Salmonella infections in the mouse model: host resistance factors and in vivo dynamics of bacterial spread and distribution in the tissues

PgtE Enzyme of Salmonella enterica Shares the Similar Biological Roles to Plasminogen Activator (Pla) in Interacting With DEC-205 (CD205), and Enhancing Host Dissemination and Infectivity by Yersinia pestis

Yersinia pestis, the cause of plague, is a newly evolved Gram-negative bacterium. Through the acquisition of the plasminogen activator (Pla), Y. pestis gained the means to rapidly disseminate throughout its mammalian hosts. It was suggested that Y. pestis utilizes Pla to interact with the DEC-205 (CD205) receptor on antigen-presenting cells (APCs) to initiate host dissemination and infection. However, the evolutionary origin of Pla has not been fully elucidated. The PgtE enzyme of Salmonella enterica, involved in host dissemination, shows sequence similarity with the Y. pestis Pla. In this study, we demonstrated that both Escherichia coli K-12 and Y. pestis bacteria expressing the PgtE-protein were able to interact with primary alveolar macrophages and DEC-205-transfected CHO cells. The interaction between PgtE-expressing bacteria and DEC-205-expressing transfectants could be inhibited by the application of an anti-DEC-205 antibody. Moreover, PgtE-expressing Y. pestis partially re-gained the ability to promote host dissemination and infection. In conclusion, the DEC-205-PgtE interaction plays a role in promoting the dissemination and infection of Y. pestis, suggesting that Pla and the PgtE of S. enterica might share a common evolutionary origin.

Structural Comparison Between MHC Classes I and II; in Evolution, a Class-II-Like Molecule Probably Came First

Structures of peptide-loaded major histocompatibility complex class I (pMHC-I) and class II (pMHC-II) complexes are similar. However, whereas pMHC-II complexes include similar-sized IIα and IIβ chains, pMHC-I complexes include a heavy chain (HC) and a single domain molecule β₂-microglobulin (β₂-m). Recently, we elucidated several pMHC-I and pMHC-II structures of primitive vertebrate species. In the present study, a comprehensive comparison of pMHC-I and pMHC-II structures helps to understand pMHC structural evolution and supports the earlier proposed—though debated—direction of MHC evolution from class II-type to class I. Extant pMHC-II structures share major functional characteristics with a deduced MHC-II-type homodimer ancestor. Evolutionary establishment of pMHC-I presumably involved important new functions such as (i) increased peptide selectivity by binding the peptides in a closed groove (ii), structural amplification of peptide ligand sequence differences by binding in a non-relaxed fashion, and (iii) increased peptide selectivity by syngeneic heterotrimer complex formation between peptide, HC, and β₂-m. These new functions were associated with structures that since their establishment in early pMHC-I have been very well conserved, including a shifted and reorganized P1 pocket (aka A pocket), and insertion of a β₂-m hydrophobic knob into the peptide binding domain β-sheet floor. A comparison between divergent species indicates better sequence conservation of peptide binding domains among MHC-I than among MHC-II, agreeing with more demanding interactions within pMHC-I complexes. In lungfishes, genes encoding fusions of all MHC-IIα and MHC-IIβ extracellular domains were identified, and although these lungfish genes presumably derived from classical MHC-II, they provide an alternative mechanistic hypothesis for how evolution from class II-type to class I may have occurred.

Association between Interferon-Lambda-3 rs12979860, TLL1 rs17047200 and DDR1 rs4618569 Variant Polymorphisms with the Course and Outcome of SARS-CoV-2 Patients

BACKGROUND

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection provides a critical host-immunological challenge.

AIM

We explore the effect of host-genetic variation in interferon-lambda-3 rs12979860, Tolloid Like-1 (TLL1) rs17047200 and Discoidin domain receptor 1(DDR1) rs4618569 on host response to respiratory viral infections and disease severity that may probe the mechanistic approach of allelic variation in virus-induced inflammatory responses.

METHODS

141 COVID-19 positive patients and 100 healthy controls were tested for interferon-lambda-3 rs12979860, TLL1 rs17047200 and DDR1 rs4618569 polymorphism by TaqMan probe-based genotyping. Different genotypes were assessed regarding the COVID-19 severity and prognosis.

RESULTS

There were statistically significant differences between the studied cases and control group with regard to the presence of comorbidities, total leucocytic count, lymphocytic count, CRP, serum LDH, ferritin and D-dimer (p < 0.01). The CC genotype of rs12979860 cytokine, the AA genotype of TLL1 rs17047200 and the AA genotype of the rs4618569 variant of DDR1 showed a higher incidence of COVID-19 compared to the others. There were significant differences between the rs4618569 variant of DDR and the outcome of the disease, with the highest mortality in AG genotype 29 (60.4%) in comparison to 16 (33.3%) and 3 (6.2%) in the AA and GG genotypes, respectively (p = 0.007*), suggesting that the A allele is associated with a poor outcome in the disease.

CONCLUSION

Among people who carry C and A alleles of SNPs IFN-λ rs12979860 and TLL1 rs17047200, respectively, the AG genotype of the DDR1 rs4618569 variant is correlated with a COVID-19 poor outcome. In those patients, the use of anti-IFN-λ 3, TLL1 and DDR1 therapy may be promising for personalized translational clinical practice.

Targeted Therapeutic Strategies in the Battle Against Pathogenic Bacteria

The emergence and rapid spread of antibiotic resistance in pathogenic bacteria constitute a global threat for public health. Despite ongoing efforts to confront this crisis, the pace of finding new potent antimicrobials is far slower than the evolution of drug resistance. The abuse of broad-spectrum antibiotics not only accelerates the formation of resistance but also imposes a burden on the intestinal microbiota, which acts a critical role in human homeostasis. As such, innovative therapeutic strategies with precision are pressingly warranted and highly anticipated. Recently, target therapies have achieved some breakthroughs by the aid of modern technology. In this review, we provide an insightful illustration of current and future medical targeted strategies, including narrow-spectrum agents, engineered probiotics, nanotechnology, phage therapy, and CRISPR-Cas9 technology. We discuss the recent advances and potential hurdles of these strategies. Meanwhile, the possibilities to mitigate the spread of resistance in these approaches are also mentioned. Altogether, a better understanding of the advantages, disadvantages, and mechanisms of action of these targeted therapies will be conducive to broadening our horizons and optimizing the existing antibacterial approaches.

Human Genetic Variation Influences Enteric Fever Progression

In the 21st century, enteric fever is still causing a significant number of mortalities, especially in high-risk regions of the world. Genetic studies involving the genome and transcriptome have revealed a broad set of candidate genetic polymorphisms associated with susceptibility to and the severity of enteric fever. This review attempted to explain and discuss the past and the most recent findings on human genetic variants affecting the progression of Salmonella typhoidal species infection, particularly toll-like receptor (TLR) 4, TLR5, interleukin (IL-) 4, natural resistance-associated macrophage protein 1 (NRAMP1), VAC14, PARK2/PACRG, cystic fibrosis transmembrane conductance regulator (CFTR), major-histocompatibility-complex (MHC) class II and class III. These polymorphisms on disease susceptibility or progression in patients could be related to multiple mechanisms in eliminating both intracellular and extracellular Salmonella typhoidal species. Here, we also highlighted the limitations in the studies reported, which led to inconclusive results in association studies. Nevertheless, the knowledge obtained through this review may shed some light on the development of risk prediction tools, novel therapies as well as strategies towards developing a personalised typhoid vaccine.

Autoimmune-prone lpr mice exhibit a prolonged but lethal infection with an attenuated Salmonella Typhimurium strain

Autoimmunity can potentially pre-dispose to, exacerbate or ameliorate pathogenic infections. The current study was designed to compare and understand the infection outcomes with Salmonella enterica serovar Typhimurium ATCC 14028s (S. Typhimurium) wild type (WT) and attenuated ΔrpoS strains, in autoimmune-prone lpr mice. C57BL/6 (B6) and B6/lpr (lpr) 6-8 weeks old mice were orally infected with S. Typhimurium WT and ΔrpoS strains. Disease outcomes were assessed with respect to survival, organ bacterial load, tissue damage and inflammation in infected mice. The acute infection stage (day 4) was examined and compared to the later stages (up to day 12) post ΔrpoS infection. S. Typhimurium WT exhibited an acute and lethal infection in both B6 and lpr mice. However, the ΔrpoS strain exhibited prolonged infection with reduced mortality in B6 mice but complete mortality in lpr mice. During late infection, bacterial load and serum IFNγ levels were higher in the ΔrpoS strain infected lpr mice compared to B6 mice. The ΔrpoS strain infected lpr mice also exhibited greater bacterial faecal shedding and greater tissue histopathological changes. Interestingly, ΔrpoS-infected B6 mice displayed minimal microbial load in the brain; however, sustained brain bacterial load was observed in ΔrpoS-infected lpr mice, corresponding to abnormal gait. Overall, S. Typhimurium ΔrpoS is competent in establishing infection but compromised in sustaining it. Nonetheless, lpr mice are less efficient in controlling this attenuated infection. The findings from the study demonstrate that genetic pre-disposition to autoimmunity is sufficient for greater host susceptibility to infection by attenuated S. Typhimurium strains.

Engineering probiotics as living diagnostics and therapeutics for improving human health

The gut microbiota that inhabit our gastrointestinal tract are well known to play an important role in maintaining human health in many aspects, including facilitating the digestion and absorption of nutrients, protecting against pathogens and regulating immune system. Gut microbiota dysbiosis is associated with a lot of diseases, such as inflammatory bowel disease, allergy, obesity, cardiovascular and neurodegenerative diseases and cancers. With the increasing knowledge of the microbiome, utilization of probiotic bacteria in modulating gut microbiota to prevent and treat a large number of disorders and diseases has gained much interest. In recent years, aided by the continuous development of tools and techniques, engineering probiotic microbes with desired characteristics and functionalities to benefit human health has made significant progress. In this paper, we summarize the recent advances in design and construction of probiotics as living diagnostics and therapeutics for probing and treating a series of diseases including metabolic disorders, inflammation and pathogenic bacteria infections. We also discuss the current challenges and future perspectives in expanding the application of probiotics for disease treatment and detection. We intend to provide insights and ideas for engineering of probiotics to better serve disease therapy and human health.

Within-host spatiotemporal dynamics of systemic Salmonella infection during and after antimicrobial treatment

Objectives

We determined the interactions between efficacy of antibiotic treatment, pathogen growth rates and between-organ spread during systemic Salmonella infections.

Methods

We infected mice with isogenic molecularly tagged subpopulations of either a fast-growing WT or a slow-growing ΔaroC Salmonella strain. We monitored viable bacterial numbers and fluctuations in the proportions of each bacterial subpopulation in spleen, liver, blood and mesenteric lymph nodes (MLNs) before, during and after the cessation of treatment with ampicillin and ciprofloxacin.

Results

Both antimicrobials induced a reduction in viable bacterial numbers in the spleen, liver and blood. This reduction was biphasic in infections with fast-growing bacteria, with a rapid initial reduction followed by a phase of lower effect. Conversely, a slow and gradual reduction of the bacterial load was seen in infections with the slow-growing strain, indicating a positive correlation between bacterial net growth rates and the efficacy of ampicillin and ciprofloxacin. The viable numbers of either bacterial strain remained constant in MLNs throughout the treatment with a relapse of the infection with WT bacteria occurring after cessation of the treatment. The frequency of each tagged bacterial subpopulation was similar in the spleen and liver, but different from that of the MLNs before, during and after treatment.

Conclusions

In Salmonella infections, bacterial growth rates correlate with treatment efficacy. MLNs are a site with a bacterial population structure different to those of the spleen and liver and where the total viable bacterial load remains largely unaffected by antimicrobials, but can resume growth after cessation of treatment.

Exonuclease III-aided autonomous cascade signal amplification: a facile and universal DNA biosensing platform for ultrasensitive electrochemical detection of S. typhimurium

A novel electrochemical biosensor based on exonuclease III-aided autonomous cascade signal amplification for the ultrasensitive and highly specific detection of S. typhimurium.

Animal Models for Salmonellosis: Applications in Vaccine Research

Salmonellosis remains an important cause of human disease worldwide. While there are several licensed vaccines for Salmonella enterica serovar Typhi, these vaccines are generally ineffective against other Salmonella serovars. Vaccines that target paratyphoid and nontyphoidal Salmonella serovars are very much in need. Preclinical evaluation of candidate vaccines is highly dependent on the availability of appropriate scientific tools, particularly animal models. Many different animal models exist for various Salmonella serovars, from whole-animal models to smaller models, such as those recently established in insects. Here, we discuss various mouse, rat, rabbit, calf, primate, and insect models for Salmonella infection, all of which have their place in research. However, choosing the right model is imperative in selecting the best vaccine candidates for further clinical testing. In this minireview, we summarize the various animal models that are used to assess salmonellosis, highlight some of the advantages and disadvantages of each, and discuss their value in vaccine development.

Single-cell level methods for studying the effect of antibiotics on bacteria during infection

Considerable evidence about phenotypic heterogeneity among bacteria during infection has accumulated during recent years. This heterogeneity has to be considered if the mechanisms of infection and antibiotic action are to be understood, so we need to implement existing and find novel methods to monitor the effects of antibiotics on bacteria at the single-cell level. This review provides an overview of methods by which this aim can be achieved. Fluorescence label-based methods and Raman scattering as a label-free approach are discussed in particular detail. Other label-free methods that can provide single-cell level information, such as impedance spectroscopy and surface plasmon resonance, are briefly summarized. The advantages and disadvantages of these different methods are discussed in light of a challenging in vivo environment.

Urinary tract infection of mice to model human disease: Practicalities, implications and limitations

Urinary tract infections (UTIs) are among the most common bacterial infections in humans. Murine models of human UTI are vital experimental tools that have helped to elucidate UTI pathogenesis and advance knowledge of potential treatment and infection prevention strategies. Fundamentally, several variables are inherent in different murine models, and understanding the limitations of these variables provides an opportunity to understand how models may be best applied to research aimed at mimicking human disease. In this review, we discuss variables inherent in murine UTI model studies and how these affect model usage, data analysis and data interpretation. We examine recent studies that have elucidated UTI host-pathogen interactions from the perspective of gene expression, and review new studies of biofilm and UTI preventative approaches. We also consider potential standards for variables inherent in murine UTI models and discuss how these might expand the utility of models for mimicking human disease and uncovering new aspects of pathogenesis.

Gold Nanoparticle Labeling Based ICP-MS Detection/Measurement of Bacteria, and Their Quantitative Photothermal Destruction

Bacteria such as Salmonella and E. coli present a great challenge in public health care in today's society. Protection of public safety against bacterial contamination and rapid diagnosis of infection require simple and fast assays for the detection and elimination of bacterial pathogens. After utilizing Salmonella DT104 as an example bacterial strain for our investigation, we report a rapid and sensitive assay for the qualitative and quantitative detection of bacteria by using antibody affinity binding, popcorn shaped gold nanoparticle (GNPOPs) labeling, surfance enchanced Raman spectroscopy (SERS), and inductively coupled plasma mass spectrometry (ICP-MS) detection. For qualitative analysis, our assay can detect Salmonella within 10 min by Raman spectroscopy; for quantitative analysis, our assay has the ability to measure as few as 100 Salmonella DT104 in a 1 mL sample (100 CFU/mL) within 40 min. Based on the quantitative detection, we investigated the quantitative destruction of Salmonella DT104, and the assay's photothermal efficiency in order to reduce the amount of GNPOPs in the assay to ultimately to eliminate any potential side effects/toxicity to the surrounding cells in vivo. Results suggest that our assay may serve as a promising candidate for qualitative and quantitative detection and elimination of a variety of bacterial pathogens.

Immunity to intestinal pathogens: lessons learned from Salmonella

Salmonella are a common source of food- or water-borne infection and cause a wide range of clinical disease in human and animal hosts. Salmonella are relatively easy to culture and manipulate in a laboratory setting, and the infection of laboratory animals induces robust innate and adaptive immune responses. Thus, immunologists have frequently turned to Salmonella infection models to expand understanding of host immunity to intestinal pathogens. In this review, I summarize current knowledge of innate and adaptive immunity to Salmonella and highlight features of this response that have emerged from recent studies. These include the heterogeneity of the antigen-specific T-cell response to intestinal infection, the prominence of microbial mechanisms to impede T- and B-cell responses, and the contribution of non-cognate pathways for elicitation of T-cell effector functions. Together, these different issues challenge an overly simplistic view of host-pathogen interaction during mucosal infection, but also allow deeper insight into the real-world dynamic of protective immunity to intestinal pathogens.

Mortality and translocation assay to study the protective capacity of Bifidobacterium lactis INL1 against Salmonella Typhimurium infection in mice

The mouse has been largely used for the study of the protective capacity of probiotics against intestinal infections caused by Salmonella. In this work we aimed at comparing the mortality and translocation assay for the study of the protective capacity of the human breast milk-derived strain Bifidobacterium animalis subsp. lactis INL1 on a model of gut infection by Salmonella enterica subsp. enterica serovar Typhimurium. Different doses of S. Typhimurium FUNED and B. animalis subsp. lactis INL 1 were administered to Balb/c mice in a mortality or a translocation assay. The survival of the control group in the mortality assay resulted to be variable along experiments, and then we preferred to use a translocation assay where the preventive administration of 109 cfu of bifidobacteria/mouse for 10 consecutive days significantly reduced the number of infected animals and the levels of translocation to liver and spleen, with enhanced secretory immunoglobulin A and interleukin 10 production in the small and large intestine, respectively. Ten days of B. animalis subsp. lactis strain INL1 administration to mice significantly reduced both the incidence and the severity of Salmonella infection in a mouse model of translocation. This work provided the first evidence that a translocation assay, compared to a mortality assay, could be more useful to study the protective capacity of probiotics against Salmonella infection, as more information can be obtained from mice and less suffering is conferred to animals due to the fact that the mortality assay is shorter than the latter. These facts are in line with the guidelines of animal research recently established by the National Centre for the Replacement, Refinement & Reduction of Animals in Research.

Systemic responses of BALB/c mice to Salmonella typhimurium infection

Salmonella typhimurium is a bacterial pathogen that poses a great threat to humans and animals. In order to discover hosts' responses to S. typhimurium infection, we collected and analyzed biofluids and organ tissues from mice which had ingested S. typhimurium. We employed (1)H NMR spectroscopy coupled with multivariate data analysis and immunological techniques. The results indicate that infection leads to a severe impact on mice spleen and ileum, which are characterized by splenomegaly and edematous villi, respectively. We found that increased levels of itaconic acid were correlated with the presence of splenomegaly during infection and may play an important role in Salmonella-containing vacuole acidification. In addition, metabonomic analyses of urine displayed the development of salmonellosis in mice, which is characterized by dynamic changes in energy metabolism. Furthermore, we found that the presence of S. typhimurium activated an anti-oxidative response in infected mice. We also observed changes in the gut microbial co-metabolites (hippurate, TMAO, TMA, methylamine). This investigation sheds much needed light on the host-pathogen interactions of S. typhimurium, providing further information to deepen our understanding of the long co-evolution process between hosts and infective bacteria.

Genetic dissection of the ity3 locus identifies a role for ncf2 co-expression modules and suggests selp as a candidate gene underlying the ity3.2 locus

Typhoid fever and salmonellosis, which are caused by Salmonella typhi and typhimurium, respectively, are responsible for significant morbidity and mortality in both developed and developing countries. We model typhoid fever using mice infected with Salmonella typhimurium, which results in a systemic disease, whereby the outcome of infection is variable in different inbred strains of mice. This model recapitulates several clinical aspects of the human disease and allows the study of the host response to Salmonella typhimurium infection in vivo. Previous work in our laboratory has identified three loci (Ity, Ity2, and Ity3) in the wild-derived MOLF/Ei mice influencing survival after infection with Salmonella typhimurium. Fine mapping of the Ity3 locus indicated that two sub-loci contribute collectively to the susceptibility of B6.MOLF-Ity/Ity3 congenic mice to Salmonella infection. In the current paper, we provided further evidence supporting a role for Ncf2 (neutrophil cytosolic factor 2 a subunit of NADPH oxidase) as the gene underlying the Ity3.1 sub-locus. Gene expression profiling indicated that the Ity3.1 sub-locus defined a global gene expression signature with networks articulated around Ncf2. Furthermore, based on differential expression and complementation analysis using Selp (selectin-P) knock-out mice, Selp was identified as a strong candidate gene for the Ity3.2 sub-locus.

Oxidoreductases that act as conditional virulence suppressors in Salmonella enterica serovar Typhimurium

In Salmonella enterica serovar Typhimurium, oxidoreductases of the thioredoxin superfamily contribute to bacterial invasiveness, intracellular replication and to the virulence in BALB/c mice as well as in the soil nematode Caenorhabditis elegans. The scsABCD gene cluster, present in many but not all enteric bacteria, codes for four putative oxidoreductases of the thioredoxin superfamily. Here we have analyzed the potential role of the scs genes in oxidative stress tolerance and virulence in S. Typhimurium. An scsABCD deletion mutant showed moderate sensitization to the redox-active transition metal ion copper and increased protein carbonylation upon exposure to hydrogen peroxide. Still, the scsABCD mutant was not significantly affected for invasiveness or intracellular replication in respectively cultured epithelial or macrophage-like cells. However, we noted a significant copper chloride sensitivity of SPI1 T3SS mediated invasiveness that strongly depended on the presence of the scs genes. The scsABCD deletion mutant was not attenuated in animal infection models. In contrast, the mutant showed a moderate increase in its competitive index upon intraperitoneal challenge and enhanced invasiveness in small intestinal ileal loops of BALB/c mice. Moreover, deletion of the scsABCD genes restored the invasiveness of a trxA mutant in epithelial cells and its virulence in C. elegans. Our findings thus demonstrate that the scs gene cluster conditionally affects virulence and underscore the complex interactions between oxidoreductases of the thioredoxin superfamily in maintaining host adaptation of S. Typhimurium.

Exploring the immune response of porcine mesenteric lymph nodes to Salmonella enterica serovar Typhimurium: an analysis of transcriptional changes, morphological alterations and pathogen burden

Infections caused by Salmonella enterica serovar Typhimurium (S. typhimurium) cause important economic problems in the swine industry and threaten the integrity of a safe and healthy food supply. Controlling the prevalence of Salmonella in pig production requires a thorough knowledge of the response processes that occurs in the gut associated immune tissues. To explore the in vivo porcine response to S. typhimurium, MLN samples from four control pigs and twelve infected animals at 1, 2 and 6 days post infection (dpi) were collected to quantify the mRNA expression of gene coding for 42 innate immune-related molecules. In addition, the presence of S. typhimurium in MLN was examined and its effect on tissue micro-anatomy. Higher S. typhimurium loads were observed at 2dpi, triggering an innate immune response, marked by a substantial infiltration of phagocytes and up-regulation of pro-inflammatory genes. Such response resulted in a significant decrease in pathogen burden in MLN at 6dpi, although Salmonella could not be completely eliminated from tissue. Furthermore, our results suggest that in porcine infections, S. typhimurium might interferes with dendritic cell-T cell interactions and this strategy could be involved in the conversion of Salmonella infected pigs to a carrier state.

Study of the effect exerted by fructo-oligosaccharides from yacon (Smallanthus sonchifolius) root flour in an intestinal infection model with Salmonella Typhimurium

Beneficial effects of prebiotics like inulin and fructo-oligosaccharides (FOS) have been proven in health and nutrition. Yacon (Smallanthus sonchifolius), an Andean crop, contains FOS (50–70 % of its dry weight) and, therefore, is considered a prebiotic. Commercial FOS can up-regulate total secretory IgA (S-IgA) in infant mice, prevent infection with Salmonella in swine or enhance immune response for Salmonella vaccine in a mouse model. Previously, we found that administration of yacon root flour regulates gut microbiota balance and has immunomodulatory effects without inflammatory responses. The aim of the present paper is to analyse if yacon prevents enteric infection caused by a strain of Salmonella enteritidis serovar Typhimurium (S. Typhimurium) in a mouse model. BALB/c mice were supplemented with yacon flour (45 d), challenged with S. Typhimurium and killed to study pathogen translocation, total and specific IgA production by ELISA, presence of IgA and other cytokines and Toll-like receptor 4 (TLR4) and clustor of differentiation 206 (CD206) receptors positive cells by immunofluorescence and histological changes. Yacon flour administration had a protective effect from 15 to 30 d of treatment. We found a peak of total S-IgA production without translocation of the pathogen for these periods. At 30 d, there was an increase in IL-6 and macrophage inflammatory proteins-1α+ cells and expression of the receptors CD206 and TLR4. Yacon flour did not have incidence in pathogen-specific S-IgA production. Longer periods (45 d) of administration had no protective effect. Therefore, yacon can prevent enteric infection caused by S. Typhimurium when given up to 30 d; this effect would be mediated by enhancing non-specific immunity, such as total S-IgA, that improves the immunological intestinal barrier.

The in vivo extracellular life of facultative intracellular bacterial parasites: role in pathogenesis

Classically labeled facultative intracellular pathogens are characterized by the ability to have an intracellular phase in the host, which is required for pathogenicity, while capable of extracellular growth in vitro. The ability of these bacteria to replicate in cell-free conditions is usually assessed by culture in artificial bacteriological media. However, the extracellular growth ability of these pathogens may also be expressed by a phase of extracellular infection in the natural setting of the host with pathologic consequences, an ability that adds to the pathogenic potential of the infectious agent. This infective capability to grow in the extracellular sites of the host represents an additional virulence attribute of those pathogens which may lead to severe outcomes. Here we discuss examples of infectious diseases where the in vivo infective extracellular life is well documented, including infections by Francisella tularensis, Yersinia pestis, Burkholderia pseudomallei, Burkholderia cenocepacia, Salmonella enterica serovar Typhimurium and Edwardsiella tarda. The occurrence of a phase of systemic dissemination with extracellular multiplication during progressive infections by facultative intracellular bacterial pathogens has been underappreciated, with most studies exclusively centered on the intracellular phase of the infections. The investigation of the occurrence of a dual lifestyle in the host among bacterial pathogens in general should be extended and likely will reveal more cases of infectious diseases with a dual infective intracellular/extracellular pattern.

Identification of a common immune signature in murine and human systemic Salmonellosis

Despite the importance of Salmonella infections in human and animal health, the target antigens of Salmonella-specific immunity remain poorly defined. We have previously shown evidence for antibody-mediating protection against invasive Salmonellosis in mice and African children. To generate an overview of antibody targeting in systemic Salmonellosis, a Salmonella proteomic array containing over 2,700 proteins was constructed and probed with immune sera from Salmonella-infected mice and humans. Analysis of multiple inbred mouse strains identified 117 antigens recognized by systemic antibody responses in murine Salmonellosis. Importantly, many of these antigens were independently identified as target antigens using sera from Malawian children with Salmonella bacteremia, validating the study of the murine model. Furthermore, vaccination with SseB, the most prominent antigenic target in Malawian children, provided mice with significant protection against Salmonella infection. Together, these data uncover an overlapping immune signature of disseminated Salmonellosis in mice and humans and provide a foundation for the generation of a protective subunit vaccine.

Phenotypic pattern-based assay for dynamically monitoring host cellular responses to Salmonella infections

The interaction between mammalian host cells and bacteria is a dynamic process, and the underlying pathologic mechanisms are poorly characterized. Limited information describing the host-bacterial interaction is based mainly on studies using label-based endpoint assays that detect changes in cell behavior at a given time point, yielding incomplete information. In this paper, a novel, label-free, real-time cell-detection system based on electronic impedance sensor technology was adapted to dynamically monitor the entire process of intestinal epithelial cells response to Salmonella infection. Changes in cell morphology and attachment were quantitatively and continuously recorded following infection. The resulting impedance-based time-dependent cell response profiles (TCRPs) were compared to standard assays and showed good correlation and sensitivity. Biochemical assays further suggested that TCRPs were correlated with cytoskeleton-associated morphological dynamics, which can be largely attenuated by inhibitions of actin and microtubule polymerization. Collectively, our data indicate that cell-electrode impedance measurements not only provide a novel, real-time, label-free method for investigating bacterial infection but also help advance our understanding of host responses in a more physiological and continuous manner that is beyond the scope of current endpoint assays.

Immunity to salmonellosis

Salmonella enterica is a genetically broad species harboring isolates that display considerable antigenic heterogeneity and significant differences in virulence potential. Salmonella generally exhibit an invasive potential and they can survive for extended periods within cells of the immune system. They cause acute or chronic infections that can be local (e.g. gastroenteritis) or systemic (e.g. typhoid). In vivo Salmonella infections are complex with multiple arms of the immune system being engaged. Both humoral and cellular responses can be detected and characterized, but full protective immunity is not always induced, even following natural infection. The murine model has proven to be a fertile ground for exploring immune mechanisms and observations in the mouse have often, although not always, correlated with those in other infectable species, including humans. Host genetic studies have identified a number of mammalian genes that are central to controlling infection, operating both in innate and acquired immune pathways. Vaccines, both oral and parenteral, are available or under development, and these have been used with some success to explore immunity in both model systems and clinically in humans.

Soluble flagellin, FliC, induces an Ag-specific Th2 response, yet promotes T-bet-regulated Th1 clearance of Salmonella typhimurium infection

Clearance of disseminated Salmonella infection requires bacterial-specific Th1 cells and IFN-γ production, and Th1-promoting vaccines are likely to help control these infections. Consequently, vaccine design has focused on developing Th1-polarizing adjuvants or Ag that naturally induce Th1 responses. In this study, we show that, in mice, immunization with soluble, recombinant FliC protein flagellin (sFliC) induces Th2 responses as evidenced by Ag-specific GATA-3, IL-4 mRNA, and protein induction in CD62L(lo) CD4(+) T cells without associated IFN-γ production. Despite these Th2 features, sFliC immunization can enhance the development of protective Th1 immunity during subsequent Salmonella infection in an Ab-independent, T-cell-dependent manner. Salmonella infection in sFliC-immunized mice resulted in augmented Th1 responses, with greater bacterial clearance and increased numbers of IFN-γ-producing CD4(+) T cells, despite the early induction of Th2 features to sFliC. The augmented Th1 immunity after sFliC immunization was regulated by T-bet although T-bet is dispensable for primary responses to sFliC. These findings show that there can be flexibility in T-cell responses to some subunit vaccines. These vaccines may induce Th2-type immunity during primary immunization yet promote Th1-dependent responses during later infection. This suggests that designing Th1-inducing subunit vaccines may not always be necessary since this can occur naturally during subsequent infection.

Dynamics of growth and dissemination of Salmonella in vivo

The last decade has witnessed increasing research on dissemination of bacterial pathogens in their hosts and on the processes that underlie bacterial spread and growth during organ colonization. Here, we discuss work on the mouse model of human typhoid fever caused by Salmonella enterica serovar Typhimurium. This has revealed the use of several routes of systemic dissemination that result in colonization and growth within the spleen and liver, the major sites of bacterial proliferation. We also highlight techniques that enable in vivo analysis of the infecting population at the spatiotemporal and single cell levels. These approaches have provided more detailed insights into the events underlying the dynamics of Salmonella replication, spread and clearance within host organs and tissues.

Global analysis of the eukaryotic pathways and networks regulated by Salmonella typhimurium in mouse intestinal infection in vivo

BACKGROUND

Acute enteritis caused by Salmonella is a public health concern. Salmonella infection is also known to increase the risk of inflammatory bowel diseases and cancer. Therefore, it is important to understand how Salmonella works in targeting eukaryotic pathways in intestinal infection. However, the global physiological function of Salmonella typhimurium in intestinal mucosa in vivo is unclear. In this study, a whole genome approach combined with bioinformatics assays was used to investigate the in vivo genetic responses of the mouse colon to Salmonella. We focused on the intestinal responses in the early stage (8 hours) and late stage (4 days) after Salmonella infection.

RESULTS

Of the 28,000 genes represented on the array, our analysis of mRNA expression in mouse colon mucosa showed that a total of 856 genes were expressed differentially at 8 hours post-infection. At 4 days post-infection, a total of 7558 genes were expressed differentially. 23 differentially expressed genes from the microarray data was further examined by real-time PCR. Ingenuity Pathways Analysis identified that the most significant pathway associated with the differentially expressed genes in 8 hours post-infection is oxidative phosphorylation, which targets the mitochondria. At the late stage of infection, a series of pathways associated with immune and inflammatory response, proliferation, and apoptosis were identified, whereas the oxidative phosphorylation was shut off. Histology analysis confirmed the biological role of Salmonella, which induced a physiological state of inflammation and proliferation in the colon mucosa through the regulation of multiple signaling pathways. Most of the metabolism-related pathways were targeted by down-regulated genes, and a general repression process of metabolic pathways was observed. Network analysis supported IFN-γ and TNF-α function as mediators of the immune/inflammatory response for host defense against pathogen.

CONCLUSION

Our study provides novel genome-wide transcriptional profiling data on the mouse colon mucosa's response to the Salmonella typhimurium infection. Building the pathways and networks of interactions between these genes help us to understand the complex interplay in the mice colon during Salmonella infection, and further provide new insights into the molecular cascade, which is mobilized to combat Salmonella-associated colon infection in vivo.

Interference of Bifidobacterium choerinum or Escherichia coli Nissle 1917 with Salmonella Typhimurium in gnotobiotic piglets correlates with cytokine patterns in blood and intestine

The colonization, translocation and protective effect of two intestinal bacteria - PR4 (pig commensal strain of Bifidobacterium choerinum) or EcN (probiotic Escherichia coli strain Nissle 1917) - against subsequent infection with a virulent LT2 strain of Salmonella enterica serovar Typhimurium were studied in gnotobiotic pigs after oral association. The clinical state of experimental animals correlated with bacterial translocation and levels of inflammatory cytokines [a chemokine, interleukin (IL)-8, a proinflammatory cytokine, tumour necrosis factor (TNF)-α and an anti-inflammatory cytokine, IL-10] in plasma and intestinal lavages. Gnotobiotic pigs orally mono-associated with either PR4 or EcN thrived, and bacteria were not found in their blood. No significant inflammatory cytokine response was observed. Mono-association with Salmonella caused devastating septicaemia characterized by high levels of IL-10 and TNF-α in plasma and TNF-α in the intestine. Di-associated gnotobiotic pigs were given PR4 or EcN for 24 h. Subsequently, they were infected orally with Salmonella and euthanized 24 h later. Pigs associated with bifidobacteria before Salmonella infection suffered from severe systemic infection and mounted similar cytokine responses as pigs infected with Salmonella alone. In contrast, EcN interfered with translocation of Salmonella into mesenteric lymph nodes and systemic circulation. Pigs pre-associated with EcN thrived and their clinical condition correlated with the absence of IL-10 in their plasma and a decrease of TNF-α in plasma and ileum.

Role for neutrophils in host immune responses and genetic factors that modulate resistance to Salmonella enterica serovar typhimurium in the inbred mouse strain SPRET/Ei

Infection with Salmonella enterica serovar Typhimurium is a complex disease in which the host-bacterium interactions are strongly influenced by genetic factors of the host. We demonstrate that SPRET/Ei, an inbred mouse strain derived from Mus spretus, is resistant to S. Typhimurium infections. The kinetics of bacterial proliferation, as well as histological examinations of tissue sections, suggest that SPRET/Ei mice can control bacterial multiplication and spreading despite significant attenuation of the cytokine response. The resistance of SPRET/Ei mice to S. Typhimurium infection is associated with increased leukocyte counts in the circulation and enhanced neutrophil influx into the peritoneum during the course of infection. A critical role of neutrophils was confirmed by neutrophil depletion: neutropenic SPRET/Ei mice were sensitive to infection with S. Typhimurium and showed much higher bacterial loads. To identify genes that modulate the natural resistance of SPRET/Ei mice to S. Typhimurium infection, we performed a genome-wide study using an interspecific backcross between C3H/HeN and SPRET/Ei mice. The results of this analysis demonstrate that at least two loci, located on chromosomes 6 and 11, affect survival following lethal infection with S. Typhimurium. These two loci contain several interesting candidate genes which may have important implications for the search for genetic factors controlling Salmonella infections in humans and for our understanding of complex host-pathogen interactions in general.

B7-H1 (programmed cell death ligand 1) is required for the development of multifunctional Th1 cells and immunity to primary, but not secondary, Salmonella infection

Robust Ab and CD4 T cell responses are required for the resolution of Salmonella infection in susceptible mice. In this study, we examined the role of B7-H1 (programmed cell death ligand 1) in resistance to primary Salmonella infection. Infected B7-H1-deficient mice had significantly higher bacterial burdens at day 21 and day 35 postinfection compared with wild-type mice, demonstrating that B7-H1 plays an important role in immunity to Salmonella. B7-H1-deficient and wild-type mice both generated Salmonella-specific IgM and IgG2c Ab responses to infection, and clonal expansion of endogenous and adoptively transferred Salmonella-specific CD4 T cells was similar in both groups. However, although Salmonella-specific IFN-gamma-producing Th1 CD4 T cells were generated in Salmonella-infected B7-H1-deficient mice, these cells did not expand to the level observed in wild-type mice. Furthermore, fewer multifunctional Th1 cells that simultaneously secreted IFN-gamma, TNF-alpha, and IL-2 were detected in Salmonella-infected B7-H1-deficient mice. Together, these data demonstrate that B7-H1 is required for the generation of multifunctional Th1 responses and optimal protective immunity to primary Salmonella infection.

Cholinergic stimulation of the immune system protects against lethal infection by Salmonella enterica serovar Typhimurium

SUMMARY

The cholinergic nervous system has been demonstrated to attenuate the inflammatory response during sepsis via the inhibitory action of acetylcholine (ACh) on macrophages. These findings were largely based on experimental sepsis models using endotoxin as the inducing agent. Herein, however, we report that the specific inhibition of acetylcholinesterase (AChE) renders animals more resistant to infection by a virulent strain of Salmonella enterica serovar Typhimurium, a Gram-negative enteric pathogen. Inhibition of AChE was induced by a subchronic exposure to paraoxon, a potent anti-cholinesterase metabolite of the organophosphorous compound parathion. Our findings indicate that inhibition of AChE enhanced survival of infected mice in a dose-dependent fashion and this correlated with efficient control of bacterial proliferation in target organs. Immunologically, inhibition of AChE enabled the animals to mount a more effective inflammatory anti-microbial response, and to secrete higher levels of interleukin-12, a key T helper type 1-promoting cytokine. The ACh-induced enhancement in resistance to infection was abrogated by co-administration of an oxime which can reactivate AChE. Hence, in a model of Gram-negative bacterial infection, cholinergic stimulation is shown to enhance the anti-microbial immune response leading to effective control of bacterial proliferation and enhanced animal survival.

Toll-like receptor 4 signalling through MyD88 is essential to control Salmonella enterica serovar typhimurium infection, but not for the initiation of bacterial clearance

Toll-like receptor-4 (TLR4) is important in protection against lethal Salmonella enterica serovar Typhimurium (S. Typhimurium) infection. Control of the early stages of sublethal S. Typhimurium infection in mice depends on TLR4-dependent activation of macrophages and natural killer (NK) cells to drive an inflammatory response. TLR4 signals through the adapter proteins Mal/MyD88 and TRIF-related adaptor molecule (TRAM)/TIR-domain-containing adaptor-inducing interferon-b (TRIF). In the mouse typhoid model we showed that TLR4 and MyD88, but not Mal or TRIF, are essential for the control of exponential S. Typhimurium growth. TRIF(-/-) mice have a higher bacterial load in comparison with wild-type mice during a sublethal infection because TRIF is important for bacterial killing during the first day of systemic disease. Minimal pro-inflammatory responses were induced by S. Typhimurium infection of macrophages from TLR4(-/-), MyD88(-/-) and TRIF(-/-) mice in vitro. Pro-inflammatory responses from Mal(-/-) macrophages were similar to those from wild-type cells. The pro-inflammatory responses of TRIF(-/-) macrophages were partially restored by the addition of interferon-gamma (IFN-gamma), and TRIF(-/-) mice produced markedly enhanced IFN-gamma levels, in comparison to wild-type mice, probably explaining why bacterial growth can be controlled in these mice. TLR4(-/-), MyD88(-/-), TRIF(-/-) and Mal(-/-) mice all initiated clearance of S. Typhimurium, suggesting that TLR4 signalling is not important in driving bacterial clearance in comparison to its critical role in controlling early bacterial growth in mouse typhoid.

Molecular mechanisms of Salmonella virulence and host resistance

Salmonella species can cause typhoid fever and gastroenteritis in humans and pose a global threat to human health. In order to establish a successful infection, Salmonella utilize a large number of genes encoding a variety of virulence factors. Different animal models of infection have been used to better understand the mechanisms underlying each disease including cattle, rodents, and nematodes. To date, a number of different bacterial virulence factors have been identified using such animal models, most of which are secreted by two type three secretion systems (T3SS) encoded within Salmonella pathogenicity islands (SPI) 1 and 2. These proteins alter various host cell pathways, facilitating the invasion of epithelial cells during infection, as well as the survival and replication of Salmonella inside phagocytic cells. On the other hand, host genetics and resistance also play a role in the susceptibility to Salmonella infection. The natural resistance-associated macrophage protein 1 (Nramp1), for example, is critical for host defense, since mice lacking Nramp1 fail to control bacterial replication and succumb to low doses of S. Typhimurium. In this chapter, we analyze the different pathogen and host factors that play a role in the dynamic interaction between Salmonella and its host and their impact on disease.

Bacterial growth rate and host factors as determinants of intracellular bacterial distributions in systemic Salmonella enterica infections

Bacteria of the species Salmonella enterica cause a range of life-threatening diseases in humans and animals worldwide. The within-host quantitative, spatial, and temporal dynamics of S. enterica interactions are key to understanding how immunity acts on these infections and how bacteria evade immune surveillance. In this study, we test hypotheses generated from mathematical models of in vivo dynamics of Salmonella infections with experimental observation of bacteria at the single-cell level in infected mouse organs to improve our understanding of the dynamic interactions between host and bacterial mechanisms that determine net growth rates of S. enterica within the host. We show that both bacterial and host factors determine the numerical distributions of bacteria within host cells and thus the level of dispersiveness of the infection.

Comprehensive identification of Salmonella enterica serovar typhimurium genes required for infection of BALB/c mice

Genes required for infection of mice by Salmonella Typhimurium can be identified by the interrogation of random transposon mutant libraries for mutants that cannot survive in vivo. Inactivation of such genes produces attenuated S. Typhimurium strains that have potential for use as live attenuated vaccines. A quantitative screen, Transposon Mediated Differential Hybridisation (TMDH), has been developed that identifies those members of a large library of transposon mutants that are attenuated. TMDH employs custom transposons with outward-facing T7 and SP6 promoters. Fluorescently-labelled transcripts from the promoters are hybridised to whole-genome tiling microarrays, to allow the position of the transposon insertions to be determined. Comparison of microarray data from the mutant library grown in vitro (input) with equivalent data produced after passage of the library through mice (output) enables an attenuation score to be determined for each transposon mutant. These scores are significantly correlated with bacterial counts obtained during infection of mice using mutants with individual defined deletions of the same genes. Defined deletion mutants of several novel targets identified in the TMDH screen are effective live vaccines.

Vaccine-induced antibody isotypes are skewed by impaired CD4 T cell and invariant NKT cell effector responses in MyD88-deficient mice

The requirement for TLR signaling in the initiation of an Ag-specific Ab response is controversial. In this report we show that a novel OVA-expressing recombinant Salmonella vaccine (Salmonella-OVA) elicits a Th1-biased cell-mediated and serum Ab response upon oral or i.p. immunization of C57BL/6 mice. In MyD88(-/-) mice, Th1-dependent Ab responses are greatly reduced while Th2-dependent Ab isotypes are elevated in response to oral and i.p., but not s.c. footpad, immunization. When the T effector response to oral vaccination is examined we find that activated, adoptively transferred Ag-specific CD4(+) T cells accumulate in the draining lymph nodes, but fail to produce IFN-gamma, in MyD88(-/-) mice. Moreover, CD1d tetramer staining shows that invariant NKT cells are activated in response to oral Salmonella-OVA vaccination in wild-type, but not MyD88(-/-), mice. Treatment with neutralizing Ab to CD1d reduces the OVA-specific Ab response only in MyD88-sufficient wild-type mice, suggesting that both Ag-specific CD4 T cell and invariant NKT cell effector responses to Salmonella-OVA vaccination are MyD88 dependent. Taken together, our data indicate that the type of adaptive immune response generated to this live attenuated vaccine is regulated by both the presence of MyD88-mediated signals and vaccination route, which may have important implications for future vaccine design.

Successful treatment of bacterial infection hinders development of acquired immunity

Antibiotics are routinely used to control bacterial infection, but the acquisition of acquired immunity following successful treatment has rarely been examined. We developed a model that allows visualization of acquired immunity during and following antibiotic treatment of typhoid. Pathogen-specific humoral and cellular immune responses were activated rapidly in antibiotic-treated mice, but were not sustained after successful antibiotic treatment and did not confer protection to secondary infection. In marked contrast, pathogen-specific Th1 and Ab responses matured over several weeks following immunization with a live vaccine strain. The deficiency in protective immunity following antibiotic treatment could be overcome by administering flagellin during antibiotic therapy. Thus, development of protective immunity is hindered by rapid therapeutic elimination of bacteria, but can be overcome by providing additional inflammatory and/or antigenic stimuli.

Dexamethasone modulates Salmonella enterica serovar Typhimurium infection in vivo independently of the glucocorticoid-inducible protein annexin-A1

Salmonella enterica serovar Typhimurium (S. Typhimurium) infection causes an inflammatory response through activation of Toll-like receptor 4 by lipopolysaccharide. Dexamethasone, a glucocorticoid analogue, suppresses inflammatory responses by many mechanisms including inhibition of the lipopolysaccharide-induced production of proinflammatory mediators. There is little information on the effect of glucocorticoids on murine salmonellosis. In this study, we treated susceptible BALB/c mice by subcutaneous implantation of slow-release dexamethasone pellets before infection with S. Typhimurium. Dexamethasone promotes bacterial growth early in infection and induces a dose-dependent increase in bacterial growth within mouse livers and spleens. The bacterial load in organs from infected placebo-treated mice was lower than that in dexamethasone-treated mice. Glucocorticoids inhibit lipopolysaccharide-induced inflammation partially through the steroid-inducible protein annexin-A1 (ANXA1). Infection of wild-type and ANXA1 knock-out mice with S. Typhimurium led to similar organ bacterial loads. ANXA1 also did not affect the bacterial load in organs from infected dexamethasone-treated mice. This suggests that glucocorticoids, independently of ANXA1, accelerate S. Typhimurium growth in vivo in susceptible BALB/c mice.

A dynamic view of the spread and intracellular distribution of Salmonella enterica

The events that determine the dynamics of proliferation, spread and distribution of microbial pathogens within their hosts are surprisingly heterogeneous and poorly understood. We contend that understanding these phenomena at a sophisticated level with the help of mathematical models is a prerequisite for the development of truly novel, targeted preventative measures and drug regimes. We describe here recent studies of Salmonella enterica infections in mice which suggest that bacteria resist the antimicrobial environment inside host cells and spread to new sites, where infection foci develop, and thus avoid local escalation of the adaptive immune response. We further describe implications for our understanding of the pathogenic mechanism inside the host.

Tracking the dynamics of salmonella specific T cell responses

Over the last decade, significant advances have been made in the methodology for studying immune responses in vivo. It is now possible to follow almost every aspect of pathogen-specific immunity using in vivo models that incorporate physiological infectious doses and natural routes of infection. This new ability to study immunity in a relevant physiological context will greatly expand our understanding of the dynamic interplay between host and pathogen. Visualizing the resolution of primary infection and the development of long-term immunological memory should also aid the development of new vaccines and therapeutics for infectious diseases. In this review, we will describe the application of in vivo visualization technology to Salmonella infection, describe our current understanding of Salmonella-specific immunity, and discuss some unanswered questions that remain in this model.

Salmonella-containing vacuoles display centrifugal movement associated with cell-to-cell transfer in epithelial cells

Intracellular Salmonella enterica serovar Typhimurium (serovar Typhimurium) occupies a Salmonella-containing vacuole (SCV) where bacterial effector proteins are secreted into the host cell using type III secretion systems (T3SS). Cytoskeletal motor proteins and T3SS-delivered effector proteins facilitate SCV positioning to juxtanuclear positions where bacterial replication occurs. Here, we show that this characteristic SCV positioning is not maintained by all SCVs during infection of HeLa cells. Notably, juxtanuclear SCV localization that occurs by 8 to 14 h postinfection is followed by significant centrifugal displacement of a subset of SCVs toward the host cell periphery by 24 h postinfection. This novel phenotype requires bacterial protein synthesis, a functional Salmonella pathogenicity island 2 (SPI-2)-encoded T3SS, intact microtubules, and kinesin-1 motor protein. Bacteria lacking PipB2, a kinesin-recruiting T3SS effector, did not exhibit centrifugal displacement and remained at juxtanuclear positions throughout 24 h of infection. While levels of the SPI-2 effectors PipB2 and SifA increased during 24 h postinfection, a corresponding decrease in levels of the SPI-1 T3SS effectors SipA and SopB, both known to mediate juxtanuclear SCV positioning, was observed. A fluorescence-based assay indicated that wild-type serovar Typhimurium transferred from infected to uninfected epithelial cells while strains deficient in SPI-2 T3SS secretion or PipB2 did not. Our results reveal a novel SCV phenotype implicated in the cell-to-cell spread of serovar Typhimurium during infection.

The role of microorganisms in coral bleaching

Coral bleaching is the disruption of the symbiosis between the coral host and its endosymbiotic algae. The prevalence and severity of the disease have been correlated with high seawater temperature. During the last decade, the major hypothesis to explain coral bleaching is that high water temperatures cause irreversible damage to the symbiotic algae resulting in loss of pigment and/or algae from the holobiont. Here, we discuss the evidence for an alternative but not mutually exclusive concept, the microbial hypothesis of coral bleaching.

Gene expression profiling in chicken heterophils with Salmonella enteritidis stimulation using a chicken 44 K Agilent microarray

Background

Salmonella enterica serovar Enteritidis (SE) is one of the most common food-borne pathogens that cause human salmonellosis and usually results from the consumption of contaminated poultry products. The mechanism of SE resistance in chickens remains largely unknown. Previously, heterophils isolated from broilers with different genetic backgrounds (SE-resistant [line A] and -susceptible [line B]) have been shown to be important in defending against SE infections. To dissect the interplay between heterophils and SE infection, we utilized large-scale gene expression profiling.

Results

The results showed more differentially expressed genes were found between different lines than between infection (SE-treated) and non-infection (control) samples within line. However, the numbers of expressed immune-related genes between these two comparisons were dramatically different. More genes related to immune function were down-regulated in line B than line A. The analysis of the immune-related genes indicated that SE infection induced a stronger, up-regulated gene expression of line heterophils A than line B, and these genes include several components in the Toll-like receptor (TLR) signaling pathway, and genes involved in T-helper cell activation.

Conclusion

We found: (1) A divergent expression pattern of immune-related genes between lines of different genetic backgrounds. The higher expression of immune-related genes might be more beneficial to enhance host immunity in the resistant line; (2) a similar TLR regulatory network might exist in both lines, where a possible MyD88-independent pathway may participate in the regulation of host innate immunity; (3) the genes exclusively differentially expressed in line A or line B with SE infection provided strong candidates for further investigating SE resistance and susceptibility. These findings have laid the foundation for future studies of TLR pathway regulation and cellular modulation of SE infection in chickens.

Hemophagocytic macrophages harbor Salmonella enterica during persistent infection

Salmonella enterica subspecies can establish persistent, systemic infections in mammals, including human typhoid fever. Persistent S. enterica disease is characterized by an initial acute infection that develops into an asymptomatic chronic infection. During both the acute and persistent stages, the bacteria generally reside within professional phagocytes, usually macrophages. It is unclear how salmonellae can survive within macrophages, cells that evolved, in part, to destroy pathogens. Evidence is presented that during the establishment of persistent murine infection, macrophages that contain S. enterica serotype Typhimurium are hemophagocytic. Hemophagocytic macrophages are characterized by the ingestion of non-apoptotic cells of the hematopoietic lineage and are a clinical marker of typhoid fever as well as certain other infectious and genetic diseases. Cell culture assays were developed to evaluate bacterial survival in hemophagocytic macrophages. S. Typhimurium preferentially replicated in macrophages that pre-phagocytosed viable cells, but the bacteria were killed in macrophages that pre-phagocytosed beads or dead cells. These data suggest that during persistent infection hemophagocytic macrophages may provide S. Typhimurium with a survival niche.

Caspase-3-dependent phagocyte death during systemic Salmonella enterica serovar Typhimurium infection of mice

Growth of Salmonella enterica in mammalian tissues results from continuous spread of bacteria to new host cells. Our previous work indicated that infective S. enterica are liberated from host cells via stochastic necrotic burst independently of intracellular bacterial numbers. Here we report that liver phagocytes can undergo apoptotic caspase-3-mediated cell death in vivo, with apoptosis being a rare event, more prevalent in heavily infected cells. The density-dependent apoptotic cell death is likely to constitute an alternative mechanism of bacterial spread as part of a bet-hedging strategy, ensuring an ongoing protective intracellular environment in which some bacteria can grow and persist.

Entry route of Salmonella typhimurium directs the type of induced immune response

Secretory immunoglobulin A (SIgA) represents a first line of defense against mucosal pathogens by limiting their entrance. By using different strains of Salmonella typhimurium that target the two mechanisms of bacterial entry (microfold cell [M cell]- or dendritic cell-mediated), we demonstrated here that the distribution of bacteria after oral infection directed the type of induced immune response. M cell-penetrating invasive, but not noninvasive, S. typhimurium was found in large numbers in Peyer's patches (PPs), leading to the activation of immune cells and the release of fecal IgA. In contrast, both strains of bacteria were equally capable of reaching the mesenteric lymph node and the spleen and inducing IgG responses. These data suggest that PPs are absolutely required for the initiation of an IgA response to Salmonella, whereas they are dispensable for a systemic response. This compartmentalization could allow the fast generation of both mucosal and systemic acquired immunity to pathogens.

The Salmonella Pathogenicity Island 2 regulator ssrA promotes reproductive tract but not intestinal colonization in chickens

Using a deletion mutant in the regulator of SPI-2, ssrA, we investigated the role of SPI-2 in invasion, intestinal colonization and reproductive tract infection of chickens by Salmonella Enteritidis. The ssrA mutant was fully invasive in phagocytic and non-phagocytic cells but failed to persist within chicken macrophages. The ability of Salmonella Enteritidis to cause disease in orally infected 1-day-old chicks was not altered when ssrA was deleted. Furthermore, caecal colonization was not affected, while spleen and liver showed reduced colonization. Following intra-peritoneal and intravenous infection of 1-day-old chicks, internal organ colonization was strongly reduced. After intravenous inoculation in adult laying hens bacterial numbers of the ssrA mutant were significantly lower in oviducts and ovaries as compared to the wild type strain. The chickens showed less reproductive tract lesions and the recovery of egg production were faster compared to the wild type strain infected chickens. These findings indicate that the SPI-2 regulator ssrA promotes reproductive tract colonization, but is not essential for intestinal colonization of chickens with the host non-specific serotype Enteritidis.