Construction of pSPI12-cured Salmonella enterica serovar Pullorum and identification of IpaJ as an immune response modulator

Identification of Salmonella Pullorum Factors Affecting Immune Reaction in Macrophages from the Avian Host

The host-specific Salmonella serovar S. Pullorum (SP) modulates the chicken immune response to a Th2-biased response associated with persistent infection. This is different from the Th1-biased immune response induced by the genetically close serovar, S. Enteritidis (SE). Based on core genome differences between SP and SE, we used three complementary bioinformatics approaches to identify SP genes, which may be important for stimulation of the immune response. Defined mutants were constructed in selected genes, and the infection potential and ability of mutants to stimulate cytokine production in avian derived HD11 macrophages were determined. Deletion of large genomic regions unique to SP did not change infection potential nor immune stimulation significantly. Mutants in genes with conserved single nucleotide polymorphisms (SNPs) between the two serovars in the region 100 bp upstream of the start codon (conserved upstream SNPs [CuSNPs]) such as sseE, osmB, tolQ, a putative immune antigen, and a putative persistent infection factor, exhibited differences in induction of inflammatory cytokines compared to wild-type SP, suggesting a possible role of these CuSNPs in immune regulation. Single nucleotide SP mutants correcting for the CuSNP difference were constructed in the upstream region of sifA and pipA. The SNP corrected pipA mutant expressed pipA at a higher level than the wild-type SP strain, and the mutant differentially caused upregulation of proinflammatory cytokines. It suggests that this CuSNP is important for the suppression of proinflammatory responses. In conclusion, this study has identified putative immune stimulating factors of relevance to the difference in infection dynamics between SP and SE in avian macrophages. IMPORTANCE Salmonella Pullorum is host specific to avian species, where it causes life-threatening infection in young birds. It is unknown why it is host restricted and causes systemic disease, rather than gastroenteritis normally seen with Salmonella. In the present study, we identified genes and single nucleotide polymorphisms (SNPs; relative to the broad-host-range type Salmonella Enteritidis), which affected survival and immune induction in macrophages from hens suggesting a role in development of the host specific infection. Further studies of such genes may enable understanding of which genetic factors determine the development of host specific infection by S. Pullorum. In this study, we developed an in silico approach to predict candidate genes and SNPs for development of the host-specific infection and the specific induction of immunity associated with this infection. This study flow can be used in similar studies in other clades of bacteria.

The Salmonella T3SS1 effector IpaJ is regulated by ItrA and inhibits the MAPK signaling pathway

Invasion plasmid antigen J (IpaJ) is a protein with cysteine protease activity that is present in Salmonella and Shigella species. Salmonella enterica serovar Pullorum uses IpaJ to inhibit the NF-κB pathway and the subsequent inflammatory response, resulting in bacterial survival in host macrophages. In the present study, we performed a DNA pull-down assay and EMSA and identified ItrA, a new DeoR family transcriptional regulator that could control the expression of IpaJ by directly binding to the promoter of ipaJ. The deletion of itrA inhibited the transcription of ipaJ in Salmonella. Tn-Seq revealed that two regulators of Salmonella pathogenicity island 1 (SPI-1), namely HilA and HilD, regulated the secretion of IpaJ. The deletion of hilA, hilD or SPI-1 inhibited the secretion of IpaJ in both cultured medium and Salmonella-infected cells. In contrast, the strain with the deletion of ssrB (an SPI-2 regulator-encoding gene) displayed normal IpaJ secretion, indicating that IpaJ is an effector of the SPI-1-encoded type III secretion system (T3SS1). To further demonstrate the role of IpaJ in host cells, we performed quantitative phosphoproteomics and compared the fold changes in signaling molecules in HeLa cells infected with wild-type S. Pullorum C79-13 with those in HeLa cells infected with the ipaJ-deleted strain C79-13ΔpSPI12. Both phosphoproteomics and Western blot analyses revealed that p-MEK and p-ERK molecules were increased in C79-13ΔpSPI12- and C79-13ΔpSPI12-pipaJ(C45A)-infected cells; and Co-IP assays demonstrated that IpaJ interacts with Ras to reduce its ubiquitination, indicating that IpaJ can inhibit the activation of the MAPK signaling pathway.

SteE Enhances the Virulence of Salmonella Pullorum in Chickens by Regulating the Inflammation Response

Salmonella enterica serovar Pullorum (S. Pullorum) is a host-specific pathogen, which causes acute gastroenteritis with high mortality in poultry. However, the association between steE, encoded by type III secretion system 2, and Salmonella virulence is not well-understood. To elucidate the functions of steE in S. Pullorum, ΔsteE strain was constructed using the λ-Red recombination technology. Compared to that in the wild-type, the deletion of steE in S. Pullorum reduced bacterial invasion, proliferation, and late apoptosis in the infected HD-11 cells. In addition, we analyzed the mRNA expression levels of effector genes and cytokines by qRT-PCR. SteE was associated with the regulation of various effector genes and inflammatory cytokines in HD-11 cells during S. Pullorum infection. The wild-type effector steE promoted the expression of anti-inflammatory cytokines (IL-4 and IL-10) and reduced that of pro-inflammatory cytokines (IL-1β, IL-6, and IL-12) compared to that in the ΔsteE-infected HD-11 cells and chicken spleens. Results from the chicken infection model showed that the deletion of steE resulted in significantly decreased colonization and long-term survival of the bacteria and alleviated pathological lesions compared to those in the wild-type. Further, steE increased the virulence of S. Pullorum in chickens by regulating the expression of inflammatory cytokines. Our findings provide insights into the persistent infection and autoimmunity associated with steE in S. Pullorum.

The role of PhoP/PhoQ two component system in regulating stress adaptation in Cronobacter sakazakii

Cronobacter sakazakii is an opportunistic foodborne bacterial pathogen that shows resistance to multiple stress conditions. The PhoP/PhoQ two component system is a key regulatory mechanism of stress response and virulence in various bacteria, but its role in C. sakazakii has not been thoroughly studied. In this study, we found the PhoP/PhoQ system in C. sakazakii ATCC BAA-894 enhanced bacterial growth in conditions with low Mg²⁺, acid pH, and the presence of polymyxin B. Moreover, the ΔphoPQ strain significantly reduced survival following exposure to heat, high osmotic pressure, oxidative or bile salts compared with WT strain. Furthermore, the RNA-seq analysis indicated that 1029 genes were upregulated and 979 genes were downregulated in ΔphoPQ strain. The bacterial secretion system, flagella assembly, beta-Lactam resistance and two-component system pathways were significantly downregulated, while the ABC transporters and microbial metabolism in diverse environments pathways were upregulated. qRT-PCR analysis further confirmed that twelve genes associated with stress tolerance were positively regulated by the PhoP/PhoQ system. Therefore, these findings suggest that the PhoP/PhoQ system is an important regulatory mechanism for C. sakazakii to resist various environmental stress.

Function Characterization of Endogenous Plasmids in Cronobacter sakazakii and Identification of p-Coumaric Acid as Plasmid-Curing Agent

Virulence traits and antibiotic resistance are frequently provided by genes located on plasmids. However, experimental verification of the functions of these genes is often lacking due to a lack of related experimental technology. In the present study, an integrated suicide vector was used to efficiently and specifically delete a bacterial endogenous plasmid in Cronobacter sakazakii. The pESA3 plasmid was removed from C. sakazakii BAA-894, and we confirmed that this plasmid contributes to the invasion and virulence of this strain. In addition, the pGW1 plasmid was expunged from C. sakazakii GZcsf-1, and we confirmed that this plasmid confers multidrug resistance. We further screened plasmid-curing agents and found that p-coumaric acid had a remarkable effect on the curing of pESA3 and pGW1 at sub-inhibitory concentrations. Our study investigated the contribution of endogenous plasmids pESA3 and pGW1 by constructing plasmid-cured strains using suicide vectors and suggested that p-coumaric acid can be a safe and effective plasmid-curing agent for C. sakazakii.

Campylobacter fetus Induced Proinflammatory Response in Bovine Endometrial Epithelial Cells

Campylobacter fetus subsp. fetus is the causal agent of sporadic abortion in bovines and infertility that produces economic losses in livestock. In many infectious diseases, the immune response has an important role in limiting the invasion and proliferation of bacterial pathogens. Innate immune sensing of microorganisms is mediated by pattern-recognition receptors (PRRs) that identify pathogen-associated molecular patterns (PAMPs) and induces the secretion of several proinflammatory cytokines, like IL-1β, TNF-α, and IL-8. In this study, the expression of IL-1β, TNF-α, IL-8, and IFN-γ in bovine endometrial epithelial cells infected with C. fetus and Salmonella Typhimurium (a bacterial invasion control) was analyzed. The results showed that expression levels of IL-1β and IL-8 were high at the beginning of the infection and decreased throughout the intracellular period. Unlike in this same assay, the expression levels of IFN-γ increased through time and reached the highest peak at 4 hours post infection. In cells infected with S. Typhimurium, the results showed that IL8 expression levels were highly induced by infection but not IFN-γ. In cells infected with S. Typhimurium or C. fetus subsp. fetus, the results showed that TNF-α expression did not show any change during infection. A cytoskeleton inhibition assay was performed to determine if cytokine expression was modified by C. fetus subsp. fetus intracellular invasion. IL-1β and IL-8 expression were downregulated when an intracellular invasion was avoided. The results obtained in this study suggest that bovine endometrial epithelial cells could recognize C. fetus subsp. fetus resulting in early proinflammatory response.

The SPI-19 encoded T6SS is required for Salmonella Pullorum survival within avian macrophages and initial colonization in chicken dependent on inhibition of host immune response

SalmonellaPathogenicity Island 19 (SPI-19) encoded type VI secretion system (T6SS) is a virulence factor present in few serotypes of S. enterica, including S. Dublin, S. Gallinarum and S. Pullorum. Comparative genomic sequence analysis revealed that the gene clusters of SPI-19 showed high homology to T6SS2 locus from avian pathogenic Escherichia coli, implying the similar T6SS locus is potentially related to the host adaption of both pathogens. Deletion of SPI-19 in S. Pullorum caused the dramatically decreased invasion into chicken LMH epithelial cells and HD-11 macrophages, and affected survival of Salmonella within both cells. In addition, deletion of SPI-19 caused the decreased colonization of S. Pullorum in chicken liver, spleen, ileum, and cecum at the initial infection stage, and induced rapid bacterial clearance. However, the SPI-19/T6SS had no effect on bacterial killing activity and induction of cytotoxicity to HD-11 macrophages. Further analysis demonstrated SPI-19/T6SS was involved in mediating the inhibition of host Th1 and Th2 immune responses, resulting in persistent colonization of S. Pullorum in hosts.

rOmpF and OMVs as efficient subunit vaccines against Salmonella enterica serovar Enteritidis infections in poultry farms

Salmonella enterica serovar Enteritidis remains the most prevalent serotype causing human salmonellosis through the consumption of contaminated foods, especially poultry products. The development of a subunit vaccine against S. Enteritidis can not only protect chickens against Salmonella infection in the poultry industry but also cut the transmission sources. In this study, both the expressed recombinant outer membrane protein F (rOmpF) and extracted outer membrane vesicles (OMVs) were developed as subunit vaccines against S. Enteritidis challenge in chickens. Immunization with the subunit vaccine could induce not only antibody production but also strong cell-mediated immune response. Both rOmpF plus QuilA adjuvant and OMVs alone had highly protective efficacy against S. Enteritidis challenge and rapidly decreased the colonization of bacteria in chicken. These findings revealed the potential application of rOmpF and OMVs as subunit vaccines in the poultry industry.

A bioinformatic approach to identify core genome difference between Salmonella Pullorum and Salmonella Enteritidis

S. Pullorum and S. Enteritidis are closely related in genetic terms, but they show very different pathogenicity and host range. S. Enteritidis infects many different hosts, usually causing acute gastroenteritis, while S. Pullorum is restricted to avian, where it causes systemic disease in young animals. The reason why they differ in host range and pathogenicity is unknown. The core-genome denotes those genes that are present in all strains within a clade, and in the present work, an automated bioinformatics workflow was developed and applied to identify core-genome differences between these two serovars with the aim to identify genome features associated with host specificity of S. Pullorum. Results showed that S. Pullorum unique coding sequences (CDS) were mainly concentrated in three regions not present in S. Enteritidis, suggesting that such CDS were taken up probably during the separation of the two types from their common ancestor. One of the unique regions encoded Pathogenicity Islands 19 (SPI-19), which encodes a type VI secretion system (T6SS). Single-nucleotide polymorphism (SNP) analysis identified 1791 conserved SNPs in coding sequences between the two serovars, including several SNPs located in a type IV secretion system (T4SS). Analyzing of 100 bp regions upstream of coding sequences identified 443 conserved SNPs between the two serovars, including SNP variations in type III secretion system effector (T3SE). In conclusion, this analysis has identified genetic features encoding putative factors controlling host-specificity in S. Pullorum. The novel bioinformatic workflow and associated scripts can directly be applied to other bacteria to uncover the genome difference between clades.

The Invasion Plasmid Antigen J (IpaJ) from Salmonella Inhibits NF-κB Activation by Suppressing IκBα Ubiquitination

Salmonella enterica serovar Pullorum is the pathogen of pullorum disease, which leads to severe economic losses in many developing countries. In contrast to the strong inflammatory response induced by Salmonella enterica serovar Typhimurium and Salmonella enterica serovar Enteritidis, S Pullorum causes systemic infection with little inflammation. The effector proteins secreted by Salmonella often play a crucial role in modulating host signal transduction and cellular processes to the pathogen's advantage. In the present study, the invasion plasmid antigen J (IpaJ) protein specifically identified in S Pullorum was found to significantly inhibit activation of the key proinflammatory transcription factor, NF-κB, which was induced by tumor necrosis factor alpha (TNF-α), interleukin-1β (IL-1β), and lipopolysaccharide (LPS). IpaJ inhibited the NF-κB pathway in cells infected with S Pullorum through the stabilization of IκBα. Deletion of ipaJ in S Pullorum caused a significantly increased level of ubiquitinated IκBα that was subsequently degraded by the proteasome in HeLa cells. Moreover, IpaJ was efficient in the prevention of NF-κB translocation to the nucleus and ultimately interfered with the secretion of the proinflammatory cytokines IL-1β, IL-6, and IL-8 in infected HeLa cells. Additionally, the transformation of ipaJ into S Enteritidis decreased the secretion of proinflammatory cytokines in HeLa cells through suppression of the NF-κB pathway. The infection of chicken peripheral blood monocyte-derived macrophages (chMDM) confirmed that ipaJ-deleted S Pullorum induced a stronger expression of proinflammatory cytokines than the wild-type and complementary strains. In summary, the present study revealed that IpaJ functions as an important anti-inflammatory protein involved in S Pullorum infection through inhibition of the NF-κB pathway and the subsequent inflammatory response.

Evaluation of the Safety and Protection Efficacy of spiC and nmpC or rfaL Deletion Mutants of Salmonella Enteritidis as Live Vaccine Candidates for Poultry Non-Typhoidal Salmonellosis

Salmonella enterica serovar Enteritidis (S. Enteritidis) is a host-ranged pathogen that can infect both animals and humans. Poultry and poultry products are the main carriers of S. Enteritidis, which can be transmitted to humans through the food chain. To eradicate the prevalence of S. Enteritidis in poultry farms, it is necessary to develop novel vaccines against the pathogen. In this study, we constructed two vaccine candidates, CZ14-1∆spiC∆nmpC and CZ14-1∆spiC∆rfaL, and evaluated their protective efficacy. Both mutant strains were much less virulent than the parental strain, as determined by the 50% lethal dose (LD50) for three-day-old specific-pathogen free (SPF) White Leghorns and Hyline White chickens. Immunization with the mutant candidates induced highly specific humoral immune responses and expression of cytokines IFN-γ, IL-1β, and IL-6. In addition, the mutant strains were found to be persistent for almost three weeks post-infection. The survival percentages of chickens immunized with CZ14-1∆spiC∆nmpC and CZ14-1∆spiC∆rfaL reached 80% and 75%, respectively, after challenge with the parental strain. Overall, these results demonstrate that the two mutant strains can be developed as live attenuated vaccines.

Application of Monoclonal Antibodies Developed Against the IpaJ Protein for Detection of Chickens Infected With Salmonella enterica Serovar Pullorum Using Competitive ELISA

Pullorum disease remains an epidemic in the poultry industry in China. The causing pathogen is a host-restricted Salmonella enterica serovar Pullorum, which can spread through both horizontal and vertical transmissions. To eradicate the pullorum disease from poultry farms, it is necessary to specifically monitor the prevalence of the bacterial infection in adult chicks. In this study, we constructed a new competitive ELISA method based on the development of monoclonal antibodies (MAbs) against a specific immunogen of S. Pullorum, IpaJ protein. In total, eight MAbs against IpaJ were prepared using the purified recombinant His-IpaJ protein as the immunogen. Characterization of the eight MAbs demonstrated that 4G5 can be used as the competitive antibody in ELISA. A competitive ELISA was subsequently developed using purified MBP-IpaJ as the capture (0.5 μg/ml) and the HRP-labeled 4G5 (0.14 μg/ml) as the competitive antibody, respectively. A specificity test demonstrated that the ELISA assay can differentiate antisera of S. Pullorum-infected chickens from that of S. Gallinarum and S. Enteritidis. Furthermore, 4 out of 200 clinical antisera collected from a poultry farm were detected to be S. Pulloram positive using this method. The plate agglutination test (PAT) and the previously established indirect ELISA confirmed that these positive antisera reacted specifically with S. Pullorum. We propose that the established competitive ELISA assay based on MAb against IpaJ protein, is a novel and quick method that can detect S. Pullroum infection in the poultry industry.

Purification of recombinant IpaJ to develop an indirect ELISA-based method for detecting Salmonella enterica serovar Pullorum infections in chickens

Background

Salmonella enterica serovar Pullorum is a host-restricted serotype causing infection in poultry. The pathogen can not only cause acute infection in young chicks with high mortality and morbidity, but also persist in adult chickens without evident clinical symptoms and lead to vertical transmission. To eradicate S. Pullorum in poultry farms, it is necessary to establish an efficient method to monitor the prevalence of the pathogen in adult chickens. The protein IpaJ is a specific immunogen in S. Pullorum and is not detected in closely related serotypes, such as S. Gallinarum and S. Enteritidis.

Results

In the present study, IpaJ was expressed as a recombinant fusion protein MBP-IpaJ in E. coli. The purified MBP-IpaJ was used as a coating antigen to develop an indirect ELISA assay, which was applied to the detection of S. Pullorum infection in chickens. The indirect ELISA assay demonstrated that antibodies produced against IpaJ were detectable in antisera of chickens infected with S. Pullorum in the second week, stably increased until the tenth week, and persisted at a high level in the following two weeks. Furthermore, the ELISA method detected four positive samples out of 200 clinical antiserum samples collected from a poultry farm, and the positive samples were confirmed to be reacted with S. Pullorum using the standard plate agglutination test.

Conclusions

The established indirect ELISA using the IpaJ protein is a novel method for specific detection of S. Pullorum infection, and contribute to eradication of pullorum disease in the poultry industry.

Electronic supplementary material

The online version of this article (10.1186/s12917-018-1753-0) contains supplementary material, which is available to authorized users.