The capsule encoding the viaB locus reduces interleukin-17 expression and mucosal innate responses in the bovine intestinal mucosa during infection with Salmonella enterica serotype Typhi

Comparing Campylobacter jejuni to three other enteric pathogens in OligoMM12 mice reveals pathogen-specific host and microbiota responses

Campylobacter jejuni, non-typhoidal Salmonella spp., Listeria monocytogenes and enteropathogenic/enterohemorrhagic Escherichia coli (EPEC/EHEC) are leading causes of food-borne illness worldwide. Citrobacter rodentium has been used to model EPEC and EHEC infection in mice. The gut microbiome is well-known to affect gut colonization and host responses to many food-borne pathogens. Recent progress has established gnotobiotic mice as valuable models to study how microbiota affect the enteric infections by S. Typhimurium, C. rodentium and L. monocytogenes. However, for C. jejuni, we are still lacking a suitable gnotobiotic mouse model. Moreover, the limited comparability of data across laboratories is often negatively affected by variations between different research facilities or murine microbiotas. In this study, we applied the standardized gnotobiotic OligoMM12 microbiota mouse model and compared the infections in the same facility. We provide evidence of robust colonization and significant pathological changes in OligoMM12 mice following infection with these pathogens. Moreover, we offer insights into pathogen-specific host responses and metabolite signatures, highlighting the advantages of a standardized mouse model for direct comparisons of factors influencing the pathogenesis of major food-borne pathogens. Notably, we reveal for the first time that C. jejuni stably colonizes OligoMM12 mice, triggering inflammation. Additionally, our comparative approach successfully identifies pathogen-specific responses, including the detection of genes uniquely associated with C. jejuni infection in humans. These findings underscore the potential of the OligoMM12 model as a versatile tool for advancing our understanding of food-borne pathogen interactions.

Pathogenic diversification of the gut commensal Providencia alcalifaciens via acquisition of a second type III secretion system

Providencia alcalifaciens is a Gram-negative bacterium found in various water and land environments and organisms, including insects and mammals. Some P. alcalifaciens strains encode gene homologs of virulence factors found in pathogenic Enterobacterales members, such as Salmonella enterica serovar Typhimurium and Shigella flexneri. Whether these genes are pathogenic determinants in P. alcalifaciens is not known. In this study, we investigated P. alcalifaciens-host interactions at the cellular level, focusing on the role of two type III secretion systems (T3SS) belonging to the Inv-Mxi/Spa family. T3SS1b is widespread in Providencia spp. and encoded on the chromosome. A large plasmid that is present in a subset of P. alcalifaciens strains, primarily isolated from diarrheal patients, encodes for T3SS1a. We show that P. alcalifaciens 205/92 is internalized into eukaryotic cells, lyses its internalization vacuole, and proliferates in the cytosol. This triggers caspase-4-dependent inflammasome responses in gut epithelial cells. The requirement for the T3SS1a in entry, vacuole lysis, and cytosolic proliferation is host cell type-specific, playing a more prominent role in intestinal epithelial cells than in macrophages or insect cells. In a bovine ligated intestinal loop model, P. alcalifaciens colonizes the intestinal mucosa and induces mild epithelial damage with negligible fluid accumulation in a T3SS1a- and T3SS1b-independent manner. However, T3SS1b was required for the rapid killing of Drosophila melanogaster. We propose that the acquisition of two T3SS has allowed P. alcalifaciens to diversify its host range, from a highly virulent pathogen of insects to an opportunistic gastrointestinal pathogen of animals.

Pathogenic diversification of the gut commensal Providencia alcalifaciens via acquisition of a second type III secretion system

Providencia alcalifaciens is a Gram-negative bacterium found in a wide variety of water and land environments and organisms. It has been isolated as part of the gut microbiome of animals and insects, as well as from stool samples of patients with diarrhea. Specific P. alcalifaciens strains encode gene homologs of virulence factors found in other pathogenic members of the same Enterobacterales order, such as Salmonella enterica serovar Typhimurium and Shigella flexneri. Whether these genes are also pathogenic determinants in P. alcalifaciens is not known. Here we have used P. alcalifaciens 205/92, a clinical isolate, with in vitro and in vivo infection models to investigate P. alcalifaciens -host interactions at the cellular level. Our particular focus was the role of two type III secretion systems (T3SS) belonging to the Inv-Mxi/Spa family. T3SS 1b is widespread in Providencia spp. and encoded on the chromosome. T3SS 1a is encoded on a large plasmid that is present in a subset of P. alcalifaciens strains, which are primarily isolates from diarrheal patients. Using a combination of electron and fluorescence microscopy and gentamicin protection assays we show that P. alcalifaciens 205/92 is internalized into eukaryotic cells, rapidly lyses its internalization vacuole and proliferates in the cytosol. This triggers caspase-4 dependent inflammasome responses in gut epithelial cells. The requirement for the T3SS 1a in entry, vacuole lysis and cytosolic proliferation is host-cell type specific, playing a more prominent role in human intestinal epithelial cells as compared to macrophages. In a bovine ligated intestinal loop model, P. alcalifaciens colonizes the intestinal mucosa, inducing mild epithelial damage with negligible fluid accumulation. No overt role for T3SS 1a or T3SS 1b was seen in the calf infection model. However, T3SS 1b was required for the rapid killing of Drosophila melanogaster . We propose that the acquisition of two T3SS by horizontal gene transfer has allowed P. alcalifaciens to diversify its host range, from a highly virulent pathogen of insects to an opportunistic gastrointestinal pathogen of animals.

A TLR4 ligand-based adjuvant for promoting the immunogenicity of typhoid subunit vaccines

None of the typhoid Vi Polysaccharide (ViPS) subunit vaccines incorporate adjuvants, and the immunogenicity of ViPS vaccines (e.g. Typbar TCV® and Typhim Vi®) is in part due to associated TLR4 ligands such as endotoxin present in these vaccines. Since endotoxin content in vaccines is variable and kept very low due to inherent toxicity, it was hypothesized that incorporating a defined amount of a non-toxic TLR4-ligand such as monophosphoryl lipid A in ViPS vaccines would improve their immunogenicity. To test this hypothesis, a monophosphoryl lipid A-based adjuvant formulation named Turbo was developed. Admixing Turbo with Typbar TCV® (ViPS-conjugated to tetanus toxoid) increased the levels of anti-ViPS IgM, IgG1, IgG2b, IgG2a/c, and IgG3 in inbred and outbred mice. In infant mice, a single immunization with Turbo adjuvanted Typbar TCV® resulted in a significantly increased and durable IgG response and improved the control of bacterial burden compared to mice immunized without Turbo. Similarly, when adjuvanted with Turbo, the antibody response and control of bacteremia were also improved in mice immunized with Typhim Vi®, an unconjugated vaccine. The immunogenicity of unconjugated ViPS is inefficient in young mice and is lost in adult mice when immunostimulatory ligands in ViPS are removed. Nevertheless, when adjuvanted with Turbo, poorly immunogenic ViPS induced a robust IgG response in young and adult mice, and this was observed even under antigen-limiting conditions. These data suggest that incorporation of Turbo as an adjuvant will make typhoid vaccines more immunogenic regardless of their intrinsic immunogenicity or conjugation status and maximize the efficacy across all ages.

The Lack of Natural IgM Increases Susceptibility and Impairs Anti-Vi Polysaccharide IgG Responses in a Mouse Model of Typhoid

Circulating IgM present in the body prior to any apparent Ag exposure is referred to as natural IgM. Natural IgM provides protective immunity against a variety of pathogens. Salmonella enterica serovar Typhi (S. Typhi) is the causative agent of typhoid fever in humans. Because mice are not permissive to S. Typhi infection, we employed a murine model of typhoid using S. enterica serovar Typhimurium expressing the Vi polysaccharide (ViPS) of S. Typhi (S. Typhimurium strain RC60) to evaluate the role of natural IgM in pathogenesis. We found that natural mouse IgM binds to S. Typhi and S. Typhimurium. The severity of S. Typhimurium infection in mice is dependent on presence of the natural resistance-associated macrophage protein 1 (Nramp1) allele; therefore, we infected mice deficient in secreted form of IgM (sIgM) on either a Nramp1-resistant (129S) or -susceptible (C57BL/6J) background. We found that the lack of natural IgM results in a significantly increased susceptibility and an exaggerated liver pathology regardless of the route of infection or the Nramp1 allele. Reconstitution of sIgM-/- mice with normal mouse serum or purified polyclonal IgM restored the resistance to that of sIgM+/+ mice. Furthermore, immunization of sIgM-/- mice with heat-killed S. Typhi induced a significantly reduced anti-ViPS IgG and complement-dependent bactericidal activity against S. Typhi in vitro, compared with that of sIgM+/+ mice. These findings indicate that natural IgM is an important factor in reducing the typhoid severity and inducing an optimal anti-ViPS IgG response to vaccination.

Distribution of Ciprofloxacin- and Azithromycin-Resistant Genes among Salmonella Typhi Isolated from Human Blood

Context

Salmonella Typhi has developed resistance to different groups of antibiotics.

Aims

The purpose of the present study was to assess the distribution of ciprofloxacin- and azithromycin-resistant genes among Salmonella Typhi isolated from human blood.

Settings and Design

This cross-sectional study was conducted in the Department of Microbiology of a tertiary care hospital in Bangladesh from July 2019-June 2020.

Subjects and Methods

Clinically suspected enteric fever patients, irrespective of age and gender, who attended the laboratory of the Department of Microbiology and outpatient department of Medicine of tertiary care hospital. Blood culture and sensitivity tests were done. The positive growth of Salmonella Typhi was identified by Gram staining, colony morphology, and biochemical test. Then, Salmonella Typhi was identified by using Salmonella-specific antisera. Final identification was made by using 16s rRNA by polymerase chain reaction (PCR). PCR was also done to detect quinolone and azithromycin resistance genes.

Results

A total number of 83 samples yielded positive cultures, of which 50 isolated organisms were identified as Salmonella species; however, among these isolates, Salmonella Typhi was detected in 40 (48.2%) isolates. Among 12 ciprofloxacin-resistant isolates, 8 (66.67%) were positive for the gyrA gene, 1 (8.33%) was positive for the qnrB gene and qnrS gene, 2 (16.67%) were positive for aac (6´)-Ib-cr. Among 12 azithromycin-resistant isolates, 2 (16.66%) were positive for mphA and mefA genes, respectively.

Conclusion

In conclusion, the gyrA, aac (6´)-Ib-cr, mphA, and mefA genes are found for the first time in tertiary care hospitals from the quinolones and azithromycin-resistant Salmonella Typhi.

Dual RNA sequencing reveals dendritic cell reprogramming in response to typhoidal Salmonella invasion

Salmonella enterica represent a major disease burden worldwide. S. enterica serovar Typhi (S. Typhi) is responsible for potentially life-threatening Typhoid fever affecting 10.9 million people annually. While non-typhoidal Salmonella (NTS) serovars usually trigger self-limiting diarrhoea, invasive NTS bacteraemia is a growing public health challenge. Dendritic cells (DCs) are key professional antigen presenting cells of the human immune system. The ability of pathogenic bacteria to subvert DC functions and prevent T cell recognition contributes to their survival and dissemination within the host. Here, we adapted dual RNA-sequencing to define how different Salmonella pathovariants remodel their gene expression in tandem with that of infected DCs. We find DCs harness iron handling pathways to defend against invading Salmonellas, which S. Typhi is able to circumvent by mounting a robust response to nitrosative stress. In parallel, we uncover the alternative strategies invasive NTS employ to impair DC functions.

Salmonella Paratyphi A Outer Membrane Vesicles Displaying Vi Polysaccharide as a Multivalent Vaccine against Enteric Fever

Typhoid and paratyphoid fevers have a high incidence worldwide and coexist in many geographical areas, especially in low-middle-income countries (LMIC) in South and Southeast Asia. There is extensive consensus on the urgent need for better and affordable vaccines against systemic Salmonella infections. Generalized modules for membrane antigens (GMMA), outer membrane exosomes shed by Salmonella bacteria genetically manipulated to increase blebbing, resemble the bacterial surface where protective antigens are displayed in their native environment. Here, we engineered S Paratyphi A using the pDC5-viaB plasmid to generate GMMA displaying the heterologous S Typhi Vi antigen together with the homologous O:2 O antigen. The presence of both Vi and O:2 was confirmed by flow cytometry on bacterial cells, and their amount was quantified on the resulting vesicles through a panel of analytical methods. When tested in mice, such GMMA induced a strong antibody response against both Vi and O:2, and these antibodies were functional in a serum bactericidal assay. Our approach yielded a bivalent vaccine candidate able to induce immune responses against different Salmonella serovars, which could benefit LMIC residents and travelers.

A Salmonella Typhi RNA thermosensor regulates virulence factors and innate immune evasion in response to host temperature

Sensing and responding to environmental signals is critical for bacterial pathogens to successfully infect and persist within hosts. Many bacterial pathogens sense temperature as an indication they have entered a new host and must alter their virulence factor expression to evade immune detection. Using secondary structure prediction, we identified an RNA thermosensor (RNAT) in the 5' untranslated region (UTR) of tviA encoded by the typhoid fever-causing bacterium Salmonella enterica serovar Typhi (S. Typhi). Importantly, tviA is a transcriptional regulator of the critical virulence factors Vi capsule, flagellin, and type III secretion system-1 expression. By introducing point mutations to alter the mRNA secondary structure, we demonstrate that the 5' UTR of tviA contains a functional RNAT using in vitro expression, structure probing, and ribosome binding methods. Mutational inhibition of the RNAT in S. Typhi causes aberrant virulence factor expression, leading to enhanced innate immune responses during infection. In conclusion, we show that S. Typhi regulates virulence factor expression through an RNAT in the 5' UTR of tviA. Our findings demonstrate that limiting inflammation through RNAT-dependent regulation in response to host body temperature is important for S. Typhi's "stealthy" pathogenesis.

Vaccination and Infection of Swine With Salmonella Typhimurium Induces a Systemic and Local Multifunctional CD4+ T-Cell Response

The gram-negative facultative intracellular bacteria Salmonella Typhimurium (STM) often leads to subclinical infections in pigs, but can also cause severe enterocolitis in this species. Due to its high zoonotic potential, the pathogen is likewise dangerous for humans. Vaccination with a live attenuated STM strain (Salmoporc) is regarded as an effective method to control STM infections in affected pig herds. However, information on the cellular immune response of swine against STM is still scarce. In this study, we investigated the T-cell immune response in pigs that were vaccinated twice with Salmoporc followed by a challenge infection with a virulent STM strain. Blood- and organ-derived lymphocytes (spleen, tonsils, jejunal and ileocolic lymph nodes, jejunum, ileum) were stimulated in vitro with heat-inactivated STM. Subsequently, CD4⁺ T cells present in these cell preparations were analyzed for the production of IFN-γ, TNF-α, and IL-17A by flow cytometry and Boolean gating. Highest frequencies of STM-specific cytokine-producing CD4⁺ T cells were found in lamina propria lymphocytes of jejunum and ileum. Significant differences of the relative abundance of cytokine-producing phenotypes between control group and vaccinated + infected animals were detected in most organs, but dominated in gut and lymph node-residing CD4⁺ T cells. IL-17A producing CD4⁺ T cells dominated in gut and gut-draining lymph nodes, whereas IFN-γ/TNF-α co-producing CD4⁺ T cells were present in all locations. Additionally, the majority of cytokine-producing CD4⁺ T cells had a CD8α⁺CD27^- phenotype, indicative of a late effector or effector memory stage of differentiation. In summary, we show that Salmonella-specific multifunctional CD4⁺ T cells exist in vaccinated and infected pigs, dominate in the gut and most likely contribute to protective immunity against STM in the pig.

Hyperexpression of type III secretion system of Salmonella Typhi linked to a higher cytotoxic effect to monocyte-derived macrophages by activating inflammasome

Inflammasome activation is an important host response to infectious diseases, but the difference in inflammasome activation between typhoid fever and non-typhoidal Salmonella infection has been rarely studied. To determine whether inflammasome activation in macrophages after S. Typhi and S. Typhimurium infection is different, we measured pyroptosis, caspase-1 activation, and IL-1β secretion in monocyte-derived macrophages infected with S. Typhi or S. Typhimurium both in vitro and ex vivo. The role of Vi capsule and virulence genes in Salmonella pathogenicity island-1 (SPI-1), belonging to type III secretion system, was also examined. S. Typhi caused more pyroptosis, caspase-1 activation, and IL-1β production than S. Typhimurium did, predominantly within 2 h of infection, in the context of high number of infecting bacteria. Mutagenesis and complementation experiments confirmed that SPI-1 effectors but not Vi were associated with greater inflammasome activation. The expression levels of invA and hilA were significantly higher in S. Typhi than in S. Typhimurium at early log phase in SPI-1 environment. Thus, S. Typhi, relative to its non-typhoidal counterpart, S. Typhimurium, induces greater SPI-1-dependent inflammasome activation in monocyte-derived macrophages. This finding may explain why S. Typhi causes a hyperinflammatory state at bacteremic stage in typhoid fever.

Salmonella Extracellular Polymeric Substances Modulate Innate Phagocyte Activity and Enhance Tolerance of Biofilm-Associated Bacteria to Oxidative Stress

Salmonella enterica serovar Typhi causes 14.3 million acute cases of typhoid fever that are responsible for 136,000 deaths each year. Chronic infections occur in 3%-5% of those infected and S. Typhi persists primarily in the gallbladder by forming biofilms on cholesterol gallstones, but how these bacterial communities evade host immunity is not known. Salmonella biofilms produce several extracellular polymeric substances (EPSs) during chronic infection, which are hypothesized to prevent pathogen clearance either by protecting biofilm-associated bacteria from direct humoral attack or by modulating innate phagocyte interaction with biofilms. Using wild-type and EPS-deficient planktonic and biofilm Salmonella, the direct attack hypothesis was tested by challenging biofilms with human serum and antimicrobial peptides. Biofilms were found to be tolerant to these molecules, but these phenotypes were independent of the tested EPSs. By examining macrophage and neutrophil responses, new roles for biofilm-associated capsular polysaccharides and slime polysaccharides were identified. The S. Typhi Vi antigen was found to modulate innate immunity by reducing macrophage nitric oxide production and neutrophil reactive oxygen species (ROS) production. The slime polysaccharides colanic acid and cellulose were found to be immune-stimulating and represent a key difference between non-typhoidal serovars and typhoidal serovars, which do not express colanic acid. Furthermore, biofilm tolerance to the exogenously-supplied ROS intermediates hydrogen peroxide (H₂O₂) and hypochlorite (ClO) indicated an additional role of the capsular polysaccharides for both serovars in recalcitrance to H₂O₂ but not ClO, providing new understanding of the stalemate that arises during chronic infections and offering new directions for mechanistic and clinical studies.

Relative Contributions of Extracellular and Internalized Bacteria to Early Macrophage Proinflammatory Responses to Streptococcus pneumoniae

Both intracellular immune sensing and extracellular innate immune sensing have been implicated in initiating macrophage proinflammatory cytokine responses to Streptococcus pneumoniae The S. pneumoniae capsule, a major virulence determinant, prevents phagocytosis, and we hypothesized that this would reduce activation of host innate inflammatory responses by preventing activation of intracellular proinflammatory signaling pathways. We investigated this hypothesis in human monocyte-derived macrophages stimulated with encapsulated or isogenic unencapsulated mutant S. pneumoniae Unexpectedly, despite strongly inhibiting bacterial internalization, the capsule resulted in enhanced inflammatory cytokine production by macrophages infected with S. pneumoniae Experiments using purified capsule material and a Streptococcus mitis mutant expressing an S. pneumoniae serotype 4 capsule indicated these differences required whole bacteria and were not due to proinflammatory effects of the capsule itself. Transcriptional profiling demonstrated relatively few differences in macrophage gene expression profiles between infections with encapsulated S. pneumoniae and those with unencapsulated S. pneumoniae, largely limited to reduced expression of proinflammatory genes in response to unencapsulated bacteria, predicted to be due to reduced activation of the NF-κB family of transcription factors. Blocking S. pneumoniae internalization using cytochalasin D had minimal effects on the inflammatory response to S. pneumoniae Experiments using murine macrophages indicated that the affected genes were dependent on Toll-like receptor 2 (TLR2) activation, although not through direct stimulation of TLR2 by capsule polysaccharide. Our data demonstrate that the early macrophage proinflammatory response to S. pneumoniae is mainly dependent on extracellular bacteria and reveal an unexpected proinflammatory effect of encapsulated S. pneumoniae that could contribute to disease pathogenesis.IMPORTANCE Multiple extra- and intracellular innate immune receptors have been identified that recognize Streptococcus pneumoniae, but the relative contributions of intra- versus extracellular bacteria to the inflammatory response were unknown. We have shown that intracellular S. pneumoniae contributes surprisingly little to the inflammatory responses, with production of important proinflammatory cytokines largely dependent on extracellular bacteria. Furthermore, although we expected the S. pneumoniae polysaccharide capsule to block activation of the host immune system by reducing bacterial internalization and therefore activation of intracellular innate immune receptors, there was an increased inflammatory response to encapsulated compared to unencapsulated bacteria, which is likely to contribute to disease pathogenesis.

Mechanisms to Evade the Phagocyte Respiratory Burst Arose by Convergent Evolution in Typhoidal Salmonella Serovars

Typhoid fever caused by Salmonella enterica serovar (S.) Typhi differs in its clinical presentation from gastroenteritis caused by S. Typhimurium and other non-typhoidal Salmonella serovars. The different clinical presentations are attributed in part to the virulence-associated capsular polysaccharide (Vi antigen) of S. Typhi, which prevents phagocytes from triggering a respiratory burst by preventing antibody-mediated complement activation. Paradoxically, the Vi antigen is absent from S. Paratyphi A, which causes a disease that is indistinguishable from typhoid fever. Here, we show that evasion of the phagocyte respiratory burst by S. Paratyphi A required very long O antigen chains containing the O2 antigen to inhibit antibody binding. We conclude that the ability to avoid the phagocyte respiratory burst is a property distinguishing typhoidal from non-typhoidal Salmonella serovars that was acquired by S. Typhi and S. Paratyphi A independently through convergent evolution.

Musca domestica Cecropin (Mdc) Alleviates Salmonella typhimurium-Induced Colonic Mucosal Barrier Impairment: Associating With Inflammatory and Oxidative Stress Response, Tight Junction as Well as Intestinal Flora

Salmonella typhimurium, a Gram-negative food-borne pathogen, induces impairment in intestinal mucosal barrier function frequently. The injury is related to many factors such as inflammation, oxidative stress, tight junctions and flora changes in the host intestine. Musca domestica cecropin (Mdc), a novel antimicrobial peptide containing 40 amino acids, has potential antibacterial, anti-inflammatory, and immunological functions. It remains unclear exactly whether and how Mdc reduces colonic mucosal barrier damage caused by S. typhimurium. Twenty four 6-week-old male mice were divided into four groups: normal group, control group (S. typhimurium-challenged), Mdc group, and ceftriaxone sodium group (Cs group). HE staining and transmission electron microscopy (TEM) were performed to observe the morphology of the colon tissues. Bacterial load of S. typhimurium in colon, liver and spleen were determined by bacterial plate counting. Inflammatory factors were detected by enzyme linked immunosorbent assay (ELISA). Oxidative stress levels in the colon tissues were also analyzed. Immunofluorescence analysis, RT-PCR, and Western blot were carried out to examine the levels of tight junction and inflammatory proteins. The intestinal microbiota composition was assessed via 16s rDNA sequencing. We successfully built and evaluated an S. typhimurium-infection model in mice. Morphology and microcosmic change of the colon tissues confirmed the protective qualities of Mdc. Mdc could inhibit colonic inflammation and oxidative stress. Tight junctions were improved significantly after Mdc administration. Interestingly, Mdc ameliorated intestinal flora imbalance, which may be related to the improvement of tight junction. Our results shed a new light on protective effects and mechanism of the antimicrobial peptide Mdc on colonic mucosal barrier damage caused by S. typhimurium infection. Mdc is expected to be an important candidate for S. typhimurium infection treatment.

Model of Persistent Salmonella Infection: Salmonella enterica Serovar Pullorum Modulates the Immune Response of the Chicken from a Th17-Type Response towards a Th2-Type Response

Salmonella enterica infection affects a wide range of animals and humans, and a small number of serovars cause typhoid-like infections, one characteristic of which is persistent infection in convalescents. Avian-specific S. enterica serovar Pullorum produces systemic disease in young chickens, which is followed by a carrier state in convalescent birds, leading to infection of the ovary at sexual maturity and vertical transmission. However, the immunological basis of persistent infection remains unclear. S. enterica serovar Enteritidis is taxonomically closely related but does not show this characteristic. Differences in the immune responses between S Pullorum and S Enteritidis were compared by using Salmonella-infected chicken monocyte-derived macrophages (chMDMs) and CD4⁺ T lymphocytes that had been cocultured with infected chMDMs or chicken splenocytes in vitro and also in 2-day-old chickens in vivo In comparison with S Enteritidis, S Pullorum-infected chMDMs showed reduced mRNA expression levels of interleukin-12α (IL-12α) and IL-18 and stimulated the proliferation of Th2 lymphocytes, with reduced expression of gamma interferon (IFN-γ) and IL-17 and increased expression levels of IL-4 and IL-13 There was little evidence of clonal anergy or immune suppression induced by S Pullorum in vitro. S Pullorum also increased the levels of expression of IL-4 and decreased the levels of IFN-γ in the spleen and cecal tonsil of infected birds. This suggests that S Pullorum is able to modulate host immunity from a dominant IFN-γ-producing Th17 response toward a Th2 response, which may promote persistent infection in chickens. S Pullorum in chickens is presented as a good model of the typhoid group to study persistent infection.

An Unmutated IgM Response to the Vi Polysaccharide of Salmonella Typhi Contributes to Protective Immunity in a Murine Model of Typhoid

T cell-dependent B cell responses typically develop in germinal centers. Abs generated during such responses are isotype switched and have a high affinity to the Ag because of somatic hypermutation of Ab genes. B cell responses to purified polysaccharides are T cell independent and do not result in the formation of bona fide germinal centers, and the dominant Ab isotype produced during such responses is IgM with very few or no somatic mutations. Activation-induced cytidine deaminase (AID) is required for both somatic hypermutation and Ig isotype switching in humans and mice. To test the extent to which unmutated polysaccharide-specific IgM confers protective immunity, we immunized wildtype and AID^-/- mice with either heat-killed Salmonella enterica serovar Typhi (S. Typhi) or purified Vi polysaccharide (ViPS). We found that wildtype and AID^-/- mice immunized with heat-killed S. Typhi generated similar anti-ViPS IgM responses. As expected, wildtype, but not AID^-/- mice generated ViPS-specific IgG. However, the differences in the Ab-dependent killing of S. Typhi mediated by the classical pathway of complement activation were not statistically significant. In ViPS-immunized wildtype and AID^-/- mice, the ViPS-specific IgM levels and S. Typhi bactericidal Ab titers at 7 but not at 28 d postimmunization were also comparable. To test the protective immunity conferred by these immunizations, mice were challenged with a chimeric S. Typhimurium strain expressing ViPS. Compared with their naive counterparts, immunized wildtype and AID^-/- mice exhibited significantly reduced bacterial burden regardless of the route of infection. These data indicate that an unmutated IgM response to ViPS contributes to protective immunity to S. Typhi.

Salmonella Typhi Colonization Provokes Extensive Transcriptional Changes Aimed at Evading Host Mucosal Immune Defense During Early Infection of Human Intestinal Tissue

Commensal microorganisms influence a variety of host functions in the gut, including immune response, glucose homeostasis, metabolic pathways and oxidative stress, among others. This study describes how Salmonella Typhi, the pathogen responsible for typhoid fever, uses similar strategies to escape immune defense responses and survive within its human host. To elucidate the early mechanisms of typhoid fever, we performed studies using healthy human intestinal tissue samples and "mini-guts," organoids grown from intestinal tissue taken from biopsy specimens. We analyzed gene expression changes in human intestinal specimens and bacterial cells both separately and after colonization. Our results showed mechanistic strategies that S. Typhi uses to rearrange the cellular machinery of the host cytoskeleton to successfully invade the intestinal epithelium, promote polarized cytokine release and evade immune system activation by downregulating genes involved in antigen sampling and presentation during infection. This work adds novel information regarding S. Typhi infection pathogenesis in humans, by replicating work shown in traditional cell models, and providing new data that can be applied to future vaccine development strategies.

An evaluation of purified Salmonella Typhi protein antigens for the serological diagnosis of acute typhoid fever

Objectives

The diagnosis of typhoid fever is a challenge. Aiming to develop a typhoid diagnostic we measured antibody responses against Salmonella Typhi (S. Typhi) protein antigens and the Vi polysaccharide in a cohort of Bangladeshi febrile patients.

Methods

IgM against 12 purified antigens and the Vi polysaccharide was measured by ELISA in plasma from patients with confirmed typhoid fever (n = 32), other confirmed infections (n = 17), and healthy controls (n = 40). ELISAs with the most specific antigens were performed on plasma from 243 patients with undiagnosed febrile disease.

Results

IgM against the S. Typhi protein antigens correlated with each other (rho > 0.8), but not against Vi (rho < 0.6). Typhoid patients exhibited higher IgM against 11/12 protein antigens and Vi than healthy controls and those with other infections. Vi, PilL, and CdtB exhibited the greatest sensitivity and specificity. Specificity and sensitivity was improved when Vi was combined with a protein antigen, generating sensitivities and specificities of 0.80 and >0.85, respectively. Applying a dynamic cut-off to patients with undiagnosed febrile disease suggested that 34–58% had an IgM response indicative of typhoid.

Conclusions

We evaluated the diagnostic potential of several S. Typhi antigens; our assays give good sensitivity and specificity, but require further assessment in differing patient populations.

Vi capsular polysaccharide: Synthesis, virulence, and application

Vi capsular polysaccharide, a linear homopolymer of α-1,4-linked N-acetylgalactosaminuronate, is characteristically produced by Salmonella enterica serovar Typhi. The Vi capsule covers the surface of the producing bacteria and serves as an virulence factor via inhibition of complement-mediated killing and promoting resistance against phagocytosis. Furthermore, Vi also represents a predominant protective antigen and plays a key role in the development of vaccines against typhoid fever. Herein, we reviewed the latest advances associated with the Vi polysaccharide, from its synthesis and transport within bacterial cells, mechanisms involved in virulence, immunological characteristics, and applications in vaccine, as well as its purification and detection methods.

Salmonella enterica serovar Typhi and gallbladder cancer: a case-control study and meta-analysis

In Chile, where gallbladder cancer (GBC) rates are high and typhoid fever was endemic until the 1990s, we evaluated the association between Salmonella enterica serovar Typhi (S. Typhi) antibodies and GBC. We tested 39 GBC cases, 40 gallstone controls, and 39 population-based controls for S. Typhi Vi antibodies and performed culture and quantitative polymerase chain reaction for the subset with bile, gallstone, tissue, and stool samples available. We calculated gender and education-adjusted odds ratios (ORs) and 95% confidence intervals (CIs) for the association with GBC. We also conducted a meta-analysis of >1000 GBC cases by combining our results with previous studies. GBC cases were more likely to have high Vi antibody titer levels than combined controls (OR: 4.0, 95% CI: 0.9-18.3), although S. Typhi was not recovered from bile, gallstone, tissue, or stool samples. In our meta-analysis, the summary relative risk was 4.6 (95% CI: 3.1-6.8, Pheterogeneity =0.6) for anti-Vi and 5.0 (95% CI: 2.7-9.3, Pheterogeneity  = 0.2) for bile or stool culture. Our results are consistent with the meta-analysis. Despite differences in study methods (e.g., S. Typhi detection assay), most studies found a positive association between S. Typhi and GBC. However, the mechanism underlying this association requires further investigation.

Salmonella Extracellular Matrix Components Influence Biofilm Formation and Gallbladder Colonization

Salmonella enterica serovar Typhi, the causative agent of typhoid fever in humans, forms biofilms encapsulated by an extracellular matrix (ECM). Biofilms facilitate colonization and persistent infection in gallbladders of humans and mouse models of chronic carriage. Individual roles of matrix components have not been completely elucidated in vitro or in vivo To examine individual functions, strains of Salmonella enterica serovar Typhimurium, the murine model of S Typhi, in which various ECM genes were deleted or added, were created to examine biofilm formation, colonization, and persistence in the gallbladder. Studies show that curli contributes most significantly to biofilm formation. Expression of Vi antigen decreased biofilm formation in vitro and virulence and bacterial survival in vivo without altering the examined gallbladder pro- or anti-inflammatory cytokines. Oppositely, loss of all ECM components (ΔwcaM ΔcsgA ΔyihO ΔbcsE) increased virulence and bacterial survival in vivo and reduced gallbladder interleukin-10 (IL-10) levels. Colanic acid and curli mutants had the largest defects in biofilm-forming ability and contributed most significantly to the virulence increase of the ΔwcaM ΔcsgA ΔyihO ΔbcsE mutant strain. While the ΔwcaM ΔcsgA ΔyihO ΔbcsE mutant was not altered in resistance to complement or growth in macrophages, it attached and invaded macrophages better than the wild-type (WT) strain. These data suggest that ECM components have various levels of importance in biofilm formation and gallbladder colonization and that the ECM diminishes disseminated disease in our model, perhaps by reducing cell attachment/invasion and dampening inflammation by maintaining/inducing IL-10 production. Understanding how ECM components aid acute disease and persistence could lead to improvements in therapeutic treatment of typhoid fever patients.

The inflammatory inception of gallbladder cancer

Gallbladder cancer is a lethal disease with notable geographical variations worldwide and a predilection towards women. Its main risk factor is prolonged exposure to gallstones, although bacterial infections and other inflammatory conditions are also associated. The recurrent cycles of gallbladder epithelium damage and repair enable a chronic inflammatory environment that promotes progressive morphological impairment through a metaplasia-dysplasia-carcinoma, along with cumulative genome instability. Inactivation of TP53, which is mutated in over 50% of GBC cases, seems to be the earliest and one of the most important carcinogenic pathways involved. Increased cell turnover and oxidative stress promote early alteration of TP53, cell cycle deregulation, apoptosis and replicative senescence. In this review, we will discuss evidence for the role of inflammation in gallbladder carcinogenesis obtained through epidemiological studies, genome-wide association studies, experimental carcinogenesis, morphogenetic studies and comparative studies with other inflammation-driven malignancies. The evidence strongly supports chronic, unresolved inflammation as the main carcinogenic mechanism of gallbladder cancer, regardless of the initial etiologic trigger. Given this central role of inflammation, evaluation of the potential for GBC prevention removing causes of inflammation or using anti-inflammatory drugs in high-risk populations may be warranted.

Loss of Multicellular Behavior in Epidemic African Nontyphoidal Salmonella enterica Serovar Typhimurium ST313 Strain D23580

Nontyphoidal Salmonella enterica serovar Typhimurium is a frequent cause of bloodstream infections in children and HIV-infected adults in sub-Saharan Africa. Most isolates from African patients with bacteremia belong to a single sequence type, ST313, which is genetically distinct from gastroenteritis-associated ST19 strains, such as 14028s and SL1344. Some studies suggest that the rapid spread of ST313 across sub-Saharan Africa has been facilitated by anthroponotic (person-to-person) transmission, eliminating the need for Salmonella survival outside the host. While these studies have not ruled out zoonotic or other means of transmission, the anthroponotic hypothesis is supported by evidence of extensive genomic decay, a hallmark of host adaptation, in the sequenced ST313 strain D23580. We have identified and demonstrated 2 loss-of-function mutations in D23580, not present in the ST19 strain 14028s, that impair multicellular stress resistance associated with survival outside the host. These mutations result in inactivation of the KatE stationary-phase catalase that protects high-density bacterial communities from oxidative stress and the BcsG cellulose biosynthetic enzyme required for the RDAR (red, dry, and rough) colonial phenotype. However, we found that like 14028s, D23580 is able to elicit an acute inflammatory response and cause enteritis in mice and rhesus macaque monkeys. Collectively, these observations suggest that African S. Typhimurium ST313 strain D23580 is becoming adapted to an anthroponotic mode of transmission while retaining the ability to infect and cause enteritis in multiple host species.

IMPORTANCE

The last 3 decades have witnessed an epidemic of invasive nontyphoidal Salmonella infections in sub-Saharan Africa. Genomic analysis and clinical observations suggest that the Salmonella strains responsible for these infections are evolving to become more typhoid-like with regard to patterns of transmission and virulence. This study shows that a prototypical African nontyphoidal Salmonella strain has lost traits required for environmental stress resistance, consistent with an adaptation to a human-to-human mode of transmission. However, in contrast to predictions, the strain remains capable of causing acute inflammation in the mammalian intestine. This suggests that the systemic clinical presentation of invasive nontyphoidal Salmonella infections in Africa reflects the immune status of infected hosts rather than intrinsic differences in the virulence of African Salmonella strains. Our study provides important new insights into the evolution of host adaptation in bacterial pathogens.

Differential Killing of Salmonella enterica Serovar Typhi by Antibodies Targeting Vi and Lipopolysaccharide O:9 Antigen

Salmonella enterica serovar Typhi expresses a capsule of Vi polysaccharide, while most Salmonella serovars, including S. Enteritidis and S. Typhimurium, do not. Both S. Typhi and S. Enteritidis express the lipopolysaccharide O:9 antigen, yet there is little evidence of cross-protection from anti-O:9 antibodies. Vaccines based on Vi polysaccharide have efficacy against typhoid fever, indicating that antibodies against Vi confer protection. Here we investigate the role of Vi capsule and antibodies against Vi and O:9 in antibody-dependent complement- and phagocyte-mediated killing of Salmonella. Using isogenic Vi-expressing and non-Vi-expressing derivatives of S. Typhi and S. Typhimurium, we show that S. Typhi is inherently more sensitive to serum and blood than S. Typhimurium. Vi expression confers increased resistance to both complement- and phagocyte-mediated modalities of antibody-dependent killing in human blood. The Vi capsule is associated with reduced C3 and C5b-9 deposition, and decreased overall antibody binding to S. Typhi. However, purified human anti-Vi antibodies in the presence of complement are able to kill Vi-expressing Salmonella, while killing by anti-O:9 antibodies is inversely related to Vi expression. Human serum depleted of antibodies to antigens other than Vi retains the ability to kill Vi-expressing bacteria. Our findings support a protective role for Vi capsule in preventing complement and phagocyte killing of Salmonella that can be overcome by specific anti-Vi antibodies, but only to a limited extent by anti-O:9 antibodies.

Comparative growth analysis of capsulated (Vi+) and acapsulated (Vi-) Salmonella typhi isolates in human blood

Salmonella enterica serovar Typhi (S. Typhi) is a human restricted pathogen. It biosynthesizes a virulence capsular polysaccharide named as Vi antigen. S. Typhi regulates expression of genes involved in the biosynthesis of Vi antigen in response to osmolarity. Beside Vi-positive isolates, Vi-negative (acapsulated) isolates are also pathogenic. However, Vi-positive isolates are more prevalent. The present study was planned to investigate comparative growth of Vi-positive and Vi-negative S. Typhi isolates in an ex vivo human whole blood model. Four isolates of each type were tested for growth in human whole blood and in an enrichment medium (Tryptic soy broth-TSB) as a control. It was found that capsulated (Vi-positive) strains formed smooth circular colonies and grew with shorter lag and generation time than Vi-negative isolates. Overall growth pattern of S. Typhi isolates both in vitro and ex vivo conditions showed that Vi-positive isolates grew at a faster rate. Especially in human blood, the lag time of acapsulated isolates was almost doubled as compared to capsulated S. Typhi isolates. It was also observed that Vi-negative isolates reduced in number up to 81 % during the first 12 hours of incubation in human whole blood. Interestingly, both types of isolates had similar growth curve in TSB indicating that Vi capsule is dispensable for bacterial growth in vitro. This study shows for the first time that absence of capsular antigen retards the growth of Vi-negative isolates on initial contact with human blood, but with passage of time they adjust themselves according to the new environment.

The Pyromaniac Inside You: Salmonella Metabolism in the Host Gut

A metabolically diverse microbial community occupies all available nutrient-niches in the lumen of the mammalian intestine, making it difficult for pathogens to establish themselves in this highly competitive environment. Salmonella serovars sidestep the competition by using their virulence factors to coerce the host into creating a novel nutrient-niche. Inflammation-derived nutrients available in this new niche support a bloom of Salmonella serovars, thereby ensuring transmission of the pathogen to the next susceptible host by the fecal-oral route. Here we review the anaerobic food chain that characterizes resident gut-associated microbial communities along with the winning metabolic strategy Salmonella serovars use to edge out competing microbes in the inflamed intestine.

Now you see me, now you don't: the interaction of Salmonella with innate immune receptors

Salmonella enterica serovars are associated with an estimated 1 million deaths annually and are also useful model organisms for investigating the mechanisms of host-bacterium interactions. The insights gained from studies on non-typhoidal Salmonella (NTS) serovars have provided a fascinating overview of the mechanisms by which the innate immune system detects and responds to bacterial pathogens. However, specific virulence factors and changes in virulence gene regulation in S. enterica subsp. enterica serovar Typhi alter the innate immune responses to this pathogen. In this Review, we compare and contrast the interactions of S. Typhi and NTS serovars with host innate immune receptors and discuss why the disease manifestations associated with S. Typhi infection differ considerably from those associated with the closely related NTS serovars.

An O antigen capsule modulates bacterial pathogenesis in Shigella sonnei

Shigella is the leading cause for dysentery worldwide. Together with several virulence factors employed for invasion, the presence and length of the O antigen (OAg) of the lipopolysaccharide (LPS) plays a key role in pathogenesis. S. flexneri 2a has a bimodal OAg chain length distribution regulated in a growth-dependent manner, whereas S. sonnei LPS comprises a monomodal OAg. Here we reveal that S. sonnei, but not S. flexneri 2a, possesses a high molecular weight, immunogenic group 4 capsule, characterized by structural similarity to LPS OAg. We found that a galU mutant of S. sonnei, that is unable to produce a complete LPS with OAg attached, can still assemble OAg material on the cell surface, but a galU mutant of S. flexneri 2a cannot. High molecular weight material not linked to the LPS was purified from S. sonnei and confirmed by NMR to contain the specific sugars of the S. sonnei OAg. Deletion of genes homologous to the group 4 capsule synthesis cluster, previously described in Escherichia coli, abolished the generation of the high molecular weight OAg material. This OAg capsule strongly affects the virulence of S. sonnei. Uncapsulated knockout bacteria were highly invasive in vitro and strongly inflammatory in the rabbit intestine. But, the lack of capsule reduced the ability of S. sonnei to resist complement-mediated killing and to spread from the gut to peripheral organs. In contrast, overexpression of the capsule decreased invasiveness in vitro and inflammation in vivo compared to the wild type. In conclusion, the data indicate that in S. sonnei expression of the capsule modulates bacterial pathogenesis resulting in balanced capabilities to invade and persist in the host environment.

The flagellar regulator TviA reduces pyroptosis by Salmonella enterica serovar Typhi

To discern virulent from innocuous microbes, the innate immune system senses events associated with bacterial access to immunoprivileged sites such as the host cell cytosol. One such pathway is triggered by the cytosolic delivery of flagellin, the major subunit of the flagellum, by bacterial secretion systems. This leads to inflammasome activation and subsequent proinflammatory cell death (pyroptosis) of the infected phagocyte. In this study, we demonstrate that the causative agent of typhoid fever, Salmonella enterica serovar Typhi, can partially subvert this critical innate immune recognition event. The transcriptional regulator TviA, which is absent from Salmonella serovars associated with human gastroenteritis, repressed the expression of flagellin during infection of human macrophage-like (THP-1) cells. This mechanism allowed S. Typhi to dampen inflammasome activation, leading to reduced interleukin-1β (IL-1β) secretion and diminished cell death. Likewise, the introduction of the tviA gene in nontyphoidal Salmonella enterica serovar Typhimurium reduced flagellin-induced pyroptosis. These data suggest that gene regulation of virulence factors enables S. Typhi to evade innate immune recognition by concealing a pathogen-induced process from being sensed by the inflammasome.

The Vi capsular polysaccharide enables Salmonella enterica serovar typhi to evade microbe-guided neutrophil chemotaxis

Salmonella enterica serovar Typhi (S. Typhi) causes typhoid fever, a disseminated infection, while the closely related pathogen S. enterica serovar Typhimurium (S. Typhimurium) is associated with a localized gastroenteritis in humans. Here we investigated whether both pathogens differ in the chemotactic response they induce in neutrophils using a single-cell experimental approach. Surprisingly, neutrophils extended chemotactic pseudopodia toward Escherichia coli and S. Typhimurium, but not toward S. Typhi. Bacterial-guided chemotaxis was dependent on the presence of complement component 5a (C5a) and C5a receptor (C5aR). Deletion of S. Typhi capsule biosynthesis genes markedly enhanced the chemotactic response of neutrophils in vitro. Furthermore, deletion of capsule biosynthesis genes heightened the association of S. Typhi with neutrophils in vivo through a C5aR-dependent mechanism. Collectively, these data suggest that expression of the virulence-associated (Vi) capsular polysaccharide of S. Typhi obstructs bacterial-guided neutrophil chemotaxis.

Salmonella enterica Serovar Typhi conceals the invasion-associated type three secretion system from the innate immune system by gene regulation

Delivery of microbial products into the mammalian cell cytosol by bacterial secretion systems is a strong stimulus for triggering pro-inflammatory host responses. Here we show that Salmonella enterica serovar Typhi (S. Typhi), the causative agent of typhoid fever, tightly regulates expression of the invasion-associated type III secretion system (T3SS-1) and thus fails to activate these innate immune signaling pathways. The S. Typhi regulatory protein TviA rapidly repressed T3SS-1 expression, thereby preventing RAC1-dependent, RIP2-dependent activation of NF-κB in epithelial cells. Heterologous expression of TviA in S. enterica serovar Typhimurium (S. Typhimurium) suppressed T3SS-1-dependent inflammatory responses generated early after infection in animal models of gastroenteritis. These results suggest that S. Typhi reduces intestinal inflammation by limiting the induction of pathogen-induced processes through regulation of virulence gene expression.

Bacterial pathogens translocate effector proteins into the cytoplasm of host cells to manipulate the mammalian host. These processes, e.g. the stimulation of small regulatory GTPases, activate the innate immune system and induce pro-inflammatory responses aimed at clearing invading microbes from the infected tissue. Here we show that strict regulation of virulence gene expression can be used as a strategy to limit the induction of inflammatory responses while retaining the ability to manipulate the host. Upon entry into host tissue, Salmonella enterica serovar Typhi, the causative agent of typhoid fever, rapidly represses expression of a virulence factor required for entering tissue to avoid detection by the host innate immune surveillance. This tight control of virulence gene expression enables the pathogen to deploy a virulence factor for epithelial invasion, while preventing the subsequent generation of pro-inflammatory responses in host cells. We conclude that regulation of virulence gene expression contributes to innate immune evasion during typhoid fever by concealing a pattern of pathogenesis.

Coinfection with an intestinal helminth impairs host innate immunity against Salmonella enterica serovar Typhimurium and exacerbates intestinal inflammation in mice

Salmonella enterica serovar Typhimurium is a Gram-negative food-borne pathogen that is a major cause of acute gastroenteritis in humans. The ability of the host to control such bacterial pathogens may be influenced by host immune status and by concurrent infections. Helminth parasites are of particular interest in this context because of their ability to modulate host immune responses and because their geographic distribution coincides with those parts of the world where infectious gastroenteritis is most problematic. To test the hypothesis that helminth infection may negatively regulate host mucosal innate immunity against bacterial enteropathogens, a murine coinfection model was established by using the intestinal nematode Heligmosomoides polygyrus and S. Typhimurium. We found that mice coinfected with S. Typhimurium and H. polygyrus developed more severe intestinal inflammation than animals infected with S. Typhimurium alone. The enhanced susceptibility to Salmonella-induced intestinal injury in coinfected mice was found to be associated with diminished neutrophil recruitment to the site of bacterial infection that correlated with decreased expression of the chemoattractants CXCL2/macrophage inflammatory protein 2 (MIP-2) and CXCL1/keratinocyte-derived chemokine (KC), poor control of bacterial replication, and exacerbated intestinal inflammation. The mechanism of helminth-induced inhibition of MIP-2 and KC expression involved interleukin-10 (IL-10) and, to a lesser extent, IL-4 and IL-13. Ly6G antibody-mediated depletion of neutrophils reproduced the adverse effects of H. polygyrus on Salmonella infection. Our results suggest that impaired neutrophil recruitment is an important contributor to the enhanced severity of Salmonella enterocolitis associated with helminth coinfection.

Comparative analysis of Salmonella genomes identifies a metabolic network for escalating growth in the inflamed gut

The Salmonella genus comprises a group of pathogens associated with illnesses ranging from gastroenteritis to typhoid fever. We performed an in silico analysis of comparatively reannotated Salmonella genomes to identify genomic signatures indicative of disease potential. By removing numerous annotation inconsistencies and inaccuracies, the process of reannotation identified a network of 469 genes involved in central anaerobic metabolism, which was intact in genomes of gastrointestinal pathogens but degrading in genomes of extraintestinal pathogens. This large network contained pathways that enable gastrointestinal pathogens to utilize inflammation-derived nutrients as well as many of the biochemical reactions used for the enrichment and biochemical discrimination of Salmonella serovars. Thus, comparative genome analysis identifies a metabolic network that provides clues about the strategies for nutrient acquisition and utilization that are characteristic of gastrointestinal pathogens. IMPORTANCE While some Salmonella serovars cause infections that remain localized to the gut, others disseminate throughout the body. Here, we compared Salmonella genomes to identify characteristics that distinguish gastrointestinal from extraintestinal pathogens. We identified a large metabolic network that is functional in gastrointestinal pathogens but decaying in extraintestinal pathogens. While taxonomists have used traits from this network empirically for many decades for the enrichment and biochemical discrimination of Salmonella serovars, our findings suggest that it is part of a "business plan" for growth in the inflamed gastrointestinal tract. By identifying a large metabolic network characteristic of Salmonella serovars associated with gastroenteritis, our in silico analysis provides a blueprint for potential strategies to utilize inflammation-derived nutrients and edge out competing gut microbes.

Spatial segregation of virulence gene expression during acute enteric infection with Salmonella enterica serovar Typhimurium

To establish a replicative niche during its infectious cycle between the intestinal lumen and tissue, the enteric pathogen Salmonella enterica serovar Typhimurium requires numerous virulence genes, including genes for two type III secretion systems (T3SS) and their cognate effectors. To better understand the host-pathogen relationship, including early infection dynamics and induction kinetics of the bacterial virulence program in the context of a natural host, we monitored the subcellular localization and temporal expression of T3SS-1 and T3SS-2 using fluorescent single-cell reporters in a bovine, ligated ileal loop model of infection. We observed that the majority of bacteria at 2 h postinfection are flagellated, express T3SS-1 but not T3SS-2, and are associated with the epithelium or with extruding enterocytes. In epithelial cells, S. Typhimurium cells were surrounded by intact vacuolar membranes or present within membrane-compromised vacuoles that typically contained numerous vesicular structures. By 8 h postinfection, T3SS-2-expressing bacteria were detected in the lamina propria and in the underlying mucosa, while T3SS-1-expressing bacteria were in the lumen. Our work identifies for the first time the temporal and spatial regulation of T3SS-1 and -2 expression during an enteric infection in a natural host and provides further support for the concept of cytosolic S. Typhimurium in extruding epithelium as a mechanism for reseeding the lumen.

The pathogenic bacterium Salmonella enterica serovar Typhimurium invades and persists within host cells using distinct sets of virulence genes. Genes from Salmonella pathogenicity island 1 (SPI-1) are used to initiate contact and facilitate uptake into nonphagocytic host cells, while genes within SPI-2 allow the pathogen to colonize host cells. While many studies have identified bacterial virulence determinants in animal models of infection, very few have focused on virulence gene expression at the single-cell level during an in vivo infection. To better understand when and where bacterial virulence factors are expressed during an acute enteric infection of a natural host, we infected bovine jejunal-ileal loops with S. Typhimurium cells harboring fluorescent transcriptional reporters for SPI-1 and -2 (PinvF and PssaG, respectively). After a prescribed time of infection, tissue and luminal fluid were collected and analyzed by microscopy. During early infection (≤2 h), bacteria within both intact and compromised membrane-bound vacuoles were observed within the epithelium, with the majority expressing SPI-1. As the infection progressed, S. Typhimurium displayed differential expression of the SPI-1 and SPI-2 regulons, with the majority of tissue-associated bacteria expressing SPI-2 and the majority of lumen-associated bacteria expressing SPI-1. This underscores the finding that Salmonella virulence gene expression changes as the pathogen transitions from one anatomical location to the next.

Salmonella vaccines: lessons from the mouse model or bad teaching?

Salmonella enterica subsp. enterica includes several very important human serovars including Typhi, Paratyphi, Typhimurium and Enteritidis. These bacteria cause a significant global burden of disease, typically classified into enteric fever, gastroenteritis and, more recently, invasive non-typhoidal salmonellosis (iNTS). Vaccines have been developed for one of these serovars, S. Typhi and the recent increase in iNTS cases has resulted in a push to develop new vaccines that will inhibit disease by S. Typhimurium and S. Enteritidis, the most common iNTS S. enterica serovars. The development of new human vaccines has been informed by studies in the murine model of typhoid fever based on S. Typhimurium infections of very 'sensitive' (Nramp-1(S)) mice, which has some obvious deficiencies, not the least that antibodies protect humans against S. Typhi infection but are only weakly protective in 'sensitive' mice infected with S. Typhimurium. S. Typhimurium also lacks Vi, the target of protective antibodies in typhoid fever. Notwithstanding these deficiencies, the murine model has identified a very complex series of innate and adaptive immune responses to infection that might be exploited to develop new vaccines. Equally, advances in understanding the pathogenesis of infection, through pathogenomics and more sophisticated animal models will likely contribute to the development of novel immunogens.

Host specificity of bacterial pathogens

Most pathogens are able to infect multiple hosts but some are highly adapted to a single-host species. A detailed understanding of the basis of host specificity can provide important insights into molecular pathogenesis, the evolution of pathogenic microbes, and the potential for pathogens to cross the species barrier to infect new hosts. Comparative genomics and the development of humanized mouse models have provided important new tools with which to explore the basis of generalism and specialism. This review will examine host specificity of bacterial pathogens with a focus on generalist and specialist serovars of Salmonella enterica.

Prevalence and genetic analysis of phenotypically Vi- negative Salmonella typhi isolates in children from Kathmandu, Nepal

The Vi capsular polysaccharide (ViPS) protects Salmonella enterica subspecies enterica serotype Typhi (S.Typhi) in vivo by multiple mechanisms. Recent microbiological reports from typhoid endemic countries suggest that acapsulate S.Typhi may occur in nature and contribute to clinical typhoid fever that is indistinguishable from disease caused by capsulate strains. The prevalence and genetic basis of ViPS-negative S.Typhi isolates in children from Kathmandu, Nepal, were tested in 68 isolates. Although 5.9% of isolates tested negative for capsular expression by slide agglutination tests, a novel multiplex PCR assay and individual PCR analyses demonstrated the presence of all 14 genes responsible for the synthesis, transportation and regulation of the ViPS. These data suggest that phenotypically acapsulate S.Typhi may not have a genetic basis for the same.

Loss of very-long O-antigen chains optimizes capsule-mediated immune evasion by Salmonella enterica serovar Typhi

Expression of capsular polysaccharides is a variable trait often associated with more-virulent forms of a bacterial species. For example, typhoid fever is caused by the capsulated Salmonella enterica serovar Typhi, while nontyphoidal Salmonella serovars associated with gastroenteritis are noncapsulated. Here we show that optimization of the immune evasive properties conferred by the virulence-associated (Vi) capsular polysaccharide involved an additional alteration to the cell envelope of S. Typhi, namely inactivation of the fepE gene, encoding the regulator of very-long O-antigen chains. Introduction of the capsule-encoding viaB locus into the nontyphoidal S. enterica serovar Typhimurium reduced complement deposition in vitro and intestinal inflammation in a mouse colitis model. However, both phenotypes were markedly enhanced when the viaB locus was introduced into an S. Typhimurium fepE mutant, which lacks very-long O-antigen chains. Collectively, these data suggest that during the evolution of the S. Typhi lineage, loss of very-long O-antigen chains by pseudogene formation was an adaptation to maximize the anti-inflammatory properties of the Vi capsular polysaccharide.

Genomic comparison illustrates that acquisition of virulence factors by horizontal gene transfer is an important contributor to the evolution of enteric pathogens. Acquisition of complex virulence traits commonly involves horizontal transfer of a large gene cluster, and integration of the gene cluster into the host genome results in the formation of a pathogenicity island. Acquisition of the virulence-associated (Vi) capsular polysaccharide encoded by SPI7 (Salmonella pathogenicity island 7) was accompanied in the human-adapted Salmonella enterica serovar Typhi by inactivation of the fepE gene, encoding the regulator of very-long O-antigen chains. We show that the resulting loss of very-long O-antigen chains was an important mechanism for maximizing immune evasion mediated by the Vi capsular polysaccharide. These data suggest that successful incorporation of a capsular polysaccharide requires changes in the cell envelope of the hosting pathogen.

The polysaccharide capsule of Campylobacter jejuni modulates the host immune response

Campylobacter jejuni is a major cause of bacterial diarrheal disease worldwide. The organism is characterized by a diversity of polysaccharide structures, including a polysaccharide capsule. Most C. jejuni capsules are known to be decorated nonstoichiometrically with methyl phosphoramidate (MeOPN). The capsule of C. jejuni 81-176 has been shown to be required for serum resistance, but here we show that an encapsulated mutant lacking the MeOPN modification, an mpnC mutant, was equally as sensitive to serum killing as the nonencapsulated mutant. A nonencapsulated mutant, a kpsM mutant, exhibited significantly reduced colonization compared to that of wild-type 81-176 in a mouse intestinal colonization model, and the mpnC mutant showed an intermediate level of colonization. Both mutants were associated with higher levels of interleukin 17 (IL-17) expression from lamina propria CD4(+) cells than from cells from animals infected with 81-176. In addition, reduced levels of Toll-like receptor 4 (TLR4) and TLR2 activation were observed following in vitro stimulation of human reporter cell lines with the kpsM and mpnC mutants compared to those with wild-type 81-176. The data suggest that the capsule polysaccharide of C. jejuni and the MeOPN modification modulate the host immune response.

Typhoid fever: "you can't hit what you can't see"

The host restricts dissemination of invasive enteric pathogens, such as non-typhoidal Salmonella serovars, by mounting acute inflammatory responses characterized by the recruitment of neutrophils. However, some enteric pathogens, such as Salmonella enterica serovar Typhi (S. typhi), can bypass these defenses and cause an invasive bloodstream infection known as typhoid fever. Recent studies on virulence mechanisms of S. typhi suggest that tight regulation of virulence gene expression during the transition from the intestinal lumen into the intestinal mucosa enables this pathogen to evade detection by the innate immune system, thereby penetrating defenses that prevent bacterial dissemination. This example illustrates how the outcome of host pathogen interaction at the intestinal mucosal interface can alter the clinical presentation and dictate the disease outcome.

Innate immune response to Salmonella typhimurium, a model enteric pathogen

The innate immune system provides the first line of defense against invading microorganisms by inducing a variety of inflammatory and antimicrobial responses. These responses are particularly important in the gastrointestinal tract, where the needs for efficient nutrient uptake and host defense collide. Many pathogens have evolved to specifically colonize the intestine, causing millions of cases of enteric infections a year. A paradigm of an enteric pathogen is Salmonella enterica, a gram-negative bacterium that causes a wide range of gastrointestinal and systemic diseases. Infections with Salmonella enterica serovar Typhimurium (S. typhimurium) lead to an acute intestinal inflammation in human and animal hosts, as a result of the bacterium invading the mucosa. A distinctive feature of Salmonella is that it has not only adapted to survive in a strong inflammatory environment, but it also uses this adaptation as a strategy to gain a growth advantage over the intestinal microbiota. We will use the model organism S. typhimurium to discuss the innate immune mechanisms employed by the mammalian gastrointestinal system and how the pathogen responds and subverts these mechanisms. In particular, we focus on the recognition of extra- and intra-cellular Salmonellae by germline-encoded pattern recognition receptors of the TLR and NLR families, and how Salmonella might profit from the activation of these receptors.

Salmonella type III effector SpvC, a phosphothreonine lyase, contributes to reduction in inflammatory response during intestinal phase of infection

Salmonella phosphothreonine lyase SpvC inactivates the dual-phosphorylated host mitogen-activated protein kinases (MAPK) through β-elimination. While SpvC can be secreted in vitro by both Salmonella pathogenicity island (SPI)-1 and SPI-2 type III secretion systems (T3SSs), translocation of this protein into the host cell cytosol has only been demonstrated by SPI-2 T3SS. In this study, we show that SpvC can be delivered into the host cell cytoplasm by both SPI-1 and SPI-2 T3SSs. Dephosphorylation of the extracellular signal-regulated protein kinases (ERK) was detected in an SPI-1 T3SS-dependent manner 2 h post infection. Using a mouse model for Salmonella enterocolitis, which was treated with streptomycin prior to infection, we observed that mice infected with Salmonella enterica serovar Typhimurium strains lacking the spvC gene showed pronounced colitis when compared with mice infected with the wild-type strain 1 day after infection. The effect of SpvC on the development of colitis was characterized by reduced mRNA levels of the pro-inflammatory cytokines and chemokines, and reduced inflammation with less infiltration of neutrophils. Furthermore, the reduction in inflammation by SpvC resulted in increased bacterial dissemination in spleen of mice infected with Salmonella. Collectively, our findings suggest that SpvC exerts as an anti-inflammatory effector and the attenuation of intestinal inflammatory response by SpvC is involved in systemic infection of Salmonella.

Role of SPI-1 secreted effectors in acute bovine response to Salmonella enterica Serovar Typhimurium: a systems biology analysis approach

Salmonella enterica Serovar Typhimurium (S. Typhimurium) causes enterocolitis with diarrhea and polymorphonuclear cell (PMN) influx into the intestinal mucosa in humans and calves. The Salmonella Type III Secretion System (T3SS) encoded at Pathogenicity Island I translocates Salmonella effector proteins SipA, SopA, SopB, SopD, and SopE2 into epithelial cells and is required for induction of diarrhea. These effector proteins act together to induce intestinal fluid secretion and transcription of C-X-C chemokines, recruiting PMNs to the infection site. While individual molecular interactions of the effectors with cultured host cells have been characterized, their combined role in intestinal fluid secretion and inflammation is less understood. We hypothesized that comparison of the bovine intestinal mucosal response to wild type Salmonella and a SipA, SopABDE2 effector mutant relative to uninfected bovine ileum would reveal heretofore unidentified diarrhea-associated host cellular pathways. To determine the coordinated effects of these virulence factors, a bovine ligated ileal loop model was used to measure responses to wild type S. Typhimurium (WT) and a ΔsipA, sopABDE2 mutant (MUT) across 12 hours of infection using a bovine microarray. Data were analyzed using standard microarray analysis and a dynamic bayesian network modeling approach (DBN). Both analytical methods confirmed increased expression of immune response genes to Salmonella infection and novel gene expression. Gene expression changes mapped to 219 molecular interaction pathways and 1620 gene ontology groups. Bayesian network modeling identified effects of infection on several interrelated signaling pathways including MAPK, Phosphatidylinositol, mTOR, Calcium, Toll-like Receptor, CCR3, Wnt, TGF-β, and Regulation of Actin Cytoskeleton and Apoptosis that were used to model of host-pathogen interactions. Comparison of WT and MUT demonstrated significantly different patterns of host response at early time points of infection (15 minutes, 30 minutes and one hour) within phosphatidylinositol, CCR3, Wnt, and TGF-β signaling pathways and the regulation of actin cytoskeleton pathway.

Intestinal and chronic infections: Salmonella lifestyles in hostile environments

The main disease syndromes caused by Salmonella serovars in immunocompetent individuals are gastroenteritis and typhoid fever. These syndromes differ with regard to the host niches in which Salmonella serovars grow and survive to ensure their transmission. During gastroenteritis, non-typhoidal Salmonella serovars such as Salmonella enterica serovar Typhimurium (S. Typhimurium) use their virulence factors to elicit acute intestinal inflammation, thereby creating a novel luminal niche. Reactive oxygen species produced by phagocytes in the intestinal lumen oxidize endogenous sulfur compounds to produce a new respiratory electron acceptor, tetrathionate. Respiration of tetrathionate confers a growth advantage to S. Typhimurium over competing microbes. This growth advantage ensures transmission of the pathogen by the faecal-oral route. In typhoid fever, S. enterica serovar Typhi (S. Typhi) establishes a chronic infection in the gall bladder, and perhaps in additional niches. Studies using the mouse model of typhoid fever suggest that survival and proliferation in the gall bladder may involve several strategies. Invasion of the gallbladder epithelium and formation of biofilms on gallstones may protect the pathogen from the bactericidal activities of bile salts. In the gallbladder lumen, activation of bile defence responses may permit survival of planktonic Salmonella cells. Individuals developing chronic carriage after an episode of typhoid fever can transmit the disease for the remainder of their lives by shedding the pathogen through the cystic duct. Shedding promotes S. Typhi transmission to new susceptible hosts. Here we review Salmonella virulence strategies for growth and survival in host niches that represent reservoirs for transmission.

Host response transcriptional profiling reveals extracellular components and ABC (ATP-binding cassette) transporters gene enrichment in typhoid fever-infected Nigerian children

Background

Salmonella enterica serovar Typhi (S. Typhi) is a human-specific pathogen that causes typhoid fever, and remains a global health problem especially in developing countries. Its pathogenesis is complex and host response is poorly understood. In Africa, typhoid fever can be a major cause of morbidity in young infected children. The onset of the illness is insidious and clinical diagnosis is often unreliable. Gold standard blood culture diagnostic services are limited, thus rapid, sensitive, and affordable diagnostic test is essential in poor-resourced clinical settings. Routine typhoid fever vaccination is highly recommended but currently licensed vaccines provide only 55-75% protection. Recent epidemiological studies also show the rapid emergence of multi-drug resistant S. Typhi strains. High-throughput molecular technologies, such as microarrays, can dissect the molecular mechanisms of host responses which are S. Typhi-specific to provide a comprehensive genomic component of immunological responses and suggest new insights for diagnosis and treatment.

Methods

Global transcriptional profiles of S. Typhi-infected young Nigerian children were obtained from their peripheral blood and compared with that of other bacteremic infections using Agilent gene expression microarrays. The host-response profiles of the same patients in acute vs. convalescent phases were also determined. The top 96-100 differentially-expressed genes were identified and four genes were validated by quantitative real-time PCR. Gene clusters were obtained and functional pathways were predicted by DAVID (Database for Annotation, Visualization and Integrated Discovery).

Results

Transcriptional profiles from S. Typhi-infected children could be distinguished from those of other bacteremic infections. Enriched gene clusters included genes associated with extracellular peptides/components such as lipocalin (LCN2) and systemic immune response which is atypical in bacterial invasion. Distinct gene expression profiles can also be obtained from acute vs. convalescent phase during typhoid fever infection. We found novel down-regulation of ABC (ATP-binding cassette) transporters genes such as ABCA7, ABCC5, and ABCD4 and ATPase activity as the highest enriched pathway.

Conclusions

We identified unique extracellular components and ABC transporters gene enrichments in typhoid fever-infected Nigerian children, which have never been reported. These enriched gene clusters may represent novel targeted pathways to improve diagnostic, prognostic, therapeutic and next-generation vaccine strategies for typhoid fever in Africa.