Divergent roles of Salmonella pathogenicity island 2 and metabolic traits during interaction of S. enterica serovar typhimurium with host cells

Destruction of the brush border by Salmonella enterica sv. Typhimurium subverts resorption by polarized epithelial cells

Salmonella enterica serovar Typhimurium is an invasive, facultative intracellular gastrointestinal pathogen that destroys the brush border of polarized epithelial cells (PEC). The brush border is critical for the functions of PEC because it resorbs nutrients from the intestinal lumen and builds a physical barrier to infecting pathogens. The manipuation of PEC during infection by Salmonella was investigated by live-cell imaging and ultrastructural analysed of the brush border. We demonstrate that the destruction of the brush border by Salmonella significantly reduces the resorption surface of PEC along with the abrogation of endocytosis at the apical side of PEC. Both these changes in the physiology of PEC were associated with the translocation of type III secretion system effector protein SopE. Additionally, the F-actin polymerization rate at the apical side of PEC was highly altered by SopE, indicating that reduced endocytosis observed in infected PEC is related to the manipulation of F-actin polymerization mediated by SopE and, to a lesser extent, by effectors SopE2 or SipA. We further observed that in the absence of SopE, Salmonella effaced microvilli and induced reticular F-actin by bacterial accumulation during prolonged infection periods. In contrast to strains translocating SopE, strains lacking SopE did not alter resorption by PEC. Finally, we observed that after engulfment of Salmonella, ezrin was lost from the apical side of PEC and found later in early endosomes containing Salmonella. Our observations suggest that the destruction of the brush border by Salmonella may contribute to the pathogenesis of diarrhea.

Manipulation of microvillar proteins during Salmonella enterica invasion results in brush border effacement and actin remodeling

Enterocyte invasion by the gastrointestinal pathogen Salmonella enterica is accompanied by loss of brush border and massive remodeling of the actin cytoskeleton, leading to microvilli effacement and formation of membrane ruffles. These manipulations are mediated by effector proteins translocated by the Salmonella Pathogenicity Island 1-encoded type III secretion system (SPI1-T3SS). To unravel the mechanisms of microvilli effacement and contribution of SPI1-T3SS effector proteins, the dynamics of host-pathogen interactions was analyzed using live cell imaging (LCI) of polarized epithelial cells (PEC) expressing LifeAct-GFP. PEC were infected with S. enterica wild-type and mutant strains with defined defects in SPI1-T3SS effector proteins, and pharmacological inhibition of actin assembly were applied. We identified that microvilli effacement involves two distinct mechanisms: i) F-actin depolymerization mediated by villin and ii), the consumption of cytoplasmic G-actin by formation of membrane ruffles. By analyzing the contribution of individual SPI1-T3SS effector proteins, we demonstrate that SopE dominantly triggers microvilli effacement and formation of membrane ruffles. Furthermore, SopE via Rac1 indirectly manipulates villin, which culminates in F-actin depolymerization. Collectively, these results indicate that SopE has dual functions during F-actin remodeling in PEC. While SopE-Rac1 triggers F-actin polymerization and ruffle formation, activation of PLCγ and villin by SopE depolymerizes F-actin in PEC. These results demonstrate the key role of SopE in destruction of the intestinal barrier during intestinal infection by Salmonella.

Controlling of Bacterial Virulence: Evaluation of Anti-Virulence Activities of Prazosin against Salmonella enterica

Salmonella enterica is a Gram-negative orofecal transmitted pathogen that causes a wide diversity of local and systemic illnesses. Salmonella enterica utilizes several interplayed systems to regulate its invasion and pathogenesis: namely, quorum sensing (QS) and type three secretion system (T3SS). In addition, S. enterica could sense the adrenergic hormones in the surroundings that enhance its virulence. The current study aimed to evaluate the ability of α-adrenoreceptor antagonist prazosin to mitigate the virulence of S. enterica serovar Typhimurium. The prazosin effect on biofilm formation and the expression of sdiA, qseC, qseE, and T3SS-type II encoding genes was evaluated. Furthermore, the prazosin intracellular replication inside macrophage and anti-virulence activity was evaluated in vivo against S. typhimurium. The current finding showed a marked prazosin ability to compete on SdiA and QseC and downregulate their encoding genes. Prazosin significantly downregulated the virulence factors encoding genes and diminished the biofilm formation, intracellular replication inside macrophages, and in vivo protected mice. To sum up, prazosin showed significant inhibitory activities against QS, T3SS, and bacterial espionage, which documents its considered anti-virulence activities.

Terazosin Interferes with Quorum Sensing and Type Three Secretion System and Diminishes the Bacterial Espionage to Mitigate the Salmonella Typhimurium Pathogenesis

Salmonella enterica is an invasive intracellular pathogen and hires diverse systems to manipulate its survival in the host cells. Salmonella could eavesdrop on the host cells, sensing and responding to the produced adrenergic hormones and other neurotransmitters, which results in the augmentation of its virulence and establishes its accommodation in host cells. The current study aims to assess the anti-virulence effect of α-adrenergic antagonist terazosin on S. Typhimurium. Our findings show that terazosin significantly reduced S. Typhimurium adhesion and biofilm formation. Furthermore, terazosin significantly decreased invasion and intracellular replication of S. Typhimurium. Interestingly, in vivo, terazosin protected the mice from S. Typhimurium pathogenesis. To understand the terazosin anti-virulence activity, its effect on quorum sensing (QS), bacterial espionage, and type three secretion system (T3SS) was studied. Strikingly, terazosin competed on the membranal sensors that sense adrenergic hormones and down-regulated their encoding genes, which indicates the ability of terazosin to diminish the bacterial eavesdropping on the host cells. Moreover, terazosin significantly reduced the Chromobacterium violaceum QS-controlled pigment production and interfered with the QS receptor Lux-homolog Salmonella SdiA, which indicates the possible terazosin-mediated anti-QS activity. Furthermore, terazosin down-regulated the expression of T3SS encoding genes. In conclusion, terazosin may mitigate S. Typhimurium virulence owing to its hindering QS and down-regulating T3SS encoding genes besides its inhibition of bacterial espionage.

SLAMF8 Downregulates Mouse Macrophage Microbicidal Mechanisms via PI3K Pathways

Signaling lymphocytic activation molecule family 8 (SLAMF8) is involved in the negative modulation of NADPH oxidase activation. However, the impact of SLAMF8 downregulation on macrophage functionality and the microbicide mechanism remains elusive. To study this in depth, we first analyzed NADPH oxidase activation pathways in wild-type and SLAMF8-deficient macrophages upon different stimulus. Herein, we describe increased phosphorylation of the Erk1/2 and p38 MAP kinases, as well as increased phosphorylation of NADPH oxidase subunits in SLAMF8-deficient macrophages. Furthermore, using specific inhibitors, we observed that specific PI3K inhibition decreased the differences observed between wild-type and SLAMF8-deficient macrophages, stimulated with either PMA, LPS, or Salmonella typhimurium infection. Consequently, SLAMF8-deficient macrophages also showed increased recruitment of small GTPases such as Rab5 and Rab7, and the p47^phox subunit to cytoplasmic Salmonella, suggesting an impairment of Salmonella-containing vacuole (SCV) progression in SLAMF8-deficient macrophages. Enhanced iNOS activation, NO production, and IL-6 expression were also observed in the absence of SLAMF8 upon Salmonella infection, either in vivo or in vitro, while overexpression of SLAMF8 in RAW264.7 macrophages showed the opposite phenotype. In addition, SLAMF8-deficient macrophages showed increased activation of Src kinases and reduced SHP-1 phosphate levels upon IFNγ and Salmonella stimuli in comparison to wild-type macrophages. In agreement with in vitro results, Salmonella clearance was augmented in SLAMF8-deficient mice compared to that in wild-type mice. Therefore, in conclusion, SLAMF8 intervention upon bacterial infection downregulates mouse macrophage activation, and confirmed that SLAMF8 receptor could be a potential therapeutic target for the treatment of severe or unresolved inflammatory conditions.

Alteration of Salmonella enterica Virulence and Host Pathogenesis through Targeting sdiA by Using the CRISPR-Cas9 System

Salmonella enterica is a common cause of many enteric infections worldwide and is successfully engineered to deliver heterologous antigens to be used as vaccines. Clustered Regularly Interspaced Short Palindromic Repeats (CRISPRs) RNA-guided Cas9 endonuclease is a promising genome editing tool. In the current study, a CRISPR-Cas9 system was used to target S.enterica sdiA that encodes signal molecule receptor SdiA and responds to the quorum sensing (QS) signaling compounds N-acylhomoserine lactones (AHLs). For this purpose, sdiA was targeted in both S.enterica wild type (WT) and the ΔssaV mutant strain, where SsaV has been reported to be an essential component of SPI2-T3SS. The impact of sdiA mutation on S. enterica virulence was evaluated at both early invasion and later intracellular replication in both the presence and absence of AHL. Additionally, the influence of sdiA mutation on the pathogenesis S. enterica WT and mutants was investigated in vivo, using mice infection model. Finally, the minimum inhibitory concentrations (MICs) of various antibiotics against S. enterica strains were determined. Present findings show that mutation in sdiA significantly affects S.enterica biofilm formation, cell adhesion and invasion. However, sdiA mutation did not affect bacterial intracellular survival. Moreover, in vivo bacterial pathogenesis was markedly lowered in S.enterica ΔsdiA in comparison with the wild-type strain. Significantly, double-mutant sdiA and ssaV attenuated the S. enterica virulence and in vivo pathogenesis. Moreover, mutations in selected genes increased Salmonella susceptibility to tested antibiotics, as revealed by determining the MICs and MBICs of these antibiotics. Altogether, current results clearly highlight the importance of the CRISPR-Cas9 system as a bacterial genome editing tool and the valuable role of SdiA in S.enterica virulence. The present findings extend the understanding of virulence regulation and host pathogenesis of Salmonellaenterica.

Ciprofloxacin interferes with Salmonella Typhimurium intracellular survival and host virulence through repression of Salmonella pathogenicity island-2 (SPI-2) genes expression

Current study aims to characterize the influence of sub-minimum inhibitory concentration (sub-MIC) of ciprofloxacin on Salmonella intracellular survival and host virulence. Herein, Salmonella resistance patterns to various antibiotics were in agreement with those reported in previous studies. Moreover, intracellular survival of both ciprofloxacin-sensitive and -resistant Salmonella was markedly reduced upon treatment with sub-MIC of ciprofloxacin as determined by gentamicin protection assay. These findings were further confirmed using immunostaining indicating an inhibitory effect of sub-MIC of ciprofloxacin on Salmonella intracellular survival. RT-qPCR revealed that expression of genes encoding Salmonella type three secretion system (TTSS) decreased upon bacterial exposure to sub-MIC of ciprofloxacin. Furthermore, bacterial exposure to sub-MIC of ciprofloxacin significantly reduced expression of both sifA and sifB, which are important for Salmonella filaments formation within the host. Treatment of Salmonella with sub-MIC of ciprofloxacin reduced bacterial capacity to kill mice infection models. A lower mortality rate was observed in mice injected with Salmonella treated with sub-MIC of ciprofloxacin as compared with mice inoculated with untreated bacteria. Collectively, current findings indicate that, in addition to its bactericidal potential, sub-MIC of ciprofloxacin could inhibit Salmonella intracellular survival, virulence genes expression as well as host pathogenesis, providing another mechanism for ciprofloxacin in limiting Salmonella host infection.

Tracking of an Oral Salmonella-Based Vaccine for Type 1 Diabetes in Non-obese Diabetic Mice

Type 1 diabetes (T1D) arises secondary to immune-driven destruction of pancreatic β-cells and manifests as insulin-deficient hyperglycemia. We showed that oral vaccination with live attenuated Salmonella, which simultaneously delivers autoantigens and a TGFβ expression vector to immune cells in the gut mucosa, provides protection against the progression of T1D in non-obese diabetic (NOD) mice. In this study we employed the Sleeping Beauty (SB) transposon system that is composed of a transposase and transposon encoding the td-Tomato to express red fluorescent protein (RFP) to permanently mark the cells that take up the Salmonella vaccine. After animal vaccination, the transposon labeled-dendritic cells (DCs) with red fluorescence appeared throughout the secondary lymphoid tissues. Furthermore, Sleeping Beauty containing tgfβ1 gene (SB-tgfβ1) co-expressed TGFβ and RFP. The labeled DCs were detected predominantly in Peyer's patches (PP) and mesenteric lymph nodes (MLN) and expressed CD103 surface marker. CD103+ DCs induced tolerogenic effects and gut homing. TGFβ significantly increased programmed death-ligand-1 (PDL-1 or CD274) expression in the DCs in the MLN and PP of treated mice. Also, TGFβ increased cytotoxic T-lymphocyte-associated protein-4 (CTLA-4) levels in CD4+ cells in MLN and PP. Interestingly, DCs increased in all lymphatic organs of mice vaccinated with oral live Salmonella-based vaccine expressing preproinsulin (PPI), in combination with TGFβ, IL10, and subtherapeutic-doses of anti-CD3 mAb compared with vehicle-treated mice. These DCs are mostly tolerogenic in MLN and PP. Furthermore the DCs obtained from vaccine-treated but not vehicle-treated mice suppressed in vitro T cell proliferation. These data suggest that the MLN and the PP are a central hub for the beneficial anti-diabetic effects of an oral Salmonella-based vaccine prevention of diabetes in rodents.

Analysis of the capacity of Salmonella enterica Typhimurium to infect the human Placenta

INTRODUCTION

Salmonella species are gram-negative facultative intracellular bacteria that are common causes of foodborne illness in North America. Infections by Salmonella during pregnancy are a significant cause of fetal loss in domestic livestock, and fetal and maternal mortality in mice. Furthermore, Salmonella infection is associated with miscarriage, stillbirth and preterm birth in pregnant women. Despite these collective associations, the extent to which Salmonella can infect the human placenta has not been investigated.

METHODS

Human placental villous explants from several gestational ages were exposed to Salmonella enterica serovar Typhimurium (STm) ex vivo. Infection was assessed by colony forming unit assay and whole mount immunofluorescence (WMIF).

RESULTS

Viable bacteria were recovered from placental villous explants of all gestational ages tested, but the bacterial burden was highest in 1st trimester explants. Bacterial numbers did not change appreciably with time post-infection in explants from any gestational age examined, suggesting that STm does not proliferate in placental villi. Exposure of villous explants to STm strains defective for the type III secretion systems revealed that Salmonella pathogenicity island 1 is essential for optimal invasion. In contrast to placental explants, STm infected and proliferated within villous cytotrophoblast cells isolated from term placentas. WMIF demonstrated that STm was restricted primarily to the syncytiotrophoblast layer in infected placentas.

DISCUSSION

Our study demonstrates that STm can invade into the syncytiotrophoblast but does not subsequently proliferate. Thus, the syncytiotrophoblast may function as a barrier to STm infection of the fetus.

The metabolic pathways utilized by Salmonella Typhimurium during infection of host cells

Only relatively recently has research on the metabolism of intracellular bacterial pathogens within their host cells begun to appear in the published literature. This reflects in part the experimental difficulties encountered in separating host metabolic processes from those of the resident pathogen. One of the most genetically tractable and thoroughly studied intracellular bacterial pathogens, Salmonella enterica serovar Typhimurium (S. Typhimurium), has been at the forefront of metabolic studies within eukaryotic host cells. In this review, we offer a synthesis of what has been discovered to date regarding the metabolic adaptation of S. Typhimurium to survival and growth within the infected host. We discuss many studies in the context of techniques used, types of host cells, how host metabolites contribute to intracellular survival and proliferation of the pathogen and how bacterial metabolism affects the virulence and persistence of the pathogen.

speG Is Required for Intracellular Replication of Salmonella in Various Human Cells and Affects Its Polyamine Metabolism and Global Transcriptomes

The speG gene has been reported to regulate polyamine metabolism in Escherichia coli and Shigella, but its role in Salmonella remains unknown. Our preliminary studies have revealed that speG widely affects the transcriptomes of infected in vitro M and Caco-2 cells and that it is required for the intracellular replication of Salmonella enterica serovar Typhimurium (S. Typhimurium) in HeLa cells. In this study, we demonstrated that speG plays a time-dependent and cell type-independent role in the intracellular replication of S. Typhimurium. Moreover, high-performance liquid chromatography (HPLC) of four major polyamines demonstrated putrescine, spermine, and cadaverine as the leading polyamines in S. Typhimurium. The deletion of speG significantly increased the levels of the three polyamines in intracellular S. Typhimurium, suggesting the inhibitory effect of speG on the biosynthesis of these polyamines. The deletion of speG was associated with elevated levels of these polyamines in the attenuated intracellular replication of S. Typhimurium in host cells. This result was subsequently validated by the dose-dependent suppression of intracellular proliferation after the addition of the polyamines. Furthermore, our RNA transcriptome analysis of S. Typhimurium SL1344 and its speG mutant outside and inside Caco-2 cells revealed that speG regulates the genes associated with flagellar biosynthesis, fimbrial expression, and functions of types III and I secretion systems. speG also affects the expression of genes that have been rarely reported to correlate with polyamine metabolism in Salmonella, including those associated with the periplasmic nitrate reductase system, glucarate metabolism, the phosphotransferase system, cytochromes, and the succinate reductase complex in S. Typhimurium in the mid-log growth phase, as well as those in the ilv-leu and histidine biosynthesis operons of intracellular S. Typhimurium after invasion in Caco-2 cells. In the present study, we characterized the phenotypes and transcriptome effects of speG in S. Typhimurium and reviewed the relevant literature to facilitate a more comprehensive understanding of the potential role of speG in the polyamine metabolism and virulence regulation of Salmonella.

Intracellular Salmonella metabolism

Growth of Salmonella inside infected host cells is a key aspect of their ability to cause local enteritis or systemic disease. This growth depends on exploitation of host nutrients through a large Salmonella metabolism network with hundreds of metabolites and enzymes. Studies in cell culture infection models are unravelling more and more of the underlying molecular and cellular mechanisms but also show striking Salmonella metabolic plasticity depending on host cell line and experimental conditions. In vivo studies have revealed a qualitatively diverse, but quantitatively poor, host-Salmonella nutritional interface, which on one side makes Salmonella fitness largely resilient against metabolic perturbations, but on the other side severely limits Salmonella biomass generation and growth rates. This review discusses goals and techniques for studying Salmonella intracellular metabolism, summarises main results and implications, and proposes key issues that could be addressed in future studies.

The global strategy employed by Xanthomonas oryzae pv. oryzae to conquer low-oxygen tension

Xanthomonas oryzae pv. oryzae (Xoo) is a notorious rice pathogen that causes bacterial leaf blight (BLB), a destructive rice disease. Low-oxygen tension in the xylem vessels of rice stresses Xoo during infection. In this study, differentially expressed proteins under normoxic and hypoxic conditions were identified using high-performance liquid chromatography (HPLC) coupled with LC-MS/MS to investigate the global effects of low oxygen environment on Xoo PXO99^A. A statistically validated list of 187 (normoxia) and 140 (hypoxia) proteins with functional assignments was generated, allowing the reconstruction of central metabolic pathways. Ten proteins involved in aromatic amino acid biosynthesis, glycolysis, butanoate metabolism, propanoate metabolism and biological adhesion were significantly modulated under low-oxygen tension. The genes encoded by these proteins were in-frame deleted, and three of them were determined to be required for full virulence in Xoo. The contributions of these three genes to important virulence-associated functions, including extracellular polysaccharide, cell motility and antioxidative ability, are presented.

BIOLOGICAL SIGNIFICANCE

To study how Xanthomonas oryzae pv. oryzae (Xoo) conquers low-oxygen tension in the xylem of rice, we identified differentially expressed proteins under normoxic and hypoxia. We found 140 proteins that uniquely expressed under the hypoxia were involved in 33 metabolism pathways. We identified 3 proteins were required for full virulence in Xoo and related to the ability of extracellular polysaccharide, cell motility, and antioxidative. This study is helpful for broadening our knowledge of the metabolism processed of Xoo in the xylem of rice.

Salmonella Typhimurium metabolism affects virulence in the host - A mini-review

Salmonella enterica remains an important food borne pathogen in all regions of the world with S. Typhimurium as one of the most frequent serovars causing food borne disease. Since the majority of human cases are caused by food of animal origin, there has been a high interest in understanding how S. Typhimurium interacts with the animal host, mostly focusing on factors that allow it to breach host barriers and to manipulate host cells to the benefit of itself. Up to recently, such studies have ignored the metabolic factors that allow the bacteria to multiply in the host, but this is changing rapidly, and we are now beginning to understand that virulence and metabolism in the host are closely linked. The current review highlights which metabolic factors that are essential for Salmonella Typhimurium growth in the intestine, in cultured epithelial and macrophage-like cell lines, at systemic sites during invasive salmonellosis, and during long term asymptomatic colonization of the host. It also points to the limitations in our current knowledge, most notably that most studies have been carried out with few well-characterized laboratory strains, that we do not know how much the in vivo metabolism differs between serotypes, and that most results are based on challenges in the mouse model of infection. It will be very important to realize whether the current understanding of Salmonella metabolism in the host is true for all serotypes and all possible hosts.

Intracellular Salmonella induces aggrephagy of host endomembranes in persistent infections

Xenophagy has been studied in epithelial cells infected with Salmonella enterica serovar Typhimurium (S. Typhimurium). Distinct autophagy receptors target this pathogen to degradation after interacting with ubiquitin on the surface of cytosolic bacteria, and the phagophore- and autophagosome-associated protein MAP1LC3/LC3. Glycans exposed in damaged phagosomal membranes and diacylglycerol accumulation in the phagosomal membrane also trigger S. Typhimurium xenophagy. How these responses control intraphagosomal and cytosolic bacteria remains poorly understood. Here, we examined S. Typhimurium interaction with autophagy in fibroblasts, in which the pathogen displays limited growth and does not escape into the cytosol. Live-cell imaging microscopy revealed that S. Typhimurium recruits late endosomal or lysosomal compartments that evolve into a membranous aggregate connected to the phagosome. Active dynamics and integrity of the phagosomal membrane are requisite to induce such aggregates. This membranous structure increases over time to become an aggresome that engages autophagy machinery at late infection times (> 6 h postentry). The newly formed autophagosome harbors LC3 and the autophagy receptor SQSTM1/p62 but is devoid of ubiquitin and the receptor CALCOCO2/NDP52. Live-cell imaging showed that this autophagosome captures and digests within the same vacuole the aggresome and some apposed intraphagosomal bacteria. Other phagosomes move away from the aggresome and avoid destruction. Thus, host endomembrane accumulation resulting from activity of intracellular S. Typhimurium stimulates a novel type of aggrephagy that acts independently of ubiquitin and CALCOCO2, and destroys only a few bacteria. Such selective degradation might allow the pathogen to reduce its progeny and, as a consequence, to establish persistent infections.

Role of host cell-derived amino acids in nutrition of intracellular Salmonella enterica

The facultative intracellular pathogen Salmonella enterica resides in a specific membrane-bound compartment termed the Salmonella-containing vacuole (SCV). Despite being segregated from access to metabolites in the host cell cytosol, Salmonella is able to efficiently proliferate within the SCV. We set out to unravel the nutritional supply of Salmonella in the SCV with focus on amino acids. We studied the availability of amino acids by the generation of auxotrophic strains for alanine, asparagine, aspartate, glutamine, and proline in a macrophage cell line (RAW264.7) and an epithelial cell line (HeLa) and examined access to extracellular nutrients for nutrition. Auxotrophies for alanine, asparagine, or proline attenuated intracellular replication in HeLa cells, while aspartate, asparagine, or proline auxotrophies attenuated intracellular replication in RAW264.7 macrophages. The different patterns of intracellular attenuation of alanine- or aspartate-auxotrophic strains support distinct nutritional conditions in HeLa cells and RAW264.7 macrophages. Supplementation of medium with individual amino acids restored the intracellular replication of mutant strains auxotrophic for asparagine, proline, or glutamine. Similarly, a mutant strain deficient in succinate dehydrogenase was complemented by the extracellular addition of succinate. Complementation of the intracellular replication of auxotrophic Salmonella by external amino acids was possible if bacteria were proficient in the induction of Salmonella-induced filaments (SIFs) but failed in a SIF-deficient background. We propose that the ability of intracellular Salmonella to redirect host cell vesicular transport provides access of amino acids to auxotrophic strains and, more generally, is essential to continuously supply bacteria within the SCV with nutrients.

Salmonella-how a metabolic generalist adopts an intracellular lifestyle during infection

The human-pathogenic bacterium Salmonella enterica adjusts and adapts to different environments while attempting colonization. In the course of infection nutrient availabilities change drastically. New techniques, "-omics" data and subsequent integration by systems biology improve our understanding of these changes. We review changes in metabolism focusing on amino acid and carbohydrate metabolism. Furthermore, the adaptation process is associated with the activation of genes of the Salmonella pathogenicity islands (SPIs). Anti-infective strategies have to take these insights into account and include metabolic and other strategies. Salmonella infections will remain a challenge for infection biology.

The Salmonella Typhimurium effector protein SopE transiently localizes to the early SCV and contributes to intracellular replication

Salmonella enterica serovar Typhimurium (S. Tm) is a facultative intracellular pathogen that induces entry into non-phagocytic cells by a Type III secretion system (TTSS) and cognate effector proteins. Upon host cell entry, S. Tm expresses a second TTSS and subverts intracellular trafficking to create a replicative niche - the Salmonella-containing vacuole (SCV). SopE, a guanidyl exchange factor (GEF) for Rac1 and Cdc42, is translocated by the TTSS-1 upon host cell contact and promotes entry through triggering of actin-dependent ruffles. After host cell entry, the bulk of SopE undergoes proteasomal degradation. Here we show that a subfraction is however detectable on the nascent SCV membrane up to ∼ 6 h post infection. Membrane localization of SopE and the closely related SopE2 differentially depend on the Rho-GTPase-binding GEF domain, and to some extent involves also the unstructured N-terminus. SopE localizes transiently to the early SCV, dependent on continuous synthesis and secretion by the TTSS-1 during the intracellular state. Mutant strains lacking SopE or SopE2 are attenuated in early intracellular replication, while complementation restores this defect. Hence, the present study reveals an unanticipated role for SopE and SopE2 in establishing the Salmonella replicative niche, and further emphasizes the importance of entry effectors in later stages of host-cell manipulation.

Salmonella enterica invasion of polarized epithelial cells is a highly cooperative effort

The invasion of polarized epithelial cells by Salmonella enterica requires the cooperative activity of the Salmonella pathogenicity island 1 (SPI1)-encoded type III secretion system (T3SS) and the SPI4-encoded adhesin SiiE. The invasion of polarized cells is more efficient than that of nonpolarized cells, and we observed the formation of clusters of bacteria on infected cells. Here we demonstrate that the invasion of polarized cells is a highly cooperative activity. Using a novel live-cell imaging approach, we visualized the cooperative entry of multiple bacteria into ruffles induced on the apical surfaces of polarized cells. The induction of membrane ruffles by activity of Salmonella enables otherwise noninvasive mutant strains to enter polarized host cells. Bacterial motility and chemotaxis were of lower importance for cooperativity in polarized-cell invasion. We propose that cooperative invasion is a key factor for the very efficient entry into polarized cells and a factor contributing to epithelial damage and intestinal inflammation.

Pathogenesis of human enterovirulent bacteria: lessons from cultured, fully differentiated human colon cancer cell lines

Hosts are protected from attack by potentially harmful enteric microorganisms, viruses, and parasites by the polarized fully differentiated epithelial cells that make up the epithelium, providing a physical and functional barrier. Enterovirulent bacteria interact with the epithelial polarized cells lining the intestinal barrier, and some invade the cells. A better understanding of the cross talk between enterovirulent bacteria and the polarized intestinal cells has resulted in the identification of essential enterovirulent bacterial structures and virulence gene products playing pivotal roles in pathogenesis. Cultured animal cell lines and cultured human nonintestinal, undifferentiated epithelial cells have been extensively used for understanding the mechanisms by which some human enterovirulent bacteria induce intestinal disorders. Human colon carcinoma cell lines which are able to express in culture the functional and structural characteristics of mature enterocytes and goblet cells have been established, mimicking structurally and functionally an intestinal epithelial barrier. Moreover, Caco-2-derived M-like cells have been established, mimicking the bacterial capture property of M cells of Peyer's patches. This review intends to analyze the cellular and molecular mechanisms of pathogenesis of human enterovirulent bacteria observed in infected cultured human colon carcinoma enterocyte-like HT-29 subpopulations, enterocyte-like Caco-2 and clone cells, the colonic T84 cell line, HT-29 mucus-secreting cell subpopulations, and Caco-2-derived M-like cells, including cell association, cell entry, intracellular lifestyle, structural lesions at the brush border, functional lesions in enterocytes and goblet cells, functional and structural lesions at the junctional domain, and host cellular defense responses.

The role of the st313-td gene in virulence of Salmonella Typhimurium ST313

Multidrug-resistant Salmonella enterica serovar Typhimurium ST313 has emerged in sub-Saharan Africa causing severe infections in humans. Therefore, it has been speculated that this specific sequence type, ST313, carries factors associated with increased pathogenicity. We assessed the role in virulence of a gene with a yet unknown function, st313-td, detected in ST313 through comparative genomics. Additionally, the structure of the genomic island ST313-GI, harbouring the gene was determined. The gene st313-td was cloned into wild type S. Typhimurium 4/74 (4/74-C) as well as knocked out in S. Typhimurium ST313 02–03/002 (Δst313-td) followed by complementation (02-03/002-C). Δst313-td was less virulent in mice following i.p. challenge than the wild type and this phenotype could be partly complemented in trans, indicating that st313-td plays a role during systemic infection. The gene st313-td was shown not to affect invasion of cultured epithelial cells, while the absence of the gene significantly affects uptake and intracellular survival within macrophages. The gene st313-td was proven to be strongly associated to invasiveness, harboured by 92.5% of S. Typhimurium blood isolates (n = 82) and 100% of S. Dublin strains (n = 50) analysed. On the contrary, S. Typhimurium isolates of animal and food origin (n = 82) did not carry st313-td. Six human, non-blood isolates of S. Typhimurium from Belarus, China and Nepal harboured the gene and belonged to sequence types ST398 and ST19. Our data showed a global presence of the st313-td gene and in other sequence types than ST313. The gene st313-td was shown to be expressed during logarithmic phase of growth in 14 selected Salmonella strains carrying the gene. This study reveals that st313-td plays a role in S. Typhimurium ST313 pathogenesis and adds another chapter to understanding of the virulence of S. Typhimurium and in particular of the emerging sequence type ST313.

Evaluation of nanoparticles as endocytic tracers in cellular microbiology

The study of pathogen interactions with eukaryotic host cells requires the introduction of fluorescent probes to visualize processes such as endocytosis, intracellular transport or host cell manipulation by the pathogen. Here, three types of fluorescent nanoparticles (NPs), i.e. Rhodamine-labeled polymethacrylate (PMA) NPs, silica NPs and gold NPs, were employed to label the host cellular endolysosomal system and monitor manipulations by the pathogen Salmonella enterica. Using live cell imaging, we investigated the performance of NPs in cellular uptake, labeling of endocytic vesicles and lysosomes, as well as interaction with the pathogen. We show that fluorescent gold and silica, but not PMA NPs appropriately label host cell structures and efficiently track rearrangements of the host endosomal system by the activities of intracellular Salmonella. Silica NPs slightly aggregated and located in Salmonella-induced compartments as isolated dots, while gold NPs distributed uniformly inside such structures. Both silica and gold NPs exhibited no adverse impact on either host cells or pathogens, and are versatile tools for infection biology.

The putative thiosulfate sulfurtransferases PspE and GlpE contribute to virulence of Salmonella Typhimurium in the mouse model of systemic disease

The phage-shock protein PspE and GlpE of the glycerol 3-phosphate regulon of Salmonella enterica serovar Typhimurium are predicted to belong to the class of thiosulfate sulfurtransferases, enzymes that traffic sulfur between molecules. In the present study we demonstrated that the two genes contribute to S. Typhimurium virulence, as a glpE and pspE double deletion strain showed significantly decreased virulence in a mouse model of systemic infection. However, challenge of cultured epithelial cells and macrophages did not reveal any virulence-associated phenotypes. We hypothesized that their contribution to virulence could be in sulfur metabolism or by contributing to resistance to nitric oxide, oxidative stress, or cyanide detoxification. In vitro studies demonstrated that glpE but not pspE was important for resistance to H₂O₂. Since the double mutant, which was the one affected in virulence, was not affected in this assay, we concluded that resistance to oxidative stress and the virulence phenotype was most likely not linked. The two genes did not contribute to nitric oxid stress, to synthesis of essential sulfur containing amino acids, nor to detoxification of cyanide. Currently, the precise mechanism by which they contribute to virulence remains elusive.

Salmonella utilizes D-glucosaminate via a mannose family phosphotransferase system permease and associated enzymes

Salmonella enterica is a globally significant bacterial food-borne pathogen that utilizes a variety of carbon sources. We report here that Salmonella enterica subsp. enterica serovar Typhimurium (S. Typhimurium) uses d-glucosaminate (2-amino-2-deoxy-d-gluconic acid) as a carbon and nitrogen source via a previously uncharacterized mannose family phosphotransferase system (PTS) permease, and we designate the genes encoding the permease dgaABCD (d-glucosaminate PTS permease components EIIA, EIIB, EIIC, and EIID). Two other genes in the dga operon (dgaE and dgaF) were required for wild-type growth of S. Typhimurium with d-glucosaminate. Transcription of dgaABCDEF was dependent on RpoN (σ(54)) and an RpoN-dependent activator gene we designate dgaR. Introduction of a plasmid bearing dgaABCDEF under the control of the lac promoter into Escherichia coli strains DH5α, BL21, and JM101 allowed these strains to grow on minimal medium containing d-glucosaminate as the sole carbon and nitrogen source. Biochemical and genetic data support a catabolic pathway in which d-glucosaminate, as it is transported across the cell membrane, is phosphorylated at the C-6 position by DgaABCD. DgaE converts the resulting d-glucosaminate-6-phosphate to 2-keto-3-deoxygluconate 6-phosphate (KDGP), which is subsequently cleaved by the aldolase DgaF to form glyceraldehyde-3-phosphate and pyruvate. DgaF catalyzes the same reaction as that catalyzed by Eda, a KDGP aldolase in the Entner-Doudoroff pathway, and the two enzymes can substitute for each other in their respective pathways. Examination of the Integrated Microbial Genomes database revealed that orthologs of the dga genes are largely restricted to certain enteric bacteria and a few species in the phylum Firmicutes.

Differential modulation of intracellular survival of cytosolic and vacuolar pathogens by curcumin

Curcumin, a principal component of turmeric, acts as an immunomodulator regulating the host defenses in response to a diseased condition. The role of curcumin in controlling certain infectious diseases is highly controversial. It is known to alleviate symptoms of Helicobacter pylori infection and exacerbate that of Leishmania infection. We have evaluated the role of curcumin in modulating the fate of various intracellular bacterial pathogens. We show that pretreatment of macrophages with curcumin attenuates the infections caused by Shigella flexneri (clinical isolates) and Listeria monocytogenes and aggravates those caused by Salmonella enterica serovar Typhi CT18 (a clinical isolate), Salmonella enterica serovar Typhimurium, Staphylococcus aureus, and Yersinia enterocolitica. Thus, the antimicrobial nature of curcumin is not a general phenomenon. It modulated the intracellular survival of cytosolic (S. flexneri and L. monocytogenes) and vacuolar (Salmonella spp., Y. enterocolitica, and S. aureus) bacteria in distinct ways. Through colocalization experiments, we demonstrated that curcumin prevented the active phagosomal escape of cytosolic pathogens and enhanced the active inhibition of lysosomal fusion by vacuolar pathogens. A chloroquine resistance assay confirmed that curcumin retarded the escape of the cytosolic pathogens, thus reducing their inter- and intracellular spread. We have demonstrated that the membrane-stabilizing activity of curcumin is crucial for its differential effect on the virulence of the bacteria.

Salmonella enterica serovars Typhimurium and Typhi as model organisms: revealing paradigm of host-pathogen interactions

The lifestyle of intracellular pathogens has always questioned the skill of a microbiologist in the context of finding the permanent cure to the diseases caused by them. The best tool utilized by these pathogens is their ability to reside inside the host cell, which enables them to easily bypass the humoral immunity of the host, such as the complement system. They further escape from the intracellular immunity, such as lysosome and inflammasome, mostly by forming a protective vacuole-bound niche derived from the host itself. Some of the most dreadful diseases are caused by these vacuolar pathogens, for example, tuberculosis by Mycobacterium or typhoid fever by Salmonella. To deal with such successful pathogens therapeutically, the knowledge of a host-pathogen interaction system becomes primarily essential, which further depends on the use of a model system. A well characterized pathogen, namely Salmonella, suits the role of a model for this purpose, which can infect a wide array of hosts causing a variety of diseases. This review focuses on various such aspects of research on Salmonella which are useful for studying the pathogenesis of other intracellular pathogens.