Salmonella enterica serovar gallinarum requires ppGpp for internalization and survival in animal cells

Enhanced anti-cancer efficacy of arginine deaminase expressed by tumor-seeking Salmonella Gallinarum

Amino acid deprivation, particularly of nonessential amino acids that can be synthesized by normal cells but not by cancer cells with specific defects in the biosynthesis pathway, has emerged as a potential strategy in cancer therapeutics. In normal cells, arginine is synthesized from citrulline in two steps via two enzymes: argininosuccinate synthetase (ASS1) and argininosuccinate lyase. Several cancer cells exhibit arginine auxotrophy due to the loss or down-regulation of ASS1. These cells undergo starvation-induced cell death in the presence of arginine-degrading enzymes such as arginine deaminase (ADI). Thus, ADI has emerged as a potential therapeutic in cancer therapy. However, the use of ADI has two major disadvantages: ADI of bacterial origin is strongly antigenic in mammals, and ADI has a short circulation half-life (∼5 h). In this study, we engineered tumor-targeting Salmonella Gallinarum to express and secrete ADI and deployed this strain into mice implanted with ASS1-defective mouse colorectal cancer (CT26) through an intravenous route. A notable antitumor effect was observed, suggesting that the disadvantages were overcome as ADI was expressed constitutively by tumor-targeting bacteria. A combination with chloroquine, which inhibits the induction of autophagy, further enhanced the effect. Anti-cancer effect of Salmonella Gallinarum expressing an arginine deiminase (ADI) on arginine-dependent tumors in situ.

Salmonella Gallinarum mgtC mutant shows a delayed fowl typhoid progression in chicken

Salmonella Gallinarum (SG) provokes fowl typhoid, an infectious disease of acute clinical course that affects gallinaceous of any age and leads to high mortality rates. During the typhoid-like systemic infection of S. Typhimurium (STM) in mice, the bacterium expresses the mgtC gene, which is encoded in the Salmonella Pathogenecity Island - 3 (SPI-3). In this serovar, the function is linked to bacterial replication within macrophages, and its absence attenuates the pathogen. We hypothesized that deleting mgtC from SG genome would alter the microorganism pathogenicity in susceptible commercial poultry in a similar manner. Thus, the present study sought to elucidate the importance of mgtC on SG pathogenicity. For this, a mgtC-mutant lacking S. Gallinarum mutant was constructed (SG ΔmgtC). Its ability to replicate in medium that mimicries the mgtC-related intracellular environment of macrophages as well as in primary macrophages from chicken was evaluated. Moreover, the infection of susceptible chickens was performed to elucidate its pathogenicity and the elicited immune responses by measuring key interleukins by qRT-PCR and the population of macrophages and lymphocytes T CD4+ and CD8+ by means of immunohistochemistry. It was observed that mgtC was required for S. Gallinarum replication in acidified low-Mg2+ media and survival within macrophages. However, unlike its requirement for initial phase of STM infection in mice, lower bacterial counts were only observed at the late stage of macrophage infection without affecting the citotoxicity. Experiments showed that knocking-out the mgtC gene neither altered bacterial uptake by macrophages nor affects bacterial counts in liver and spleen and total chicken mortality. However, plotting a survival curve and analyzing the clinical-pathologic conditions, it was observed a slower progression of the disease in chickens infected by SG ΔmgtC compared to those challenged by the wild-type strain. Furthermore, the mRNA expression of IFN-γ and LITAF were similar between the infected chickens, but higher than in the uninfected group. The same was observed in macrophages and lymphocytes T CD4+ populations. On the other hand, the presence of lymphocytes T CD8+ was increased in the initial phase of the disease provoked by the wild-type strain over the mutant strain. We concluded that the role of mgtC in Fowl Typhoid in susceptible chickens differs from the role in typhoid-like infections in mammals. Thus, the deletion of mgtC gene from S. Gallinarum genome does not affect the overall pathogenicity, but slightly alters the pathogenesis.

Bacterial cancer therapy using the attenuated fowl-adapted Salmonella enterica serovar Gallinarum

We report here a novel anti-cancer therapy based on an avian-host-specific serotype Salmonella enterica serovar Gallinarum (S. Gallinarum) deficient in ppGpp synthesis. To monitor the tumor targeting, a bioluminescent ΔppGpp S. Gallinarum was constructed and injected intravenously into mice bearing syngeneic and human xenograft tumors. Strong bioluminescent signals were detected specifically in all grafted tumors at 2 days post-injection (dpi). The bacterial counts in normal and tumor tissue at 1 dpi revealed that ΔppGpp S. Gallinarum reached >108 CFU/g in tumor tissue and 106-107 CFU/g in endothelial organs; counts were much lower in other organs. At 16 dpi, ΔppGpp S. Gallinarum counts in tumor tissue decreased to ∼106 CFU/g, while those in the other organs became undetectable. A strong anti-cancer effect was observed after the injection of ΔppGpp S. Gallinarum into BALB/c mice grafted with CT26 colon cancer cells. This could be attributed to reduced virulence, which allowed the administration of at least a 10-fold greater dose (108 CFU) of ΔppGpp S. Gallinarum than other attenuated strains of S. enterica serovar Typhimurium (≤107 CFU). An advantage of the avian-specific S. Gallinarum as a cancer therapeutic should be a reduced capacity to cause infections or harm in humans.

Unraveling the role of type 1 fimbriae in Salmonella pathogenesis: insights from a comparative analysis of Salmonella Enteritidis and Salmonella Gallinarum

Significant differences in pathogenicity between Salmonella Enteritidis and Salmonella Gallinarum exist despite the fact that S. Gallinarum is a direct descendant of S. Enteritidis. It was hypothesized that such various properties may be in part the result of differences in structure and functions of type 1 fimbriae (T1Fs). In S. Enteritidis, T1Fs bind to oligomannosidic structures carried by host cell glycoproteins and are called mannose-sensitive T1Fs (MST1F). In S. Gallinarum, T1Fs lost ability to bind such carbohydrate chains, and were named mannose-resistant MRT1Fs (MRT1F). Therefore, the present study was undertaken to evaluate the role of MST1Fs and MRT1Fs in the adhesion, invasion, intracellular survival and cytotoxicity of S. Enteritidis and S. Gallinarum toward chicken intestinal CHIC8-E11cells and macrophage-like HD11 cells. Using mutant strains: S. Enteritidis fimH::kan and S. Gallinarum fimH::kan devoid of T1Fs and in vitro assays the following observations were made. MST1Fs have a significant impact on the chicken cell invasion by S. Enteritidis as MST1F-mediated adhesion facilitates direct and stable contact of bacteria with host cells, in contrast to MRT1Fs expressed by S. Gallinarum. MST1Fs as well as MRT1Fs did not affected intracellular viability of S. Enteritidis and S. Gallinarum. However, absolute numbers of intracellular viable wild-type S. Enteritidis were significantly higher than S. Enteritidis fimH::kan mutant and wild-type S. Gallinarum and S. Gallinarum fimH::kan mutant. These differences, reflecting the numbers of adherent and invading bacteria, underline the importance of MST1Fs in the pathogenicity of S. Enteritidis infections. The cytotoxicity of wild-type S. Enteritidis and its mutant devoid of MST1Fs to HD11 cells was essentially the same, despite the fact that the number of viable intracellular bacteria was significantly lower in the mutated strain. Using HD11 cells with similar number of intracellular wild-type S. Enteritidis and S. Enteritidis fimH::kan mutant, it was found that the lack of MST1Fs did not affect directly the cytotoxicity, suggesting that the increase in cytotoxicity of S. Enteritidis devoid of MST1Fs may be associated with crosstalk between T1Fs and other virulence factors.

Constitutive Expression of a Cytotoxic Anticancer Protein in Tumor-Colonizing Bacteria

Bacterial cancer therapy is a promising next-generation modality to treat cancer that often uses tumor-colonizing bacteria to deliver cytotoxic anticancer proteins. However, the expression of cytotoxic anticancer proteins in bacteria that accumulate in the nontumoral reticuloendothelial system (RES), mainly the liver and spleen, is considered detrimental. This study examined the fate of the Escherichia coli strain MG1655 and an attenuated strain of Salmonella enterica serovar Gallinarum (S. Gallinarum) with defective ppGpp synthesis after intravenous injection into tumor-bearing mice (~10⁸ colony forming units/animal). Approximately 10% of the injected bacteria were detected initially in the RES, whereas approximately 0.01% were in tumor tissues. The bacteria in the tumor tissue proliferated vigorously to up to 10⁹ colony forming units/g tissue, whereas those in the RES died off. RNA analysis revealed that tumor-associated E. coli activated rrnB operon genes encoding the rRNA building block of ribosome needed most during the exponential stage of growth, whereas those in the RES expressed substantially decreased levels of this gene and were cleared soon presumably by innate immune systems. Based on this finding, we engineered ΔppGpp S. Gallinarum to express constitutively a recombinant immunotoxin comprising TGFα and the Pseudomonas exotoxin A (PE38) using a constitutive exponential phase promoter, the ribosomal RNA promoter rrnB P1. The construct exerted anticancer effects on mice grafted with mouse colon (CT26) or breast (4T1) tumor cells without any notable adverse effects, suggesting that constitutive expression of cytotoxic anticancer protein from rrnB P1 occurred only in tumor tissue.

The Evolution and Future of Targeted Cancer Therapy: From Nanoparticles, Oncolytic Viruses, and Oncolytic Bacteria to the Treatment of Solid Tumors

While many classes of chemotherapeutic agents exist to treat solid tumors, few can generate a lasting response without substantial off-target toxicity despite significant scientific advancements and investments. In this review, the paths of development for nanoparticles, oncolytic viruses, and oncolytic bacteria over the last 20 years of research towards clinical translation and acceptance as novel cancer therapeutics are compared. Novel nanoparticle, oncolytic virus, and oncolytic bacteria therapies all start with a common goal of accomplishing therapeutic drug activity or delivery to a specific site while avoiding off-target effects, with overlapping methodology between all three modalities. Indeed, the degree of overlap is substantial enough that breakthroughs in one therapeutic could have considerable implications on the progression of the other two. Each oncotherapeutic modality has accomplished clinical translation, successfully overcoming the potential pitfalls promising therapeutics face. However, once studies enter clinical trials, the data all but disappears, leaving pre-clinical researchers largely in the dark. Overall, the creativity, flexibility, and innovation of these modalities for solid tumor treatments are greatly encouraging, and usher in a new age of pharmaceutical development.

(p)ppGpp synthetases are required for the pathogenicity of Salmonella Pullorum in chickens

Salmonella Pullorum is a pathogen specific to birds that can cause Pullorum disease in young chickens and lead to considerable economic losses in the poultry industry. During transmission and infection, S. Pullorum will encounter various environmental stresses and host defenses. The stringent response is an important adaptation response induced by (p)ppGpp, and in Salmonella, (p)ppGpp is synthesized by two (p)ppGpp synthetases, RelA and SpoT. To investigate the role of (p)ppGpp synthetases in the adaptation and pathogenicity of S. Pullorum, a (p)ppGpp synthetases mutant (ΔrelAΔspoT) was constructed, and its physiological phenotypes and pathogenicity, as well as transcription profiling, were compared with the parent strain. The ΔrelAΔspoT mutant showed decreased ability to form biofilms, and reduced resistance to acidic, alkaline, high osmolarity and H₂O₂ conditions. The internalization of the ΔrelAΔspoT mutant into host cells in vitro and its lethality and colonization abilities within young chickens were also significantly reduced. RNA sequencing showed that the (p)ppGpp synthetases did not only affect the classic stringent response, such as inhibition of DNA replication and protein synthesis, but also controlled the expression of many virulence factors, in particular, the Salmonella pathogenicity island 1 (SPI-1) and SPI-2 type III secretion systems (T3SSs), and adhesion factors. These results suggest that the (p)ppGpp synthetases are required for the pathogenicity of S. Pullorum by affecting its stress response and the expression of the virulence factors.

Survival of the Fittest: The Relationship of (p)ppGpp With Bacterial Virulence

The signaling nucleotide (p)ppGpp has been the subject of intense research in the past two decades. Initially discovered as the effector molecule of the stringent response, a bacterial stress response that reprograms cell physiology during amino acid starvation, follow-up studies indicated that many effects of (p)ppGpp on cell physiology occur at levels that are lower than those needed to fully activate the stringent response, and that the repertoire of enzymes involved in (p)ppGpp metabolism is more diverse than initially thought. Of particular interest, (p)ppGpp regulation has been consistently linked to bacterial persistence and virulence, such that the scientific pursuit to discover molecules that interfere with (p)ppGpp signaling as a way to develop new antimicrobials has grown substantially in recent years. Here, we highlight contemporary studies that have further supported the intimate relationship of (p)ppGpp with bacterial virulence and studies that provided new insights into the different mechanisms by which (p)ppGpp modulates bacterial virulence.

Bacteria-triggered tumor-specific thrombosis to enable potent photothermal immunotherapy of cancer

We discovered that attenuated Salmonella after intravenous injection would proliferate within various types of solid tumors but show rapid clearance in normal organs, without rendering notable toxicity. Bacteria-induced inflammation would trigger thrombosis in the infected tumors by destroying tumor blood vessels. Six types of tested tumors would all turn into darkened color with strong near-infrared absorbance, as observed by photoacoustic imaging. Under laser irradiation, those bacterial-infected tumors would be effectively ablated. Because of the immune-stimulation function, such bacteria-based photothermal therapy (PTT) would subsequently trigger antitumor immune responses, which could be further enhanced by immune checkpoint blockade to effectively suppress the growth of abscopal tumors. A robust immune memory effect to reject rechallenged tumors is also observed after bacteria-based PTT. Our work demonstrates that bacteria by themselves could act as a tumor-specific PTT agent to enable photoimmunotherapy cancer therapy to inhibit tumor metastasis and recurrence.

Epithelial invasion by Salmonella Typhi using STIV-Met interaction

Typhoid is a life-threatening febrile illness that affects ~24.2 million people worldwide and is caused by the intracellular bacteria Salmonella Typhi (S. Typhi). Intestinal epithelial invasion by S. Typhi is essential for the establishment of successful infection and is traditionally believed to depend on Salmonella pathogenicity island 1-encoded type 3 secretion system 1 (T3SS-1). We had previously reported that bacterial outer membrane protein T2942/STIV functions as a standalone invasin and contributes to the pathogenesis of S. Typhi by promoting epithelial invasion independent of T3SS-1 (Cell Microbiol, 2015). Here, we show that STIV, by using its 20-amino-acid extracellular loop, interacts with receptor tyrosine kinase, Met, of host intestinal epithelial cells. This interaction leads to Met phosphorylation and activation of a downstream signalling cascade, involving Src, phosphatidylinositol 3-kinase/Akt, and Rac1, which culminates into localized actin polymerisation and bacterial engulfment by the cell. Inhibition of Met tyrosine kinase activity severely limited intestinal invasion and systemic infection by S. Typhi in vivo, highlighting the importance of this invasion pathway in disease progression. This is the first report elucidating the mechanism of T3SS-1-independent epithelial invasion of S. Typhi, and this crucial host-pathogen interaction may be targeted therapeutically to restrict pathogenesis.

Tumour-targeting bacteria engineered to fight cancer

Recent advances in targeted therapy and immunotherapy have once again raised the hope that a cure might be within reach for many cancer types. Yet, most late-stage cancers are either insensitive to the therapies to begin with or develop resistance later. Therapy with live tumour-targeting bacteria provides a unique option to meet these challenges. Compared with most other therapeutics, the effectiveness of tumour-targeting bacteria is not directly affected by the 'genetic makeup' of a tumour. Bacteria initiate their direct antitumour effects from deep within the tumour, followed by innate and adaptive antitumour immune responses. As microscopic 'robotic factories', bacterial vectors can be reprogrammed following simple genetic rules or sophisticated synthetic bioengineering principles to produce and deliver anticancer agents on the basis of clinical needs. Therapeutic approaches using live tumour-targeting bacteria can either be applied as a monotherapy or complement other anticancer therapies to achieve better clinical outcomes. In this Review, we summarize the potential benefits and challenges of this approach. We discuss how live bacteria selectively induce tumour regression and provide examples to illustrate different ways to engineer bacteria for improved safety and efficacy. Finally, we share our experience and insights on oncology clinical trials with tumour-targeting bacteria, including a discussion of the regulatory issues.

Cell mass-dependent expression of an anticancer protein drug by tumor-targeted Salmonella

Bacterial cancer therapy relies on the properties of certain bacterial species capable of targeting and proliferating within solid malignancies. If these bacteria could be loaded with antitumor proteins, the efficacy of this approach could be greatly increased. However, because most antitumor proteins are also toxic to normal tissue, they must be expressed by bacteria that specifically target and exclusively localize to tumor tissue. As a strategy for treating solid malignancies, we recently evaluated L-asparaginase (L-ASNase) delivered by tumor-targeted Salmonella. In this system, L-ASNase was expressed under the control of the araBAD promoter (PBAD) of the E. coli arabinose operon, which is induced by injection of L-arabinose. Here, we further improved the performance of recombinant Salmonella in cancer therapy by exploiting the quorum-sensing (QS) system, which uses cell mass-dependent auto-induction logic. This approach obviates the necessity of monitoring intratumoral bacterial status and inducing cargo protein expression by administration of an exogenous compound. Recombinant Salmonella in tumors expressed and secreted active L-ASNase in a cell mass-dependent manner, yielding significant anticancer effects. These results suggest that expression of a therapeutic protein under the control of the QS system represents a promising engineering platform for the production of recombinant proteins in vivo.

Three-dimensional organotypic co-culture model of intestinal epithelial cells and macrophages to study Salmonella enterica colonization patterns

Three-dimensional models of human intestinal epithelium mimic the differentiated form and function of parental tissues often not exhibited by two-dimensional monolayers and respond to Salmonella in key ways that reflect in vivo infections. To further enhance the physiological relevance of three-dimensional models to more closely approximate in vivo intestinal microenvironments encountered by Salmonella, we developed and validated a novel three-dimensional co-culture infection model of colonic epithelial cells and macrophages using the NASA Rotating Wall Vessel bioreactor. First, U937 cells were activated upon collagen-coated scaffolds. HT-29 epithelial cells were then added and the three-dimensional model was cultured in the bioreactor until optimal differentiation was reached, as assessed by immunohistochemical profiling and bead uptake assays. The new co-culture model exhibited in vivo-like structural and phenotypic characteristics, including three-dimensional architecture, apical-basolateral polarity, well-formed tight/adherens junctions, mucin, multiple epithelial cell types, and functional macrophages. Phagocytic activity of macrophages was confirmed by uptake of inert, bacteria-sized beads. Contribution of macrophages to infection was assessed by colonization studies of Salmonella pathovars with different host adaptations and disease phenotypes (Typhimurium ST19 strain SL1344 and ST313 strain D23580; Typhi Ty2). In addition, Salmonella were cultured aerobically or microaerobically, recapitulating environments encountered prior to and during intestinal infection, respectively. All Salmonella strains exhibited decreased colonization in co-culture (HT-29-U937) relative to epithelial (HT-29) models, indicating antimicrobial function of macrophages. Interestingly, D23580 exhibited enhanced replication/survival in both models following invasion. Pathovar-specific differences in colonization and intracellular co-localization patterns were observed. These findings emphasize the power of incorporating a series of related three-dimensional models within a study to identify microenvironmental factors important for regulating infection.

Protective efficacy and immune responses by homologous prime-booster immunizations of a novel inactivated Salmonella Gallinarum vaccine candidate

Purpose

Salmonella enterica serovar Gallinarum (SG) ghost vaccine candidate was recently constructed. In this study, we evaluated various prime-boost vaccination strategies using the candidate strain to optimize immunity and protection efficacy against fowl typhoid.

Materials and Methods

The chickens were divided into five groups designated as group A (non-immunized control), group B (orally primed and boosted), group C (primed orally and boosted intramuscularly), group D (primed and boosted intramuscularly), and group E (primed intramuscularly and boosted orally). The chickens were primed with the SG ghost at 7 days of age and were subsequently boosted at the fifth week of age. Post-immunization, the plasma IgG and intestinal secretory IgA (sIgA) levels, and the SG antigen-specific lymphocyte stimulation were monitored at weekly interval and the birds were subsequently challenged with a virulent SG strain at the third week post-second immunization.

Results

Chickens in group D showed an optimized protection with significantly increased plasma IgG, sIgA, and lymphocyte stimulation response compared to all groups. The presence of CD4⁺ and CD8⁺ T cells and monocyte/macrophage (M/M) in the spleen, and splenic expression of cytokines such as interferon γ (IFN-γ) and interleukin 6 (IL-6) in the immunized chickens were investigated. The prime immunization induced significantly higher splenic M/M population and mRNA levels of IFN-γ whereas the booster showed increases of splenic CD4⁺ and CD8⁺ T-cell population and IL-6 cytokine in mRNA levels.

Conclusion

Our results indicate that the prime immunization with the SG ghost vaccine induced Th1 type immune response and the booster elicited both Th1- and Th2-related immune responses.

The role of rpoS, hmp, and ssrAB in Salmonella enterica Gallinarum and evaluation of a triple-deletion mutant as a live vaccine candidate in Lohmann layer chickens

Salmonella enterica Gallinarum (SG) causes fowl typhoid (FT), a septicemic disease in avian species. We constructed deletion mutants lacking the stress sigma factor RpoS, the nitric oxide (NO)-detoxifying flavohemoglobin Hmp, and the SsrA/SsrB regulator to confirm the functions of these factors in SG. All gene products were fully functional in wild-type (WT) SG whereas mutants harboring single mutations or a combination of rpoS, hmp, and ssrAB mutations showed hypersusceptibility to H₂O₂, loss of NO metabolism, and absence of Salmonella pathogenicity island (SPI)-2 expression, respectively. A triple-deletion mutant, SGΔ3 (SGΔrpoSΔhmpΔssrAB), was evaluated for attenuated virulence and protection efficacy in two-week-old Lohmann layer chickens. The SGΔ3 mutant did not cause any mortality after inoculation with either 1 × 10⁶ or 1 × 10⁸ colony-forming units (CFUs) of bacteria. Significantly lower numbers of salmonellae were recovered from the liver and spleen of chickens inoculated with the SGΔ3 mutant compared to chickens inoculated with WT SG. Vaccination with the SGΔ3 mutant conferred complete protection against challenge with virulent SG on the chickens comparable to the group vaccinated with a conventional vaccine strain, SG9R. Overall, these results indicate that SGΔ3 could be a promising candidate for a live Salmonella vaccine against FT.

An AIL family protein promotes type three secretion system-1-independent invasion and pathogenesis of Salmonella enterica serovar Typhi

Adhesion and invasion of Intestinal Epithelial Cells (IECs) are critical for the pathogenesis of Salmonella Typhi, the aetiological agent of human typhoid fever. While type three secretion system-1 (T3SS-1) is a major invasion apparatus of Salmonella, independent invasion mechanisms were described for non-typhoidal Salmonellae. Here, we show that T2942, an AIL-like protein of S. Typhi Ty2 strain, is required for adhesion and invasion of cultured IECs. That invasion was T3SS-1 independent was proved by ectopic expression of T2942 in the non-invasive E. coli BL21 and double-mutant Ty2 (Ty2Δt2942ΔinvG) strains. Laminin and fibronectin were identified as the host-binding partners of T2942 with higher affinity for laminin. Standalone function of T2942 was confirmed by cell adhesion of the recombinant protein, while the protein or anti-T2942 antiserum blocked adhesion/invasion of S. Typhi, indicating specificity. A 20-amino acid extracellular loop was required for invasion, while several loop regions of T2942 contributed to adhesion. Further, T2942 cooperates with laminin-binding T2544 for adhesion and T3SS-1 for invasion. Finally, T2942 was required and synergistically worked with T3SS-1 for pathogenesis of S. Typhi in mice. Considering wide distribution of T2942 among clinical strains, the protein or the 20-mer peptide may be suitable for vaccine development.

Generation of a safety enhanced Salmonella Gallinarum ghost using antibiotic resistance free plasmid and its potential as an effective inactivated vaccine candidate against fowl typhoid

A safety enhanced Salmonella Gallinarum (SG) ghost was constructed using an antibiotic resistance gene free plasmid and evaluated its potential as fowl typhoid (FT) vaccine candidate. The antibiotic resistance free pYA3342 plasmid possesses aspartate semialdehyde dehydrogenase gene which is complimentary to the deletion of the chromosomal asd gene in the bacterial host. This plasmid was incorporated with a ghost cassette containing the bacteriophage PhiX174 lysis gene E, designated as pJHL101. The plasmid pJHL101 was transformed into a two virulence genes-deleted SG. The SG ghosts with tunnel formation and loss of cytoplasmic contents were observed by scanning electron microscopy and transmission electron microscopy. The cell viability of the culture solution was decreased to 0% at 24h after the induction of gene E expression by an increase in temperature from 37°C to 42°C. The safety and protective efficacy of the SG ghost vaccine was further examined in chickens which were divided into three groups: group A (non-immunized control), group B (orally immunized), and group C (intramuscularly immunized). The birds were immunized at 7d of age. No clinical symptoms associated with FT such as anorexia, depression and greenish diarrhea were observed in the immunized chickens. Upon challenge with a virulent SG strain at 3 week post-immunization, the chickens immunized with the SG ghost via various routes were efficiently protected, as shown by significantly lower mortality and post-mortem lesions in comparison with control group. In addition, all the immunized chickens showed significantly higher antibody responses accompanied by a potent antigen-specific lymphocyte proliferative response along with significantly increased numbers of CD4⁺ and CD8⁺ T lymphocytes. Overall, our results provide a promising approach of generating SG ghosts using the antibiotic resistance free plasmid in order to prepare a non-living bacterial vaccine candidate which could be environmentally safe yet efficient to prevent FT in chickens.

A novel balanced-lethal host-vector system based on glmS

During the last decade, an increasing number of papers have described the use of various genera of bacteria, including E. coli and S. typhimurium, in the treatment of cancer. This is primarily due to the facts that not only are these bacteria capable of accumulating in the tumor mass, but they can also be engineered to deliver specific therapeutic proteins directly to the tumor site. However, a major obstacle exists in that bacteria because the plasmid carrying the therapeutic gene is not needed for bacterial survival, these plasmids are often lost from the bacteria. Here, we report the development of a balanced-lethal host-vector system based on deletion of the glmS gene in E. coli and S. typhimurium. This system takes advantage of the phenotype of the GlmS(-) mutant, which undergoes lysis in animal systems that lack the nutrients required for proliferation of the mutant bacteria, D-glucosamine (GlcN) or N-acetyl-D-glucosamine (GlcNAc), components necessary for peptidoglycan synthesis. We demonstrate that plasmids carrying a glmS gene (GlmS(+)p) complemented the phenotype of the GlmS(-) mutant, and that GlmS(+) p was maintained faithfully both in vitro and in an animal system in the absence of selection pressure. This was further verified by bioluminescent signals from GlmS (+)pLux carried in bacteria that accumulated in grafted tumor tissue in a mouse model. The signal was up to several hundred-fold stronger than that from the control plasmid, pLux, due to faithful maintenance of the plasmid. We believe this system will allow to package a therapeutic gene onto an expression plasmid for bacterial delivery to the tumor site without subsequent loss of plasmid expression as well as to quantify bioluminescent bacteria using in vivo imaging by providing a direct correlation between photon flux and bacterial number.

Pathogenicity of dodecyltrimethylammonium chloride-resistant Salmonella enterica

Salmonella infection causes a self-limiting gastroenteritis in humans but can also result in a life-threatening invasive disease, especially in old, young, and/or immunocompromised patients. The prevalence of antimicrobial and multidrug-resistant Salmonella has increased worldwide since the 1980s. However, the impact of antimicrobial resistance on the pathogenicity of Salmonella strains is not well described. In our study, a microarray was used to screen for differences in gene expression between a parental strain and a strain of Salmonella enterica serovar Enteritidis with reduced susceptibility (SRS) to the widely used antimicrobial sanitizer dodecyltrimethylammonium chloride (DTAC). Three of the genes, associated with adhesion, invasion, and intracellular growth (fimA, csgG, and spvR), that showed differences in gene expression of 2-fold or greater were chosen for further study. Real-time reverse transcriptase PCR (real-time RT-PCR) was used to confirm the microarray data and to compare the expression levels of these genes in the parental strain and four independently derived SRS strains. All SRS strains showed lower levels of gene expression of fimA and csgG than those of the parental strain. Three of the four SRS strains showed lower levels of spvR gene expression while one SRS strain showed higher levels of spvR gene expression than those of the parental strain. Transmission electron microscopy determined that fimbriae were absent in the four SRS strains but copiously present in the parental strain. All four SRS strains demonstrated a significantly reduced ability to invade tissue culture cells compared to the parental strains, suggesting reduced pathogenicity of the SRS strains.

The type VI secretion system encoded in Salmonella pathogenicity island 19 is required for Salmonella enterica serotype Gallinarum survival within infected macrophages

Salmonella enterica serotype Gallinarum is the causative agent of fowl typhoid, a disease characterized by high morbidity and mortality that causes major economic losses in poultry production. We have reported that S. Gallinarum harbors a type VI secretion system (T6SS) encoded in Salmonella pathogenicity island 19 (SPI-19) that is required for efficient colonization of chicks. In the present study, we aimed to characterize the SPI-19 T6SS functionality and to investigate the mechanisms behind the phenotypes previously observed in vivo. Expression analyses revealed that SPI-19 T6SS core components are expressed and produced under in vitro bacterial growth conditions. However, secretion of the structural/secreted components Hcp1, Hcp2, and VgrG to the culture medium could not be determined, suggesting that additional signals are required for T6SS-dependent secretion of these proteins. In vitro bacterial competition assays failed to demonstrate a role for SPI-19 T6SS in interbacterial killing. In contrast, cell culture experiments with murine and avian macrophages (RAW264.7 and HD11, respectively) revealed production of a green fluorescent protein-tagged version of VgrG soon after Salmonella uptake. Furthermore, infection of RAW264.7 and HD11 macrophages with deletion mutants of SPI-19 or strains with genes encoding specific T6SS core components (clpV and vgrG) revealed that SPI-19 T6SS contributes to S. Gallinarum survival within macrophages at 20 h postuptake. SPI-19 T6SS function was not linked to Salmonella-induced cytotoxicity or cell death of infected macrophages, as has been described for other T6SS. Our data indicate that SPI-19 T6SS corresponds to a novel tool used by Salmonella to survive within host cells.

The stringent response of Staphylococcus aureus and its impact on survival after phagocytosis through the induction of intracellular PSMs expression

The stringent response is initiated by rapid (p)ppGpp synthesis, which leads to a profound reprogramming of gene expression in most bacteria. The stringent phenotype seems to be species specific and may be mediated by fundamentally different molecular mechanisms. In Staphylococcus aureus, (p)ppGpp synthesis upon amino acid deprivation is achieved through the synthase domain of the bifunctional enzyme RSH (RelA/SpoT homolog). In several firmicutes, a direct link between stringent response and the CodY regulon was proposed. Wild-type strain HG001, rsh(Syn), codY and rsh(Syn), codY double mutants were analyzed by transcriptome analysis to delineate different consequences of RSH-dependent (p)ppGpp synthesis after induction of the stringent response by amino-acid deprivation. Under these conditions genes coding for major components of the protein synthesis machinery and nucleotide metabolism were down-regulated only in rsh positive strains. Genes which became activated upon (p)ppGpp induction are mostly regulated indirectly via de-repression of the GTP-responsive repressor CodY. Only seven genes, including those coding for the cytotoxic phenol-soluble modulins (PSMs), were found to be up-regulated via RSH independently of CodY. qtRT-PCR analyses of hallmark genes of the stringent response indicate that an RSH activating stringent condition is induced after uptake of S. aureus in human polymorphonuclear neutrophils (PMNs). The RSH activity in turn is crucial for intracellular expression of psms. Accordingly, rsh(Syn) and rsh(Syn), codY mutants were less able to survive after phagocytosis similar to psm mutants. Intraphagosomal induction of psmα1-4 and/or psmβ1,2 could complement the survival of the rsh(Syn) mutant. Thus, an active RSH synthase is required for intracellular psm expression which contributes to survival after phagocytosis.

Multiplicity of Salmonella entry mechanisms, a new paradigm for Salmonella pathogenesis

The Salmonella enterica species includes about 2600 diverse serotypes, most of which cause a wide range of food- and water-borne diseases ranging from self-limiting gastroenteritis to typhoid fever in both humans and animals. Moreover, some serotypes are restricted to a few animal species, whereas other serotypes are able to infect plants as well as cold- and warm-blooded animals. An essential feature of the pathogenicity of Salmonella is its capacity to cross a number of barriers requiring invasion of a large variety of phagocytic and nonphagocytic cells. The aim of this review is to describe the different entry pathways used by Salmonella serotypes to enter different nonphagocytic cell types. Until recently, it was accepted that Salmonella invasion of eukaryotic cells required only the type III secretion system (T3SS) encoded by the Salmonella pathogenicity island-1. However, recent evidence shows that Salmonella can cause infection in a T3SS-1-independent manner. Currently, two outer membrane proteins Rck and PagN have been clearly identified as Salmonella invasins. As Rck mediates a Zipper-like entry mechanism, Salmonella is therefore the first bacterium shown to be able to induce both Zipper and Trigger mechanisms to invade host cells. In addition to these known entry pathways, recent data have shown that unknown entry routes could be used according to the serotype, the host and the cell type considered, inducing either Zipper-like or Trigger-like entry processes. The new paradigm presented here should change our classic view of Salmonella pathogenicity. It could also modify our understanding of the mechanisms leading to the different Salmonella-induced diseases and to Salmonella-host specificity.

Comparative analysis of two phenotypically-similar but genomically-distinct Burkholderia cenocepacia-specific bacteriophages

Background

Genomic analysis of bacteriophages infecting the Burkholderia cepacia complex (BCC) is an important preliminary step in the development of a phage therapy protocol for these opportunistic pathogens. The objective of this study was to characterize KL1 (vB_BceS_KL1) and AH2 (vB_BceS_AH2), two novel Burkholderia cenocepacia-specific siphoviruses isolated from environmental samples.

Results

KL1 and AH2 exhibit several unique phenotypic similarities: they infect the same B. cenocepacia strains, they require prolonged incubation at 30°C for the formation of plaques at low titres, and they do not form plaques at similar titres following incubation at 37°C. However, despite these similarities, we have determined using whole-genome pyrosequencing that these phages show minimal relatedness to one another. The KL1 genome is 42,832 base pairs (bp) in length and is most closely related to Pseudomonas phage 73 (PA73). In contrast, the AH2 genome is 58,065 bp in length and is most closely related to Burkholderia phage BcepNazgul. Using both BLASTP and HHpred analysis, we have identified and analyzed the putative virion morphogenesis, lysis, DNA binding, and MazG proteins of these two phages. Notably, MazG homologs identified in cyanophages have been predicted to facilitate infection of stationary phase cells and may contribute to the unique plaque phenotype of KL1 and AH2.

Conclusions

The nearly indistinguishable phenotypes but distinct genomes of KL1 and AH2 provide further evidence of both vast diversity and convergent evolution in the BCC-specific phage population.

Glycerol supplementation enhances L. reuteri's protective effect against S. Typhimurium colonization in a 3-D model of colonic epithelium

The probiotic effects of Lactobacillus reuteri have been speculated to partly depend on its capacity to produce the antimicrobial substance reuterin during the reduction of glycerol in the gut. In this study, the potential of this process to protect human intestinal epithelial cells against infection with Salmonella enterica serovar Typhimurium was investigated. We used a three-dimensional (3-D) organotypic model of human colonic epithelium that was previously validated and applied to study interactions between S. Typhimurium and the intestinal epithelium that lead to enteric salmonellosis. Using this model system, we show that L. reuteri protects the intestinal cells against the early stages of Salmonella infection and that this effect is significantly increased when L. reuteri is stimulated to produce reuterin from glycerol. More specifically, the reuterin-containing ferment of L. reuteri caused a reduction in Salmonella adherence and invasion (1 log unit), and intracellular survival (2 log units). In contrast, the L. reuteri ferment without reuterin stimulated growth of the intracellular Salmonella population with 1 log unit. The short-term exposure to reuterin or the reuterin-containing ferment had no observed negative impact on intestinal epithelial cell health. However, long-term exposure (24 h) induced a complete loss of cell-cell contact within the epithelial aggregates and compromised cell viability. Collectively, these results shed light on a potential role for reuterin in inhibiting Salmonella-induced intestinal infections and may support the combined application of glycerol and L. reuteri. While future in vitro and in vivo studies of reuterin on intestinal health should fine-tune our understanding of the mechanistic effects, in particular in the presence of a complex gut microbiota, this the first report of a reuterin effect on the enteric infection process in any mammalian cell type.

Construction of a Salmonella Gallinarum ghost as a novel inactivated vaccine candidate and its protective efficacy against fowl typhoid in chickens

In order to develop a novel, safe and immunogenic fowl typhoid (FT) vaccine candidate, a Salmonella Gallinarum ghost with controlled expression of the bacteriophage PhiX174 lysis gene E was constructed using pMMP99 plasmid in this study. The formation of the Salmonella Gallinarum ghost with tunnel formation and loss of cytoplasmic contents was observed by scanning electron microscopy and transmission electron microscopy. No viable cells were detectable 24 h after the induction of gene E expression by an increase in temperature from 37 °C to 42 °C. The safety and protective efficacy of the Salmonella Gallinarum ghost vaccine was tested in chickens that were divided into four groups: group A (non-immunized control), group B (orally immunized), group C (subcutaneously immunized) and group D (intramuscularly immunized). The birds were immunized at day 7 of age. None of the immunized animals showed any adverse reactions such as abnormal behavior, mortality, or signs of FT such as anorexia, depression, or diarrhea. These birds were subsequently challenged with a virulent Salmonella Gallinarum strain at 3 weeks post-immunization (wpi). Significant protection against the virulent challenge was observed in all immunized groups based on mortality and post-mortem lesions compared to the non-immunized control group. In addition, immunization with the Salmonella Gallinarum ghosts induced significantly high systemic IgG response in all immunized groups. Among the groups, orally-vaccinated group B showed significantly higher levels of secreted IgA. A potent antigen-specific lymphocyte activation response along with significantly increased percentages of CD4⁺ and CD8⁺ T lymphocytes found in all immunized groups clearly indicate the induction of cellular immune responses. Overall, these findings suggest that the newly constructed Salmonella Gallinarum ghost appears to be a safe, highly immunogenic, and efficient non-living bacterial vaccine candidate that protects against FT.

Selection of Salmonella enterica serovar Typhi genes involved during interaction with human macrophages by screening of a transposon mutant library

The human-adapted Salmonella enterica serovar Typhi (S. Typhi) causes a systemic infection known as typhoid fever. This disease relies on the ability of the bacterium to survive within macrophages. In order to identify genes involved during interaction with macrophages, a pool of approximately 10⁵ transposon mutants of S. Typhi was subjected to three serial passages of 24 hours through human macrophages. Mutants recovered from infected macrophages (output) were compared to the initial pool (input) and those significantly underrepresented resulted in the identification of 130 genes encoding for cell membrane components, fimbriae, flagella, regulatory processes, pathogenesis, and many genes of unknown function. Defined deletions in 28 genes or gene clusters were created and mutants were evaluated in competitive and individual infection assays for uptake and intracellular survival during interaction with human macrophages. Overall, 26 mutants had defects in the competitive assay and 14 mutants had defects in the individual assay. Twelve mutants had defects in both assays, including acrA, exbDB, flhCD, fliC, gppA, mlc, pgtE, typA, waaQGP, SPI-4, STY1867-68, and STY2346. The complementation of several mutants by expression of plasmid-borne wild-type genes or gene clusters reversed defects, confirming that the phenotypic impairments within macrophages were gene-specific. In this study, 35 novel phenotypes of either uptake or intracellular survival in macrophages were associated with Salmonella genes. Moreover, these results reveal several genes encoding molecular mechanisms not previously known to be involved in systemic infection by human-adapted typhoidal Salmonella that will need to be elucidated.

Global analysis of the Staphylococcus aureus response to mupirocin

In the present study, we analyzed the response of S. aureus to mupirocin, the drug of choice for nasal decolonization. Mupirocin selectively inhibits the bacterial isoleucyl-tRNA synthetase (IleRS), leading to the accumulation of uncharged isoleucyl-tRNA and eventually the synthesis of (p)ppGpp. The alarmone (p)ppGpp induces the stringent response, an important global transcriptional and translational control mechanism that allows bacteria to adapt to nutritional deprivation. To identify proteins with an altered synthesis pattern in response to mupirocin treatment, we used the highly sensitive 2-dimensional gel electrophoresis technique in combination with mass spectrometry. The results were complemented by DNA microarray, Northern blot, and metabolome analyses. Whereas expression of genes involved in nucleotide biosynthesis, DNA metabolism, energy metabolism, and translation was significantly downregulated, expression of isoleucyl-tRNA synthetase, the branched-chain amino acid pathway, and genes with functions in oxidative-stress resistance (ahpC and katA) and putative roles in stress protection (the yvyD homologue SACOL0815 and SACOL1759 and SACOL2131) and transport processes was increased. A comparison of the regulated genes to known regulons suggests the involvement of the global regulators CodY and SigB in shaping the response of S. aureus to mupirocin. Of particular interest was the induced transcription of genes encoding virulence-associated regulators (i.e., arlRS, saeRS, sarA, sarR, sarS, and sigB), as well as genes directly involved in the virulence of S. aureus (i.e., fnbA, epiE, epiG, and seb).

Analysis of interactions of Salmonella type three secretion mutants with 3-D intestinal epithelial cells

The prevailing paradigm of Salmonella enteropathogenesis based on monolayers asserts that Salmonella pathogenicity island-1 Type Three Secretion System (SPI-1 T3SS) is required for bacterial invasion into intestinal epithelium. However, little is known about the role of SPI-1 in mediating gastrointestinal disease in humans. Recently, SPI-1 deficient nontyphoidal Salmonella strains were isolated from infected humans and animals, indicating that SPI-1 is not required to cause enteropathogenesis and demonstrating the need for more in vivo-like models. Here, we utilized a previously characterized 3-D organotypic model of human intestinal epithelium to elucidate the role of all characterized Salmonella enterica T3SSs. Similar to in vivo reports, the Salmonella SPI-1 T3SS was not required to invade 3-D intestinal cells. Additionally, Salmonella strains carrying single (SPI-1 or SPI-2), double (SPI-1/2) and complete T3SS knockout (SPI-1/SPI-2: flhDC) also invaded 3-D intestinal cells to wildtype levels. Invasion of wildtype and TTSS mutants was a Salmonella active process, whereas non-invasive bacterial strains, bacterial size beads, and heat-killed Salmonella did not invade 3-D cells. Wildtype and T3SS mutants did not preferentially target different cell types identified within the 3-D intestinal aggregates, including M-cells/M-like cells, enterocytes, or Paneth cells. Moreover, each T3SS was necessary for substantial intracellular bacterial replication within 3-D cells. Collectively, these results indicate that T3SSs are dispensable for Salmonella invasion into highly differentiated 3-D models of human intestinal epithelial cells, but are required for intracellular bacterial growth, paralleling in vivo infection observations and demonstrating the utility of these models in predicting in vivo-like pathogenic mechanisms.

Pathogenicity of SG 9R, a rough vaccine strain against fowl typhoid

SG 9R, a rough vaccine strain of Salmonella gallinarum, has been used for the prevention of fowl typhoid and paratyphoid in the world despite the presence of residual virulence. SG 9R-like rough strains have been recently isolated from fowl typhoid cases; however, molecular markers to differentiate SG 9R from field strains are not well-characterized and the molecular mechanisms of SG 9R residual virulence are unclear. Therefore, we analyzed LPS biosynthesis (rfa gene cluster) and virulence genes (spv, SPI-2) of both SG 9R and S. gallinarum rough field strains. SG 9R carried a unique nonsense mutation in rfaJ (TCA to TAA) and a shared rfaZ mutation (G-deletion) by rough and smooth S. gallinarum strains. SG 9R also carried intact SPI-2 and spvC, B, A, and R (except deleted spvD). SG 9R-like rough strains (n=10) carried identical mutations in virulence-related genes to SG 9R. SG 9R and SG 9R-like rough strains did not demonstrate significant mortality or liver lesions under normal conditions. However, fowl typhoid was successfully reproduced in the present study by SG 9R inoculation to 1-day-old male brown layer chicks per os following starvation. Therefore, the LPS defect may be one of the major mechanisms of SG 9R attenuation, and the possession of intact SPI-2, spvC, B, A, and R virulence genes may be associated with residual SG 9R virulence.

Immune response induced by ppGpp-defective Salmonella enterica serovar Gallinarum in chickens

To protect chickens from typhoid caused by Salmonella enterica serovar Gallinarum (S. Gallinarum), the attenuated 9R strain has been used in the field as a vaccine. However, safety concerns have been raised because the mutations in 9R are undefined while its efficacy is still a question under debate. A global regulator, ppGpp, synthesized by RelA and SpoT, has been shown to induce various virulence genes in S. Gallinarum (Jeong et al., 2008). In this study, two mutant strains defective in ppGpp-synthesis were constructed in wild-type S. Gallinarum (ΔppGpp) and 9R strain (9R-ΔppGpp) backgrounds and tested as live vaccines in chickens. After oral inoculation, the LD(50) values of ΔppGpp and 9R-ΔppGpp were approximately 5×10(10) colony forming unit (CFU) similarly as 9R strain, which was ∼10(5)-fold higher than that of the wildtype S. Gallinarum strain. Immunological analyses revealed immunization with either of the two attenuated ppGpp-defective strains induced significant antibody responses, the production of antibody-secreting B cells in blood, proliferation of CD4+ and CD8+ T cells in the spleen, and splenic expression of proinflammatory cytokines, such as IFN-γ and TGF-β4, at levels comparable to the 9R strain. Chickens immunized with the mutants (1×10(8) CFU) were 80% protected against oral challenge with 1×10(9) wild-type virulent bacteria (4,000-fold LD(50) dose), similar to the level of protection achieved by 9R immunization. Based on these data, live attenuated ΔppGpp-defective strains may serve as novel vaccines to control fowl typhoid in chickens.

Contribution of the type VI secretion system encoded in SPI-19 to chicken colonization by Salmonella enterica serotypes Gallinarum and Enteritidis

Salmonella Gallinarum is a pathogen with a host range specific to poultry, while Salmonella Enteritidis is a broad host range pathogen that colonizes poultry sub-clinically but is a leading cause of gastrointestinal salmonellosis in humans and many other species. Despite recent advances in our understanding of the complex interplay between Salmonella and their hosts, the molecular basis of host range restriction and unique pathobiology of Gallinarum remain largely unknown. Type VI Secretion System (T6SS) represents a new paradigm of protein secretion that is critical for the pathogenesis of many Gram-negative bacteria. We recently identified a putative T6SS in the Salmonella Pathogenicity Island 19 (SPI-19) of Gallinarum. In Enteritidis, SPI-19 is a degenerate element that has lost most of the T6SS functions encoded in the island. In this work, we studied the contribution of SPI-19 to the colonization of Salmonella Gallinarum strain 287/91 in chickens. Non-polar deletion mutants of SPI-19 and the clpV gene, an essential T6SS component, colonized the ileum, ceca, liver and spleen of White Leghorn chicks poorly compared to the wild-type strain after oral inoculation. Return of SPI-19 to the ΔSPI-19 mutant, using VEX-Capture, complemented this colonization defect. In contrast, transfer of SPI-19 from Gallinarum to Enteritidis resulted in transient increase in the colonization of the ileum, liver and spleen at day 1 post-infection, but at days 3 and 5 post-infection a strong colonization defect of the gut and internal organs of the experimentally infected chickens was observed. Our data indicate that SPI-19 and the T6SS encoded in this region contribute to the colonization of the gastrointestinal tract and internal organs of chickens by Salmonella Gallinarum and suggest that degradation of SPI-19 T6SS in Salmonella Enteritidis conferred an advantage in colonization of the avian host.

ppGpp conjures bacterial virulence

Like for all microbes, the goal of every pathogen is to survive and replicate. However, to overcome the formidable defenses of their hosts, pathogens are also endowed with traits commonly associated with virulence, such as surface attachment, cell or tissue invasion, and transmission. Numerous pathogens couple their specific virulence pathways with more general adaptations, like stress resistance, by integrating dedicated regulators with global signaling networks. In particular, many of nature's most dreaded bacteria rely on nucleotide alarmones to cue metabolic disturbances and coordinate survival and virulence programs. Here we discuss how components of the stringent response contribute to the virulence of a wide variety of pathogenic bacteria.

Physiology, pathogenicity and immunogenicity of lon and/or cpxR deleted mutants of Salmonella Gallinarum as vaccine candidates for fowl typhoid

To construct a novel live vaccine candidate for fowl typhoid (FT) caused by Salmonella Gallinarum (SG), the lon and cpxR genes that are related to host-pathogen interaction were deleted from a wild type SG using the allelic exchange method. The mutants were grown normally, as was the wild type. The biochemical properties of the mutants remained very similar to those of the wild-type, while JOL914 (Δlon) and JOL916 (ΔlonΔcpxR) were mucoid. Extracellular polysaccharide increased 30.6-, 1.3-, and 46.2-fold in JOL914, JOL915 (ΔcpxR), and JOL916, respectively. Dot-blot analysis demonstrated significant increases of FimA expression at 6.77-, 2.33-, and 3.90-fold for JOL914, JOL915, and JOL916, respectively. Internalizations of JOL914, JOL915, and JOL916, in chicken abdominal macrophages, were increased at 4.65-, 0.50-, and 2.72-fold, respectively. Virulences of JOL914, JOL915 and JOL916, analyzed by LD₅₀ using 1-week-old chickens, were attenuated approximately at 10¹-, 10¹-, and > 10³-fold, respectively. The oral inoculations of 2 × 10⁷ cfu of the wild type, JOL914, JOL915 and JOL916 caused 55.6, 16.7, 22.2, and 0.0% mortality, respectively. Significantly moderate gross lesions of the liver and spleen were observed in the JOL916 group compared to the other groups. An induced immune response and significant peripheral mononuclear proliferation reaction were observed in the JOL916 group. At the protection against the wild type challenge, JOL916 offered 100% protection. Thus, the results of this study suggest that JOL916 among the mutants studied represented the safest and most effective live vaccine candidate against FT.

The molecular alarmone (p)ppGpp mediates stress responses, vancomycin tolerance, and virulence in Enterococcus faecalis

The stringent response is a global bacterial response to stress that is mediated by accumulation of the alarmone (p)ppGpp. In this study, treatment with mupirocin was shown to induce high levels of (p)ppGpp production in Enterococcus faecalis, indicating that this nosocomial pathogen can mount a classic stringent response. In addition, (p)ppGpp was found to accumulate in cells subjected to heat shock, alkaline shock, and inhibitory concentrations of vancomycin. Sequence analysis of the E. faecalis genome indicated that (p)ppGpp synthesis is catalyzed by the bifunctional synthetase/hydrolase RelA and the RelQ small synthase. The (p)ppGpp profiles of DeltarelA, DeltarelQ, and DeltarelAQ strains revealed that RelA is the major enzyme responsible for the accumulation of (p)ppGpp during antibiotic or physical stresses, while RelQ appears to be responsible for maintaining basal levels of alarmone during homeostatic growth. Compared to its parent, the DeltarelA strain was more susceptible to several stress conditions, whereas complete elimination of (p)ppGpp in a DeltarelAQ double mutant restored many of the stress-sensitive phenotypes of DeltarelA. Interestingly, growth curves and time-kill studies indicated that tolerance to vancomycin is enhanced in the DeltarelA strain but diminished in the DeltarelQ and DeltarelAQ strains. Finally, virulence of the DeltarelAQ strain but not of the DeltarelA or DeltarelQ strain was significantly attenuated in the Caenorhabditis elegans model. Taken together, these results indicate that (p)ppGpp pools modulate environmental stress responses, vancomycin tolerance, and virulence in this important nosocomial pathogen.

A dual-signal regulatory circuit activates transcription of a set of divergent operons in Salmonella typhimurium

We present a molecular mechanism for signal transduction that activates transcription of the SlyA regulon in Salmonella typhimurium. We demonstrate that SlyA mediates transcriptional activation in response to guanosine tetraphosphate, ppGpp, according to the following observations: (i) in vivo transcription of SlyA-dependent genes is repressed when ppGpp is absent; this transcription can be restored by overproducing SlyA; (ii) in vivo dimerization and binding of SlyA to the target promoter are facilitated in the presence of ppGpp; and (iii) in vitro SlyA binding to the target promoter is enhanced when ppGpp is supplemented. Thus, ppGpp must be the cytoplasmic component that stimulates SlyA regulatory function by interacting directly with this regulator in Salmonella. This signaling domain, integrated by the PhoP/PhoQ 2-component system that activates slyA transcription by sensing Mg(2+), forms feedforward loops that regulate chromosomal loci identified through a motif search over the S. typhimurium genome. Many such loci are divergent operons, each formed by 2 neighboring genes in which transcription of these 2 loci proceeds in opposite directions. Both genes, however, are controlled by PhoP and SlyA through a single shared PhoP box and SlyA box present in their intergenic regions. A substitution in either box sequence causes a simultaneous cessation of transcription of a divergent operon, pagD-pagC, equivalent to the phenotype in a phoP or slyA mutant. We also identified several chromosomal loci that possess pagC-type genes without the cognate pagD-type genes. Therefore, our results provide a molecular basis for the understanding of SlyA-dependent phenotypes associated with Salmonella virulence.