Role of SPI-1 in the interactions of Salmonella Typhimurium with porcine macrophages

Salmonella pathogenicity island-14 is a critical virulence factor responsible for systemic infection in chickens caused by Salmonella gallinarum

Salmonella enterica serovar Gallinarum (S. gallinarum) is an important host-specific pathogen that causes fowl typhoid, a severe systemic, septicemic, and fatal infection, in chickens. S. gallinarum causes high morbidity and mortality in chickens and poses a significant burden and economic losses to the poultry industry in many developing countries. However, the virulence factors and mechanisms of S. gallinarum-induced systemic infection in chickens remain poorly understood. In this study, we constructed a Salmonella pathogenicity island-14 (SPI-14) mutant strain (mSPI-14) of S. gallinarum and evaluated the pathogenicity of mSPI-14 in the chicken systemic infection model. The mSPI-14 exhibited the same level of bacterial growth and morphological characteristics but significantly reduced resistance to bile acids compared with the wild-type (WT) strain in vitro. The virulence of mSPI-14 was significantly attenuated in the chicken oral infection model in vivo. Chickens infected with WT showed typical clinical symptoms of fowl typhoid, with all birds succumbing to the infection within 6 to 9 days post-inoculation, and substantial increases in bacterial counts and significant pathological changes in the liver and spleen were observed. In contrast, all mSPI-14-infected chickens survived, the bacterial counts in the organs were significantly lower, and no significant pathological changes were observed in the liver and spleen. The expression of interleukin (IL)-1β, IL-12, CXCLi1, tumor necrosis factor (TNF)-α, and interferon (IFN)-γ in the liver of mSPI-14-infected chickens were significantly lower than those in the WT-infected chickens. These results indicate that SPI-14 is a crucial virulence factor in systemic infection of chickens, and avirulent mSPI-14 could be used to develop a new attenuated live vaccine to prevent S. gallinarum infection in chickens.

Phenotypic characterization of swine peripheral blood monocyte-derived macrophages and ex vivo infection with Salmonella enterica serovar Typhimurium

Macrophages are critical mediators of the inflammatory process, playing a relevant role in the pathogenesis of Salmonella Typhimurium. The protocols for isolation, culture, and differentiation of monocytes into macrophages and their interaction with Salmonella are well established in humans and murine models, but little information is available in swine. The aims of this study were to establish an efficient protocol for macrophage culture and to evaluate the interaction of the invA mutant strain and the wild type (WT) Salmonella Typhimurium with porcine macrophages. Peripheral blood monocyte-derived macrophages from pigs were obtained, separated by density-gradient centrifugation, and cultured in Teflon vials for 10 days. After the differentiation period, cultures consisted of 92.4% CD14⁺ cells. In addition, these cells showed phagocytic ability, demonstrated by the presence of the same amount of WT and invA mutant Salmonella Typhimurium 1 h after interaction with macrophages. The early cytotoxic effect was Salmonella pathogenicity island (SPI)-[1]dependent, in which log-phase WT strains were more efficient (p < 0.01) than the invA mutant strain at inducing the death of macrophages.

SipA Activation of Caspase-3 Is a Decisive Mediator of Host Cell Survival at Early Stages of Salmonella enterica Serovar Typhimurium Infection

Salmonella invasion protein A (SipA) is a dual-function effector protein that plays roles in both actin polymerization and caspase-3 activation in intestinal epithelial cells. To date its function in other cell types has remained largely unknown despite its expression in multiple cell types and its extracellular secretion during infection. Here we show that in macrophages SipA induces increased caspase-3 activation early in infection. This activation required a threshold level of SipA linked to multiplicity of infection and may be a limiting factor controlling bacterial numbers in infected macrophages. In polymorphonuclear leukocytes, SipA or other Salmonella pathogenicity island 1 effectors had no effect on induction of caspase-3 activation either alone or in the presence of whole bacteria. Tagging of SipA with the small fluorescent phiLOV tag, which can pass through the type three secretion system, allowed visualization and quantification of caspase-3 activation by SipA-phiLOV in macrophages. Additionally, SipA-phiLOV activation of caspase-3 could be tracked in the intestine through multiphoton laser scanning microscopy in an ex vivo intestinal model. This allowed visualization of areas where the intestinal epithelium had been compromised and demonstrated the potential use of this fluorescent tag for in vivo tracking of individual effectors.

Roles of Hcp family proteins in the pathogenesis of the porcine extraintestinal pathogenic Escherichia coli type VI secretion system

Hcp (hemolysin-coregulated protein) is considered a vital component of the functional T6SS (Type VI Secretion System), which is a newly discovered secretion system. Our laboratory has previously sequenced the whole genome of porcine extraintestinal pathogenic E. coli (ExPEC) strain PCN033, and identified an integrated T6SS encoding three different hcp family genes. In this study, we first identified a functional T6SS in porcine ExPEC strain PCN033, and demonstrated that the Hcp family proteins were involved in bacterial competition and the interactions with other cells. Interestingly, the three Hcp proteins had different functions. Hcp2 functioned predominantly in bacterial competition; all three proteins were involved in the colonization of mice; and Hcp1 and Hcp3 were predominantly contributed to bacterial-eukaryotic cell interactions. We showed an active T6SS in porcine ExPEC strain PCN033, and the Hcp family proteins had different functions in their interaction with other bacteria or host cells.

Host Stress Drives Salmonella Recrudescence

Host stress is well known to result in flare-ups of many bacterial, viral and parasitic infections. The mechanism by which host stress is exploited to increase pathogen loads, is poorly understood. Here we show that Salmonella enterica subspecies enterica serovar Typhimurium employs a dedicated mechanism, driven by the scsA gene, to respond to the host stress hormone cortisol. Through this mechanism, cortisol increases Salmonella proliferation inside macrophages, resulting in increased intestinal infection loads in DBA/2J mice. ScsA directs overall Salmonella virulence gene expression under conditions that mimic the intramacrophagic environment of Salmonella, and stimulates the host cytoskeletal alterations that are required for increased Salmonella proliferation inside cortisol exposed macrophages. We thus provide evidence that in a stressed host, the complex interplay between a pathogen and its host endocrine and innate immune system increases intestinal pathogen loads to facilitate pathogen dispersal.

HtpG contributes to Salmonella Typhimurium intestinal persistence in pigs

Salmonellaenterica subspecies enterica serovar Typhimurium (Salmonella Typhimurium) contamination of pork, is one of the major sources of human salmonellosis. The bacterium is able to persist and hide in asymptomatic carrier animals, generating a reservoir for Salmonella transmission to other animals and humans. Mechanisms involved in Salmonella persistence in pigs remain poorly understood. In the present study, we demonstrate that the SalmonellahtpG gene, encoding a homologue of the eukaryotic heat shock protein 90, contributes to Salmonella Typhimurium persistence in intestine-associated tissues of pigs, but not in the tonsils. HtpG does not seem to play an important role during the acute phase of infection. The contribution to persistence was shown to be associated with htpG-dependent Salmonella invasion and survival in porcine enterocytes and macrophages. These results reveal the role of HtpG as a virulence factor contributing to Salmonella persistence in pigs.

Induction of seroconversion and persistence of Salmonella Typhimurium in pigs are strain dependent

Foodborne salmonellosis is one of the most important bacterial zoonotic diseases worldwide. Salmonella Typhimurium is the serovar most frequently isolated from persistently infected slaughter pigs in Europe. Salmonella Typhimurium pathogenesis is host species specific. In addition, differences in in vitro behaviour of Salmonella Typhimurium strains have also been described, which may be reflected by a different course of infection within a host species. We compared the course of a Salmonella Typhimurium infection in pigs, using two Salmonella Typhimurium strains that were able to interfere with MHC II expression on porcine macrophages to a different extent in vitro. After experimental inoculation, blood and faecal samples from all pigs were collected at regular time points. At 40 days post inoculation (pi), animals were euthanized and tissue samples were bacteriologically analysed. The proportion of serologically positive piglets at 33 days pi was significantly higher in pigs that were inoculated with the strain that did not downregulate MHC II expression in vitro. Furthermore, this strain was less frequently shed and isolated in lower numbers from tonsils and ileocaecal lymph nodes than the strain that was able to markedly downregulate MHC II expression in vitro. We thus found that the delayed onset of seroconversion after oral inoculation of piglets with a particular Salmonella Typhimurium strain coincided with higher faecal shedding and increased persistence. Strain specific differences in Salmonella pathogenesis might thus have repercussions on the serological detection of Salmonella Typhimurium infections in pigs.

Salmonella Typhimurium induces SPI-1 and SPI-2 regulated and strain dependent downregulation of MHC II expression on porcine alveolar macrophages

Foodborne salmonellosis is one of the most important bacterial zoonotic diseases worldwide. Salmonella Typhimurium is the serovar most frequently isolated from persistently infected slaughter pigs in Europe. Circumvention of the host's immune system by Salmonella might contribute to persistent infection of pigs. In the present study, we found that Salmonella Typhimurium strain 112910a specifically downregulated MHC II, but not MHC I, expression on porcine alveolar macrophages in a Salmonella pathogenicity island (SPI)-1 and SPI-2 dependent way. Salmonella induced downregulation of MHC II expression and intracellular proliferation of Salmonella in macrophages were significantly impaired after opsonization with Salmonella specific antibodies prior to inoculation. Furthermore, the capacity to downregulate MHC II expression on macrophages differed significantly among Salmonella strains, independently of strain specific differences in invasion capacity, Salmonella induced cytotoxicity and altered macrophage activation status. The fact that strain specific differences in MHC II downregulation did not correlate with the extent of in vitro SPI-1 or SPI-2 gene expression indicates that other factors are involved in MHC II downregulation as well. Since Salmonella strain dependent interference with the pig's immune response through downregulation of MHC II expression might indicate that certain Salmonella strains are more likely to escape serological detection, our findings are of major interest for Salmonella monitoring programs primarily based on serology.

T-2 toxin induced Salmonella Typhimurium intoxication results in decreased Salmonella numbers in the cecum contents of pigs, despite marked effects on Salmonella-host cell interactions

The mycotoxin T-2 toxin and Salmonella Typhimurium infections pose a significant threat to human and animal health. Interactions between both agents may result in a different outcome of the infection. Therefore, the aim of the presented study was to investigate the effects of low and relevant concentrations of T-2 toxin on the course of a Salmonella Typhimurium infection in pigs. We showed that the presence of 15 and 83 μg T-2 toxin per kg feed significantly decreased the amount of Salmonella Typhimurium bacteria present in the cecum contents, and a tendency to a reduced colonization of the jejunum, ileum, cecum, colon and colon contents was noticed. In vitro, proteomic analysis of porcine enterocytes revealed that a very low concentration of T-2 toxin (5 ng/mL) affects the protein expression of mitochondrial, endoplasmatic reticulum and cytoskeleton associated proteins, proteins involved in protein synthesis and folding, RNA synthesis, mitogen-activated protein kinase signaling and regulatory processes. Similarly low concentrations (1-100 ng/mL) promoted the susceptibility of porcine macrophages and intestinal epithelial cells to Salmonella Typhimurium invasion, in a SPI-1 independent manner. Furthermore, T-2 toxin (1-5 ng/mL) promoted the translocation of Salmonella Typhimurium over an intestinal porcine epithelial cell monolayer. Although these findings may seem in favour of Salmonella Typhimurium, microarray analysis showed that T-2 toxin (5 ng/mL) causes an intoxication of Salmonella Typhimurium, represented by a reduced motility and a downregulation of metabolic and Salmonella Pathogenicity Island 1 genes. This study demonstrates marked interactions of T-2 toxin with Salmonella Typhimurium pathogenesis, resulting in bacterial intoxication.

Antibody binding to porcine sialoadhesin reduces phagocytic capacity without affecting other macrophage effector functions

Sialoadhesin (Sn) is a macrophage-restricted endocytic receptor involved in cell-cell, cell-matrix and cell-pathogen interactions. Recently, porcine Sn (pSn) was shown to be involved in signaling and lately Sn is gaining interest as a potential target for immunotherapy. However, little is known about the effect of ligand binding to Sn on macrophage effector functions. In this study, we tested the effect of antibody binding to pSn on macrophage viability, phagocytosis of microspheres, uptake and processing of soluble antigens, reactive oxygen/nitrogen species production, MHC I and MHC II cell surface expression and cytokine production. This was done by treatment of porcine primary alveolar macrophages with the pSn-specific mAb 41D3, or an isotype-matched control mAb. No significant effect on most effector functions under study was observed, except for a significant reduction of phagocytosis. Thus, antibody binding to pSn can downregulate phagocytosis, which could have implications on homeostasis, infectious and immune diseases, and immunotherapy.

Association of attenuated mutants of Salmonella enterica serovar Enteritidis with porcine peripheral blood leukocytes

In this study, we were interested in the association of attenuated mutants of Salmonella enterica serovar Enteritidis with subpopulations of porcine white blood cells (WBC). The mutants included those with inactivated aroA, phoP, rfaL, rfaG, rfaC and fliC genes and a mutant with five major pathogenicity islands removed (ΔSPI1-5 mutant). Using flow cytometry, we did not observe any difference in the interactions of the wild-type S. Enteritidis, aroA and phoP mutants with WBC. ΔSPI1-5 and fliC mutants had a minor defect in their association with granulocytes and monocytes, but not with T- or B-lymphocytes. All three rfa mutants associated with granulocytes, monocytes and B-lymphocytes more than the wild-type S. Enteritidis did. Electron microscopy confirmed that the association correlated with the intracellular presence of S. Enteritidis and that the Salmonella-containing vacuole in the WBC infected with the rfa mutants, unlike all other strains, did not develop into a spacious phagosome. Intact lipopolysaccharide, but not the type III secretion system encoded by SPI-1, SPI-2 or the flagellar operon, is important for the initial interaction of S. Enteritidis with porcine leukocytes. This information can be used for the design of live Salmonella vaccines preferentially targeting particular cell types including cancer or tumor cells.

Salmonella Typhimurium LPS mutations for use in vaccines allowing differentiation of infected and vaccinated pigs

Contaminated pork is a major source of human salmonellosis and the serovar most frequently isolated from pigs is Salmonella Typhimurium. Vaccination could contribute greatly to controlling Salmonella infections in pigs. However, pigs vaccinated with the current vaccines cannot be discriminated from infected pigs with the LPS-based serological tests used in European Salmonella serosurveillance programmes. We therefore examined which LPS encoding genes of Salmonella Typhimurium can be deleted to allow differentiation of infected and vaccinated pigs (DIVA), without affecting the vaccine strain's protective capacity. For this purpose, deletion mutants in Salmonella strain 112910a, used as vaccine strain, were constructed in the LPS encoding genes: ΔrfbA, ΔrfaL, ΔrfaJ, ΔrfaI, ΔrfaG and ΔrfaF. Primary inoculation of BALB/c mice with the parent strain, ΔrfaL, ΔrfbA or ΔrfaJ strain but not the ΔrfaG, ΔrfaF or ΔrfaI strain protected significantly against subsequent infection with the virulent Salmonella Typhimurium strain NCTC12023. Immunization of piglets with the ΔrfaJ or ΔrfaL mutants resulted in the induction of a serological response lacking detectable antibodies against LPS. This allowed a clear differentiation between sera from pigs immunized with the ΔrfaJ or ΔrfaL strains and sera from pigs infected with their isogenic wild type strain. In conclusion, applying deletions in the rfaJ or the rfaL gene in Salmonella Typhimurium strain 112910a allows differentiation of infected and vaccinated pigs in an LPS based ELISA without reducing the strain's protective capacities in mice.

SPI-1-encoded type III secretion system of Salmonella enterica is required for the suppression of porcine alveolar macrophage cytokine expression

Genes localized at Salmonella pathogenicity island-1 (SPI-1) are involved in Salmonella enterica invasion of host non-professional phagocytes. Interestingly, in macrophages, SPI-1-encoded proteins, in addition to invasion, induce cell death via activation of caspase-1 which also cleaves proIL-1β and proIL-18, precursors of 2 proinflammatory cytokines. In this study we were therefore interested in whether SPI-1-encoded type III secretion system (T3SS) may influence proinflammatory response of macrophages. To test this hypothesis, we infected primary porcine alveolar macrophages with wild-type S. Typhimurium and S. Enteritidis and their isogenic SPI-1 deletion mutants. ΔSPI1 mutants of both serovars invaded approx. 5 times less efficiently than the wild-type strains and despite this, macrophages responded to the infection with ΔSPI1 mutants by increased expression of proinflammatory cytokines IL-1β, IL-8, TNFα, IL-23α and GM-CSF. Identical macrophage responses to that induced by the ΔSPI1 mutants were also observed to the infection with sipB but not the sipA mutant. The hilA mutant exhibited an intermediate phenotype between the ΔSPI1 mutant and the wild-type S. Enteritidis. Our results showed that the SPI-1-encoded T3SS is required not only for cell invasion but in macrophages also for the suppression of early proinflammatory cytokine expression.

Efficacy of a novel virulence gene-deleted Salmonella Typhimurium vaccine for protection against Salmonella infections in growing piglets

We have previously developed a novel attenuated Salmonella Typhimurium (S. Typhimurium) ΔcpxR Δlon vaccine. This study was carried out to examine whether this vaccine could effectively protect growing piglets against Salmonella infection. Attenuated S. Typhimurium secreting the B subunit of Escherichia coli heat-labile enterotoxin was also used as a mucosal adjuvant. Pregnant sows in groups A and B were primed and boosted with the vaccine and mucosal adjuvant, whereas sows in groups C, D and E received PBS. Piglets in groups A and C were intramuscularly primed with formalin-inactivated vaccine and orally boosted with live vaccine, while piglets in groups B, D and E received PBS. Piglets in groups A, B, C, and D were challenged with a wild type virulent S. Typhimurium at the 11th weeks of age. Colostrum sIgA and IgG titers in vaccinated groups A and B sows were approximately 50 and 40 times higher than those of non-vaccinated groups C, D and E sows (P<0.001). Serum IgG titers of group A piglets were also significantly higher than those of groups D and E piglets during the study (P<0.001). Furthermore, no clinical signs were observed in group A piglets during the entire experimental period after the challenge, while diarrhea was observed in many of the piglets in groups B, C, and D. No Salmonella was isolated from fecal samples of the groups A and C piglets on day 14 after challenge, whereas the challenge strain was isolated from several piglets in groups B and D. These results indicate that vaccination of the piglets with the vaccine and mucosal adjuvant in addition to vaccination of their sows induced effective protection against Salmonella infections in the growing piglets.

The mycotoxin deoxynivalenol promotes uptake of Salmonella Typhimurium in porcine macrophages, associated with ERK1/2 induced cytoskeleton reorganization

Both the mycotoxin deoxynivalenol (DON) and Salmonella Typhimurium are major issues in swine production. This study aimed at examining the interaction between DON and Salmonella Typhimurium at the level of the porcine innate immune system, represented by macrophages. First, we assessed the direct cytotoxic effect of DON on porcine macrophages. Incubation with 0.25 microg/mL of DON or higher resulted in a significant cytotoxic effect after 24 h of incubation. Secondly, the direct toxic effect of DON on the growth and on the expression of Salmonella pathogenicity island 1 (SPI-1) and SPI-2 virulence genes of Salmonella Typhimurium was determined. At low non-cytotoxic concentrations, as can be found in the serum of pigs, DON did not have any effect on either growth or virulence gene expression of Salmonella Typhimurium. However, when the invasion and intracellular survival of Salmonella Typhimurium in macrophages preexposed to 0.025 microg/mL of DON was examined, DON significantly promoted the uptake of Salmonella Typhimurium into macrophages. The enhanced uptake coincided with marked F-actin reorganization of the cells, which was due to the activation of extracellular signal-regulated protein kinase 1/2 (ERK1/2). These results suggest that low but relevant concentrations of DON modulate the innate immune system and could thus increase the susceptibility of pigs to infections with Salmonella Typhimurium.

Porcine in vitro and in vivo models to assess the virulence of Salmonella enterica serovar Typhimurium for pigs

Salmonella Typhimurium infections in pigs pose an important human health hazard. One promising control measure is the development of live attenuated vaccine strains using defined knockout mutants. Preferably, screening of candidate knockout vaccine strains for attenuation should first be done in models allowing testing of a large number of strains. Thereafter, a limited number of selected strains should be further characterized in an experimental infection model in pigs. The aim of the present study was to develop such models. The invasive and proliferative characteristics of S. Typhimurium were assessed in both a non-polarized and a polarized porcine intestinal epithelial cell line. Neutrophils obtained from porcine blood were used to study the capacity of Salmonella to withstand killing by these phagocytes. The ability to induce an intestinal inflammatory response was investigated in a terminal intestinal loop model. The systemic phase of infection was mimicked by studying the uptake and intracellular survival of S. Typhimurium in porcine pulmonary alveolar macrophages and peripheral blood monocytes. These models should allow screening for attenuated strains. For further characterization, an experimental infection model was established, providing extensive data on the course of an oral infection and the optimal time points for colonization (day 5 postinoculation [pi]) and persistency (days 21–28 pi) in pigs. In conclusion, screening for virulence of S. Typhimurium strains with subsequent confirmation for a subset of strains in a well-defined experimental infection model would significantly reduce the number of experimental pigs required.

Coated fatty acids alter virulence properties of Salmonella Typhimurium and decrease intestinal colonization of pigs

Salmonella Typhimurium infections in pigs are a major source of human foodborne salmonellosis. To reduce the number of infected pigs, acidification of feed or drinking water is a common practice. The aim of the present study was to determine whether some frequently used short- (SCFA) and medium-chain fatty acids (MCFA) are able to alter virulence gene expression and to decrease Salmonella Typhimurium colonization and shedding in pigs using well established and controlled in vitro and in vivo assays. Minimal inhibitory concentrations (MIC) of 4 SCFA (formic acid, acetic acid, propionic acid and butyric acid) and 2 MCFA (caproic and caprylic acid) were determined using 54 porcine Salmonella Typhimurium field strains. MIC values increased at increasing pH-values and were two to eight times lower for MCFA than for SCFA. Expression of virulence gene fimA was significantly lower when bacteria were grown in LB-broth supplemented with sub-MIC concentrations of caproic or caprylic acid (2 mM). Expression of hilA and invasion in porcine intestinal epithelial cells was significantly lower when bacteria were grown in LB-broth containing sub-MIC concentrations of butyric acid or propionic acid (10 mM) and caproic or caprylic acid (2 mM). When given as feed supplement to pigs experimentally infected with Salmonella Typhimurium, coated butyric acid decreased the levels of faecal shedding and intestinal colonization, but had no influence on the colonization of tonsils, spleen and liver. Uncoated fatty acids, however, did not influence fecal shedding, intestinal or tonsillar colonization in pigs. In conclusion, supplementing feed with certain coated fatty acids, such as butyric acid, may help to reduce the Salmonella load in pigs.

Non-typhoidal Salmonella infections in pigs: a closer look at epidemiology, pathogenesis and control

Contaminated pork is an important source of Salmonella infections in humans. The increasing multiple antimicrobial resistance associated with pork-related serotypes such as Salmonella Typhimurium and Salmonella Derby may become a serious human health hazard in the near future. Governments try to anticipate the issue of non-typhoidal Salmonella infections in pork by starting monitoring programmes and coordinating control measures worldwide. A thorough knowledge of how these serotypes interact with the porcine host should form the basis for the development and optimisation of these monitoring and control programmes. During recent years, many researchers have focussed on different aspects of the pathogenesis of non-typhoidal Salmonella infections in pigs. The present manuscript reviews the importance of pigs and pork as a source for salmonellosis in humans and discusses commonly accepted and recent insights in the pathogenesis of non-typhoidal Salmonella infections in pigs, with emphasis on Salmonella Typhimurium, and to relate this knowledge to possible control measures.

Cytokine response of porcine cell lines to Salmonella enterica serovar typhimurium and its hilA and ssrA mutants

Salmonella enterica serovar Typhimurium (S. Typhimurium) is a facultative intracellular bacterium which can infect and colonize pigs. After contact with enterocytes and macrophages, S. Typhimurium induces production of cytokines thus triggering the innate immune response. In this study we evaluated the cytokine response of two porcine cell lines, IPI-2I and 3D4/31, of epithelial or macrophage origins, respectively, to the wild-type S. Typhimurium and its hilA and ssrA mutants. We observed that the 3D4/31 cell line essentially did not respond to S. Typhimurium infection when a medium with foetal calf serum was used. However when the 3D4 cell line was incubated overnight in the presence of porcine serum, it efficiently responded to the wild-type strain and the ssrA mutant but not to the noninvasive hilA mutant as measured by mRNA quantification of TNF-alpha, IL-8 and GM-CSF by the real-time RT-PCR. In IPI-2I, all the cytokines were also induced by the wild-type S. Typhimurium and the ssrA mutant although the induction of TNF-alpha was lower than that induced by the wild-type strain. The hilA mutant was unable to induce any of the cytokines tested. The ssrA mutant can therefore be considered as more suitable for further vaccine development as the stimulation of innate immune response is important for animal protection against a challenge with virulent strains.

Role in virulence and protective efficacy in pigs of Salmonella enterica serovar Typhimurium secreted components identified by signature-tagged mutagenesis

Salmonella enterica serovar Typhimurium (S. Typhimurium) is a zoonotic enteric pathogen of worldwide importance and pigs are a significant reservoir of human infection. Signature-tagged transposon mutagenesis (STM) was used to identify genes required by S. Typhimurium to colonize porcine intestines. A library of 1045 signature-tagged mutants of S. Typhimurium ST4/74 Nal(R) was screened following oral inoculation of pigs in duplicate. A total of 119 attenuating mutations were identified in 95 different genes, many of which encode known or putative secreted or surface-anchored molecules. A large number of attenuating mutations were located within Salmonella pathogenicity islands (SPI)-1 and -2, confirming important roles for type III secretion systems (T3SS)-1 and -2 in intestinal colonization of pigs. Roles for genes encoded in other pathogenicity islands and islets, including the SPI-6-encoded Saf atypical fimbriae, were also identified. Given the role of secreted factors and the protection conferred against other pathogens by vaccination with extracellular and type III secreted proteins, the efficacy of a secreted protein vaccine from wild-type S. Typhimurium following intramuscular vaccination of pigs was evaluated. Serum IgG responses against type III secreted proteins were induced following vaccination and a significant reduction in faecal excretion of S. Typhimurium was observed in the acute phase of infection compared to mock-vaccinated animals. Vaccination with secreted proteins from an isogenic S. Typhimurium prgH mutant produced comparable levels of protection to vaccination with the preparation from the parent strain, indicating that protection was not reliant on T3SS-1 secreted proteins. The data provide valuable information for the control of Salmonella in pigs.

Salmonella Typhimurium SPI-1 genes promote intestinal but not tonsillar colonization in pigs

Salmonella Pathogenicity Island 1 (SPI-1) genes are indispensable for virulence of Salmonella Typhimurium in several animal species. The role of SPI-1 in the pathogenesis of Salmonella Typhimurium infections of pigs, however, is not well described. The interactions of a porcine Salmonella Typhimurium field strain and its isogenic mutants with disruptions in the SPI-1 genes hilA, sipA and sipB with porcine intestinal epithelial cells were characterized in vitro and in a ligated intestinal loop model in pigs. HilA and SipB were essential in the invasion of porcine intestinal epithelial cells in vitro. A sipA mutant was impaired for invasion using a polarized cell line, but fully invasive in a non-polarized cell line. All SPI-1 mutants induced a significant decrease in influx of neutrophils in the porcine intestinal loop model compared with the wild type strain. Pigs were orally inoculated with 10(8) colony forming units of both the wild type Salmonella Typhimurium strain and its isogenic sipB::kan mutant strain. The sipB mutant strain was significantly impaired to invade the intestinal, but not the tonsillar tissue, one day after inoculation and was unable to efficiently colonize the intestines and the GALT, but not the tonsils, 3 days after inoculation. This study shows that SPI-1 plays a crucial role in the invasion and colonization of the porcine gut and in the induction of influx of neutrophils towards the intestinal lumen, but not in the colonization of the tonsils.