Exponential-phase expression of spvA of the Salmonella typhimurium virulence plasmid: induction in intracellular salts medium and intracellularly in mice and cultured mammalian cells

Role of RpoS in Regulating Stationary Phase Salmonella Typhimurium Pathogenesis-Related Stress Responses under Physiological Low Fluid Shear Force Conditions

The discovery that biomechanical forces regulate microbial virulence was established with the finding that physiological low fluid shear (LFS) forces altered gene expression, stress responses, and virulence of the enteric pathogen Salmonella enterica serovar Typhimurium during the log phase. These log phase LFS-induced phenotypes were independent of the master stress response regulator, RpoS (σ^S). Given the central importance of RpoS in regulating stationary-phase stress responses of S. Typhimurium cultured under conventional shake flask and static conditions, we examined its role in stationary-phase cultures grown under physiological LFS. We constructed an isogenic rpoS mutant derivative of wild-type S. Typhimurium and compared the ability of these strains to survive in vitro pathogenesis-related stresses that mimic those encountered in the infected host and environment. We also compared the ability of these strains to colonize (adhere, invade, and survive within) human intestinal epithelial cell cultures. Unexpectedly, LFS-induced resistance of stationary-phase S. Typhimurium cultures to acid and bile salts stresses did not rely on RpoS. Likewise, RpoS was dispensable for stationary-phase LFS cultures to adhere to and survive within intestinal epithelial cells. In contrast, the resistance of these cultures to challenges of oxidative and thermal stresses, and their invasion into intestinal epithelial cells was influenced by RpoS. These findings expand our mechanistic understanding of how physiological fluid shear forces modulate stationary-phase S. Typhimurium physiology in unexpected ways and provide clues into microbial mechanobiology and nuances of Salmonella responses to microenvironmental niches in the infected host.

IMPORTANCE Bacterial pathogens respond dynamically to a variety of stresses in the infected host, including physical forces of fluid flow (fluid shear) across their surfaces. While pathogens experience wide fluctuations in fluid shear during infection, little is known about how these forces regulate microbial pathogenesis. This is especially important for stationary-phase bacterial growth, which is a critical period to understand microbial resistance, survival, and infection potential, and is regulated in many bacteria by the general stationary-phase stress response protein RpoS. Here, we showed that, unlike conventional culture conditions, several stationary-phase Salmonella pathogenic stress responses were not impacted by RpoS when bacteria were cultured under fluid shear conditions relevant to those encountered in the intestine of the infected host. These findings offer new insight into how physiological fluid shear forces encountered by Salmonella during infection might impact pathogenic responses in unexpected ways that are relevant to their disease-causing ability.

Impact of the Resistance Responses to Stress Conditions Encountered in Food and Food Processing Environments on the Virulence and Growth Fitness of Non-Typhoidal Salmonellae

The success of Salmonella as a foodborne pathogen can probably be attributed to two major features: its remarkable genetic diversity and its extraordinary ability to adapt. Salmonella cells can survive in harsh environments, successfully compete for nutrients, and cause disease once inside the host. Furthermore, they are capable of rapidly reprogramming their metabolism, evolving in a short time from a stress-resistance mode to a growth or virulent mode, or even to express stress resistance and virulence factors at the same time if needed, thanks to a complex and fine-tuned regulatory network. It is nevertheless generally acknowledged that the development of stress resistance usually has a fitness cost for bacterial cells and that induction of stress resistance responses to certain agents can trigger changes in Salmonella virulence. In this review, we summarize and discuss current knowledge concerning the effects that the development of resistance responses to stress conditions encountered in food and food processing environments (including acid, osmotic and oxidative stress, starvation, modified atmospheres, detergents and disinfectants, chilling, heat, and non-thermal technologies) exerts on different aspects of the physiology of non-typhoidal Salmonellae, with special emphasis on virulence and growth fitness.

Bimodal Expression of the Salmonella Typhimurium spv Operon

The well-studied spv operon of Salmonellatyphimurium is important for causing full virulence in mice and both the regulation and function of the Spv proteins have been characterized extensively over the past several decades. Using quantitative single-cell fluorescence microscopy, we demonstrate the spv regulon to display a bimodal expression pattern that originates in the bimodal expression of the SpvR activator. The spv expression pattern is influenced by growth conditions and the specific Styphimurium strain used, but does not require Salmonella-specific virulence regulators. By monitoring real-time promoter kinetics, we reveal that SpvA has the ability to impart negative feedback on spvABCD expression without affecting spvR expression. Together, our data suggest that the SpvA protein counteracts the positive feedback loop imposed by SpvR, and could thus be responsible for dampening spvABCD expression and coordinating virulence protein production in time. The results presented here yield new insights in the intriguing regulation of the spv operon and adds this operon to the growing list of virulence factors exhibiting marked expression heterogeneity in Styphimurium.

Molecular characterization of antibiotic resistant Salmonella Typhimurium and Salmonella Kentucky isolated from pre- and post-chill whole broilers carcasses

There is conflicting data regarding whether commercial chilling has any effect on persistence of Salmonella serovars, including antibiotic resistant variants, on chicken carcasses. A total of 309 Salmonella Typhimurium and Salmonella Kentucky isolates recovered from pre- and post-chill whole broiler carcasses were characterized for genetic relatedness using Pulsed Field Gel Electrophoresis (PFGE) and for the presence of virulence factors (invA, pagC, spvC) by PCR and for aerobactin and colicin production by bioassays. A subset of these isolates (n = 218) displaying resistance to either sulfisoxazole and/or ceftiofur [S. Typhimurium (n = 66) and S. Kentucky (n = 152)] were further tested for the presence of associated antibiotic resistance elements (class-I integrons and blaCMY genes) by PCR. All 145 ceftiofur resistant S. Kentucky and S. Typhimurium isolates possessed blaCMY genes. Class-I integrons were only detected in 6.1% (n = 4/66) of sulfisoxazole resistant S. Typhimurium isolates. The PFGE analysis revealed the presence of genetically diverse populations within the recovered isolates but clusters were generally concordant with serotypes and antimicrobial resistance profiles. At a 100% pattern similarity index, thirty-six percent of the undistinguishable S. Typhimurium and 22% of the undistinguishable S. Kentucky isolates were recovered from the same chilling step. All isolates possessed the invA and pagC genes, but only 1.4%possessed spvC. Irrespective of the chilling step, there was a significant difference (P < 0.05) in the production of aerobactin and colicin between S. Typhimurium and S. Kentucky isolates. Taken together, these results indicate that chilling impacted the recovery of particular Salmonella clonal groups but had no effect on the presence of class-I integrons, blaCMY genes, and tested virulence factors.

Stationary-Phase Gene Regulation in Escherichia coli §

In their stressful natural environments, bacteria often are in stationary phase and use their limited resources for maintenance and stress survival. Underlying this activity is the general stress response, which in Escherichia coli depends on the σS (RpoS) subunit of RNA polymerase. σS is closely related to the vegetative sigma factor σ70 (RpoD), and these two sigmas recognize similar but not identical promoter sequences. During the postexponential phase and entry into stationary phase, σS is induced by a fine-tuned combination of transcriptional, translational, and proteolytic control. In addition, regulatory "short-cuts" to high cellular σS levels, which mainly rely on the rapid inhibition of σS proteolysis, are triggered by sudden starvation for various nutrients and other stressful shift conditons. σS directly or indirectly activates more than 500 genes. Additional signal input is integrated by σS cooperating with various transcription factors in complex cascades and feedforward loops. Target gene products have stress-protective functions, redirect metabolism, affect cell envelope and cell shape, are involved in biofilm formation or pathogenesis, or can increased stationary phase and stress-induced mutagenesis. This review summarizes these diverse functions and the amazingly complex regulation of σS. At the molecular level, these processes are integrated with the partitioning of global transcription space by sigma factor competition for RNA polymerase core enzyme and signaling by nucleotide second messengers that include cAMP, (p)ppGpp, and c-di-GMP. Physiologically, σS is the key player in choosing between a lifestyle associated with postexponential growth based on nutrient scavenging and motility and a lifestyle focused on maintenance, strong stress resistance, and increased adhesiveness. Finally, research with other proteobacteria is beginning to reveal how evolution has further adapted function and regulation of σS to specific environmental niches.

Genome expression analyses revealing the modulation of the Salmonella Rcs regulon by the attenuator IgaA

Intracellular growth attenuator A (IgaA) was identified as a Salmonella enterica regulator limiting bacterial growth inside fibroblasts. Genetic evidence further linked IgaA to repression of the RcsCDB regulatory system, which responds to envelope stress. How IgaA attenuates this system is unknown. Here, we present genome expression profiling data of S. enterica serovar Typhimurium igaA mutants grown at high osmolarity and displaying exacerbated Rcs responses. Transcriptome data revealed that IgaA attenuates gene expression changes requiring phosphorylated RcsB (RcsB~P) activity. Some RcsB-regulated genes, yciGFE and STM1862 (pagO)-STM1863-STM1864, were equally expressed in wild-type and igaA strains, suggesting a maximal expression at low levels of RcsB ~P. Other genes, such as metB, ypeC, ygaC, glnK, glnP, napA, glpA, and nirB, were shown for the first time and by independent methods to be regulated by the RcsCDB system. Interestingly, IgaA-deficient strains with reduced RcsC or RcsD levels exhibited different Rcs responses and distinct virulence properties. spv virulence genes were differentially expressed in most of the analyzed strains. spvA expression required RcsB and IgaA but, unexpectedly, was also impaired upon stimulation of the RcsC-->RcsD-->RcsB phosphorelay. Overproduction of either RcsB(+) or a nonphosphorylatable RcsB(D56Q) variant in strains displaying low spvA expression unveiled that both dephosphorylated RcsB and RcsB~P are required for optimal spvA expression. Taken together, our data support a model with IgaA attenuating the RcsCDB system by favoring the switch of RcsB~P to the dephosphorylated state. This role of IgaA in constantly fine-tuning the RcsB~P/RcsB ratio may ensure the proper expression of important virulence factors, such as the Spv proteins.

Instability of the Salmonella RcsCDB signalling system in the absence of the attenuator IgaA

IgaA is a Salmonella enterica membrane protein that attenuates the response of the RcsCDB signalling system to envelope stress. This protein is essential unless the RcsCDB system is inactivated, suggesting that IgaA may constantly adjust the magnitude of the response. Such a functional link is also supported by the concurrence of the igaA and rcsD-rcsB-rcsC loci in genomes of enteric bacteria and the selection of spontaneous mutations in the RcsCDB system following IgaA deprivation. However, the exact nature of the spontaneous mutations rendering IgaA dispensable remains undefined. In this work, we examined how the transduction of an igaA null allele affects the status of the RcsCDB system. Loss of RcsCDB response was registered in approximately 90 % of the IgaA-defective clones, which failed to produce the capsule material positively regulated by this system. About half of these non-mucoid clones suppressed the loss of IgaA with large deletions encompassing variable regions of the rcsD-rcsB-rcsC locus. Unexpectedly, mucoid transductants were also reproducibly obtained and indicated the capacity of S. enterica to retain a functional RcsCDB system in the absence of IgaA. Decreased levels of either RcsC or RcsD were shown in 'mucoid' clones lacking IgaA and displaying low responsiveness to stimuli. Taken together, these data demonstrate that the stability and responsiveness of the RcsCDB system relies on its attenuator IgaA. The type of suppressions found also support a model with IgaA controlling the level of signal flowing through RcsC and RcsD.

SpvC is a Salmonella effector with phosphothreonine lyase activity on host mitogen-activated protein kinases

SpvC is encoded by the Salmonella virulence plasmid. We have investigated the biochemical function of SpvC and the mechanism by which it is secreted by bacteria and translocated into infected macrophages. We constructed a strain carrying a deletion in spvC and showed that the strain is attenuated for systemic virulence in mice. SpvC can be secreted in vitro by either the SPI-1 or SPI-2 type III secretion systems. Cell biological and genetic experiments showed that translocation of the protein into the cytosol of macrophages by intracellular bacteria is dependent on the SPI-2 T3SS. Using antibodies specific to phospho-amino acids and mass spectrometry we demonstrate that SpvC has phosphothreonine lyase activity on full-length phospho-Erk (pErk) and a synthetic 13-amino-acid phospho-peptide containing the TXY motif. A Salmonella strain expressing spvC from a plasmid downregulated cytokine release from infected cells.

The BarA-UvrY two-component system regulates virulence in avian pathogenic Escherichia coli O78:K80:H9

The BarA-UvrY two-component system (TCS) in Escherichia coli is known to regulate a number of phenotypic traits. Both in vitro and in vivo assays, including the chicken embryo lethality assay, showed that this TCS regulates virulence in avian pathogenic E. coli (APEC) serotype O78:K80:H9. A number of virulence determinants, such as the abilities to adhere, invade, persist within tissues, survive within macrophages, and resist bactericidal effects of serum complement, were compromised in mutants lacking either the barA or uvrY gene. The reduced virulence was attributed to down regulation of type 1 and Pap fimbriae, reduced exopolysaccharide production, and increased susceptibility to oxidative stress. Our results indicate that BarA-UvrY regulates virulence properties in APEC and that the chicken embryo lethality assay can be used as a surrogate model to determine virulence determinants and their regulation in APEC strains.

Polynucleotide phosphorylase negatively controls spv virulence gene expression in Salmonella enterica

Mutational inactivation of the cold-shock-associated exoribonuclease polynucleotide phosphorylase (PNPase; encoded by the pnp gene) in Salmonella enterica serovar Typhimurium was previously shown to enable the bacteria to cause chronic infection and to affect the bacterial replication in BALB/c mice (M. O. Clements et al., Proc. Natl. Acad. Sci. USA 99:8784-8789, 2002). Here, we report that PNPase deficiency results in increased expression of Salmonella plasmid virulence (spv) genes under in vitro growth conditions that allow induction of spv expression. Furthermore, whole-genome microarray-based transcriptome analyses of bacteria growing inside murine macrophage-like J774.A.1 cells revealed six genes as being significantly up-regulated in the PNPase-deficient background, which included spvABC, rtcB, entC, and STM2236. Mutational inactivation of the spvR regulator diminished the increased expression of spv observed in the pnp mutant background, implying that PNPase acts upstream of or at the level of SpvR. Finally, competition experiments revealed that the growth advantage of the pnp mutant in BALB/c mice was dependent on spvR as well. Combined, our results support the idea that in S. enterica PNPase, apart from being a regulator of the cold shock response, also functions in tuning the expression of virulence genes and bacterial fitness during infection.

Azithromycin inhibits the formation of flagellar filaments without suppressing flagellin synthesis in Salmonella enterica serovar typhimurium

The present study shows that a sub-MIC of the macrolide antibiotic azithromycin (AZM) diminishes the virulence function of Salmonella enterica serovar Typhimurium. We first constructed an AZM-resistant strain (MS248) by introducing ermBC, an erythromycin ribosome methylase gene, into serovar Typhimurium. The MIC of AZM for MS248 exceeded 100 microg/ml. Second, we managed to determine the efficacy with which a sub-MIC of AZM reduced the virulence of MS248 in vitro. On the one hand, AZM (10 microg/ml) in the culture medium was unable to inhibit the total protein synthesis, growth rate, or survival within macrophages of MS248. On the other hand, AZM (10 microg/ml) reduced MS248's swarming and swimming motilities in addition to its invasive activity in Henle-407 cells. Electron micrographs revealed no flagellar filaments on the surface of MS248 after overnight growth in L broth supplemented with AZM (10 microg/ml). However, immunoblotting analysis showed that flagellin (FliC) was fully synthesized within the bacterial cells in the presence of AZM (10 microg/ml). In contrast, the same concentration of AZM reduced the export of FliC to the culture medium. These results indicate that a sub-MIC of AZM was able to affect the formation of flagellar filaments, specifically by reducing the amount of flagellin exported from bacterial cells, but it was not involved in suppressing the synthesis of flagellin. Unfortunately, AZM treatment was ineffective against murine salmonellosis caused by MS248.

Molecular analysis of Salmonella isolates recovered from processed Turkey carcasses

This study was carried out to determine the prevalence of some virulence characteristics associated with Salmonella isolates recovered from processed turkey carcasses in the Midwestern region of the United States. A total of 94 Salmonella isolates recovered from turkey carcasses from two processing plants (A and B) were examined to determine the prevalence of invA, pagC, and spvC genes. Bioassays also were used to evaluate aerobactin and colicin production. All isolates (100%) were positive for the presence of invA and pagC but were negative for spvC. Overall, 19.1% of all isolates tested were positive for aerobactin production, and 25.5% of all isolates were positive for colicin. Aerobactin and colicin production differed among isolates recovered from the two plants; more isolates from plant B produced these compounds. The Salmonella isolates examined in this study possess significant potential for causing human illness.

The bacterial adaptive response gene, barA, encodes a novel conserved histidine kinase regulatory switch for adaptation and modulation of metabolism in Escherichia coli

Histidine kinases are important prokaryotic determinants of cellular adaptation to environmental conditions, particularly stress. The highly conserved histidine kinase, BarA, encoded by the bacterial adaptive response gene, barA, is a member of the family of tripartite histidine kinases, and is involved in stress adaptation. BarA has been implicated to play a role during infection of epithelial cells. Homologues and orthologues of BarA have been found in pathogenic yeast, fungi, mould and in plants. The primary aim of this review is to assimilate evidence present in the current literature linking the role of BarA in stress response, and to support it with preliminary experimental evidence indicating that, it is indeed a global response regulator. In particular, the review focuses on the unusual domain structure of the BarA protein, its role in oxidative, weak acid, and osmotic stress responses and its role in biofilm formation. A preliminary genomic approach to identify downstream genes regulated by the BarA signaling pathway, using DNA microarray, is reported. The results demonstrate that BarA plays a global response regulatory role in cell division, carbon metabolism, iron metabolism and pili formation. The evolutionary significance of these types of histidine kinase sensors is reviewed in light of their roles in pathogenesis.

Salmonella enterica serotype Choleraesuis: epidemiology, pathogenesis, clinical disease, and treatment

Nontyphoid Salmonella strains are important causes of reportable food-borne infection. Among more than 2,000 serotypes, Salmonella enterica serotype Choleraesuis shows the highest predilection to cause systemic infections in humans. The most feared complication of serotype Cholearesuis bacteremia in adults is the development of mycotic aneurysm, which previously was almost uniformally fatal. The advances in diagnostic techniques, surgical care, and antimicrobial therapy have greatly improved the survival of these patients. However, the recent emergence of serotype Choleraesuis that is resistant to ampicillin, chloramphenicol, trimethoprim-sulfamethoxazole, and, notably, fluoroquinolone antibiotics has aroused concern about the use of these agents for the empirical treatment of systemic infection caused by this organism. In view of the serious implications of the situation, the chain of transmission and mechanism of resistance should be carefully studied to reduce the spread of infection and threat to human health. To date, there are no vaccines available to prevent serotype Choleraesuis infections in humans. The availability, in the near future, of the genome sequence of serotype Cholearesuis will facilitate the development of effective vaccines as well as the discovery of new targets for novel antimicrobial agents.

Differences in the carriage and the ability to utilize the serotype associated virulence plasmid in strains of Salmonella enterica serotype Typhimurium investigated by use of a self-transferable virulence plasmid, pOG669

Most strains of Salmonella enterica subspecies enterica serotype typhimurium (S. typhimurium) naturally harbour a virulence plasmid which carries the salmonella plasmid virulence (spv) genes. However, isolates belonging to certain phage types are generally found without the plasmid. We have utilized a self-transferable virulence plasmid, pOG669 to investigate the effect of introduction of spv genes into strains of such phage types. The use of the co-integrate plasmid, pOG669, was validated on a diverse collection of strains. pOG669 was transferred into strains of serotypes that are normally associated with the possession of virulence plasmids. All strains maintained the wild type level of virulence in a mouse model, except that introduction of pOG669 restored normal virulence levels in an avirulent, plasmid free strain of S. dublin and resulted in a decrease in virulence in a strain of S. dublin from clonal line Du3. S. gallinarum did not become virulent in mice, but pOG669 was functionally interchangeable with the wild type plasmid when strains were tested in a chicken model. Strains of serotypes not normally associated with the carriage of a virulence plasmid did not increase in virulence upon the introduction of pOG669. An IncX plasmid pOG670 that was included as control was incompatible with the virulence plasmid in a strain of S. dublin, demonstrating that the common virulence plasmid of this serotype is of a different incompatibility group than other virulence plasmids. Strains of S. typhimurium from phage types that do not normally carry a virulence plasmid responded differently to attempts to introduce pOG669. No transconjugants were observed with the strains of DT5 and DT21. The introduction of pOG669 did not alter the virulence of JEO3942(DT10), DT35 and JEO3949(DT66) significantly, while DT1 and DT27 became more virulent. DT27 became as virulent as wild type C5, while logVC(10) of DT1 only increased from 4.1 to 5.7. The ability to express spv-genes was measured by use of an spvRAB'-cat fusion. Expression in S. enteritidis was found to be higher than in other serotypes tested. Only serotypes that naturally carry a virulence plasmid expressed spv-genes. The strain of DT1 expressed spv at a very low level, while expression in the strains of DT10 and DT35 was approximately 2-fold lower than in a control strain of S. typhimurium, while the level in the DT66 strain corresponded to the control strain. The plasmid pSTF9, which carried the fusion gene could not be introduced into the strains of DT5, DT21 and DT27. The RpoS level in the strains was measured indirectly by use of a katE-lacZ fusion. In the DT5 strain the level of expression was low, while the strains JEO3942(DT10), DT21, DT27 and DT35 expressed 4-5 fold the level in this strain. An internal fragment of the rpoS gene was sequenced in three strains. These all showed an identical sequence to a published S. typhimurium rpoS gene.

Extracellular secretion of the virulence plasmid-encoded ADP-ribosyltransferase SpvB in Salmonella

Nontyphoid Salmonella enterica requires the plasmid-encoded spv genes to establish successful systemic infection in experimental animals. The SpvB virulence-associated protein has recently been shown to contain the ADP-ribosyltransferase domain. SpvB ADP-ribosilates actin and depolymerizes actin filaments when expressed in cultured epithelial cells. However, spontaneous secretion or release of SpvB has not been observed under in vitro growth conditions. In the present study we investigated the secretion of SpvB from Salmonella using in vitro and in vivo assay systems. We showed that SpvB is secreted into supernatant from Salmonella strains that contain the cloned spvB gene on a plasmid when they grew in intracellular salts medium (ISM), a minimal medium mimicing the intracellular iron concentrations of eukaryotic cells. A series of mutant SpvB proteins revealed that an N-terminal region of SpvB located at amino acids 1-229 was sufficient to promote secretion into extracellular milieu. Confocal immunofluorescence microscopy also demonstrated efficient localization of the N-terminal domain of SpvB(1-360) tagged with biotinylated peptide within infected host cell cytosol but not truncated SpvB(1-179) fusion protein. In addition, mutations that inactivate genes within Salmonella pathogenicity island 1 or Salmonella pathogenicity island 2 that encode type III secretion systems (TTSS) could secrete the SpvB protein into the culture medium. These results indicate that SpvB protein is transported from the bacteria and into the host cytoplasm independent of TTSS.

Characterization of a novel intracellularly activated gene from Salmonella enterica serovar typhi

A Salmonella enterica serovar Typhi gene that is selectively up-regulated upon bacterial invasion of eukaryotic cells was characterized. The open reading frame encodes a 298-amino-acid hydrophobic polypeptide (30.8 kDa), which is predicted to be an integral membrane protein with nine membrane-spanning domains. The protein is closely related (87 to 94% reliability) to different transport and permease systems. Gene expression under laboratory conditions was relatively weak; however, sevenfold induction was observed in a high-osmolarity medium (300 mM NaCl). The growth pattern in a laboratory medium of a serovar Typhi strain Ty2 derivative containing a 735-bp in-frame deletion in this gene, named gaiA (for gene activated intracellularly), was not affected. In contrast, the mutant was partially impaired in intracellular survival in murine peritoneal macrophages, as well as in human monocyte-derived macrophages. However, in the case of human macrophages, this survival defect was modest and evident only at late infection times (24 h). Despite the distinct intracellular survival kinetics displayed in macrophages of different species, the gaiA null mutant was significantly affected in its potential to trigger apoptosis in both murine and human macrophages. Provision of the gaiA gene in trans resulted in complementation of these phenotypes. Interestingly, the absence of a functional gaiA gene caused a marked attenuation in the mouse mucin model, as shown by the increase (3 orders of magnitude) in the 50% lethal dose of the mutant strain over that of the parental strain Ty2 (P Collapse

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MESH Headings

• Amino Acid Sequence
• Animals
• Apoptosis
• Bacterial Proteins/genetics
• Bacterial Proteins/physiology
• Base Sequence
• DNA, Bacterial/genetics
• Gene Expression Regulation, Bacterial
• Genes, Bacterial
• Humans
• Macrophages/microbiology
• Macrophages/pathology
• Membrane Proteins/genetics
• Membrane Proteins/physiology
• Mice
• Molecular Sequence Data
• Mutation
• Salmonella Infections/etiology
• Salmonella Infections/microbiology
• Salmonella Infections/pathology
• Salmonella typhi/genetics
• Salmonella typhi/pathogenicity
• Virulence/genetics
• Virulence/physiology

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Affiliation(s)

Holger Basso Division of Microbiology, GBF-German Research Centre for Biotechnology, D-38124 Braunschweig, Germany
Faiza Rharbaoui
Lothar H Staendner
Eva Medina
Francisco García-Del Portillo
Carlos A Guzmán

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Salmonella intracellular proliferation: where, when and how?

Salmonella species proliferate within membrane-bound vacuoles of eukaryotic cells. Recent work has shown that macrophages are the main cell type supporting bacterial growth in vivo. In contrast, tissue culture models have traditionally described epithelial cells as the most permissive cells for bacterial growth. Unfortunately, no mechanism used by Salmonella to initiate growth within a vacuole has been characterised. Recently, it has been shown that Salmonella is capable of attenuating intracellular proliferation. This finding suggests that both the host and the pathogen contribute to a fine adjustment of the intracellular growth rate.

Effect of constitutively expressed phoP gene on the localization of Salmonella typhimurium within Mac-1 positive phagocytes

Intracellular localization of the wild-type (Spv+), the phoP-constitutively expressed strain (PhoPc), and the spv-deleted strain (Spv-) of Salmonella typhimurium was examined by the use of confocal laser scanning microscopy analysis of immunostained sections of mouse spleens after oral or subcutaneous inoculation. Only 40% of salmonellae of both the PhoPc and the Spv- strains were detected intracellularly within Mac-1 positive cells at day five after oral or day four after subcutaneous inoculation. In contrast, over 85% of salmonellae of the Spv+ strain were detected inside Mac-1 positive cells. In both inoculation trials, the splenic colony-forming unit values for the PhoPc and Spv- strains were significantly lower than the corresponding value for the Spv+ strain. These findings suggest that the constitutively expressed phoP gene of S. typhimurium attenuated virulence by limiting intracellular proliferation within mouse spleen phagocytes, and that the lack of spv genes had the same effect.

Virulence plasmid-borne spvB and spvC genes can replace the 90-kilobase plasmid in conferring virulence to Salmonella enterica serovar Typhimurium in subcutaneously inoculated mice

In a mouse model of systemic infection, the spv genes carried on the Salmonella enterica serovar Typhimurium virulence plasmid increase the replication rate of salmonellae in host cells of the reticuloendothelial system, most likely within macrophages. A nonpolar deletion in the spvB gene greatly decreased virulence but could not be complemented by spvB alone. However, a low-copy-number plasmid expressing spvBC from a constitutive lacUV5 promoter did complement the spvB deletion. By examining a series of spv mutations and cloned spv sequences, we deduced that spvB and spvC could be sufficient to confer plasmid-mediated virulence to S. enterica serovar Typhimurium. The spvBC-bearing plasmid was capable of replacing all of the spv genes, as well as the entire virulence plasmid, of serovar Typhimurium for causing systemic infection in BALB/c mice after subcutaneous, but not oral, inoculation. A point mutation in the spvBC plasmid preventing translation but not transcription of spvC eliminated the ability of the plasmid to confer virulence. Therefore, it appears that both spvB and spvC encode the principal effector factors for Spv- and plasmid-mediated virulence of serovar Typhimurium.

Constitutively expressed phoP inhibits mouse-virulence of Salmonella typhimurium in an Spv-dependent manner

In Salmonella typhimurium, the transcription of several virulence genes including spvB is regulated by the PhoP/PhoQ regulatory system. To further examine the relationship between the PhoP/PhoQ and Spv systems for virulence in mice, we examined a non-polar phoP mutation combined with different virulence plasmid genotypes for effects on virulence of S. typhimurium in the mouse model. PhoP-/Spv+ and PhoP-/Spv- mutants were not detectably recovered from the spleens of subcutaneously or orally inoculated mice. The phoP gene constitutively expressed from the lacZ promoter of a low copy number vector (phoP(C)) only partially complemented the non-polar phoP mutation for mouse-virulence in both the Spv+ and Spv- backgrounds; both PhoP(C) strains exhibited virulence equal only to a PhoP+/Spv- strain. Interestingly, in a PhoP+ background, the phoP(C) gene reduced splenic infection of the Spv+ but not Spv- salmonellae after subcutaneous or oral inoculation compared with the PhoP+ parents. Additionally, the phoP(C) gene in an Spv+ background reduced the net growth of salmonellae in macrophages in vitro; phoP(C) in an Spv- background was without effect. These data suggest that the constitutive expression of the phoP gene attenuates the virulence of S. typhimurium in mice in an Spv-dependent manner.

Use of confocal microscopy to detect Salmonella typhimurium within host cells associated with Spv-mediated intracellular proliferation

The major limitation in histological examination of orally inoculated mice of Salmonella typhimurium has been the difficulty of attaining high enough levels for immunochemical detection. This problem has been solved by the use of confocal laser scanning microscopy (CLSM) analysis, which allows detection of bacteria in the immunostained sections of mouse spleens at a minimum rate of approximately 1000 colony-forming units (cfu)/spleen. Here, we demonstrate that over 80% of salmonellae of the wild type of S. typhimurium were detected intracellularly within Mac-1 positive cells by the CLSM analysis of immunostained sections from spleens in orally or subcutaneously inoculated mice. Only 40% of salmonellae of the spv -deleted strain were detected inside Mac-1 positive cells. These data suggest that the spv genes play a key role in intracellular proliferation within phagocytes in the mouse spleen.

DNA topology and adaptation of Salmonella typhimurium to an intracellular environment

The expression of genes coding for determinants of DNA topology in the facultative intracellular pathogen Salmonella typhimurium was studied during adaptation by the bacteria to the intracellular environment of J774A.1 macrophage-like cells. A reporter plasmid was used to monitor changes in DNA supercoiling during intracellular growth. Induction of the dps and spv genes, previously shown to be induced in the macrophage, was detected, as was expression of genes coding for DNA gyrase, integration host factor and the nucleoid-associated protein H-NS. The topA gene, coding for the DNA relaxing enzyme topoisomerase I, was not induced. Reporter plasmid data showed that bacterial DNA became relaxed following uptake of S. typhimurium cells by the macrophage. These data indicate that DNA topology in S. typhimurium undergoes significant changes during adaptation to the intracellular environment. A model describing how this process may operate is discussed.

Virulence characterization of a strain of Salmonella enterica subspecies houten (subspecies IV) with chromosomal integrated Salmonella plasmid virulence (spv) genes

The Salmonella plasmid virulence genes (spv) are commonly found on plasmids contained in a small number of serotypes of Salmonella belonging to subspecies I, where they are important for survival within macrophages and the establishment of successful systemic infection. However, in this study, spv genes were detected by the polymerase chain reaction in the chromosome of a plasmid-free strain of S. IV 16:z4, z32:- (Salmonella subspecies IV). The full range of spv genes (spvR, spvA, spvB, spvC and spvD) was demonstrated, but a 216-bp deletion, accompanied by an insertion of 59-bp cryptic DNA, was present in spvA. S. IV 16:z4, z32:- was avirulent in mice and did not become virulent with the introduction of a fully functionally serotype-associated virulence plasmid (SAP) from S. typhimurium. By use of an spvRAB'-chloramphenicol acetyl transferase fusion gene, it was demonstrated that S. IV 16:z4, z32:- did not express the spv genes. Salmonella subspecies IV is monophasic, and in phylogenetic analyses it clusters distantly to Salmonella subspecies I, where all the serotypes that normally carry SAPs are found. The mechanisms by which spv genes have been transferred to this serotype remain unknown.

Use of the stationary phase inducible promoters, spv and dps, to drive heterologous antigen expression in Salmonella vaccine strains

We have investigated the ability of the growth phase regulated promoters dps and spv, to drive expression of heterologous antigens in Salmonella vaccine strains. Reporter plasmids were constructed which directed beta-galactosidase expression from dps (pDpslacZ) or spv (pSpvlacZ) and these were introduced independently into the Salmonella typhimurium vaccine strain SL3261 (aroA(-)). beta-galactosidase expression was induced 20-fold and 100-fold when broth cultures of SL3261 (pDpslacZ) or SL3261 (pSpvlacZ) respectively, entered the stationary phase of growth. Within macrophages, beta-galactosidase expression was induced 3.5-fold with SL3261 (pDpslacZ) and 7-fold with SL3261 (pSpvlacZ). The spv and dps promoters were used to drive independent expression of the C fragment domain of tetanus toxin (TetC) from plasmids harboured in S. typhimurium SL3261. Levels of anti-TetC antibodies were significantly higher in the sera of BALB/c mice perorally inoculated with SL3261 (pSpvtetC) or SL3261 (pDpstetC) compared to unvaccinated controls. This suggests that these promoter systems may be used to drive foreign antigen expression in live oral Salmonella vaccines.

The rpoS-dependent starvation-stress response locus stiA encodes a nitrate reductase (narZYWV) required for carbon-starvation-inducible thermotolerance and acid tolerance in Salmonella typhimurium

The starvation-stress response (SSR) of Salmonella typhimurium includes gene products necessary for starvation avoidance, starvation survival and virulence for this bacterium. Numerous genetic loci induced during carbon-source starvation and required for the long-term-starvation survival of this bacterium have been identified. The SSR not only protects the cell against the adverse effects of long-term starvation but also provides cross-resistance to other environmental stresses, e.g. thermal challenge (55 degrees C) or acid-pH challenge (pH 2.8). One carbon-starvation-inducible lac fusion, designated stiA was previously reported to be a sigma(S)-dependent SSR locus that is phosphate-starvation, nitrogen-starvation and H2O2 inducible, positively regulated by (p)ppGpp in a relA-dependent manner, and negatively regulated by cAMP:cAMP receptor protein complex and OxyR. We have discovered through sequence analysis and subsequent biochemical analysis that the stiA::lac fusion, and a similarly regulated lac fusion designated sti-99, lie at separate sites within the first gene (narZ) of an operon encoding a cryptic nitrate reductase (narZYWV) of unknown physiological function. In this study, it was demonstrated that narZ was negatively regulated by the global regulator Fnr during anaerobiosis. Interestingly, narZ(YWV) was required for carbon-starvation-inducible thermotolerance and acid tolerance. In addition, narZ expression was induced approximately 20-fold intracellularly in Madin-Darby canine kidney epithelial cells and 16-fold in intracellular salts medium, which is believed to mimic the intracellular milieu. Also, a narZ1 knock-out mutation increased the LD50 approximately 10-fold for S. typhimurium SL1344 delivered orally in the mouse virulence model. Thus, the previously believed cryptic and constitutive narZYWV operon is in fact highly regulated by a complex network of environmental-stress signals and global regulatory functions, indicating a central role in the physiology of starved and stressed cells.

The virulence plasmid of Salmonella typhimurium is self-transmissible

Most isolates of Salmonella enterica serovar Typhimurium contain a 90-kb virulence plasmid. This plasmid is reported to be mobilizable but nonconjugative. However, we have determined that the virulence plasmid of strains LT2, 14028, and SR-11 is indeed self-transmissible. The plasmid of strain SL1344 is not. Optimal conjugation frequency requires filter matings on M9 minimal glucose plates with a recipient strain lacking the virulence plasmid. These conditions result in a frequency of 2.9 x 10(-4) transconjugants/donor. Matings on Luria-Bertani plates, liquid matings, or matings with a recipient strain carrying the virulence plasmid reduce the efficiency by up to 400-fold. Homologs of the F plasmid conjugation genes are physically located on the virulence plasmid and are required for the conjugative phenotype.

The starvation-stress response (SSR) of Salmonella

Salmonella serovars are common etiologic agents of intestinal-based disease of animals and humans. As a result of their lifestyle, salmonellae occupy and survive in a wide range of niches where they can encounter an even broader range of environmental stresses. One of the most common stresses is starvation for an essential nutrient such as a carbon/energy (C)-source. The genetic and physiologic changes that the bacterium undergoes in response to starvation-stress are referred to as the starvation-stress response or SSR. The genetic loci whose expression increases in response to the starvation-stress compose the SSR stimulon. Several loci of the SSR stimulon have been identified in Salmonella typhimurium and grouped, based on putative or known functions or products, into transport systems, C-compound catabolic enzymes, known protective enzymes, respiratory enzyme systems, regulatory proteins, virulence loci and unclassified products. The majority of loci identified are under positive control by the rpoS-encoded sigma factor, sigma S. However, a few are under (indirect) negative control by sigma S, but only during starvation-induced stationary phase. Most of the loci identified are also under either positive or negative control by the cAMP:CRP complex. For many, additional regulatory proteins (e.g. FadR, OxyR, and RelA and others) play a role in their regulation as well. Furthermore, most of the SSR loci identified are induced during other stresses or environmental conditions. For example, some are induced during P- or N-starvation, in addition to C-starvation; some are induced by extremes in pH or osmolarity; and some are induced in the intracellular environment of epithelial cells, and/or macrophages, and/or medium designed to mimic the intracellular milieu of mammalian cells (ISM). Several SSR loci are required for long-term starvation-survival (core SSR loci), e.g. narZ, dadA, stiC and rpoS. In addition, a few of the core SSR loci are also required for stress-specific-inducible and/or C-starvation-inducible resistance to H2O2 (e.g. stiC), thermal (e.g. stiC), and/or acid pH (e.g. narZ), challenge. Interestingly, C-starved cells are resistant to challenge with the antimicrobial peptide, polymyxin B. However, this resistance mechanism(s) is different from the resistance mechanisms for H2O2 and other environmental stresses. Furthermore, a link between the SSR and Salmonella virulence can be hypothesized since the two major regulators of the SSR, sigma s and cAMP:CRP, are required for full virulence of Salmonella. Moreover, the spv (Salmonella plasmid-associated virulence) genes, required for Salmonella to cause systemic disease, are C (and P- and N-)-starvation-inducible. However, a direct link between starvation-stress and virulence has not been established conclusively.

Influence of the Salmonella typhimurium pathogenicity island 2 type III secretion system on bacterial growth in the mouse

We have investigated the in vivo growth kinetics of a Salmonella typhimurium strain (P11D10) carrying a mutation in ssaJ, a Salmonella pathogenicity island 2 (SPI2) gene encoding a component of a type III secretion system required for systemic growth in mice. Similar numbers of mutant and wild-type cells were recovered from the spleens and livers of BALB/c mice up to 8 h after inoculation by the intraperitoneal route. Thereafter, the numbers of wild-type cells continued to increase logarithmically in these organs, whereas those of P11D10 remained relatively static for several days before being cleared. Gentamicin protection experiments on spleen cell suspensions recovered from infected mice showed that viable intracellular wild-type bacteria accumulated over time but that intracellular P11D10 cells did not. Infection experiments were also performed with wild-type and P11D10 cells carrying the temperature-sensitive plasmid pHSG422 to distinguish between bacterial growth rates and killing in vivo. At 16 h postinoculation there were 10-fold more wild-type cells than mutant cells in the spleens of infected mice, but the numbers of cells of both strains carrying the nonreplicating plasmid were very similar, showing that there was little difference in the degree of killing sustained by the two strains and that the SPI2 secretion system must be required for bacterial replication, rather than survival, in vivo. The SPI2 mutant phenotype in mice is similar to that of strains carrying mutations in the Salmonella virulence plasmid spv genes. To determine if these two sets of genes interact together, a double mutant strain carrying SPI2 and spv mutations was constructed and compared with strains carrying single mutations in terms of virulence attenuation. These experiments failed to provide any evidence showing that the SPI2 and spv gene products interact together as part of the same virulence mechanism.

Regulation of the spvR gene of the Salmonella typhimurium virulence plasmid during exponential-phase growth in intracellular salts medium and at stationary phase in L broth

The authors previously showed that the SpvR-regulated spvABCD operon of the Salmonella typhimurium virulence plasmid is highly induced during exponential-phase growth by salmonellae intracellularly in mammalian cells and in a medium designed to mimic the intracellular environment of mammalian cells, intracellular salts medium (ISM), as well as at stationary phase in L broth (LB). The most relevant signal(s) for spv gene expression in vivo is not known. To elucidate the means by which salmonellae regulate the spv genes in response to the environment during the disease process, expression of the spvR gene, encoding the positive regulatory protein SpvR, was examined under these same growth conditions by using RNAse-protection analysis. spvR was expressed at a low, basal level during exponential growth in LB but was induced during exponential growth in ISM and during stationary phase in LB, the same conditions that increased expression of the spvABCD operon. Basal expression of spvR during exponential growth in LB was independent of both SpvR and the alternative sigma factor RpoS, whereas maximal induction of spvR was dependent on both SpvR and RpoS. In an RpoS- background, spvR message was decreased in stationary phase, whereas spvR exhibited residual RpoS-independent induction during exponential growth in ISM. Deletion of spvA from the virulence plasmid of S. typhimurium increased expression of spvR during stationary phase in LB, but not during exponential growth in ISM. These results suggest that expression of spvR is controlled by different regulatory factors, depending on the growth conditions encountered by the salmonellae.

Analysis of host cells associated with the Spv-mediated increased intracellular growth rate of Salmonella typhimurium in mice

The 90-kb virulence plasmid of Salmonella typhimurium encodes five spv genes which increase the growth rate of the bacteria within host cells within the first week of systemic infection of mice (P. A. Gulig and T. J. Doyle, Infect. Immun. 61:504-511, 1993). The presently described study was aimed at identifying the host cells associated with Spv-mediated virulence by manipulating the mouse host and the salmonellae. To test the effects of T cells and B cells on the Spv phenotype, salmonellae were orally inoculated into nude and SCID BALB/c mice. Relative to normal BALB/c mice, nude and SCID BALB/c mice were unaffected for splenic infection with either the Spv+ or Spv- S. typhimurium strains at 5 days postinoculation. When mice were pretreated with cyclophosphamide to induce granulocytopenia, there was a variable increase in total salmonella infection, but the relative splenic CFU of Spv+ versus Spv- S. typhimurium was not changed after oral inoculation. In contrast, depletion of macrophages from mice by treatment with cyclophosphamide plus liposomes containing dichloromethylene diphosphate resulted in equivalent virulence of Spv+ and Spv- salmonellae. To examine if the spv genes affected the growth of salmonellae in nonphagocytic cells, an invA::aphT mutation was transduced into Spv+ and Spv- S. typhimurium strains. InvA- Spv+ salmonellae were not significantly affected for splenic infection after subcutaneous inoculation compared with the wild-type strain, and InvA- Spv- salmonellae were only slightly attenuated relative to InvA+ Spv- salmonellae. Invasion-defective salmonellae still exhibited the Spv phenotype. Therefore, infection of nonphagocytes is not involved with the Spv virulence function. Taken together, these data demonstrate that macrophages are essential for suppressing the infection by Spv- S. typhimurium, by serving as the primary host cell for Spv-mediated intracellular replication and possibly by inhibiting the replication of salmonellae within other macrophages.

Systemic infection of mice by wild-type but not Spv- Salmonella typhimurium is enhanced by neutralization of gamma interferon and tumor necrosis factor alpha

The spv genes of the virulence plasmid of Salmonella typhimurium and other nontyphoidal serovars of S. enterica are involved in systemic infection by increasing the replication rate of the bacteria in host tissues beyond the intestines. We considered the possibility that the Spv virulence function is to evade suppression by the host response to infection. To examine this possibility, gamma interferon (IFN-gamma) and/or tumor necrosis factor alpha (TNF-alpha) were neutralized in BALB/c mice by intraperitoneal administration of monoclonal antibodies. Neutralization of IFN-gamma and/or TNF-alpha resulted in increased splenic infection with wild-type salmonellae after oral inoculation; however, Spv- salmonellae were defective at increasing splenic infection in cytokine-depleted mice. The use of a temperature-sensitive marker plasmid, pHSG422, indicated that neutralization of IFN-gamma caused less killing of wild-type S. typhimurium, while neutralization of TNF-alpha resulted in an increased in vivo replication rate for wild-type salmonellae. These results demonstrate that the Spv virulence function is not to evade suppression of bacterial infection normally mediated by IFN-gamma or TNF-alpha.