Characterization of translation termination mutations in the spv operon of the Salmonella virulence plasmid pSDL2

Salmonella multimutants enable efficient identification of SPI-2 effector protein function in gut inflammation and systemic colonization

Salmonella enterica spp. rely on translocation of effector proteins through the SPI-2 encoded type III secretion system (T3SS) to achieve pathogenesis. More than 30 effectors contribute to manipulation of host cells through diverse mechanisms, but interdependency or redundancy between effectors complicates the discovery of effector phenotypes using single mutant strains. Here, we engineer six mutant strains to be deficient in cohorts of SPI-2 effector proteins, as defined by their reported function. Using various animal models of infection, we show that three principle phenotypes define the functional contribution of the SPI-2 T3SS to infection. Multimutant strains deficient for intracellular replication, for manipulation of host cell defences, or for expression of virulence plasmid effectors all showed strong attenuation in vivo, while mutants representing approximately half of the known effector complement showed phenotypes similar to the wild-type parent strain. By additionally removing the SPI-1 T3SS, we find cohorts of effector proteins that contribute to SPI-2 T3SS-driven enhancement of gut inflammation. Further, we provide an example of how iterative mutation can be used to find a minimal number of effector deletions required for attenuation, and thus establish that the SPI-2 effectors SopD2 and GtgE are critical for the promotion of gut inflammation and mucosal pathology. This strategy provides a powerful toolset for simultaneous parallel screening of all known SPI-2 effectors in a single experimental context, and further facilitates the identification of the responsible effectors, and thereby provides an efficient approach to study how individual effectors contribute to disease.

The LysR-Type Transcription Regulator YhjC Promotes the Systemic Infection of Salmonella Typhimurium in Mice

Salmonella Typhimurium is a Gram-negative intestinal pathogen that can infect humans and a variety of animals, causing gastroenteritis or serious systemic infection. Replication within host macrophages is essential for S. Typhimurium to cause systemic infection. By analyzing transcriptome data, the expression of yhjC gene, which encodes a putative regulator in S. Typhimurium, was found to be significantly up-regulated after the internalization of Salmonella by macrophages. Whether yhjC gene is involved in S. Typhimurium systemic infection and the related mechanisms were investigated in this study. The deletion of yhjC reduced the replication ability of S. Typhimurium in macrophages and decreased the colonization of S. Typhimurium in mouse systemic organs (liver and spleen), while increasing the survival rate of the infected mice, suggesting that YhjC protein promotes systemic infection by S. Typhimurium. Furthermore, by using transcriptome sequencing and RT-qPCR assay, the transcription of several virulence genes, including spvD, iroCDE and zraP, was found to be down-regulated after the deletion of yhjC. Electrophoretic mobility shift assay showed that YhjC protein can directly bind to the promoter region of spvD and zraP to promote their transcription. These findings suggest that YhjC contributes to the systemic virulence of S. Typhimurium via the regulation of multiple virulence genes and YhjC could represent a promising target to control S. Typhimurium infection.

CRISPR/Cas9-Based Deletion of SpvB Gene From Salmonella gallinarum Leads to Loss of Virulence in Chicken

Salmonella Gallinarum causes fowl typhoid in poultry leading to a huge economic loss to the poultry industry. The large virulence plasmid of S. gallinarum has been associated with various systemic infections in poultry. A five-gene spanning region (spvRABCD) of 7.8 kb on the large plasmid mainly confers virulence to the bacteria. However, the exact role of these genes in virulence has not been elucidated yet. SpvB exhibits delayed cell death by preventing actin polymerization followed by apoptosis during intracellular infection. The specific role of SpvB in causing the disease is not known yet. In the current study, the SpvB gene was deleted through CRISPR/Cas9 method from a large virulent plasmid of locally isolated S. gallinarum strain (SG18). The homology-directed repair method was used for complete deletion of SpvB gene using the modified pCas9 plasmid. The SpvB-deleted S. gallinarum strain (ΔSpvB_SG18), when tested for its virulence in broiler chicken showed no diseases signs and mortality. In addition, the avirulent strain does not affect the bird’s weight and was rapidly cleared from the liver after infection. However, it cleared from the intestine only after 4–5 days, which suggests that the ΔSpvB_SG18 strain is unable to invade from the intestine to the liver. This is the first study to report a complete gene deletion from the S. gallinarum virulent plasmid and its effect. This method will be useful for the deletion of virulent genes from S. gallinarum, to study their role in pathogenesis, and to prepare an effective vaccine strain for controlling fowl typhoid in poultry.

Whole genome sequencing and protein structure analyses of target genes for the detection of Salmonella

Rapid and sensitive detection of Salmonella is a critical step in routine food quality control, outbreak investigation, and food recalls. Although various genes have been the targets in the design of rapid molecular detection methods for Salmonella, there is limited information on the diversity of these target genes at the level of DNA sequence and the encoded protein structures. In this study, we investigated the diversity of ten target genes (invA, fimA, phoP, spvC, and agfA; ttrRSBCA operon including 5 genes) commonly used in the detection and identification of Salmonella. To this end, we performed whole genome sequencing of 143 isolates of Salmonella serotypes (Enteritidis, Typhimurium, and Heidelberg) obtained from poultry (eggs and chicken). Phylogenetic analysis showed that Salmonella ser. Typhimurium was more diverse than either Enteritidis or Heidelberg. Forty-five non-synonymous mutations were identified in the target genes from the 143 isolates, with the two most common mutations as T ↔ C (15 times) and A ↔ G (13 times). The gene spvC was primarily present in Salmonella ser. Enteritidis isolates and absent from Heidelberg isolates, whereas ttrR was more conserved (0 non-synonymous mutations) than ttrS, ttrB, ttrC, and ttrA (7, 2, 2, and 7 non-synonymous mutations, respectively). Notably, we found one non-synonymous mutation (fimA-Mut.6) across all Salmonella ser. Enteritidis and Salmonella ser. Heidelberg, C → T (496 nt postion), resulting in the change at AA 166 position, Glutamine (Q) → Stop condon (TAG), suggesting that the fimA gene has questionable sites as a target for detection. Using Phyre² and SWISS-MODEL software, we predicted the structures of the proteins encoded by some of the target genes, illustrating the positions of these non-synonymous mutations that mainly located on the α-helix and β-sheet which are key elements for maintaining the conformation of proteins. These results will facilitate the development of sensitive molecular detection methods for Salmonella.

Systematic reconstruction of an effector-gene network reveals determinants of Salmonella cellular and tissue tropism

The minimal genetic requirements for microbes to survive within multiorganism communities, including host-pathogen interactions, remain poorly understood. Here, we combined targeted gene mutagenesis with phenotype-guided genetic reassembly to identify a cooperative network of SPI-2 T3SS effector genes that are sufficient for Salmonella Typhimurium (STm) to cause disease in a natural host organism. Five SPI-2 effector genes support pathogen survival within the host cell cytoplasm by coordinating bacterial replication with Salmonella-containing vacuole (SCV) division. Unexpectedly, this minimal genetic repertoire does not support STm systemic infection of mice. In vivo screening revealed a second effector-gene network, encoded by the spv operon, that expands the life cycle of STm from growth in cells to deep-tissue colonization in a murine model of typhoid fever. Comparison between Salmonella infection models suggests how cooperation between effector genes drives tissue tropism in a pathogen group.

Identification of Genetic Features for Attenuation of Two Salmonella Enteritidis Vaccine Strains and Differentiation of These From Wildtype Isolates Using Whole Genome Sequencing

Salmonella Enteritidis is a major cause of salmonellosis worldwide and more than 80% of outbreaks investigated in Europe have been associated with the consumption of poorly cooked eggs or foods containing raw eggs. Vaccination has been proven to be one of the most important measures to control Salmonella Enteritidis infections in poultry farms as it can decrease colonization of the reproductive organs and intestinal tract of laying hens, thereby reducing egg contamination. Differentiation of live vaccine from field or wild type S. Enteritidis isolates in poultry is essential for monitoring of veterinary isolates and targetting control actions. Due to decreasing costs, whole genome sequencing (WGS) is becoming a key tool for characterization of Salmonella isolates, including vaccine strains. Using WGS we described the genetic changes in the live attenuated Salmovac 440 and AviPro SALMONELLA VAC E vaccine strains and developed a method for differentiation from the wildtype S. Enteritidis strains. SNP analysis confirmed that streptomycin resistance was associated with a Lys43Arg missense mutation in the rpsL gene whilst 3 missense mutations in acrB and 1 missense mutation in acrA confer erythromycin sensitivity in AviPro SALMONELLA VAC E. Further mutations Arg242His in purK and Gly236Arg in the hisB gene were related to adenine and histidine dependencies in Salmovac 440. Unique SNPs were used to construct a database of variants for differentiation of vaccine from the wildtype isolates. Two fragments from each vaccine were represented in the database to ensure high accuracy. Each of the two selected Salmovac 440 fragments differed by 6 SNPs from the wildtype and the AviPro SALMONELLA VAC E fragments differed by 4 and 6 SNPs, respectively. CD-hit software was applied to cluster similar fragments that produced the best fit output when searched with SRST2. The developed vaccine differentiation method was tested with 1,253 genome samples including field isolates of Salmovac 440 (n = 51), field isolates of AviPro SALMONELLA VAC E (n = 13), S. Gallinarum (n = 19), S. Pullorum (n = 116), S. Enteritidis (n = 244), S. Typhimurium (n = 810) and achieved 100% sensitivity and specificity.

Detection of Virulence Plasmid-Encoded Genes in Salmonella Typhimurium and Salmonella Kentucky Isolates Recovered from Commercially Processed Chicken Carcasses

Salmonella enterica serovar Typhimurium is one of the leading causes of nontyphoidal gastroenteritis of humans in the United States. Commercially processed poultry carcasses are frequently contaminated with Salmonella serovar Kentucky in the United States. The aim of the study was to detect the Salmonella virulence plasmid containing the spv genes from Salmonella isolates recovered from commercially processed chicken carcasses. A total of 144 Salmonella isolates (Salmonella Typhimurium, n = 72 and Salmonella Kentucky, n = 72) were used for isolation of plasmids and detection of corresponding virulence genes (spvA, spvB, and spvC). Only four (5.5%) Salmonella Typhimurium isolates tested positive for all three virulence genes and hence were classified as possessing the virulence plasmid. All isolates of Salmonella Kentucky were negative for the virulence plasmid and genes. These results indicate that the virulence plasmid, which is very common among clinical isolates of Typhimurium and other Salmonella serovars (e.g., Enteritidis, Dublin, Choleraesuis, Gallinarum, Pullorum, and Abortusovis), may not be present in a significant portion of commercially processed chicken carcass isolates.

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.

A comparative survey between non-systemic Salmonella spp. (paratyphoid group) and systemic Salmonella Pullorum and S. Gallinarum with a focus on virulence genes

ABSTRACT: A comparative survey between non-systemic (paratyphoid Salmonellae) and systemic (S. Pullorum and S. Gallinarum) Salmonella strains was performed to produce a virulence gene profile for differentiation among the groups. The following virulence genes were evaluated: invA, spvC, sefC, pefA, fimY, sopB, sopE1, stn and avrA. There are substantial differences among paratyphoid Salmonellae, S. Pullorum, and S. Gallinarum regarding the genes sefC, spvC, sopE1 and avrA. A higher frequency of sefC, spvC, sopE1 and avrA genes were detected in S. Gallinarum and S. Pullorum when compared with strains from the paratyphoid group of Salmonella. These results may be useful for differentiating among different groups and serotypes.

A novel contribution of spvB to pathogenesis of Salmonella Typhimurium by inhibiting autophagy in host cells

Salmonella plasmid virulence genes (spv) are highly conserved in strains of clinically important Salmonella serovars. It is essential for Salmonella plasmid-correlated virulence, although the exact mechanism remains to be elucidated. Autophagy has been reported to play an important role in host immune responses limiting Salmonella infection. Our previous studies demonstrated that Salmonella conjugative plasmid harboring spv genes could enhance bacterial cytotoxicity by inhibiting autophagy. In the present study, we investigated whether spvB, which is one of the most important constituents of spv ORF could intervene in autophagy pathway. Murine macrophage-like cells J774A.1, human epithelial HeLa cells, and BALB/c mice infected with Salmonella Typhimurium wild type, mutant and complementary strains (carrying or free spvB or complemented only with ADP-ribosyltransferase activity of SpvB) were used in vitro and in vivo assay, respectively. To further explore the molecular mechanisms, both SpvB ectopic eukaryotic expression system and cells deficient in essential autophagy components by siRNA were generated. Results indicated that spvB could suppress autophagosome formation through its function in depolymerizing actin, and aggravate inflammatory injury of the host in response to S. Typhimurium infection. Our studies demonstrated virulence of spvB involving in inhibition of autophagic flux for the first time, which could provide novel insights into Salmonella pathogenesis, and have potential application to develop new antibacterial strategies for Salmonellosis.

Association of Three Plasmid-Encoded spv Genes Among Different Salmonella Serotypes Isolated from Different Origins

The virulence plasmid associated Salmonella plasmid virulence (spv) locus is strongly concomitant with strains that cause non typhoid bacteremia. The spv region contains three genes required for the virulence, the positive transcriptional regulator spvR and two structural genes spvB and spvC. The purpose of this study was to investigate the presence of these three genes among salmonella serotypes isolated from different sources. A collection of 60 salmonella serotypes from different sources were used. Polymerase chain reaction was carried out for the presence of these genes using specific primers. The prevalence of spvB, spvC, and spvR genes were 26 (43.3 %), 44 (73.3 %), and 28 (46.6 %), respectively. The findings revealed that the distribution of these genes was dissimilar among these serotypes. Many of the human pathogenic salmonella strains which can be transmitted by animals may have these genes and can be very injurious for public health.

Microbiological analysis of nontyphoidal Salmonella strains causing distinct syndromes of bacteremia or enteritis in HIV/AIDS patients in San Diego, California

Recurrent invasive nontyphoidal Salmonella (NTS) infection is an AIDS-defining illness that has become less common in the developed world in the era of highly active antiretroviral therapy (HAART), while it has emerged as a major public health problem in developing countries, particularly sub-Saharan Africa. We retrospectively analyzed Salmonella (NTS) infection in HIV/AIDS patients from June 2003 until December 2009 at the University of California, San Diego (UCSD), Medical Center. Bacterial isolates from all patients were tested for selected microbiological properties, including major Salmonella (NTS) virulence loci rpoS, sodCI, spvB, and sseI. Fourteen percent of all Salmonella (NTS) cases recorded at the UCSD Medical Center during this period occurred in known HIV/AIDS patients. The clinical presentations in HIV patients fell into two distinct groups, bacteremia and enteritis. There was little clinical overlap between these two syndromes. All strains were positive for the presence of the rpoS and sodCI virulence loci, and 75% of strains were positive for the presence of the spvB and sseI loci. Antibiotic susceptibility assay showed that all strains were susceptible to trimethoprim-sulfamethoxazole and ciprofloxacin. The clinical presentation did not have a clear relationship to the CD4(+) cell count. Of the bacteremic isolates, all but one isolate, drawn from a patient with substantial enteric comorbidities, had all of the virulence genes tested, but 66% of nonbacteremic, enteritis strains also contained all the tested virulence loci. In conclusion, neither patients' CD4(+) cell count nor bacterial strain properties necessarily predicted the clinical presentation of HIV/AIDS patients with Salmonella (NTS) infection, and AIDS patients can have episodes of Salmonella enteritis without dissemination.

Diarrhea and colitis in mice require the Salmonella pathogenicity island 2-encoded secretion function but not SifA or Spv effectors

We investigated the roles of Salmonella pathogenicity island 2 (SPI-2) and two SPI-2 effectors in Salmonella colitis and diarrhea in genetically resistant BALB/c.D2(Slc11a1) congenic mice with the wild-type Nramp1 locus. Wild-type Salmonella enterica serovar Typhimurium 14028s caused a pan-colitis, and the infected mice developed frank diarrhea with a doubling of the fecal water content. An ssaV mutant caused only a 26% increase in fecal water content, without producing the pathological changes of colitis, and it did not cause weight loss over a 1-week period of observation. However, two SPI-2 effector mutants, the spvB and sifA mutants, and a double spvB sifA mutant caused diarrhea and colitis, even though the sifA mutant was sensitive to killing by bone marrow-derived macrophages from BALB/c.D2 mice and was severely impaired in extraintestinal growth but not in growth in the cecum. These results demonstrate that systemic S. enterica infection and diarrhea/colitis are distinct pathogenic processes and that only the former requires spvB and sifA.

Salmonella Yoruba infection in white-tufted-ear marmoset (Callithrix jacchus)

The aim of this study was to describe a fatal salmonellosis case in a non-human female primate (Callithrix jacchus), found in the illegal pet trade in Brazil. The marmoset was sent to the quarantine section of the Guarulhos City Zoo and died in the sequence of an episode of profuse diarrhea. Necropsy findings included mucous enteritis, and liver enlargement and necrosis. Feces and liver fragments were collected for bacteriological tests, which indicated the presence of Salmonella sp.; it was subsequently characterized as pertaining to the Yoruba serotype. The susceptibility profile demonstrated resistance to tetracycline only. The strain was positive for genes that encoded the virulence factors investigated (invA, sefC, pefA and spvC). The results indicated the risk of introduction of Salmonella pathogenic serotypes in primates in captivity.

The Role of the spv Genes in Salmonella Pathogenesis

Salmonella strains cause three main types of diseases in people: gastroenteritis, enteric (typhoid) fever, and non-typhoid extra-intestinal disease with bacteremia. Genetic analysis indicates that each clinical syndrome requires distinct sets of virulence genes, and Salmonella isolates differ in their constellation of virulence traits. The spv locus is strongly associated with strains that cause non-typhoid bacteremia, but are not present in typhoid strains. The spv region contains three genes required for the virulence phenotype in mice: the positive transcriptional regulator spvR and two structural genes spvB and spvC. SpvB and SpvC are translocated into the host cell by the Salmonella pathogenicity island-2 type-three secretion system. SpvB prevents actin polymerization by ADP-ribosylation of actin monomers, while SpvC has phosphothreonine lyase activity and has been shown to inhibit MAP kinase signaling. The exact mechanisms by which SpvB and SpvC act in concert to enhance virulence are still unclear. SpvB exhibits a cytotoxic effect on host cells and is required for delayed cell death by apoptosis following intracellular infection. Strains isolated from systemic infections of immune compromised patients, particularly HIV patients, usually carry the spv locus, strongly suggesting that CD4 T cells are required to control disease due to Salmonella that are spv positive. This association is not seen with typhoid fever, indicating that the pathogenesis and immunology of typhoid have fundamental differences from the syndrome of non-typhoid bacteremia.

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.

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.

Identification of Salmonella SPI-2 secretion system components required for SpvB-mediated cytotoxicity in macrophages and virulence in mice

The Salmonella SpvB protein possesses ADP-ribosyl transferase activity. SpvB, acting as an intracellular toxin, covalently modifies monomeric actin, leading to loss of F-actin filaments in Salmonella-infected human macrophages. Using defined Salmonella mutants, different functional components of the SPI-2 type three secretion system (TTSS), ssaV, spiC, sseB, sseC, and sseD, were found to be required for SpvB-mediated actin depolymerization in human macrophages. Expression of SpvB protein in Salmonella was not affected by any of the SPI-2 mutants and the effects of these loci were not due to reduced numbers of intracellular bacteria. Interestingly, the major SPI-2 virulence effector, SifA, is not required for SpvB action. Further, caspase-3 activation is an additional marker of cytotoxicity in Salmonella-infected human macrophages. Caspase-3 activity depended on SpvB and SPI-2 TTSS function, but not on SifA. These human macrophage cell culture results were corroborated by virulence studies in mice. Using competitive infection of mice with mixed inocula of single and double mutants, spvBmut1 mutation did not have an effect independent of ssaJ mutation, essential for SPI-2 TTSS function. In contrast, competitive infection studies in mice confirmed that SpvB and SifA have independent virulence effects, as predicted by the macrophage studies.

Bacterial cytotoxins: targeting eukaryotic switches

Many bacterial cytotoxins act on eukaryotic cells by targeting the regulators that are involved in controlling the cytoskeleton or by directly modifying actin, with members of the Rho GTPase family being particularly important targets. The actin cytoskeleton, and especially the GTPase 'molecular switches' that are involved in its control, have crucial functions in innate and adaptive immunity, and have pivotal roles in the biology of infection. In this review, we briefly discuss the role of the actin cytoskeleton and the Rho GTPases in host-pathogen interactions, and review the mode of actions of bacterial protein toxins that target these components.

Targeting of the actin cytoskeleton during infection by Salmonella strains

Many bacterial pathogens produce virulence factors that alter the host cell cytoskeleton to promote infection. Salmonella strains target cellular actin in a carefully orchestrated series of interactions that promote bacterial uptake into host cells and the subsequent proliferation and intercellular spread of the organisms. The Salmonella Pathogenicity Island 1 (SPI1) locus encodes a type III protein secretion system (TTSS) that translocates effector proteins into epithelial cells to promote bacterial invasion through actin cytoskeletal rearrangements. SPI1 effectors interact directly with actin and also alter the cytoskeleton through activation of the regulatory proteins, Cdc42 and Rac, to produce membrane ruffles that engulf the bacteria. SPI1 also restores normal cellular actin dynamics through the action of another effector, SptP. A second TTSS, Salmonella Pathogenecity Island 2 (SPI2), translocates effectors that promote intracellular survival and growth, accompanied by focal actin polymerization around the Salmonella-containing vacuole (SCV). A number of Salmonella strains also carry the spv virulence locus, encoding an ADP-ribosyl transferase, the SpvB protein, which acts later during intracellular infection to depolymerize the actin cytoskeleton. SpvB produces a cytotoxic effect on infected host cells leading to apoptosis. The SpvB effect appears to promote intracellular infection and may facilitate cell-to-cell spread of the organism, thereby enhancing virulence.

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.

Taking possession: biogenesis of the Salmonella-containing vacuole

The Gram-negative pathogen Salmonella enterica can survive and replicate within a variety of mammalian cells. Regardless of the cell type, internalized bacteria survive and replicate within the Salmonella-containing vacuole, the biogenesis of which is dependent on bacterially encoded virulence factors. In particular, Type III secretion systems translocate bacterial effector proteins into the eukaryotic cell where they can specifically interact with a variety of targets. Salmonella has two distinct Type III secretion systems that are believed to have completely different functions. The SPI2 system is induced intracellularly and is required for intracellular survival in macrophages; it plays no role in invasion but is categorized as being required for Salmonella-containing vacuole biogenesis. In contrast, the SPI1 Type III secretion system is induced extracellularly and is essential for invasion of nonphagocytic cells. Its role in post-invasion processes has not been well studied. Recent studies indicate that Salmonella-containing vacuole biogenesis may be more dependent on SPI1 than previously believed. Other non-SPI2 virulence factors and the host cell itself may play critical roles in determining the intracellular environment of this facultative intracellular pathogen. In this review we discuss the recent advances in determining the mechanisms by which Salmonella regulate Salmonella-containing vacuole biogenesis and the implications of these findings.

Intracellular expression of the Salmonella plasmid virulence protein, SpvB, causes apoptotic cell death in eukaryotic cells

The spv genes carried on the Salmonella virulence plasmid are commonly associated with severe systemic infection in experimental animals. The SpvB virulence-associated protein has been shown to ADP-ribosylate actin, and this enzymatic activity is essential for virulence in mice. Here, we present evidence that intracellular expression of SpvB protein induces not only disruption of actin filaments but also apoptotic cell death in eukaryotic cells.

Genetic requirements for salmonella-induced cytopathology in human monocyte-derived macrophages

Infection of human macrophages with Salmonella enterica serovar Typhimurium or Salmonella enterica serovar Dublin produces delayed cytotoxicity characterized by cell detachment and associated apoptosis. Using a site-specific mutant in the SpvB active site, we verify that the ADP-ribosylation activity of SpvB is required for delayed cytotoxicity in human macrophages infected with Salmonella: SipB and the type III protein secretion system (TTSS) encoded by Salmonella pathogenicity island 1 (SPI1) are not involved, whereas the SPI2 TTSS is absolutely required for SpvB-dependent cytotoxicity. Furthermore, we show that infection of macrophage cultures with wild-type or sipB mutant bacteria led to a complete loss of polymerized actin in over half of the cells after 24 h. In contrast, macrophages infected with the spvB or SPI2 (ssaV or ssaJ) mutant strain retained normal F-actin filaments, despite similar numbers of intracellular bacteria. We conclude that SpvB and a functional SPI2 TTSS are essential for Salmonella-induced delayed cytotoxicity of human macrophages.

Characterization of the spv locus in Salmonella enterica serovar Arizona

Salmonella enterica serovar Arizona (S. enterica subspecies IIIa) is a common Salmonella isolate from reptiles and can cause serious systemic disease in humans. The spv virulence locus, found on large plasmids in Salmonella subspecies I serovars associated with severe infections, was confirmed to be located on the chromosome of serovar Arizona. Sequence analysis revealed that the serovar Arizona spv locus contains homologues of spvRABC but lacks the spvD gene and contains a frameshift in spvA, resulting in a different C terminus. The SpvR protein functions as a transcriptional activator for the spvA promoter, and SpvB and SpvC are highly conserved. The analysis supports the proposal that the chromosomal spv sequence more closely corresponds to the ancestral locus acquired during evolution of S. enterica, with plasmid acquisition of spv genes in the subspecies I strains involving addition of spvD and polymorphisms in spvA.

A role for the PhoP/Q regulon in inhibition of fusion between lysosomes and Salmonella-containing vacuoles in macrophages

After uptake by murine macrophages, Salmonella typhimurium is able to survive and replicate within specialized phagosomes called Salmonella-containing vacuoles (SCVs), which are segregated from the late endocytic pathway. The molecular basis of this process and the virulence factors required are not fully understood. In this study, we used confocal fluorescence microscopy to evaluate interactions between the endocytic pathway of the murine macrophage cell line RAW 264.7 and different S. typhimurium strains. The analysis was carried out using the fluid-phase marker Texas red-ovalbumin and antibodies against the lysosomal enzyme cathepsin D, the late endosomal lipid lysobisphosphatidic acid and the adaptor proteins AP-1 and AP-3. Less than 10% of wild-type SCVs were associated with these markers at 24 h after uptake by macrophages. A similar low level of association was observed for vacuoles containing mutant strains affected in the function of the Salmonella pathogenicity island (SPI)-2 type III secretion system or the virulence plasmid spv operon. However, at this time point, the proportion of vacuoles containing phoP-mutant bacteria that were associated with each of the markers ranged from 25% to 50%. These results show that the regulon controlled by the PhoP/Q two-component system makes a major contribution to trafficking of the SCV in macrophages. Segregation of SCVs from the endocytic pathway was also found to be dependent on bacterial proteins synthesized between 15 min and 4 h after uptake into macrophages. However, after this time, protein synthesis was not required to maintain the segregation of SCVs from late endosomes and lysosomes.

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.

The Salmonella spvB virulence gene encodes an enzyme that ADP-ribosylates actin and destabilizes the cytoskeleton of eukaryotic cells

ADP-ribosylating enzymes, such as cholera and diphtheria toxins, are key virulence factors for a variety of extracellular bacterial pathogens but have not been implicated previously during intracellular pathogenesis. Salmonella strains are capable of invading epithelial cells and localizing in macrophages during infection. The spvB virulence gene of Salmonella is required for human macrophage cytotoxicity in vitro and for enhancing intracellular bacterial proliferation during infection. Here, we present evidence that spvB encodes an ADP-ribosylating enzyme that uses actin as a substrate and depolymerizes actin filaments when expressed in CHO cells. Furthermore, site-directed mutagenesis demonstrates that the ADP-ribosylating activity of SpvB is essential for Salmonella virulence in mice. As spvB is expressed by Salmonella strains after invasion of epithelial cells or phagocytosis by macrophages, these results suggest that SpvB functions as an intracellular ADP-ribosylating toxin critical for the pathogenesis of Salmonella infections.

Actin is ADP-ribosylated by the Salmonella enterica virulence-associated protein SpvB

The Salmonella enterica virulence-associated protein SpvB was recently shown to contain a carboxy-terminal mono(ADP-ribosyl)transferase domain. We demonstrate here that the catalytic domain of SpvB as well bacterial extracts containing full-length SpvB modifies a 43 kDa protein from macrophage-like J774-A.1 and epithelial MDCK cells as shown by label transfer from [32P]-nicotinamide adenine dinucleotide (NAD) to the 43 kDa protein. When analysed by two-dimensional gel electrophoresis, the same protein was modified in cells infected with S. enterica serovariant Dublin strain SH9325, whereas infection with an isogenic spvB mutant strain did not result in modification. Immunoprecipitation and immunoblotting experiments using SH9325-infected cells identified the modified protein as actin. The isolated catalytic domain of SpvB mediated transfer of 32P from [32P]-NAD to actins from various sources in vitro, whereas isolated eukaryotic control proteins or bacterial proteins were not modified. In an in vitro actin polymerization assay, the isolated catalytic SpvB domain prevented the conversion of G actin into F actin. Microscopic examination of MDCK cells infected with SH9325 revealed morphological changes and loss of filamentous actin content, whereas cells infected with the spvB mutant remained virtually unaffected. We conclude that actin is a target for an SpvB-mediated modification, most probably ADP-ribosylation, and that the modification of G actin interferes with actin polymerization.

The best defense is a good offense--Salmonella deploys an ADP-ribosylating toxin

The dramatic clinical manifestations of toxigenic infections such as cholera and diphtheria occur without substantial bacterial invasion. Disease is mediated by the secretion of potent toxins that use ADP-ribosylation as the catalytic mechanism underlying their action. ADP-ribosylating toxins comprise a large family, including the cholera, diphtheria, pertussis and Escherichia coli heat-labile (LT) toxins, and all produce disease by altering key metabolic processes after transfer of an ADP-ribose moiety from NAD to specific host-cell target proteins. A new paradigm implicating ADP-ribosylation during intracellular pathogenesis is beginning to emerge from recent research in Salmonella.

Plasmid-located pathogenicity determinants of Serratia entomophila, the causal agent of amber disease of grass grub, show similarity to the insecticidal toxins of Photorhabdus luminescens

Serratia entomophila and Serratia proteamaculans cause amber disease in the grass grub Costelytra zealandica (Coleoptera: Scarabaeidae), an important pasture pest in New Zealand. Larval disease symptoms include cessation of feeding, clearance of the gut, amber coloration, and eventual death. A 115-kb plasmid, pADAP, identified in S. entomophila is required for disease causation and, when introduced into Escherichia coli, enables that organism to cause amber disease. A 23-kb fragment of pADAP that conferred disease-causing ability on E. coli and a pADAP-cured strain of S. entomophila was isolated. Using insertion mutagenesis, the pathogenicity determinants were mapped to a 17-kb region of the clone. Sequence analysis of the 17-kb region showed that the predicted products of three of the open reading frames (sepA, sepB, and sepC) showed significant sequence similarity to components of the insecticidal toxin produced by the bacterium Photorhabdus luminescens. Transposon insertions in sepA, sepB, or sepC completely abolished both gut clearance and cessation of feeding on the 23-kb clone; when recombined back into pADAP, they abolished gut clearance but not cessation of feeding. These results suggest that SepA, SepB, and SepC together are sufficient for amber disease causation by S. entomophila and that another locus also able to exert a cessation-of-feeding effect is encoded elsewhere on pADAP.

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.

The Salmonella virulence plasmid spv genes are required for cytopathology in human monocyte-derived macrophages

The pathogenesis of serious systemic Salmonella infections is characterized by survival and proliferation of bacteria inside macrophages. Infection of human monocyte-derived macrophages in vitro with S. typhimurium or S. dublin produces cytopathology characterized by detachment of cells that contain large numbers of proliferating bacteria. This cytopathology is dependent on the expression of the bacterial spv genes, a virulence locus previously shown to markedly enhance the ability of Salmonella to produce systemic disease. After 24 h of infection, macrophage cultures contain two populations of bacteria: (i) proliferating organisms present in a detached cell fraction; and (ii) a static bacterial population in macrophages remaining attached to the culture well. Mutations in either the essential transcriptional activator SpvR or the key SpvB protein markedly reduce the cytopathic effect of Salmonella infection. The spv-dependent cytopathology in macrophages exhibits characteristics of apoptosis, with release of nucleosomes into the cytoplasm, nuclear condensation and DNA fragmentation. The current findings suggest that the mechanism of the spv effect is through induction of increased cytopathology in host macrophages.

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.

Salmonella virulence plasmid. Modular acquisition of the spv virulence region by an F-plasmid in Salmonella enterica subspecies I and insertion into the chromosome of subspecies II, IIIa, IV and VII isolates

The spv operon is common to all Salmonella virulence plasmids. DNA hybridization analysis indicates that the spv region is limited in distribution to serovars of Salmonella enterica subspecies I, II, IIIa, IV, and VII and is absent from Salmonella bongori isolates. Among strains of subspecies II, IIIa, and VII, all isolates examined contained sequences that hybridized with the spv region. However, among isolates of subspecies I, DNA sequences capable of hybridizing with the spv region were found in some isolates of certain serovars. Furthermore, in isolates of subspecies I, the virulence plasmid was found in the same set of isolates as an F-related plasmid, as determined by the presence of the spv region of the virulence plasmid and the finO, traD, and repA sequences of the F-plasmid. The concordance of the virulence plasmid and all three F-plasmid sequences in subspecies I serovar Choleraesuis, Paratyphi, and Typhimurium is most easily explained if the spv region is carried in an F-related plasmid in these isolates. In contrast, among S. enterica subspecies II, IIIa, IV, and VII, the isolates that contain spv sequences did not hybridize with an F-related plasmid or any other identifiable plasmid. With the use of pulse-field gel electrophoresis, the spv region in subspecies II, IIIa, and VII was found to be encoded on the chromosome. Analysis of the phylogenetic distribution of spv among Salmonella isolates and comparative nucleotide sequence analysis of spvA and spvC suggests that the spv region was acquired very recently, after speciation of the salmonellae.

Importance of the serovar-specific plasmid for virulence of salmonella strains in calves

To evaluate the influence of serovar-specific plasmids on salmonella virulence in calves, experiments were performed involving infection, by the oral route, with mixtures of strains containing equal counts of a plasmid-carrying and a plasmid-free strain of the same serovar. The concentration ratio between the plasmid-carrying and the plasmid-free strain which had developed in the organs of the infected animals was used for a comparative evaluation of virulence and pathogenetic behaviour of the strains. While in the S. typhimurium strains studied, the presence of the plasmid was accompanied by a significantly increased colonization and multiplication of the agent in the host's body, examination of S. enteritidis and S. dublin revealed that the plasmid-free strains exhibited identical or even significantly higher bacterial counts than the plasmid-carrying strains in organs. The fact that plasmid-free salmonella strains with a high virulence for calves have been found demonstrates that the presence of a serovar-specific plasmid is not an indispensable requirement for the development of salmonellosis in calves.

Expression profile and subcellular location of the plasmid-encoded virulence (Spv) proteins in wild-type Salmonella dublin

The plasmid-encoded virulence genes (spvABCD) in nontyphoid Salmonella strains mediate lethal infections in a variety of animals. Previous studies have shown that these genes are transcriptionally regulated by stationary-phase growth. We studied the expression profile and the subcellular locations of the SpvABCD proteins in wild-type S. dublin by using polyclonal antibodies against SpvA, SpvB, SpvC, and SpvD. The cellular levels of the individual proteins were determined during growth by quantitative immunoblotting. As expected, SpvA, SpvB, SpvC, and SpvD were not detectable before the late logarithmic growth phase and appeared in the sequence SpvA, SpvB, SpvC, and SpvD. In contrast to the transcriptional regulation, however, SpvA and SpvB reached their maximal expression shortly after induction and declined during further growth whereas SpvC and SpvD expression remained high throughout the stationary phase, indicating that the Spv proteins are individually regulated at a posttranscriptional level. To localize SpvABCD within the bacteria, the cells were fractionated into the periplasmic, cytoplasmic, inner membrane, and outer membrane components. The cell fractions and the culture supernatant were analyzed by immunoblotting. SpvA was present in the outer membrane, SpvB was present in the cytoplasm and the inner membrane, and SpvC was present in the cytoplasm. SpvD was secreted into the supernatant; however, a substantial portion of this protein was also detected in the cytoplasm and membranes. The molecular weights of SpvD in the supernatant and in the cytoplasm appeared to be equal, suggesting that SpvD is not cleaved upon secretion.

The spv genes on the Salmonella dublin virulence plasmid are required for severe enteritis and systemic infection in the natural host

The pathogenic role of the spv (Salmonella plasmid virulence) genes of Salmonella dublin was determined in the natural, bovine host. Since the lack of overt signs of enteritis or enterocolitis due to Salmonella infections in mice has limited the development of a convenient experimental system to study enteric disease, we used calves to study the contribution of the spv genes to S. dublin-induced salmonellosis. Since the SpvR transcriptional regulator is required for expression of the spvABCD operon, we constructed an spvR knockout mutation in a calf-virulent strain of S. dublin. Calves were infected with the wild-type strain, an spvR mutant, and an spvR mutant containing a complementing plasmid. Calves that were infected with the wild type or the complemented spvR mutant rapidly developed severe diarrhea and became moribund. Calves that were infected with the spvR mutant showed little or no clinical signs of systemic salmonellosis and developed only mild diarrhea. The survival and growth of the wild-type strain and the spvR mutant were determined by using blood-derived bovine monocytes. Wild-type S. dublin survived and grew inside cells, while the spvR mutant did not proliferate. These results suggest that the spv genes of S. dublin promote enhanced intracellular proliferation in intestinal tissues and at extraintestinal sites in the natural host.

Central regulatory role for the RpoS sigma factor in expression of Salmonella dublin plasmid virulence genes

The plasmid virulence genes spvABCD of Salmonella spp. are regulated by SpvR and the stationary-phase sigma factor RpoS. The transcription of spv genes is induced during the post-exponential phase of bacterial growth in vitro. We sought to investigate the relationship between growth phase and RpoS in spv regulation. rpoS insertion mutations were constructed in S. dublin Lane and plasmid-cured LD842 strains, and the mutants were found to be attenuated for virulence and deficient in spv gene expression. We utilized the plasmid pBAD::rpoS to express rpoS independent of the growth phase under the control of the arabinose-inducible araBAD promoter. SpvA expression was induced within 2 h after the addition of 0.1% arabinose, even when bacteria were actively growing. This suggested that the level of RpoS, instead of the growth phase itself, controls induction of the spv genes. However, RpoS did not activate transcription of spvA in the absence of SpvR protein. Using a constitutive tet promoter to express spvR, we found that the spvA gene can be partially expressed in the rpoS mutant, suggesting that RpoS is required for SpvR synthesis. We confirmed that spvR is poorly expressed in the absence of RpoS. With an intact rpoS gene, spvR expression is not dependent on an intact spvR gene but is enhanced by spvR supplied in trans. We propose a model for Salmonella spv gene regulation in which both RpoS and SpvR are required for maximal expression at the spvR and spvA promoters.

Current perspectives in salmonellosis

Salmonellosis remains an important human and animal problem worldwide and, despite extensive research effort, many of the details of its pathogenesis are not known. While there have been recent advances in some aspects of pathogenesis, other areas are not understood. The host adaptation shown by several serotypes and the recent dramatic changes in the predominance of particular serotypes are examples. Molecular techniques using in vitro model systems have identified several genes involved in adhesion and invasion, though their function and even their relevance to disease remain poorly defined. Similarly, several potential toxins have been identified and the genes cloned, although their significance is far from clear. Some of the essential genes on the large virulence plasmids have been defined, and these are known to be necessary for the establishment of systemic infection. Two of these genes are regulatory, but the function of the other genes is unknown. A general theme has been the identification of gene systems involved in regulation of virulence. New vaccines, based on 'rational attenuation' are being designed, and these have also been used to carry heterologous antigens; such vaccines are currently undergoing trials. The improved understanding of the pathogenesis of salmonellosis may also provide a model of wide applicability to a more general understanding of bacterial pathogenesis. New techniques, including the polymerase chain reaction, are being applied to diagnose salmonellosis.

Transcriptional regulation and promoter sequence of the spvR gene of virulence plasmid pKDSC50 in Salmonella choleraesuis serovar Choleraesuis

The transcript of the spvR gene on the virulence plasmid, pKDSC50, of Salmonella choleraesuis serovar Choleraesuis was detected for the first time by Northern blot analysis, and the transcriptional regulation of the spvR gene was investigated. The transcription of the spvR was negatively regulated by spvA and spvB, and enhanced at stationary phase under control of a sigma factor RpoS (sigma 38). The spvR transcript was 2.4 kilonucleotides in Salmonella cells, and deduced to encode SpvR and SpvA, suggesting that SpvA but not SpvB is the functioning repressor in spv operon. The promoter sequence analysis revealed that spvR was transcribed from a single promoter and the 5' end of the transcript was located at 18 bp upstream from the start codon of spvR. Sequential similarity between the promoter of spvR and other sigma 38-controlled genes was not found, but the consensus sequence was found in -10 to -35 region of spvR and spvA, which may correlate to our previous data indicating that both genes were positively regulated by the SpvR protein.

Plasmid-mediated virulence genes in non-typhoid Salmonella serovars

Specific non-typhoid Salmonella serovars carry large virulence plasmids that promote sustained extra-intestinal infections. These plasmids all share a highly conserved 8-kb region containing the spv operon, consisting of the regulatory spvR locus and the four structural spvABCD genes. The SpvR protein belongs to the LysR/MetR family of transcriptional activators, and induces spvABCD expression in the stationary phase in response to nutrient limitation. spv expression also depends on the chromosomal stationary phase sigma factor RpoS (KatF), and is markedly induced when salmonellae enter eukaryotic cells. Additional plasmid genes encode complement resistance including the rck locus which is homologous to ail from Yersinia. Rck blocks formation of the complement membrane attack complex on the bacterial surface. Several loci involved in plasmid replication and stable maintenance have also been identified.

Regulation of spvR gene expression of Salmonella virulence plasmid pKDSC50 in Salmonella choleraesuis serovar Choleraesuis

The expression regulation of spvR, a regulatory gene on the virulence plasmid (pKDSC50) of Salmonella choleraesuis serovar Choleraesuis, was investigated by spvR-lacZ translational fusion. The spvR gene was found to be positively regulated by its own product, the SpvR protein, and this unusual positive autoregulation was repressed by the products of spvA and spvB, virulence-associated genes present downstream from the spvR gene. Amino acid sequence analysis revealed that the amino-terminal region of SpvB had homology with the CatM repressor protein of Acinetobacter calcoaceticus, which belongs to the MetR/LysR protein family. On the other hand, the sigma factor RpoS was required for expression of the spvR gene in the stationary phase of bacterial growth. The SpvR protein was also necessary for self-activation, suggesting that an RNA polymerase holoenzyme containing RpoS requires SpvR protein in order to recognize the spvR promoter.

Regulation of spvR, the positive regulatory gene of Salmonella plasmid virulence genes

The regulation of the spvR promoter from the Salmonella dublin virulence plasmid was monitored using promoter-reporter gene fusion constructs. Activity was dependent upon the presence of the spv region and was affected by the number of copies of the spv region present within the cell. Activity remained constant throughout exponential growth, and increased rapidly with the onset of stationary phase, under both aerobic and anaerobic conditions. Additionally, the level of spvR expression was controlled by the availability of iron, activity being greatest under low iron conditions in stationary phase. The spvA gene product negatively regulated spvR expression in a dose-dependent manner, indicating that SpvA provides a negative feedback mechanism for this operon.

Expression of the Salmonella virulence plasmid gene spvB in cultured macrophages and nonphagocytic cells

Certain serotypes of salmonellae carry virulence plasmids that greatly enhance the pathogenicity of these bacteria in experimentally infected mice. This phenotype is largely attributable to the 8-kb spv regulon. However, spv genes are not expressed while bacteria grow in vitro. We now show that spvB, which is required for virulence, is expressed rapidly after Salmonella dublin is ingested by cultured J774 and murine peritoneal macrophages and that expression is not affected by the alkalinization of intracellular vesicles. The level of induction of spvB is reduced when macrophages are pretreated with gamma interferon. spvB is also expressed in human and canine epithelial cell lines and a human hepatoma cell line. In all cases, spvB expression is dependent on the spvR gene, just as it is in stationary-phase cultures in vitro. These data suggest that spv virulence genes are expressed by intracellular salmonellae in vivo in response to a signal that is common to the intracellular compartments of cells that are invaded by salmonellae.

Molecular analysis of spv virulence genes of the Salmonella virulence plasmids

Genes on an 8 kb region common to the virulence plasmids of several serovars of Salmonella are sufficient to replace the entire plasmid in enabling systemic infection in animal models. This virulence region encompasses five genes which previously have been designated with different names from each investigating laboratory. A common nomenclature has been devised for the five genes, i.e. spv for salmonella plasmid virulence. The first gene, spvR, encodes a positive activator for the following four genes, spvABCD. DNA sequence analysis of the spv genes from Salmonella typhimurium, Salmonella dublin, and Salmonella choleraesuis demonstrated extremely high conservation of the DNA and amino acid sequences. The spv genes are induced at stationary phase and in carbon-poor media, and optimal expression is dependent on the katF locus. The virulence functions of the spv genes are not known, but these genes may increase the growth rate of salmonellae in host cells and affect the interaction of salmonellae with the host immune system.

Stress induction of the virulence proteins (SpvA, -B, and -C) from native plasmid pSDL2 of Salmonella dublin

The virulence region of the wild-type plasmid pSDL2 contained in Salmonella dublin is highly conserved among plasmids from several nontyphoid Salmonella serotypes and is essential for the development of systemic infection in BALB/c mice. Polyclonal antibodies against three proteins (SpvA, -B, and -C) expressed from a 4.1-kb EcoRI subclone of the plasmid virulence region were generated. These antibodies were used to detect expression of the Spv proteins when S. dublin was grown in vitro under stress-inducing conditions, such as nutrient deprivation and increased temperature, that the bacteria may encounter during the course of infection within the host. Glucose starvation resulted in expression of all three proteins shortly after the lag phase. When the bacteria were grown to the late-log phase without glucose, heat shock strongly induced expression of SpvA but not SpvB or SpvC. The addition of 0.2% glucose to the medium resulted in loss of expression of the proteins until the late-log to stationary phase. Iron limitation or lowered pH induced expression of the proteins during exponential growth even in the presence of glucose. Insertion mutations into the positive regulator gene spvR upstream from spvABC and insertions into spvA and spvC resulted in loss of expression of SpvA, -B, and -C, suggesting a complex regulation of expression. These studies define a variety of environmental conditions that induce expression of the Spv virulence proteins from the wild-type plasmid pSDL2 in S. dublin in vitro.