Salmonella infections in the mouse model: host resistance factors and in vivo dynamics of bacterial spread and distribution in the tissues

Mucosal delivery of a transmission-blocking DNA vaccine encoding Giardia lamblia CWP2 by Salmonella typhimurium bactofection vehicle

In this study, we investigated the use of Salmonella typhimurium (STM1 strain) as a bactofection vehicle to deliver a transmission-blocking DNA vaccine (TBDV) plasmid to the intestinal immune system. The gene encoding the full length cyst wall protein-2 (CWP2) from Giardia lamblia was subcloned into the pCDNA3 mammalian expression vector and stably introduced into S. typhimurium STM1. Eight-week-old female BALB/c mice were orally immunized every 2 weeks, for a total of three immunizations. Vaccinated and control mice were sacrificed 1 week following the last injection. Administration of the DNA vaccine led to the production of CWP2-specific cellular immune responses characterized by a mixed Th1/Th2 response. Using ELISA, antigen-specific IgA and IgG antibodies were detected in intestinal secretions. Moreover, analysis of sera demonstrated that the DNA immunization also stimulated the production of CWP2-specific IgG antibodies that were mainly of the IgG2a isotype. Finally, challenge infection with live Giardia muris cysts revealed that mice receiving the CWP2-encoding DNA vaccine were able to reduce cyst shedding by approximately 60% compared to control mice. These results demonstrate, for the first time, the development of parasite transmission-blocking immunity at the intestinal level following the administration of a mucosal DNA vaccine delivered by S. typhimurium STM1.

Activation of pro-matrix metalloproteinase-9 and degradation of gelatin by the surface protease PgtE of Salmonella enterica serovar Typhimurium

Mammalian matrix metalloproteinases (MMPs) degrade collagen networks in extracellular matrices by cleaving collagen and its denatured form gelatin, and thus enhance migration of mammalian cells. The gastrointestinal pathogen Salmonella enterica survives and grows within host macrophages and dendritic cells, and can disseminate in the host by travelling within infected host cells. Here, we report that S. enterica serovar Typhimurium activates proMMP-9 (gelatinase B) secreted by human primary macrophages, and degrades gelatin after growth within J774A.1 murine macrophage-like cells. Both proMMP-9 activation and gelatin degradation were due to expression of the Salmonella surface protease PgtE. Following intraperitoneal infection in BALB/c mice, the amount of a pgtE deletion derivative was nearly ten-fold lower in the livers and spleens of mice than the amount of wild-type S. enterica, suggesting that PgtE contributes to dissemination of Salmonella in the host. PgtE belongs to the omptin family of bacterial beta-barrel transmembrane proteases. The ortholog of PgtE in Yersinia pestis, Pla, which is central for bacterial virulence in plague, was poor in proMMP-9 activation and in gelatin degradation. To model the evolution of these activities in the omptin barrel, we performed a substitution analysis in Pla and genetically modified it into a PgtE-like gelatinase. Our results indicate that PgtE and Pla have diverged in substrate specificity, and suggest that Salmonella PgtE has evolved to functionally mimic mammalian MMPs.

Chicken cecum immune response to Salmonella enterica serovars of different levels of invasiveness

Day-old chicks are very susceptible to infections with Salmonella enterica subspecies. The gut mucosa is the initial site of host invasion and provides the first line of defense against the bacteria. To study the potential of different S. enterica serovars to invade the gut mucosa and trigger an immune response, day-old chicks were infected orally with Salmonella enterica serovar Enteritidis, S. enterica serovar Typhimurium, S. enterica serovar Hadar, or S. enterica serovar Infantis, respectively. The localization of Salmonella organisms in gut mucosa and the number of immune cells in cecum were determined by immunohistochemistry in the period between 4 h and 9 days after infection. Using quantitative real-time reverse transcription (RT)-PCR, mRNA expression of various cytokines, chemokines, and inducible nitric oxide synthase (iNOS) was examined in cecum. As a result, all S. enterica serovars were able to infect epithelial cells and the lamina propria. Notably, serovar Enteritidis showed the highest invasiveness of lamina propria tissue, whereas serovars Typhimurium and Hadar displayed moderate invasiveness and serovar Infantis hardly any invasion capabilities. Only a limited number of bacteria of all serovars were found within intestinal macrophages. Elevated numbers of granulocytes, CD8+ cells, and TCR1+ cells and mRNA expression rates for interleukin 12 (IL-12), IL-18, tumor necrosis factor alpha factor, and iNOS in cecum correlated well with the invasiveness of serovars in the lamina propria. In contrast, changes in numbers of TCR2+ and CD4+ cells and IL-2 mRNA expression seemed to be more dependent on infection of epithelial cells. The data indicate that the capability of Salmonella serovars to enter the cecal mucosa and invade lower regions affects both the level and character of the immune response in tissue.

Brucella abortus uses a stealthy strategy to avoid activation of the innate immune system during the onset of infection

BACKGROUND

To unravel the strategy by which Brucella abortus establishes chronic infections, we explored its early interaction with innate immunity.

METHODOLOGY/PRINCIPAL FINDINGS

Brucella did not induce proinflammatory responses as demonstrated by the absence of leukocyte recruitment, humoral or cellular blood changes in mice. Brucella hampered neutrophil (PMN) function and PMN depletion did not influence the course of infection. Brucella barely induced proinflammatory cytokines and consumed complement, and was strongly resistant to bactericidal peptides, PMN extracts and serum. Brucella LPS (BrLPS), NH-polysaccharides, cyclic glucans, outer membrane fragments or disrupted bacterial cells displayed low biological activity in mice and cells. The lack of proinflammatory responses was not due to conspicuous inhibitory mechanisms mediated by the invading Brucella or its products. When activated 24 h post-infection macrophages did not kill Brucella, indicating that the replication niche was not fusiogenic with lysosomes. Brucella intracellular replication did not interrupt the cell cycle or caused cytotoxicity in WT, TLR4 and TLR2 knockout cells. TNF-alpha-induction was TLR4- and TLR2-dependent for live but not for killed B. abortus. However, intracellular replication in TLR4, TLR2 and TLR4/2 knockout cells was not altered and the infection course and anti-Brucella immunity development upon BrLPS injection was unaffected in TLR4 mutant mice.

CONCLUSION/SIGNIFICANCE

We propose that Brucella has developed a stealth strategy through PAMPs reduction, modification and hiding, ensuring by this manner low stimulatory activity and toxicity for cells. This strategy allows Brucella to reach its replication niche before activation of antimicrobial mechanisms by adaptive immunity. This model is consistent with clinical profiles observed in humans and natural hosts at the onset of infection and could be valid for those intracellular pathogens phylogenetically related to Brucella that also cause long lasting infections.

Oral delivery of tumor-targeting Salmonella for cancer therapy in murine tumor models

Tumor-targeting bacteria have been investigated intensively in recent years as anticancer agents. To ensure the reliability of infection, bacteria have conventionally been injected intravenously or intraperitoneally into animals or humans. However, systemic infection of bacteria is rather inconvenient and carries the risk of obvious toxicity. Here we tested whether Salmonella typhimurium VNP20009, a tumor-targeting strain, could be administrated orally for tumor therapy. Tumor-targeting potential, antitumor effects, as well as toxicity of orally administrated VNP20009 were investigated in this study. Oral delivery of VNP20009 not only exhibited high tumor-targeting potential, but also led to a significant anticancer effect by delaying tumor growth and prolonging survival in murine tumor models. As part of combination therapy, orally administrated bacteria notably improved the antitumor effect of cyclophosphamide. In vitro and in vivo studies showed that VNP20009 significantly induced tumor cell apoptosis. No obvious toxicity was observed during the treatments with oral inoculation of VNP20009. Comparative analysis of toxicity in tumor-bearing and tumor-free mice further revealed that orally administrated Salmonella had high safety compared to conventional systemic infection of bacteria. The findings indicated that oral administration of tumor-targeting bacteria is effective and safe. This approach provides a novel avenue in the application of bacteria as a potential antitumor agent.

Fcgamma receptors are crucial for the expression of acquired resistance to virulent Salmonella enterica serovar Typhimurium in vivo but are not required for the induction of humoral or T-cell-mediated immunity

Antibodies play an important role in immunity to Salmonella enterica. Here we evaluated the requirement for Fcgamma receptors in host resistance to S. enterica using an in vivo model of systemic infection. We show that mice lacking FcgammaRI, II and III can control and clear a primary infection with S. enterica micro-organisms of low virulence, but are impaired in the expression of vaccine-induced acquired immunity to oral challenge with virulent bacteria. We also show that, in vivo, FcgammaRI, II, III(-/-) mice were able to mount efficient T-helper 1 type T-cell responses and antibody responses specific for S. enterica. The work indicates that targeting S. enterica to FcgammaR is needed for the expression of vaccine-induced acquired immunity, but is not essential for the engenderment of T- and B-cell immunity to the bacterium in vivo.

The surface protease PgtE of Salmonella enterica affects complement activity by proteolytically cleaving C3b, C4b and C5

Complement activity in mammalian serum is fundamentally based on three homologous components C3b, C4b and C5. During systemic infection, the gastrointestinal pathogen Salmonella enterica disseminates within host phagocytic cells but also extracellularly. Consequently, systemic Salmonella transiently confronts the complement system. We show here that the surface protease PgtE of S. enterica proteolytically cleaves C3b, C4b and C5 and that the expression of PgtE enhances bacterial resistance to human serum. Degradation of C3b was further enhanced by PgtE-mediated plasminogen activation.

Intracellular demography and the dynamics of Salmonella enterica infections

An understanding of within-host dynamics of pathogen interactions with eukaryotic cells can shape the development of effective preventive measures and drug regimes. Such investigations have been hampered by the difficulty of identifying and observing directly, within live tissues, the multiple key variables that underlay infection processes. Fluorescence microscopy data on intracellular distributions of Salmonella enterica serovar Typhimurium (S. Typhimurium) show that, while the number of infected cells increases with time, the distribution of bacteria between cells is stationary (though highly skewed). Here, we report a simple model framework for the intensity of intracellular infection that links the quasi-stationary distribution of bacteria to bacterial and cellular demography. This enables us to reject the hypothesis that the skewed distribution is generated by intrinsic cellular heterogeneities, and to derive specific predictions on the within-cell dynamics of Salmonella division and host-cell lysis. For within-cell pathogens in general, we show that within-cell dynamics have implications across pathogen dynamics, evolution, and control, and we develop novel generic guidelines for the design of antibacterial combination therapies and the management of antibiotic resistance.

Modeling of infection shows that heterogeneities in bacterial burden per host cell need not be explained by differences in host cell permissiveness; this has important implications for understanding the ecology of infection.

Thioredoxin 1 promotes intracellular replication and virulence of Salmonella enterica serovar Typhimurium

The effect of the cytoplasmic reductase and protein chaperone thioredoxin 1 on the virulence of Salmonella enterica serovar Typhimurium was evaluated by deleting the trxA, trxB, or trxC gene of the cellular thioredoxin system, the grxA or gshA gene of the glutathione/glutaredoxin system, or the dsbC gene coding for a thioredoxin-dependent periplasmic disulfide bond isomerase. Mutants were tested for tolerance to oxidative and nitric oxide donor substances in vitro, for invasion and intracellular replication in cultured epithelial and macrophage-like cells, and for virulence in BALB/c mice. In these experiments only the gshA mutant, which was defective in glutathione synthesis, exhibited sensitization to oxidative stress in vitro and a small decrease in virulence. In contrast, the trxA mutant did not exhibit any growth defects or decreased tolerance to oxidative or nitric oxide stress in vitro, yet there were pronounced decreases in intracellular replication and mouse virulence. Complementation analyses using defined catalytic variants of thioredoxin 1 showed that there is a direct correlation between the redox potential of thioredoxin 1 and restoration of intracellular replication of the trxA mutant. Attenuation of mouse virulence that was caused by a deficiency in thioredoxin 1 was restored by expression of wild-type thioredoxin 1 in trans but not by expression of a catalytically inactive variant. These results clearly imply that in S. enterica serovar Typhimurium, the redox-active protein thioredoxin 1 promotes virulence, whereas in vitro tolerance to oxidative stress depends on production of glutathione.

Gene expression responses to a Salmonella infection in the chicken intestine differ between lines

Poultry products are an important source of Salmonella enterica. An effective way to reduce food poisoning due to Salmonella would be to breed chickens more resistant to Salmonella. Unfortunately host responses to Salmonella are complex with many factors involved. To learn more about responses to Salmonella in young chickens, a cDNA microarray analysis was performed to compare gene expression profiles between two chicken lines under control and Salmonella infected conditions. Newly hatched chickens were orally infected with S. enterica serovar Enteritidis. Since the intestine is the first barrier the bacteria encounter after oral inoculation, intestinal gene expression was investigated at different timepoints. Differences in gene expression between the two chicken lines were found in control as well as Salmonella infected conditions. In response to the Salmonella infection a fast growing chicken broiler line induced genes that affect T-cell activation, whereas in a slow growing broiler line genes involved in macrophage activation seemed to be more affected at day 1 post-infection. At days 7 and 9 most gene expression differences between the two chicken lines were identified under control conditions, indicating a difference in the intestinal development between the two chicken lines which might be linked to the difference in Salmonella susceptibility. The findings in this study have lead to the identification of novel genes and possible cellular pathways, which are host dependent.

Effect of immune serum and role of individual Fcgamma receptors on the intracellular distribution and survival of Salmonella enterica serovar Typhimurium in murine macrophages

Immune serum has a protective role against Salmonella infections in mice, domestic animals and humans. In this study, the effect of antibody on the interaction between murine macrophages and S. enterica serovar Typhimurium was examined. Detailed analysis at the single-cell level demonstrated that opsonization of the bacteria with immune serum enhanced bacterial uptake and altered bacterial distribution within individual phagocytic cells. Using gene-targeted mice deficient in individual Fc gamma receptors it was shown that immune serum enhanced bacterial internalization by macrophages via the high-affinity immunoglobulin G (IgG) receptor, Fc gamma receptor I. Exposure of murine macrophages to S. enterica serovar Typhimurium opsonized with immune serum resulted in increased production of superoxide, leading to enhanced antibacterial functions of the infected cells. However, opsonization of bacteria with immune serum did not increase either nitric oxide production in response to S. enterica serovar Typhimurium or fusion of phagosomes with lysosomes.

Genome-wide screen for Salmonella genes required for long-term systemic infection of the mouse

A microarray-based negative selection screen was performed to identify Salmonella enterica serovar Typhimurium (serovar Typhimurium) genes that contribute to long-term systemic infection in 129X1/SvJ (Nramp1^r) mice. A high-complexity transposon-mutagenized library was used to infect mice intraperitoneally, and the selective disappearance of mutants was monitored after 7, 14, 21, and 28 d postinfection. One hundred and eighteen genes were identified to contribute to serovar Typhimurium infection of the spleens of mice by 28 d postinfection. The negatively selected mutants represent many known aspects of Salmonella physiology and pathogenesis, although the majority of the identified genes are of putative or unknown function. Approximately 30% of the negatively selected genes correspond to horizontally acquired regions such as those within Salmonella pathogenicity islands (SPI 1–5), prophages (Gifsy-1 and −2 and remnant), and the pSLT virulence plasmid. In addition, mutations in genes responsible for outer membrane structure and remodeling, such as LPS- and PhoP-regulated and fimbrial genes, were also selected against. Competitive index experiments demonstrated that the secreted SPI2 effectors SseK2 and SseJ as well as the SPI4 locus are attenuated relative to wild-type bacteria during systemic infection. Interestingly, several SPI1-encoded type III secretion system effectors/translocases are required by serovar Typhimurium to establish and, unexpectedly, to persist systemically, challenging the present description of Salmonella pathogenesis. Moreover, we observed a progressive selection against serovar Typhimurium mutants based upon the duration of the infection, suggesting that different classes of genes may be required at distinct stages of infection. Overall, these data indicate that Salmonella long-term systemic infection in the mouse requires a diverse repertoire of virulence factors. This diversity of genes presumably reflects the fact that bacteria sequentially encounter a variety of host environments and that Salmonella has evolved to respond to these selective forces in a way that permits both the bacteria and the host to survive.

Bacteria belonging to the genus Salmonella are capable of establishing a long-term systemic infection in a variety of hosts, including humans, rodents, fowl, and cattle. The ability of Salmonella to subvert the active immune response of the host represents millions of years of co-evolution and is the result of specialized virulence factors that promote long-term infection. This study describes a microarray-based genome-wide screen designed to identify genes required by Salmonella enterica serovar Typhimurium (serovar Typhimurium) to persist and replicate in the spleen and liver of mice for up to 28 days. The results demonstrate that serovar Typhimurium utilizes a diverse repertoire of virulence factors, including both known and novel virulence genes, to establish infection and to persist in the host. The authors' data further established a previously unappreciated role for Salmonella pathogenicity island 1 in maintaining a persistent systemic infection. In addition, a progressive selection against serovar Typhimurium mutants based upon the duration of the infection was observed, suggesting that certain classes of genes are required at specific times during infection and providing a foundation to further dissect Salmonella pathogenesis into distinct temporal phases.

Autophagy controls Salmonella infection in response to damage to the Salmonella-containing vacuole

Salmonella enterica serovar Typhimurium (S. Typhimurium) is a facultative intracellular pathogen that causes disease in a variety of hosts. S. Typhimurium actively invade host cells and typically reside within a membrane-bound compartment called the Salmonella-containing vacuole (SCV). The bacteria modify the fate of the SCV using two independent type III secretion systems (TTSS). TTSS are known to damage eukaryotic cell membranes and S. Typhimurium has been suggested to damage the SCV using its Salmonella pathogenicity island (SPI)-1 encoded TTSS. Here we show that this damage gives rise to an intracellular bacterial population targeted by the autophagy system during in vitro infection. Approximately 20% of intracellular S. Typhimurium colocalized with the autophagy marker GFP-LC3 at 1 h postinfection. Autophagy of S. Typhimurium was dependent upon the SPI-1 TTSS and bacterial protein synthesis. Bacteria targeted by the autophagy system were often associated with ubiquitinated proteins, indicating their exposure to the cytosol. Surprisingly, these bacteria also colocalized with SCV markers. Autophagy-deficient (atg5-/-) cells were more permissive for intracellular growth by S. Typhimurium than normal cells, allowing increased bacterial growth in the cytosol. We propose a model in which the host autophagy system targets bacteria in SCVs damaged by the SPI-1 TTSS. This serves to retain intracellular S. Typhimurium within vacuoles early after infection to protect the cytosol from bacterial colonization. Our findings support a role for autophagy in innate immunity and demonstrate that Salmonella infection is a powerful model to study the autophagy process.

Polynucleotide phosphorylase negatively controls spv virulence gene expression in Salmonella enterica

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

CD154 Is Essential for Protective Immunity in ExperimentalSalmonellaInfection: Evidence for a Dual Role in Innate and Adaptive Immune Responses

CD40-CD154 interactions are of central importance in the induction of humoral and cellular immune responses. In the present study, CD154-deficient (CD154-/-) mice were used to assess the role of CD40-CD154 interactions in regulating the immune response to a systemic Salmonella infection. Compared with C57BL/6 (CD154+/+) controls, CD154-/- mice were hypersusceptible to infection by an attenuated strain of Salmonella enterica serovar Typhimurium (S. typhimurium), as evidenced by decreased survival rate and mean time to death, which correlated with increased bacterial burden and persistence in target organs. CD154-/- mice exhibited a defect both in the production of IL-12, IFN-gamma, and NO during the acute phase of the disease and in the generation of Salmonella-specific Ab responses and Ig isotype switching. Furthermore, when CD154-/- animals were administered a sublethal dose of attenuated S. typhimurium and subsequently challenged with a virulent homologous strain, all mice succumbed to an overwhelming infection. Similar treatment of CD154+/+ mice consistently resulted in > or =90% protection. The lack of protective immunity in CD154-/- mice correlated with a decreased T cell recall response to Salmonella Ags. Significant protection against virulent challenge was conferred to presensitized CD154-/- mice by transfer of serum or T cells from immunized CD154+/+ mice. For best protection, however, a combination of immune serum and T cells was required. We conclude that intercellular communications via the CD40-CD154 pathway play a critical role in the induction of type 1 cytokine responses, memory T cell generation, Ab formation, and protection against primary as well as secondary Salmonella infections.

Intracellular Voyeurism: Examining the Modulation of Host Cell Activities bySalmonella enterica Serovar Typhimurium

Salmonella spp. can infect host cells by gaining entry through phagocytosis or by inducing host cell membrane ruffling that facilitates bacterial uptake. With its wide host range, Salmonella enterica serovar Typhimurium has proven to be an important model organism for studying intracellular bacterial pathogenesis. Upon entry into host cells, serovar Typhimurium typically resides within a membrane-bound compartment termed the Salmonella-containing vacuole (SCV). From the SCV, serovar Typhimurium can inject several effector proteins that subvert many normal host cell systems, including endocytic trafficking, cytoskeletal rearrangements, lipid signaling and distribution, and innate and adaptive host defenses. The study of these intracellular events has been made possible through the use of various imaging techniques, ranging from classic methods of transmission electron microscopy to advanced livecell fluorescence confocal microscopy. In addition, DNA microarrays have now been used to provide a "snapshot" of global gene expression in serovar Typhimurium residing within the infected host cell. This review describes key aspects of Salmonella-induced subversion of host cell activities, providing examples of imaging that have been used to elucidate these events. Serovar Typhimurium engages specific host cell machinery from initial contact with the host cell to replication within the SCV. This continuous interaction with the host cell has likely contributed to the extensive arsenal that serovar Typhimurium now possesses, including two type III secretion systems, a range of ammunition in the form of TTSS effectors, and a complex genetic regulatory network that coordinates the expression of hundreds of virulence factors.

New concepts in Salmonella virulence: the importance of reducing the intracellular growth rate in the host

The literature refers to Salmonella enterica as an intracellular bacterial pathogen that proliferates within vacuoles of mammalian cells. However, recent in vivo studies have revealed that the vast majority of infected cells contain very few intracellular bacteria (three to four organisms). Salmonella intracellular growth is also limited in cultured dendritic cells and fibroblasts, two cell types abundant in tissues located underneath the intestinal epithelium. Recently, a Salmonella factor previously known for its role as a negative regulator of intracellular growth has been shown to tightly repress certain pathogen functions upon host colonization and to be critical for virulence. The connection between virulence and the negative control of intracellular growth is further sustained by the fact that some attenuated mutants overgrow in non-phagocytic cells located in the intestinal lamina propria. These findings are changing our classical view of Salmonella as a fast growing intracellular pathogen and suggest that this pathogen may trigger responses directed to reduce the growth rate within the infected cell. These responses could play a critical role in modulating the delicate balance between disease and persistence.

Survival of Salmonella enterica serovar Typhimurium within late endosomal-lysosomal compartments of B lymphocytes is associated with the inability to use the vacuolar alternative major histocompatibility complex class I antigen-processing pathway

Gamma interferon (IFN-gamma)-activated macrophages use an alternative processing mechanism to present Salmonella antigens to CD8(+) T lymphocytes. This pathway involves processing of antigen in a vacuolar compartment followed by secretion and loading of antigenic peptides to major histocompatibility complex class I (MHC-I) molecules on macrophage cell surface and bystander cells. In this study, we have shown that B lymphocytes are not able to process Salmonella antigens using this alternative pathway. This is due to differences in Salmonella enterica serovar Typhimurium-containing vacuoles (SCV) when comparing late endosomal-lysosomal processing compartments in B lymphocytes to those in macrophages. The IFN-gamma-activated IC21 macrophage cell line and A-20 B-cell line were infected with live or dead Salmonella enterica serovar Typhimurium. The SCV in B cells were in a late endosomal-lysosomal compartment, whereas SCV in macrophages were remodeled to a non-characteristic late endosomal-lysosomal compartment over time. Despite the difference in SCV within macrophages and B lymphocytes, S. enterica serovar Typhimurium survives more efficiently within the IFN-gamma-activated B cells than in activated macrophage cell lines. Similar results were found during in vivo acute infection. We determined that a lack of remodeling of late endosomal-lysosomal compartments by live Salmonella infection in B lymphocytes is associated with the inability to use the alternative MHC-I antigen-processing pathway, providing a survival advantage to the bacterium. Our data also suggest that the B lymphocyte late endosome-lysosome environment allows the expression of Salmonella virulence mechanisms favoring B lymphocytes in addition to macrophages and dendritic cells as a reservoir during in vivo infection.

In vitro effects of essential oils fromOstericum koreanum against antibiotic-resistantSalmonella spp

The essential oil fraction of Ostericum koreanum was analyzed by GC-MS. Inhibiting activities of this oil and its main components were tested by the broth dilution assay and disk diffusion test against one antibiotic-susceptible and two resistant strains of Salmonella enteritidis and S. typhimurium, respectively. The GC-MS analysis revealed thirty-four compounds; the main components were alpha-pinene (41.12%), rho-cresol (17.99%) and 4-methylacetophenone (7.90%). The essential oil of O. koreanum and its main components were significantly effective against the tested antibiotic-susceptible strains as well as against the resistant strains of the two Salmonella species, with MICs (minimum inhibitory concentrations) ranging from 2 mg/mL to 16 mg/mL. The anti-Salmonella effects of the oils were dose-dependent on Müller-Hinton agar plates in this experiment. Additionally, checkerboard titer test results demonstrated significant combined effects of streptomycin and O. koreanum oil or cresol, one of the main components of this oil, against the two streptomycin resistant strains of S. typhimurium, with FICIs ranging from 0.12 to 0.37.

Salmonella-induced filament formation is a dynamic phenotype induced by rapidly replicating Salmonella enterica serovar typhimurium in epithelial cells

Salmonella enterica serovar Typhimurium has the fascinating ability to form tubular structures known as Salmonella-induced filaments (Sifs) in host cells. Here, we show that the prevalence of the Sif phenotype in HeLa cells is affected by host cell density, growth, and the multiplicity of infection. Sif formation was observed in cells that displayed rapid intracellular bacterial replication and was found to be dynamic, being maximal 8 to 10 h postinfection and declining thereafter. The virulence factors SpvB and SseJ were found to negatively modulate Sif formation. Our findings demonstrate the complex and dynamic nature of the Sif phenotype.

Antiprotease inactivation by Salmonella enterica released from infected macrophages

The mammalian serine protease plasmin, which has an important role in extracellular matrix degradation during cell migration, is regulated by the plasma antiprotease alpha(2)-antiplasmin (alpha(2)AP). The surface protease PgtE of Salmonella enterica serovar Typhimurium proteolytically inactivated alpha(2)AP. PgtE also activates the plasma zymogen plasminogen to plasmin, and bacteria expressing PgtE promoted degradation of extracellular matrix laminin in the presence of plasminogen and alpha(2)AP. alpha(2)AP inactivation was detected with the rough derivative of S. enterica 14028, but not with the smooth wild-type strain, suggesting that the O-antigen of lipopolysaccharide prevented contact of PgtE with the substrate molecule. After growth of S. enterica 14028 in murine J774A.1 macrophage-like cells, the infected cell lysate as well as bacteria from isolated Salmonella-containing vacuoles (SCVs) cleaved alpha(2)AP. Bacteria from SCVs produced an elevated level of PgtE and had a reduced O-antigen chain length. The lysate from S. enterica 14028-infected macrophages promoted formation of plasmin in the presence of alpha(2)AP, whereas plasmin formation by lysates from uninfected macrophages, or from macrophages infected with the pgtE-negative derivative of 14028, was inhibited by alpha(2)AP. Salmonella disseminates in the host within macrophages, which utilize plasmin for migration through tissue barriers. The results suggest that intracellular enhancement of PgtE activity in Salmonella may promote macrophage-associated proteolysis and cellular migration by altering the balance between host plasmin and alpha(2)AP.