Contribution of Salmonella typhimurium virulence factors to diarrheal disease in calves

The Salmonella enterica serotype typhimurium effector proteins SipA, SopA, SopB, SopD, and SopE2 act in concert to induce diarrhea in calves

Salmonella enterica serotype Typhimurium requires a functional type III secretion system encoded by Salmonella pathogenicity island 1 (SPI1) to cause diarrhea. We investigated the role of genes encoding secreted target proteins of the SPI1-associated type III secretion system for enteropathogenicity in calves. Salmonella serotype Typhimurium strains having mutations in sptP, avrA, sspH1, or slrP induced fluid secretion in the bovine ligated ileal loop model at levels similar to that of the wild type. In contrast, mutations in sipA, sopA, sopB, sopD, or sopE2 significantly reduced fluid accumulation in bovine ligated ileal loops at 8 h postinfection. A strain carrying mutations in sipA, sopA, sopB, sopD, and sopE2 (sipA sopABDE2 mutant) caused the same level of fluid accumulation in bovine ligated ileal loops as a strain carrying a mutation in sipB, a SPI1 gene required for the translocation of effector proteins into host cells. A positive correlation was observed between the severity of histopathological lesions detected in the ileal mucosa and the levels of fluid accumulation induced by the different mutants. After oral infection of calves, the Salmonella serotype Typhimurium sipAsopABDE2 mutant caused only mild diarrhea and was more strongly attenuated than strains having only single mutations. These data demonstrate that SipA, SopA, SopB, SopD, and SopE2 are major virulence factors responsible for diarrhea during Salmonella serotype Typhimurium infection of calves.

Characterization of Salmonella enterica derivatives harboring defined aroC and Salmonella pathogenicity island 2 type III secretion system (ssaV) mutations by immunization of healthy volunteers

The attenuation and immunogenicity of two novel Salmonella vaccine strains, Salmonella enterica serovar Typhi (Ty2 Delta aroC Delta ssaV, designated ZH9) and S. enterica serovar Typhimurium (TML Delta aroC Delta ssaV, designated WT05), were evaluated after their oral administration to volunteers as single escalating doses of 10(7), 10(8), or 10(9) CFU. ZH9 was well tolerated, not detected in blood, nor persistently excreted in stool. Six of nine volunteers elicited anti-serovar Typhi lipopolysaccharide (LPS) immunoglobulin A (IgA) antibody-secreting cell (ASC) responses, with three of three vaccinees receiving 10(8) and two of three receiving 10(9) CFU which elicited high-titer LPS-specific serum IgG. WT05 was also well tolerated with no diarrhea, although the administration of 10(8) and 10(9) CFU resulted in shedding in stools for up to 23 days. Only volunteers immunized with 10(9) CFU of WT05 mounted detectable serovar Typhimurium LPS-specific ASC responses and serum antibody responses were variable. These data indicate that mutations in type III secretion systems may provide a route to the development of live vaccines in humans and highlight significant differences in the potential use of serovars Typhimurium and Typhi.

Involvement of SipA in modulating actin dynamics during Salmonella invasion into cultured epithelial cells

Salmonella entry into epithelial host cells results from the host actin cytoskeleton reorganization that is induced by a group of bacterial proteins delivered to the host cells by the Salmonella type III secretion system. SopE, SopE2 and SopB activate CDC42 and Rac1 to intercept the signal transduction pathways involved in actin cytoskeleton rearrangements. SipA and SipC directly bind actin to modulate the actin dynamics facilitating bacterial entry. Biochemical studies have indicated that SipA decreases the critical concentration for actin polymerization and may be involved in promoting the initial actin polymerization in Salmonella-induced actin reorganization. In this report, we conducted experiments to analyze the in vivo function(s) of SipA during Salmonella invasion. SipA was found to be preferentially associated with peripheral cortical actin filaments but not stress fibres using permeabilized epithelial cells. When polarized Caco-2 cells were infected with Salmonella, actin cytoskeleton rearrangements induced by the wild-type strain had many filopodia structures that were intimately associated with the bacteria. In contrast, ruffles induced by the sipA null mutant were smoother and distant from the bacteria. We also found that the F-actin content in cells infected with the sipA mutant decreased nearly 80% as compared to uninfected cells or those infected with the wild-type Salmonella strain. Furthermore, expression of either the full-length or the SipA(459-684) actin-binding fragment induced prominent punctuate actin assembly in the cortical region of COS-1 cells. These results indicate that SipA is involved in modulating actin dynamics in cultured epithelial cells during Salmonella invasion.

Analyses of the evolutionary distribution of Salmonella translocated effectors

The type III secretion system encoded by Salmonella pathogenicity island 2 (SPI2) translocates Salmonella translocated effectors (STE) into host cells. STE are encoded by genes outside of SPI2. The distribution of STE loci within the salmonellae was investigated. In contrast to the SPI2 locus that is conserved within Salmonella enterica, STE loci show a variable distribution. In addition to other virulence determinants, the possession of various sets of STE loci may contribute to the different host ranges and pathogenic potentials of S. enterica serovars.

Morphologic and molecular characterization of Salmonella typhimurium infection in neonatal calves

The host response to Salmonella plays a major role in the outcome of infection. The present study was undertaken to further characterize Salmonella typhimurium infection in neonatal calves at both the morphologic and the molecular level using the ligated ileal loop model. Eight 4-5-week-old male Holstein calves underwent laparotomy, and loops were prepared in the ileum. The loops were either inoculated with an S. typhimurium strain pathogenic for cattle or injected with sterile LB broth as control. Samples for histology, transmission and scanning electron microscopy, and RNA extraction were collected at various time points between 5 minutes and 12 hours postinfection. Invasion of both M cells and enterocytes began at 15 minutes postinfection. No specific cell type was the main target for invasion. Intracellular bacteria were observed in the lamina propria after 1 hour postinfection. A severe acute neutrophilic response was associated with invasion of the Peyer's patches. Upregulated expression of CXC chemokines (interleukin [IL]-8, growth-related oncogenes, [GRO] alpha and gamma, and granulocyte chemotactic protein [GCP]2) was detected by reverse transcription polymerase chain reaction beginning at 1 hour postinfection. Expression of proinflammatory (IL-1beta, IL-18, and tumor necrosis factor [TNF]alpha) and anti-inflammatory (IL-10, IL-IRa, and IL-4) cytokines was also assessed. A marked increase in expression of IL-1beta was observed, whereas the profile of expression of IL-18 and TNFalpha did not change after infection. Upregulation of IL-1Ra and IL-4 but not of IL-10 was observed. These findings indicate that infection of bovine ligated ileal loops with S. typhimurium results in an acute neutrophilic inflammatory response that is associated with the upregulation of CXC chemokines (IL-8, GROalpha and gamma, and GCP2), IL-1beta, IL-IRa, and IL-4.

Salmonella interactions with host cells: type III secretion at work

The bacterial pathogen Salmonella enterica has evolved a very sophisticated functional interface with its vertebrate hosts. At the center of this interface is a specialized organelle, the type III secretion system, that directs the translocation of bacterial proteins into the host cell. Salmonella spp. encode two such systems that deliver a remarkable array of bacterial proteins capable of modulating a variety of cellular functions, including actin cytoskeleton dynamics, nuclear responses, and endocytic trafficking. Many of these bacterial proteins operate by faithful mimicry of host proteins, in some cases representing the result of extensive molecular tinkering and convergent evolution. The coordinated action of these type III secreted proteins secures the replication and survival of the bacteria avoiding overt damage to the host. The study of this remarkable pathogen is not only illuminating general paradigms in microbial pathogenesis but is also providing valuable insight into host cell functions.

Salmonella entry into host cells: the work in concert of type III secreted effector proteins

Upon contact with intestinal epithelial cells, Salmonella enterica serovar spp. inject a set of bacterial proteins into host cells via the bacterial SPI-1 type III secretion system. SopE, SopE2 and SopB, activate CDC42 and Rac to initiate actin cytoskeleton rearrangements. SipA and SipC, two Salmonella actin-binding proteins, directly modulate host actin dynamics to facilitate bacterial uptake. SptP promotes the recovery of the actin cytoskeleton rearrangements by antagonizing CDC42 and Rac. Therefore, Salmonella-induced reversible actin cytoskeleton rearrangements are the result of two coordinated steps: (i) stimulation of host signal transduction to indirectly promote actin rearrangements and (ii) direct modulation of actin dynamics.

Salmonella-induced macrophage death: the role of caspase-1 in death and inflammation

Salmonella typhimurium invades host macrophages and can induce either an almost immediate cell death or establish an intracellular niche within the phagocytic vacuole. Rapid cell death depends on the Salmonella pathogenicity island SPI1 and the host protein caspase-1, a member of the pro-apoptotic caspase family of proteases. Caspase-1-dependent cell death leads to the activation of the potent pro-inflammatory cytokines interleukin (IL)-1beta and IL-18 to produce bioactive cytokines. Animal studies indicate that the activation of these cytokines is necessary for efficient colonization of the mouse gastrointestinal tract. Salmonella that reside in the phagocytic vacuole do not cause this early cell death and can trigger a macrophage death at a much later time point. This late-phase cell death is dependent on SPI2-encoded genes and ompR.

The Salmonella pathogenicity island-1 type III secretion system

Salmonella pathogenicity island 1 (SPI1) encodes a type III secretion system that is required for virulence during the intestinal phase of infection. The expression of SPI1 genes is controlled by many global regulatory pathways that affect the expression/activity of transcriptional regulators encoded on SPI1.

Animal models of Salmonella infections: enteritis versus typhoid fever

The most common disease syndromes caused by Salmonella serotypes in humans, typhoid fever and enteritis, can be modeled using Salmonella enterica serotype Typhimurium infections in mice and calves, respectively. This article reviews murine typhoid and bovine enteritis and discusses strengths, limitations and distinctive features of these animal models.

Three-dimensional tissue assemblies: novel models for the study of Salmonella enterica serovar Typhimurium pathogenesis

The lack of readily available experimental systems has limited knowledge pertaining to the development of Salmonella-induced gastroenteritis and diarrheal disease in humans. We used a novel low-shear stress cell culture system developed at the National Aeronautics and Space Administration in conjunction with cultivation of three-dimensional (3-D) aggregates of human intestinal tissue to study the infectivity of Salmonella enterica serovar Typhimurium for human intestinal epithelium. Immunohistochemical characterization and microscopic analysis of 3-D aggregates of the human intestinal epithelial cell line Int-407 revealed that the 3-D cells more accurately modeled human in vivo differentiated tissues than did conventional monolayer cultures of the same cells. Results from infectivity studies showed that Salmonella established infection of the 3-D cells in a much different manner than that observed for monolayers. Following the same time course of infection with Salmonella, 3-D Int-407 cells displayed minimal loss of structural integrity compared to that of Int-407 monolayers. Furthermore, Salmonella exhibited significantly lower abilities to adhere to, invade, and induce apoptosis of 3-D Int-407 cells than it did for infected Int-407 monolayers. Analysis of cytokine expression profiles of 3-D Int-407 cells and monolayers following infection with Salmonella revealed significant differences in expression of interleukin 1alpha (IL-1alpha), IL-1beta, IL-6, IL-1Ra, and tumor necrosis factor alpha mRNAs between the two cultures. In addition, uninfected 3-D Int-407 cells constitutively expressed higher levels of transforming growth factor beta1 mRNA and prostaglandin E2 than did uninfected Int-407 monolayers. By more accurately modeling many aspects of human in vivo tissues, the 3-D intestinal cell model generated in this study offers a novel approach for studying microbial infectivity from the perspective of the host-pathogen interaction.

Complete genome sequence of a multiple drug resistant Salmonella enterica serovar Typhi CT18

Salmonella enterica serovar Typhi (S. typhi) is the aetiological agent of typhoid fever, a serious invasive bacterial disease of humans with an annual global burden of approximately 16 million cases, leading to 600,000 fatalities. Many S. enterica serovars actively invade the mucosal surface of the intestine but are normally contained in healthy individuals by the local immune defence mechanisms. However, S. typhi has evolved the ability to spread to the deeper tissues of humans, including liver, spleen and bone marrow. Here we have sequenced the 4,809,037-base pair (bp) genome of a S. typhi (CT18) that is resistant to multiple drugs, revealing the presence of hundreds of insertions and deletions compared with the Escherichia coli genome, ranging in size from single genes to large islands. Notably, the genome sequence identifies over two hundred pseudogenes, several corresponding to genes that are known to contribute to virulence in Salmonella typhimurium. This genetic degradation may contribute to the human-restricted host range for S. typhi. CT18 harbours a 218,150-bp multiple-drug-resistance incH1 plasmid (pHCM1), and a 106,516-bp cryptic plasmid (pHCM2), which shows recent common ancestry with a virulence plasmid of Yersinia pestis.

New Zealand white rabbit as a nonsurgical experimental model for Salmonella enterica gastroenteritis

Rabbits orally challenged with Salmonella enterica developed a dose-dependent diarrheal disease comparable to human salmonellosis. Viable Salmonella organisms recovered from the intestine and deep tissues indicate local and systemic infections. Therefore, results show that the rabbit can be used as a model for diarrheal disease and sequelae associated with salmonellosis.

Salmonella enterica serovar Gallinarum requires the Salmonella pathogenicity island 2 type III secretion system but not the Salmonella pathogenicity island 1 type III secretion system for virulence in chickens

Salmonella enterica serovar Gallinarum is a host-specific serotype that causes the severe systemic disease fowl typhoid in domestic poultry and a narrow range of other avian species but rarely causes disease in mammalian hosts. Specificity of the disease is primarily at the level of the reticuloendothelial system, but few virulence factors have been described other than the requirement for an 85-kb virulence plasmid. In this work, by making functional mutations in the type III secretion systems (TTSS) encoded by Salmonella pathogenicity island 1 (SPI-1) and SPI-2, we investigated the role of these pathogenicity islands in interactions between Salmonella serovar Gallinarum and avian cells in vitro and the role of these pathogenicity islands in virulence in chickens. The SPI-1 mutant showed decreased invasiveness into avian cells in vitro but was unaffected in its ability to persist within chicken macrophages. In contrast the SPI-2 mutant was fully invasive in nonphagocytic cells but failed to persist in macrophages. In chicken infections the SPI-2 mutant was attenuated while the SPI-1 mutant showed full virulence. In oral infections the SPI-2 mutant was not observed in the spleen or liver, and following intravenous inoculation it was cleared rapidly from these sites. SPI-2 function is required by Salmonella serovar Gallinarum for virulence, primarily through promoting survival within macrophages allowing multiplication within the reticuloendothelial system, but this does not preclude the involvement of SPI-2 in uptake from the gut to the spleen and liver. SPI-1 appears to have little effect on virulence and survival of Salmonella serovar Gallinarum in the host.

Construction and characterization of genetically defined aro omp mutants of enterotoxigenic Escherichia coli and preliminary studies of safety and immunogenicity in humans

Enterotoxigenic Escherichia coli (ETEC) is a leading cause of diarrhea in travelers to countries where the disease is endemic and causes a major disease burden in the indigenous population, particularly children. We describe here the generation and preclinical characterization of candidate strains of ETEC which are intended to provide the basis of a live attenuated oral vaccine to prevent this disease. It has been shown previously that a spontaneously arising toxin-negative variant ETEC strain, E1392/75-2A, could confer 75% protection against challenge when administered to volunteers. Unfortunately this strain induced mild diarrhea in 15% of recipients. To eliminate the unacceptable reactogenicity of strain E1392/75-2A, it was further attenuated by introducing three different combinations of defined deletion mutations into the chromosome. A mouse intranasal model of immunization was developed and used to show that all of the strains were immunogenic. Immune responses against colonization factor antigens (CFAs) were particularly strong when the bacterial inocula were grown on "CFA agar," which induces strong expression of these antigens. Two of the strains were selected for a phase I dose escalation safety study with healthy adult volunteers. Freshly grown organisms were harvested from CFA agar plates and administered to volunteers as a suspension containing from 5 x 10(7) to 5 x 10(9) CFU. The vaccine was well tolerated at all doses and induced significant immune responses in all recipients at the highest dose of either strain. The results provide the basis for further clinical evaluation of these vaccine candidates.

Salmonella-induced cell death is not required for enteritis in calves

Salmonella enterica serovar Typhimurium causes cell death in bovine monocyte-derived and murine macrophages in vitro by a sipB-dependent mechanism. During this process, SipB binds and activates caspase-1, which in turn activates the proinflammatory cytokine interleukin-1beta through cleavage. We used bovine ileal ligated loops to address the role of serovar Typhimurium-induced cell death in induction of fluid accumulation and inflammation in this diarrhea model. Twelve perinatal calves had 6- to 9-cm loops prepared in the terminal ileum. They were divided into three groups: one group received an intralumen injection of Luria-Bertani broth as a control in 12 loops. The other two groups (four calves each) were inoculated with 0.75 x 10(9) CFU of either wild-type serovar Typhimurium (strain IR715) or a sopB mutant per loop in 12 loops. Hematoxylin and eosin-stained sections were scored for inflammation, and terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end labeling (TUNEL)-positive cells were detected in situ. Fluid accumulation began at 3 h postinfection (PI). Inflammation was detected in all infected loops at 1 h PI. The area of TUNEL-labeled cells in the wild-type infected loops was significantly higher than that of the controls at 12 h PI, when a severe inflammatory response and tissue damage had already developed. The sopB mutant induced the same amount of TUNEL-positive cells as the wild type, but it was attenuated for induction of fluid secretion and inflammation. Our results indicate that serovar Typhimurium-induced cell death is not required to trigger an early inflammatory response and fluid accumulation in the ileum.

Salmonella enterica serovar-host specificity does not correlate with the magnitude of intestinal invasion in sheep

The colonization of intestinal and systemic tissues by Salmonella enterica serovars with different host specificities was determined 7 days after inoculation of 1 to 2-month-old lambs. Following oral inoculation, S. enterica serovars Abortusovis, Dublin, and Gallinarum were recovered in comparable numbers from the intestinal mucosa, but serovar Gallinarum was recovered in lower numbers than the other serovars from systemic sites. The pattern of bacterial recovery from systemic sites following intravenous inoculation was similar. The magnitude of intestinal invasion was evaluated in ovine ligated ileal loops in vivo. Serovars Dublin and Gallinarum and the broad-host-range Salmonella serovar Typhimurium were recovered in comparable numbers from ileal mucosa 3 h after loop inoculation, whereas the recovery of serovar Abortusovis was approximately 10-fold lower. Microscopic analysis of intestinal mucosae infected with serovars Typhimurium and Dublin showed dramatic morphological changes and infiltration of inflammatory cells, whereas mucosae infected with serovars Abortusovis and Gallinarum were indistinguishable from uninfected mucosae. Together these data suggest that Salmonella serovar specificity in sheep correlates with bacterial persistence at systemic sites. Intestinal invasion and avoidance of the host's intestinal inflammatory response may contribute to but do not determine the specificity of serovar Abortosovis for sheep. Intestinal invasion by serovar Abortusovis was significantly reduced after mutation of invH but was not reduced following curing of the virulence plasmid, suggesting that the Salmonella pathogenicity island 1 influences but the virulence plasmid genes do not influence the ability of serovar Abortusovis to invade the intestinal mucosa in sheep.

Salmonella host cell invasion emerged by acquisition of a mosaic of separate genetic elements, including Salmonella pathogenicity island 1 (SPI1), SPI5, and sopE2

Salmonella spp. possess a conserved type III secretion system encoded within the pathogenicity island 1 (SPI1; centisome 63), which mediates translocation of effector proteins into the host cell cytosol to trigger responses such as bacterial internalization. Several translocated effector proteins are encoded in other regions of the Salmonella chromosome. It remains unclear how this complex chromosomal arrangement of genes for the type III apparatus and the effector proteins emerged and how the different effector proteins cooperate to mediate virulence. By Southern blotting, PCR, and phylogenetic analyses of highly diverse Salmonella spp., we show here that effector protein genes located in the core of SPI1 are present in all Salmonella lineages. Surprisingly, the same holds true for several effector protein genes located in distant regions of the Salmonella chromosome, namely, sopB (SPI5, centisome 20), sopD (centisome 64), and sopE2 (centisomes 40 to 42). Our data demonstrate that sopB, sopD, and sopE2, along with SPI1, were already present in the last common ancestor of all contemporary Salmonella spp. Analysis of Salmonella mutants revealed that host cell invasion is mediated by SopB, SopE2, and, in the case of Salmonella enterica serovar Typhimurium SL1344, by SopE: a sopB sopE sopE2-deficient triple mutant was incapable of inducing membrane ruffling and was >100-fold attenuated in host cell invasion. We conclude that host cell invasion emerged early during evolution by acquisition of a mosaic of genetic elements (SPI1 itself, SPI5 [sopB], and sopE2) and that the last common ancestor of all contemporary Salmonella spp. was probably already invasive.

Salmonella enterica serovar typhimurium induces cell death in bovine monocyte-derived macrophages by early sipB-dependent and delayed sipB-independent mechanisms

It was previously demonstrated that Salmonella enterica serovar Typhimurium induces cell death with features of apoptosis in murine macrophages. Mice infected with Salmonella serovar Typhimurium develop systemic disease without diarrhea, whereas the infection in cattle and in humans is localized and characterized by diarrhea. Considering these clinical disease expression differences between mice and cattle, we investigated whether serovar Typhimurium is cytotoxic for bovine macrophages. Macrophages infected with serovar Typhimurium grown in the logarithmic phase quickly underwent cell death. Macrophages infected with stationary-phase cultures or with a mutant lacking sipB underwent no immediate cell death but did develop delayed cytotoxicity, undergoing cell death between 12 and 18 h postinfection. Both pathways were temporarily blocked by the general caspase inhibitor Z-VAD-Fmk and by the caspase 1 inhibitor Z-YVAD-Fmk. Comparisons of macrophages from cattle naturally resistant or susceptible to intracellular pathogens indicated no differences between these two genetic backgrounds in terms of susceptibility to serovar Typhimurium-induced cell death. We conclude that Salmonella serovar Typhimurium induces cell death in bovine macrophages by two distinct mechanisms, early sipB-mediated and delayed sipB-independent mechanisms.

Diverse virulence traits underlying different clinical outcomes of Salmonella infection

Salmonella strains have evolved to infect a wide variety of reptiles, birds, and mammals resulting in many different syndromes ranging from colonization and chronic carriage to acute fatal disease. Adaptation to a large number of different evolutionary niches has undoubtedly driven the high degree of phenotypic and genotypic diversity in Salmonella strains. Differences in LPS and flagellar structure generate the antigenic variation that is reflected in the more than 2,000 known serotypes. Moreover, variations of LPS structure affect the virulence of the strain. The differential expression of various fimbriae by Salmonella is likely to be due to the wide variety of mucosal surfaces that are encountered by various strains, and the host immune response may select for a different expression pattern. As with these surface structures, a variety of other important virulence determinants show a variable distribution in Salmonella strains and also serve to delineate the divergence of the Salmonella lineage from E. coli. The acquisition of the SPI-1 region may have represented the defining genetic event in the separation of the Salmonella and E. coli lineages. The SPI-1 cell invasion function allowed Salmonella to establish a separate niche in epithelial cells. The mgtC locus on SPI-3 is also present in all lineages and facilitates the adaptation of the bacteria to the low Mg2+, low pH environment of the endosome that results from SPI-1-mediated invasion. Subsequent acquisition of SPI-2 allowed Salmonella to manipulate the sorting of the endosome or phagosome, altering the intracellular environment and facilitating bacterial growth within infected cells. The ability to disseminate from the bowel and establish extraintestinal niches is promoted by the spv locus. Since Salmonella proliferates within macrophages and must avoid phagocytosis by neutrophils to establish a systemic infection, the spv genes appear to promote the macrophage phase of the disease process. Here the polymorphism of the spv locus is clearly demonstrated, since the serovars that cause most cases of nontyphoid bacteremia contain the spv genes. The absence of the spv genes from S. typhi is particularly puzzling and is a strong indication that the pathogenesis of typhoid fever is fundamentally different from that of bacteremia due to nontyphoid Salmonella. There is currently no genetic explanation for the phenotype of host adaptation or for the finding that only a few serovars cause the majority of human infections. Based on recent findings that multiple individual virulence genes have a variable distribution in Salmonella, it is unlikely that a single locus will be found to be responsible for these complex biological traits. Instead, a complicated combination of genes are likely to contribute to the overall virulence phenotype.

SifA permits survival and replication of Salmonella typhimurium in murine macrophages

SifA was originally identified as a virulence factor required for formation of Salmonella-induced filaments (Sifs), elongated tubules rich in lysosomal glycoproteins that extend from the Salmonella-containing vacuole in infected epithelial cells. Here, we demonstrate that deletion mutants of ssaR, a component of the SPI-2 type III secretion system, do not form Sifs in HeLa epithelial cells. This suggests that SifA is a translocated effector of this system, acting within host cells to form Sifs. In support of this hypothesis, transfection of HeLa cells with a vector encoding SifA fused to the green fluorescent protein caused extensive vacuolation of LAMP-1-positive compartments. Filamentous tubules that closely resembled Sifs were also observed in transfected cells, demonstrating that SifA is sufficient to initiate alteration of host cell endosomal structures. deltasifA mutants were impaired in their ability to survive/replicate in RAW 264.7 murine macrophages, a phenotype similar to ssaR mutants. Our findings suggest that SifA is an effector of the SPI-2 type III secretion system and allows colonization of murine macrophages, the host niche exploited during systemic phases of disease in these animals. A family of SifA-related proteins and their importance to Salmonella pathogenesis is also discussed.

Variable assortment of prophages provides a transferable repertoire of pathogenic determinants in Salmonella

Gene transfer between separate lineages of a bacterial pathogen can promote recombinational divergence and the emergence of new pathogenic variants. Temperate bacteriophages, by virtue of their ability to carry foreign DNA, are potential key players in this process. Our previous work has shown that representative strains of Salmonella typhimurium (LT2, ATCC14028 and SL1344) are lysogenic for two temperate bacteriophages: Gifsy-1 and Gifsy-2. Several lines of evidence suggested that both elements carry genes that contribute to Salmonella virulence. One such gene, on the Gifsy-2 prophage, codes for the [Cu, Zn] superoxide dismutase SodCI. Other putative pathogenicity determinants were uncovered more recently. These include genes for known or presumptive type III-translocated proteins and a locus, duplicated on both prophages, showing sequence similarity to a gene involved in Salmonella enteropathogenesis (pipA). In addition to Gifsy-1 and Gifsy-2, each of the above strains was found to harbour a specific set of prophages also carrying putative pathogenicity determinants. A phage released from strain LT2 and identified as phage Fels-1 carries the nanH gene and a novel sodC gene, which was named sodCIII. Strain ATCC14028 releases a lambdoid phage, named Gifsy-3, which contains the phoP/phoQ-activated pagJ gene and the gene for the secreted leucine-rich repeat protein SspH1. Finally, a phage specifically released from strain SL1344 was identified as SopEPhi. Most phage-associated loci transferred efficiently between Salmonella strains of the same or different serovars. Overall, these results suggest that lysogenic conversion is a major mechanism driving the evolution of Salmonella bacteria.

Salmonella pathogenicity island 2 influences both systemic salmonellosis and Salmonella-induced enteritis in calves

We have used signature-tagged mutagenesis to identify mutants of the host-specific Salmonella enterica serotype Dublin which were avirulent in calves and/or BALB/c mice. A mutant with a transposon insertion in the sseD gene of Salmonella pathogenicity island 2 (SPI-2), which encodes a putative secreted effector protein, was identified. This mutant was recovered from the bovine host but not from the murine host following infection with a pool of serotype Dublin mutants. However, a pure inoculum of the sseD mutant was subsequently shown to be attenuated in calves following infection either by the intravenous route or by the oral route. The sseD mutant was fully invasive for bovine intestinal mucosa but was subsequently unable to proliferate to the same numbers as the parental strain in vivo. Both the sseD mutant and a second SPI-2 mutant, with a transposon insertion in the ssaT gene, induced significantly weaker secretory and inflammatory responses in bovine ligated ileal loops than did the parental strain. These results demonstrate that SPI-2 is required by serotype Dublin for the induction of both systemic and enteric salmonellosis in calves.

Susceptibility of calves to challenge with Salmonella typhimurium 4/74 and derivatives harbouring mutations in htrA or purE

Salmonella typhimurium 4/74 is highly virulent for cattle after oral challenge, causing severe diarrhoea, which is sometimes associated with systemic spread of the micro-organism. Although susceptible to oral challenge, groups of cattle were found to be relatively resistant to subcutaneous challenge with this strain. The virulence of S. typhimurium 4/74 harbouring mutations in htrA and purE was also assessed in cattle. Although S. typhimurium 4/74 htrA and purE are attenuated following oral challenge in mice, cattle were highly susceptible to oral challenge with these mutants. As with the parent S. typhimurium 4/74 strain, cattle exhibited greater susceptibility to oral compared to subcutaneous challenge with S. typhimurium htrA and purE mutants. Following subcutaneous challenge with sublethal levels of S. typhimurium 4/74, calves produced significant levels of antibodies to S. typhimurium soluble extract. No correlation was detected between interferon gamma levels in sera and susceptibility to infection by any route. The concentrations of the acute-phase-associated protein haptoglobin were increased in the sera of five of six cattle inoculated subcutaneously, although increases in concentration were smaller in cattle inoculated orally.

A secreted Salmonella protein induces a proinflammatory response in epithelial cells, which promotes neutrophil migration

In response to Salmonella typhimurium, the intestinal epithelium generates an intense inflammatory response consisting largely of polymorphonuclear leukocytes (neutrophils, PMN) migrating toward and ultimately across the epithelial monolayer into the intestinal lumen. It has been shown that bacterial-epithelial cell interactions elicit the production of inflammatory regulators that promote transepithelial PMN migration. Although S. typhimurium can enter intestinal epithelial cells, bacterial internalization is not required for the signaling mechanisms that induce PMN movement. Here, we sought to determine which S. typhimurium factors and intestinal epithelial signaling pathways elicit the production of PMN chemoattractants by enterocytes. Our results suggest that S. typhimurium activates a protein kinase C-dependent signal transduction pathway that orchestrates transepithelial PMN movement. We show that the type III effector protein, SipA, is not only necessary but is sufficient to induce this proinflammatory response in epithelial cells. Our results force us to reconsider the long-held view that Salmonella effector proteins must be directly delivered into host cells from bacterial cells.

Salmonella pathogenicity island 1-independent induction of apoptosis in infected macrophages by Salmonella enterica serotype typhimurium

The enteric pathogen Salmonella enterica serotype Typhimurium induces apoptosis in infected macrophages. This process is rapid, specific, and depends on the type III protein secretion system encoded within Salmonella pathogenicity island 1 (SPI1). Here, we demonstrate that serotype Typhimurium can activate programmed macrophage cell death independently of SPI1. SPI1 independent induction of apoptosis in infected macrophages is observed as early as 12 to 13 h postinfection, even in the absence of intracellular bacterial replication. Delayed activation of programmed macrophage cell death is not observed with serotype Typhimurium strains mutated in ompR or SPI2. Even though SPI2 mutants have a defect in intracellular proliferation, our results indicate that long-term intracellular survival and growth are not required for delayed macrophage killing per se, since Salmonella mutants that are severely defective in intracellular growth still induce delayed apoptosis. Inactivation of genes required for either rapid or delayed induction of apoptosis results in a conditional noncytotoxic phenotype, whereas simultaneous inactivation of genes required for both rapid and delayed induction of apoptosis renders serotype Typhimurium noncytotoxic under all conditions tested. Our hypothesis is that differential activation of programmed macrophage cell death by serotype Typhimurium occurs under discrete physiological conditions at distinct locations within an infected host.

Salmonella enterica serovar typhimurium-dependent regulation of inducible nitric oxide synthase expression in macrophages by invasins SipB, SipC, and SipD and effector SopE2

When Salmonella enterica invades mammalian cells, it activates signals leading to increased expression of inflammatory mediators. One such mediator is nitric oxide (NO), which is produced under control of the enzyme inducible NO synthase (iNOS). Induction of iNOS in response to Salmonella infection has been demonstrated, but the bacterial effector molecules that regulate expression of the enzyme have not been identified. In the study reported here, an analysis of Salmonella-dependent iNOS expression in macrophages was carried out. Wild-type Salmonella strains increased the levels of both iNOS protein and mRNA in murine macrophage cell lines in an invasion-independent fashion. Mutant strains lacking a functional pathogenicity island 1-encoded type III secretion system, as well as strains lacking the invasins SipB, SipC, and SipD, were impaired in iNOS induction. Complementation experiments indicated that all three of the invasins were required for induction of iNOS expression. These results suggested that an effector protein, translocated into macrophages via the type III secretion system in a SipB-, SipC-, and SipD-dependent manner, might be the ultimate mediator of iNOS induction. In keeping with this idea, a mutant strain deficient in SopE2, a recently described homolog of SopE, was found to be impaired in the induction of iNOS expression. These observations suggest that iNOS expression is regulated by signals activated by SopE2 (and possibly SopE) and that the role of SipB, SipC, and SipD in this process is to facilitate translocation of the relevant effector.

Aggregation of host endosomes by Salmonella requires SPI2 translocation of SseFG and involves SpvR and the fms-aroE intragenic region

Salmonella-induced aggregation of host endosomal compartments into tubules, termed lgp-tubules, requires sifA and ompR. Lgp-tubules result from Salmonella-directed alteration of the endocytic system and typify the unique intracellular locale where Salmonella replicate. A high-throughput method devised to screen 11 520 MudJ mutants for loss of lgp-tubule formation identified one auxotrophic and nine prototrophic mutants. Molecular characterization identified four new loci required to alter epithelial endocytic structure. Salmonella pathogenicity island 2 (SPI2) is the locus central to the phenotype. A subset of SPI2 effectors is essential: SpiC and SseFG are required, but not SseE. A subset of apparatus proteins is also implicated: SsaJ, L, M, V and P are required. SPI2 was implicated further, as SifA shows similarity with known SPI2 translocation targets, and OmpR regulates SPI2. Another locus lies within the smf-aroE intragenic region. Lgp-tubule formation also involves a locus on the virulence plasmid pSLT. The pSLT-encoded SpvR negatively regulates an unknown repressor of the phenotype located on pSLT. Finally, disruption of carB leads to multiple auxotrophy that prevents lgp-tubule formation. This study demonstrates that lgp-tubule formation is a virulence mechanism that underlies the selective disruption of host endocytic trafficking and is associated with the formation of a replication-permissive locale.

Salmonella exploits caspase-1 to colonize Peyer's patches in a murine typhoid model

Salmonella typhimurium invades host macrophages and induces apoptosis and the release of mature proinflammatory cytokines. SipB, a protein translocated by Salmonella into the cytoplasm of macrophages, is required for activation of Caspase-1 (Casp-1, an interleukin [IL]-1beta-converting enzyme), which is a member of a family of cysteine proteases that induce apoptosis in mammalian cells. Casp-1 is unique among caspases because it also directly cleaves the proinflammatory cytokines IL-1beta and IL-18 to produce bioactive cytokines. We show here that mice lacking Casp-1 (casp-1(-/)- mice) had an oral S. typhimurium 50% lethal dose (LD(50)) that was 1,000-fold higher than that of wild-type mice. Salmonella breached the M cell barrier of casp-1(-/)- mice efficiently; however, there was a decrease in the number of apoptotic cells, intracellular bacteria, and the recruitment of polymorphonuclear lymphocytes in the Peyer's patches (PP) as compared with wild-type mice. Furthermore, Salmonella did not disseminate systemically in the majority of casp-1(-/)- mice, as demonstrated by significantly less colonization in the PP, mesenteric lymph nodes, and spleens of casp-1(-/)- mice after an oral dose of S. typhimurium that was 100-fold higher than the LD(50). The increased resistance in casp-1(-/)- animals appears specific for Salmonella infection since these mice were susceptible to colonization by another enteric pathogen, Yersinia pseudotuberculosis, which normally invades the PP. These results show that Casp-1, which is both proapoptotic and proinflammatory, is essential for S. typhimurium to efficiently colonize the cecum and PP and subsequently cause systemic typhoid-like disease in mice.

A conserved amino acid sequence directing intracellular type III secretion by Salmonella typhimurium

Type III secretion systems (TTSS) are important virulence factors that Gram-negative bacteria use to translocate proteins into the cytoplasm of eukaryotic host cells. Salmonellae encode two virulence-associated TTSS. The Salmonella pathogenicity island 1 (SPI1)-encoded TTSS is active on contact with host cells, whereas the Salmonella pathogenicity island 2 (SPI2)-encoded TTSS is expressed after phagocytosis of bacteria by host cells. Previously, no consensus signal sequence for translocation has been identified among TTSS effector proteins. In this work, seven proteins, termed Salmonella-translocated effectors (STE), are described that contain conserved amino acid sequences that direct translocation by TTSS in Salmonella typhimurium. STE that are coordinately regulated with SPI2 gene expression contain translocation signals that are recognized by the SPI2 but not by the SPI1 TTSS. STE that are constitutively expressed contain signals that direct translocation through both SPI1 and SPI2 TTSS. Of the seven STE examined, SspH1 and SspH2 have been previously shown to be translocated and involved in virulence; SlrP and SifA were identified as virulence factors, but were not previously known to be associated with TTSS; and SseI, SseJ, and SifB were previously unidentified. Three STE genes (sspH1, sspH2, and sseI) are located within temperate bacteriophages, suggesting a common mechanism for the dissemination of more recently evolved STE.

Molecular basis of Salmonella-induced enteritis

Salmonella pathogenesis is a complex and multifactorial phenomenon. Many genes required for full virulence in mice have been identified, but only a few of these have been shown to be necessary for the induction of enteritis. Likewise, at least some of the Salmonella virulence factors affecting enteritis do not appear to be required for infection of systemic sites in mice. This suggests that subsets of virulence genes influence distinct aspects of Salmonella pathogenesis. Recently, considerable progress has been made in characterizing the virulence mechanisms influencing enteritis caused by non-typhoid Salmonella spp. The Salmonella pathogenicity island-1-encoded type III secretion system mediates the translocation of secreted effector proteins into target epithelial cells. These effector proteins are key virulence factors required for Salmonella intestinal invasion and the induction of fluid secretion and inflammatory responses.

Host adaptation and the emergence of infectious disease: the Salmonella paradigm

The recent emergence of food-borne pathogens, such as Salmonella enterica serotype Enteritidis (S. enteritidis) and Escherichia coli O157:H7, has generated increasing interest in how infectious diseases can invade, persist and spread within new host populations. To alter their host range pathogens require adaptations, which ensure their circulation in a new animal population. Adaptations for circulation in different populations of vertebrate hosts seem to have been acquired multiple times within the genus Salmonella because extant Salmonella serotypes differ greatly with regard to host range. In this article, mechanisms involved in host adaptation are deduced by considering the influence of the host immune response on circulation of Salmonella serotypes within populations of vertebrate animals. This approach contributes to the identification of genes involved in host adaptation and provides new insights into the emergence of food-borne pathogens.

SspA is required for lethal Salmonella enterica serovar Typhimurium infections in calves but is not essential for diarrhea

Salmonella pathogenicity island 1 (SPI-1) encodes virulence determinants, which are important for enteropathogenicity in calves. To determine whether the Salmonella enterica serovar Typhimurium SPI-1 effector proteins SspA and SptP are important for enteropathogenicity, strains lacking these proteins were tested during oral infection of calves. Calves infected with a sptP mutant or its isogenic parent developed diarrhea and lethal morbidity. In contrast, calves infected with an sspA mutant developed diarrhea, which resolved within 10 days but did not result in mortality. The sspA mutant was recovered from bovine intestinal tissues at numbers similar to those obtained for its isogenic parent and caused marked intestinal lesions. Thus, the severity of pathological changes caused by serovar Typhimurium strains or their ability to cause diarrhea were not predictive of their ability to cause lethal morbidity in calves. We conclude that factors other than or in addition to bacterial colonization, intestinal lesions, or electrolyte loss contribute to lethal morbidity in calves infected with serovar Typhimurium.

Identification of SopE2 from Salmonella typhimurium, a conserved guanine nucleotide exchange factor for Cdc42 of the host cell

Salmonella typhimurium translocates effector proteins into host cells via the SPI1 type III secretion system to induce responses such as membrane ruffling and internalization by non-phagocytic cells. Activation of the host cellular RhoGTPase Cdc42 is thought to be a key event during internalization. The translocated Salmonella protein SopE is an activator for Cdc42. Because SopE is absent from most S. typhimurium strains it remains unclear whether all S. typhimurium strains rely on activation of Cdc42 to invade host cells. We have identified SopE2, a translocated effector protein common to all S. typhimurium strains. SopE2 is a guanine nucleotide exchange factor for Cdc42 and shows 69% sequence similarity to SopE. Analysis of S. typhimurium mutants demonstrated that SopE2 plays a role in recruitment of the actin-nucleating Arp2/3 complex to the membrane ruffles and in efficient host cell invasion. Transfection experiments showed that SopE2 is sufficient to activate host cellular Cdc42, to recruit the actin-nucleating Arp2/3 complex and to induce actin cytoskeletal rearrangements and internalization. In conclusion, as a result of SopE2 all S. typhimurium strains tested have the capacity to activate Cdc42 signalling inside host cells which is important to ensure efficient entry.

Salmonella pathogenicity island 2

Systemic infections by Salmonella enterica, such as typhoid fever, are a significant threat to human health. Recent studies indicate that the function of a type III secretion system encoded by Salmonella Pathogenicity Island 2 (SPI2) is central for the ability of S. enterica to cause systemic infections and for intracellular pathogenesis. This review summarizes approaches leading to the identification of SPI2, the molecular genetics and evolution of SPI2, and the current understanding of the regulation of gene expression. Recent studies have indicated that SPI2 is used by intracellular Salmonella to actively modify functions of the host cells. The role of SPI2 during pathogenesis of salmonellosis and current models regarding function will be discussed.

The immune responses to bacterial antigens encountered in vivo at mucosal surfaces

Mammals have evolved a sophisticated immune system for handling antigens encountered at their mucosal surfaces. The way in which mucosally delivered antigens are handled influences our ability to design effective mucosal vaccines. Live attenuated derivatives of pathogens are one route towards the development of mucosal vaccines. However, some molecules, described as mucosal immunogens, are inherently immunogenic at mucosal surfaces. Studies on mucosal immunogens may facilitate the identification of common characteristics that contribute to mucosal immunogenicity and aid the development of novel, non-living mucosal vaccines and immunostimulators.

Identification and analysis of bacterial virulence genes in vivo

Signature-tagged mutagenesis is a mutation-based screening method for the identification of virulence genes of microbial pathogens. Genes isolated by this approach fall into three classes: those with known biochemical function, those of suspected function and some whose functions cannot be predicted from database searches. A variety of in vitro and in vivo methods are available to elucidate the function of genes of the second and third classes. We describe the use of some of these approaches to study the function of the Salmonella pathogenicity island 2 type III secretion system of Salmonella typhimurium. This virulence determinant is required for intracellular survival. Secretion by this system is induced by an acidic pH, and its function may be to alter trafficking of the Salmonella-containing vacuole. Use of a temperature-sensitive non-replicating plasmid and competitive index tests with other genes show that in vivo phenotypes do not always correspond to those predicted from in vitro studies.

Identification of a putative Salmonella enterica serotype typhimurium host range factor with homology to IpaH and YopM by signature-tagged mutagenesis

The genetic basis for the host adaptation of Salmonella serotypes is currently unknown. We have explored a new strategy to identify Salmonella enterica serotype Typhimurium (S. typhimurium) genes involved in host adaptation, by comparing the virulence of 260 randomly generated signature-tagged mutants during the oral infection of mice and calves. This screen identified four mutants, which were defective for colonization of only one of the two host species tested. One mutant, which only displayed a colonization defect during the infection of mice, was further characterized. During competitive infection experiments performed with the S. typhimurium wild type, the mutant was defective for colonization of murine Peyer's patches but colonized bovine Peyer's patches at the wild-type level. No difference in virulence between wild type and mutant was observed when calves were infected orally with 10(10) CFU/animal. In contrast, the mutant possessed a sixfold increase in 50% lethal morbidity dose when mice were infected orally. The transposon in this mutant was inserted in a 2.9-kb pathogenicity islet, which is located between uvrB and yphK on the S. typhimurium chromosome. This pathogenicity islet contained a single gene, termed slrP, with homology to ipaH of Shigella flexneri and yopM of Yersinia pestis. These data show that comparative screening of signature-tagged mutants in two animal species can be used for scanning the S. typhimurium genome for genes involved in host adaptation.

Salmonella typhimurium leucine-rich repeat proteins are targeted to the SPI1 and SPI2 type III secretion systems

Salmonellae encode two virulence-associated type III secretion systems (TTSS) within Salmonella pathogenicity islands 1 and 2 (SPI1 and SPI2). Two Salmonella typhimurium genes, sspH1 and sspH2, that encode proteins similar to the Shigella flexneri and Yersinia species TTSS substrates, IpaH and YopM, were identified. SspH1 and SspH2 are proteins containing leucine-rich repeats that are differentially targeted to the SPI1 and SPI2 TTSS. sspH2 transcription was induced within RAW264.7 macrophages, and was dependent upon the SPI2-encoded regulator ssrA/ssrB. In contrast, sspH1 transcription is independent of SPI2, and is not induced after bacterial phagocytosis by eukaryotic cells. Infection of eukaryotic cells with strains expressing a SspH2-CyaA fusion protein resulted in SPI2 TTSS-dependent cAMP increases. In contrast, SspH1-CyaA-mediated cAMP increases were both SPI1 and SPI2 TTSS dependent. sspH2-like sequences were found in most Salmonella serotypes examined, whereas sspH1 was detected in only one S. typhimurium isolate, indicating that the copy number of sspH genes can be variable within Salmonella serotypes. S. typhimurium deleted for both sspH1 and sspH2 was not able to cause a lethal infection in calves, indicating that these genes participate in S. typhimurium virulence for animals.