key residues. Yet some differences between various CdtB subunit activities, and discrepancies between biochemical and cellular data, have been observed. To better characterise the role of CdtB in the induction of DNA damage, we affinity-purified wild-type and mutants of CdtB, issued from E. coli and H. ducreyi, under native and denaturing conditions. We then compared their nuclease activity by a classic in vitro assay using plasmid DNA, and two different eukaryotic assays–the first assay where host cells were transfected with a plasmid encoding CdtB, the second assay where host cells were directly transfected with purified CdtB. We show here that in vitro nuclease activities are difficult to quantify, whereas CdtB activities in host cells can be easily interpreted and confirmed the loss of function of the catalytic mutant. Our results highlight the importance of performing multiple assays while studying the effects of bacterial genotoxins, and indicate that the classic in vitro assay should be complemented with cellular assays.

Salmonella Infection in Chronic Inflammation and Gastrointestinal Cancer

Salmonella not only causes acute infections, but can also cause patients to become chronic "asymptomatic" carriers. Salmonella has been verified as a pathogenic factor that contributes to chronic inflammation and carcinogenesis. This review summarizes the acute and chronic Salmonella infection and describes the current research progress of Salmonella infection contributing to inflammatory bowel disease and cancer. Furthermore, this review explores the underlying biological mechanism of the host signaling pathways manipulated by Salmonella effector molecules. Using experimental animal models, researchers have shown that Salmonella infection is related to host biological processes, such as host cell transformation, stem cell maintenance, and changes of the gut microbiota (dysbiosis). Finally, this review discusses the current challenges and future directions in studying Salmonella infection and its association with human diseases.

Coordinated existence of multiple gangliosides is required for cartilage metabolism

OBJECTIVE

Gangliosides, ubiquitously existing membrane components that modulate transmembrane signaling and mediate cell-to-cell and cell-to-matrix interactions, are key molecules of inflammatory and neurological disorders. However, the functions of gangliosides in the cartilage degradation process remain unclear. We investigated the functional role of gangliosides in cartilage metabolism related to osteoarthritis (OA) pathogenesis.

DESIGN

We generated knockout (KO) mice by targeting the β1, 4-N-acetylgalactosaminyltransferase (GalNAcT) gene, which encodes an enzyme of major gangliosides synthesis, and the GD3 synthase (GD3S) gene, which encodes an enzyme of partial gangliosides synthesis. In vivo OA and in vitro cartilage degradation models were used to evaluate the effect of gangliosides on the cartilage degradation process.

RESULTS

The GalNAcT and GD3S KO mice developed and grew normally; nevertheless, OA changes in these mice were enhanced with aging. The GalNAcT KO mice showed significantly enhanced OA progression compared to GD3S mice in vivo. Both GalNAcT and GD3S KO mice showed severe IL-1α-induced cartilage degradation ex vivo. Phosphorylation of MAPKs was enhanced in both GalNAcT and GD3S KOs after IL-1α stimulation. Gangliosides modulated by GalNAcT or GD3S rescued an increase of MMP-13 induced by IL-1α in mice lacking GalNAcT or GD3S after exogenous replenishment in vitro.

CONCLUSION

These data show that the deletion of gangliosides in mice enhanced OA development. Moreover, the gangliosides modulated by GalNAcT are important for cartilage metabolism, suggesting that GalNAcT is a potential target molecule for the development of novel OA treatments.

Peptidoglycan editing by a specific LD-transpeptidase controls the muramidase-dependent secretion of typhoid toxin

Protein secretion mechanisms are essential for the virulence of most bacterial pathogens. Typhoid toxin is an essential virulence factor for Salmonella Typhi, the cause of typhoid fever in humans. This toxin is unique in that it is only produced within mammalian cells, and it must be trafficked to the extracellular space before intoxicating target cells. An essential and poorly understood aspect of this transport pathway is the secretion of typhoid toxin from the bacterium into the S. Typhi-containing vacuole. We show here that typhoid toxin secretion requires its translocation to the trans side of the peptidoglycan layer at the bacterial poles for subsequent release through the outer membrane. This translocation process depends on a specialized muramidase, the activity of which requires the localized editing of peptidoglycan by a specific ld-transpeptidase. These studies describe a protein export mechanism that is probably conserved in other bacterial species.

Salmonella infection - prevention and treatment by antibiotics and probiotic yeasts: a review

Global Salmonella infection, especially in developing countries, is a health and economic burden. The use of antibiotic drugs in treating the infection is proving less effective due to the alarming rise of antibiotic-resistant strains of Salmonella, the effects of antibiotics on normal gut microflora and antibiotic-associated diarrhoea, all of which bring a growing need for alternative treatments, including the use of probiotic micro-organisms. However, there are issues with probiotics, including their potential to be opportunistic pathogens and antibiotic-resistant carriers, and their antibiotic susceptibility if used as complementary therapy. Clinical trials, animal trials and in vitro investigations into the prophylactic and therapeutic efficacies of probiotics have demonstrated antagonistic properties against Salmonella and other enteropathogenic bacteria. Nonetheless, there is a need for further studies into the potential mechanisms, efficacy and mode of delivery of yeast probiotics in Salmonella infections. This review discusses Salmonella infections and treatment using antibiotics and probiotics.

Decoding a Salmonella Typhi Regulatory Network that Controls Typhoid Toxin Expression within Human Cells

Salmonella Typhi is the cause of typhoid fever, a major global health concern. An essential virulence factor of this pathogen is typhoid toxin. In contrast to most AB-type toxins, typhoid toxin is exclusively expressed by intracellular bacteria. The regulatory networks that ensure this unique gene expression pattern are unknown. Here, we developed FAST-INSeq, a genome-wide screening approach to identify S. Typhi genes required for typhoid toxin expression within infected cells. We find that typhoid toxin expression is controlled by a silencing and counter-silencing mechanism through the opposing actions of the PhoP/PhoQ two-component regulatory system and the histone-like protein H-NS. The screen also identified bacterial mutants that alter the proportion of intracellular S. Typhi that reside within an intravacuolar environment, which was essential for toxin expression. Collectively, these data describe a regulatory mechanism that allows a bacterial pathogen to exclusively express a virulence factor when located within a specific intracellular compartment.

Why Is Eradicating Typhoid Fever So Challenging: Implications for Vaccine and Therapeutic Design

Salmonella enterica serovar Typhi (S. Typhi) and S. Paratyphi, namely typhoidal Salmonellae, are the cause of (para) typhoid fever, which is a devastating systemic infectious disease in humans. In addition, the spread of multidrug-resistant (MDR) and extensively drug-resistant (XDR) S. Typhi in many low and middle-income countries poses a significant risk to human health. While currently available typhoid vaccines and therapeutics are efficacious, they have some limitations. One important limitation is the lack of controlling individuals who chronically carry S. Typhi. However, due to the strict host specificity of S. Typhi to humans, S. Typhi research is hampered. As a result, our understanding of S. Typhi pathogenesis is incomplete, thereby delaying the development and improvement of prevention and treatment strategies. Nonetheless, to better combat and contain S. Typhi, it is vital to develop a vaccine and therapy for controlling both acutely and chronically infected individuals. This review discusses how scientists are trying to combat typhoid fever, why it is so challenging to do so, which approaches show promise, and what we know about the pathogenesis of S. Typhi chronic infection.

Evolution of Salmonella within Hosts

Within-host evolution has resulted in thousands of variants of Salmonella that exhibit remarkable diversity in host range and disease outcome, from broad host range to exquisite host restriction, causing gastroenteritis to disseminated disease such as typhoid fever. Within-host evolution is a continuing process driven by genomic variation that occurs during each infection, potentiating adaptation to a new niche resulting from changes in animal husbandry, the use of antimicrobials, and emergence of immune compromised populations. We discuss key advances in our understanding of the evolution of Salmonella within the host, inferred from (i) the process of host adaptation of Salmonella pathovars in the past, and (ii) direct observation of the generation of variation and selection of beneficial traits during single infections.

Salmonella is a bacterial pathogen with remarkable diversity in its host range and pathogenicity due to past within-host evolution in vertebrate species that modified ancestral mechanisms of pathogenesis.

Variation arising during infection includes point mutations, new genes acquired through horizontal gene transfer (HGT), deletions, and genomic rearrangements.

Beneficial mutations increase in frequency within the host and, if they retain the ability to be transmitted to subsequent hosts, may become fixed in the population.

Whole-genome sequencing of sequential isolates from clinical infections reveals within-host HGT and point mutations that impact therapy and clinical management.

HGT is the primary mechanism for evolution in prokaryotes and is synergised by complex networks of transfer involving the microbiome.

Within-host evolution of Salmonella, resulting in new pathovars, can proceed in the absence of HGT.

Evolution of sialic acids: Implications in xenotransplant biology

All living cells are covered with a dense “sugar-coat” of carbohydrate chains (glycans) conjugated to proteins and lipids. The cell surface glycome is determined by a non-template driven process related to the collection of enzymes that assemble glycans in a sequential manner. In mammals, many of these glycans are topped with sialic acids (Sia), a large family of acidic sugars. The “Sialome” is highly diverse owing to various Sia types, linkage to underlying glycans, range of carriers, and complex spatial organization. Presented at the front of cells, Sia play a major role in immunity and recognition of “self” versus “non-self,” largely mediated by the siglecs family of Sia-binding host receptors. Albeit many mammalian pathogens have evolved to hijack this recognition system to avoid host immune attack, presenting a fascinating host-pathogen evolutionary arms race. Similarly, cancer cells exploit Sia for their own survival and propagation. As part of this ongoing fitness, humans lost the ability to synthesize the Sia type N-glycolylneuraminic acid (Neu5Gc), in contrast to other mammals. While this loss had provided an advantage against certain pathogens, humans are continuously exposed to Neu5Gc through mammalian-derived diet (eg, red meat), consequently generating a complex immune response against it. Circulating anti-Neu5Gc antibodies together with Neu5Gc on some human tissues mediate chronic inflammation “xenosialitis” that exacerbate various human diseases (eg, cancer and atherosclerosis). Similarly, Neu5Gc-containing xenografts are exposed to human anti-Neu5Gc antibodies with implications to sustainability. This review aimed to provide a glimpse into the evolution of Sia and their implications to xenotransplantation.

Clonal analysis of Salmonella-specific effector T cells reveals serovar-specific and cross-reactive T cell responses

To tackle the complexity of cross-reactive and pathogen-specific T cell responses against related Salmonella serovars, we used mass cytometry, unbiased single-cell cloning, live fluorescence barcoding, and T cell-receptor sequencing to reconstruct the Salmonella-specific repertoire of circulating effector CD4⁺ T cells, isolated from volunteers challenged with Salmonella enterica serovar Typhi (S. Typhi) or Salmonella Paratyphi A (S. Paratyphi). We describe the expansion of cross-reactive responses against distantly related Salmonella serovars and of clonotypes recognizing immunodominant antigens uniquely expressed by S. Typhi or S. Paratyphi A. In addition, single-amino acid variations in two immunodominant proteins, CdtB and PhoN, lead to the accumulation of T cells that do not cross-react against the different serovars, thus demonstrating how minor sequence variations in a complex microorganism shape the pathogen-specific T cell repertoire. Our results identify immune-dominant, serovar-specific, and cross-reactive T cell antigens, which should aid in the design of T cell-vaccination strategies against Salmonella.

Toxins of Locus of Enterocyte Effacement-Negative Shiga Toxin-Producing Escherichia coli

Studies on Shiga toxin-producing Escherichia coli (STEC) typically examine and classify the virulence gene profiles based on genomic analyses. Among the screened strains, a subgroup of STEC which lacks the locus of enterocyte effacement (LEE) has frequently been identified. This raises the question about the level of pathogenicity of such strains. This review focuses on the advantages and disadvantages of the standard screening procedures in virulence profiling and summarizes the current knowledge concerning the function and regulation of toxins encoded by LEE-negative STEC. Although LEE-negative STEC usually come across as food isolates, which rarely cause infections in humans, some serotypes have been implicated in human diseases. In particular, the LEE-negative E. coli O104:H7 German outbreak strain from 2011 and the Australian O113:H21 strain isolated from a HUS patient attracted attention. Moreover, the LEE-negative STEC O113:H21 strain TS18/08 that was isolated from minced meat is remarkable in that it not only encodes multiple toxins, but in fact expresses three different toxins simultaneously. Their characterization contributes to understanding the virulence of the LEE-negative STEC.

Human Genetic Markers and Structural Prediction of Plasmodium falciparum Multidrug Resistance Gene (pfmdr1) for Ligand Binding in Pregnant Women Attending General Hospital Minna

The study aims to determine the association of malaria infection with ABO blood groups and genotype and also to detect point mutations at positions 86, 184, 1034, and 1042 of the Plasmodium falciparum multidrug resistance gene (pfmdr1) in blood samples collected from pregnant women attending General Hospital Minna. Out of 250 pregnant women screened, 39 (15.60%) had malaria infection. Prevalence was higher in women, during the third trimester (46.15%), genotype AA (64.10%), and O blood group (53.84%) individuals when compared with others. There was significant (p < 0.05) decrease in Packed Cell Volume (PCV), hemoglobin (HGB), Red Blood Cells (RBC), and platelet (PLC) count in infected group when compared with noninfected group. Although, two of the isolates showed disrupted protein sequence at codon 1034–1042, no mutation was found in any of the P. falciparum isolates. Structural prediction of chemical ligand led to the identification of Neu5Acα2-3Galβ1-3/β1-4Glc/GlcNAc. This compound can theoretically bind and change the functional integrity of the pfmdr1 protein, thus providing a new window for malaria drug target.

Activation of coagulation and endothelium with concurrent impairment of anticoagulant mechanisms in patients with typhoid fever

OBJECTIVES

Typhoid fever caused by Salmonella Typhi remains a major burden worldwide. Gastrointestinal bleeding can be seen in up to 10 percent of patients and may be fatal. The coagulopathy, which may be the driver of this severe complication in patients with typhoid fever, however is ill defined. The aim of this study was to evaluate the activation of coagulation, anticoagulation, and fibrinolysis in patients with acute typhoid fever.

METHODS

Parameters of coagulation and fibrinolysis were measured in 28 hospitalized patients with culture-confirmed or PCR-confirmed typhoid fever and compared to 38 age- and sex-matched healthy volunteers.

RESULTS

Patients demonstrated activation of the coagulation system, as reflected by elevated in vitro thrombin generation and high plasma levels of fibrinogen, D-dimer and prothrombin fragment F1 + 2 in concert with consumption of coagulation factors resulting in a prolonged prothrombin-time and activated-partial-thromboplastin-time. Concurrently, the anticoagulant proteins, protein C and antithrombin, were significantly lower in comparison to healthy controls. Patients also demonstrated evidence of activation and inhibition of fibrinolysis and a marked activation of endothelial cells. The extent of coagulation activation was associated with the course of the disease, repeated testing during convalescence showed a return toward normal values.

CONCLUSIONS

Activation of coagulation is an important clinical feature of typhoid fever and is associated with severity of disease.

The Typhoid Toxin Produced by the Nontyphoidal Salmonella enterica Serotype Javiana Is Required for Induction of a DNA Damage Response In Vitro and Systemic Spread In Vivo

The Salmonella cytolethal distending toxin (S-CDT), first described as the “typhoid toxin” in Salmonella enterica subsp. enterica serotype Typhi, induces DNA damage in eukaryotic cells. Recent studies have shown that more than 40 nontyphoidal Salmonella (NTS) serotypes carry genes that encode S-CDT, yet very little is known about the activity, function, and role of S-CDT in NTS. Here we show that deletion of genes encoding the binding subunit (pltB) and a bacteriophage muramidase predicted to play a role in toxin export (ttsA) does not abolish toxin activity in the S-CDT-positive NTS Salmonella enterica subsp. enterica serotype Javiana. However, S. Javiana strains harboring deletions of both pltB and its homolog artB, had a complete loss of S-CDT activity, suggesting that S. Javiana carries genes encoding two variants of the binding subunit. S-CDT-mediated DNA damage, as determined by phosphorylation of histone 2AX (H2AX), producing phosphorylated H2AX (γH2AX), was restricted to epithelial cells in S and G₂/M phases of the cell cycle and did not result in apoptosis or cell death. Compared to mice infected with a ΔcdtB strain, mice infected with wild-type S. Javiana had significantly higher levels of S. Javiana in the liver, but not in the spleen, ileum, or cecum. Overall, we show that production of active S-CDT by NTS serotype S. Javiana requires different genes (cdtB, pltA, and either pltB or artB) for expression of biologically active toxin than those reported for S-CDT production by S. Typhi (cdtB, pltA, pltB, and ttsA). However, as in S. Typhi, NTS S-CDT influences the outcome of infection both in vitro and in vivo.

Nontyphoidal Salmonella (NTS) are a major cause of bacterial food-borne illness worldwide; however, our understanding of virulence mechanisms that determine the outcome and severity of nontyphoidal salmonellosis is incompletely understood. Here we show that S-CDT produced by NTS plays a significant role in the outcome of infection both in vitro and in vivo, highlighting S-CDT as an important virulence factor for nontyphoidal Salmonella serotypes. Our data also contribute novel information about the function of S-CDT, as S-CDT-mediated DNA damage occurs only during certain phases of the cell cycle, and the resulting damage does not induce cell death as assessed using a propidium iodide exclusion assay. Importantly, our data support that, despite having genetically similar S-CDT operons, NTS serotype S. Javiana has different genetic requirements than S. Typhi, for the production and export of active S-CDT.

In vivo tropism of Salmonella Typhi toxin to cells expressing a multiantennal glycan receptor

Typhoid fever is a life-threatening disease, but little is known about the molecular bases for its unique clinical presentation. Typhoid toxin, a unique virulence factor of Salmonella Typhi (the cause of typhoid fever), recapitulates in an animal model many symptoms of typhoid fever. Typhoid toxin binding to its glycan receptor Neu5Ac is central, but, due to the ubiquity of Neu5Ac, how typhoid toxin causes specific symptoms remains elusive. Here we show that typhoid toxin displays in vivo tropism to cells expressing multiantennal glycoprotein receptors, particularly on endothelial cells of arterioles in the brain and immune cells, which is in line with typhoid symptoms. Neu5Ac displayed by multiantennal N-glycans, rather than a single Neu5Ac, appears to serve as the high-affinity receptor, as typhoid toxin possesses five identical binding pockets per toxin. Human counterparts also express the multiantennal Neu5Ac receptor. Here we also show that mice immunized with inactive typhoid toxins and challenged with wild-type typhoid toxin presented neither the characteristic in vivo tropism nor symptoms. These mice were protected against a lethal-dose toxin challenge, but Ty21a-vaccinated mice were not. Cumulatively, these results reveal remarkable features describing how a bacterial exotoxin induces virulence exclusively in specific cells at the organismal level.