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

Visualizing the invisible: novel approaches to visualizing bacterial proteins and host-pathogen interactions

Host-pathogen interactions play a critical role in infectious diseases, and understanding the underlying mechanisms is vital for developing effective therapeutic strategies. The visualization and characterization of bacterial proteins within host cells is key to unraveling the dynamics of these interactions. Various protein labeling strategies have emerged as powerful tools for studying host-pathogen interactions, enabling the tracking, localization, and functional analysis of bacterial proteins in real-time. However, the labeling and localization of Salmonella secreted type III secretion system (T3SS) effectors in host cells poses technical challenges. Conventional methods disrupt effector stoichiometry and often result in non-specific staining. Bulky fluorescent protein fusions interfere with effector secretion, while other tagging systems such as 4Cys-FLaSH/Split-GFP suffer from low labeling specificity and a poor signal-to-noise ratio. Recent advances in state-of-the-art techniques have augmented the existing toolkit for monitoring the translocation and dynamics of bacterial effectors. This comprehensive review delves into the bacterial protein labeling strategies and their application in imaging host-pathogen interactions. Lastly, we explore the obstacles faced and potential pathways forward in the realm of protein labeling strategies for visualizing interactions between hosts and pathogens.

Salmonella Bloodstream Infections

Salmonella is a major foodborne pathogen of both animals and humans. This bacterium is responsible for considerable morbidity and mortality world-wide. Different serovars of this genus cause diseases ranging from self-limiting gastroenteritis to a potentially fatal systemic disease known as enteric fever. Gastrointestinal infections with Salmonella are usually self-limiting and rarely require medical intervention. Bloodstream infections, on the other hand, are often fatal even with hospitalization. This review describes the routes and underlying mechanisms of the extraintestinal dissemination of Salmonella and the chronic infections that sometimes result. It includes information on the pathogenicity islands and individual virulence factors involved in systemic dissemination as well as a discussion of the host factors that mediate susceptibility. Also, the major outbreaks of invasive Salmonella disease in the tropics are described.

Shift in vacuolar to cytosolic regime of infecting Salmonella from a dual proteome perspective

By applying dual proteome profiling to Salmonella enterica serovar Typhimurium (S. Typhimurium) encounters with its epithelial host (here, S. Typhimurium infected human HeLa cells), a detailed interdependent and holistic proteomic perspective on host-pathogen interactions over the time course of infection was obtained. Data-independent acquisition (DIA)-based proteomics was found to outperform data-dependent acquisition (DDA) workflows, especially in identifying the downregulated bacterial proteome response during infection progression by permitting quantification of low abundant bacterial proteins at early times of infection when bacterial infection load is low. S. Typhimurium invasion and replication specific proteomic signatures in epithelial cells revealed interdependent host/pathogen specific responses besides pointing to putative novel infection markers and signalling responses, including regulated host proteins associated with Salmonella-modified membranes.

Speaking the host language: how Salmonella effector proteins manipulate the host

Salmonella injects over 40 virulence factors, termed effectors, into host cells to subvert diverse host cellular processes. Of these 40 Salmonella effectors, at least 25 have been described as mediating eukaryotic-like, biochemical post-translational modifications (PTMs) of host proteins, altering the outcome of infection. The downstream changes mediated by an effector's enzymatic activity range from highly specific to multifunctional, and altogether their combined action impacts the function of an impressive array of host cellular processes, including signal transduction, membrane trafficking, and both innate and adaptive immune responses. Salmonella and related Gram-negative pathogens have been a rich resource for the discovery of unique enzymatic activities, expanding our understanding of host signalling networks, bacterial pathogenesis as well as basic biochemistry. In this review, we provide an up-to-date assessment of host manipulation mediated by the Salmonella type III secretion system injectosome, exploring the cellular effects of diverse effector activities with a particular focus on PTMs and the implications for infection outcomes. We also highlight activities and functions of numerous effectors that remain poorly characterized.

Quantitative proteomic screen identifies annexin A2 as a host target for Salmonella pathogenicity island-2 effectors SopD2 and PipB2

Intracellular pathogens need to establish an intracellular replicative niche to promote survival and replication within the hostile environment inside the host cell. Salmonella enterica serovar Typhimurium (S. Typhimurium) initiates formation of the unique Salmonella-containing vacuole and an extensive network of Salmonella-induced tubules in order to survive and thrive within host cells. At least six effectors secreted by the type III secretion system encoded within Salmonella pathogenicity island-2 (SPI-2), namely SifA, SopD2, PipB2, SteA, SseJ, and SseF, purportedly manipulate host cell intracellular trafficking and establish the intracellular replicative niche for S. Typhimurium. The phenotypes of these effectors are both subtle and complex, complicating elucidation of the mechanism underpinning host cell manipulation by S. Typhimurium. In this work we used stable isotope labeling of amino acids in cell culture (SILAC) and a S. Typhimurium mutant that secretes increased amounts of effectors to identify cognate effector binding partners during infection. Using this method, we identified the host protein annexin A2 (AnxA2) as a binding partner for both SopD2 and PipB2 and were able to confirm its binding to SopD2 and PipB2 by reciprocal pull down, although there was a low level of non-specific binding of SopD2-2HA and PipB2-2HA to the Ni-Sepharose beads present. We further showed that knockdown of AnxA2 altered the intracellular positioning of the Salmonella containing vacuole (SCV). This suggests that AnxA2 plays a role in the subcellular positioning of the SCV which could potentially be mediated through protein–protein interactions with either SopD2 or PipB2. This demonstrates the value of studying effector interactions using proteomic techniques and natural effector delivery during infection rather than transfection.

Murine AML12 hepatocytes allow Salmonella Typhimurium T3SS1-independent invasion and intracellular fate

Numerous studies have demonstrated the key role of the Salmonella Pathogenicity Island 1-encoded type III secretion system (T3SS1) apparatus as well as its associated effectors in the invasion and intracellular fate of Salmonella in the host cell. Several T3SS1 effectors work together to control cytoskeleton networks and induce massive membrane ruffles, allowing pathogen internalization. Salmonella resides in a vacuole whose maturation requires that the activity of T3SS1 subverts early stages of cell signaling. Recently, we identified five cell lines in which Salmonella Typhimurium enters without using its three known invasion factors: T3SS1, Rck and PagN. The present study investigated the intracellular fate of Salmonella Typhimurium in one of these models, the murine hepatocyte cell line AML12. We demonstrated that both wild-type Salmonella and T3SS1-invalidated Salmonella followed a common pathway leading to the formation of a Salmonella containing vacuole (SCV) without classical recruitment of Rho-GTPases. Maturation of the SCV continued through an acidified phase that led to Salmonella multiplication as well as the formation of a tubular network resembling Salmonella induced filaments (SIF). The fact that in the murine AML12 hepatocyte, the T3SS1 mutant induced an intracellular fate resembling to the wild-type strain highlights the fact that Salmonella Typhimurium invasion and intracellular survival can be completely independent of T3SS1.

Bacteria-based immune therapies for cancer treatment

Engineered bacterial therapies that target the tumor immune landscape offer a new class of cancer immunotherapy. Salmonella enterica and Listeria monocytogenes are two species of bacteria that have been engineered to specifically target tumors and serve as delivery vessels for immunotherapies. Therapeutic bacteria have been engineered to deliver cytokines, gene silencing shRNA, and tumor associated antigens that increase immune activation. Bacterial therapies stimulate both the innate and adaptive immune system, change the immune dynamics of the tumor microenvironment, and offer unique strategies for targeting tumors. Bacteria have innate adjuvant properties, which enable both the delivered molecules and the bacteria themselves to stimulate immune responses. Bacterial immunotherapies that deliver cytokines and tumor-associated antigens have demonstrated clinical efficacy. Harnessing the diverse set of mechanisms that Salmonella and Listeria use to alter the tumor-immune landscape has the potential to generate many new and effective immunotherapies.

Bacterial nanotubes as a manifestation of cell death

Bacterial nanotubes are membranous structures that have been reported to function as conduits between cells to exchange DNA, proteins, and nutrients. Here, we investigate the morphology and formation of bacterial nanotubes using Bacillus subtilis. We show that nanotube formation is associated with stress conditions, and is highly sensitive to the cells' genetic background, growth phase, and sample preparation methods. Remarkably, nanotubes appear to be extruded exclusively from dying cells, likely as a result of biophysical forces. Their emergence is extremely fast, occurring within seconds by cannibalizing the cell membrane. Subsequent experiments reveal that cell-to-cell transfer of non-conjugative plasmids depends strictly on the competence system of the cell, and not on nanotube formation. Our study thus supports the notion that bacterial nanotubes are a post mortem phenomenon involved in cell disintegration, and are unlikely to be involved in cytoplasmic content exchange between live cells.

Multiple Salmonella-pathogenicity island 2 effectors are required to facilitate bacterial establishment of its intracellular niche and virulence

The pathogenesis of Salmonella Typhimurium depends on the bacterium’s ability to survive and replicate within host cells. The formation and maintenance of a unique membrane-bound compartment, termed the Salmonella-containing vacuole (SCV), is essential for S. Typhimurium pathogenesis. SCV-bound S. Typhimurium induces formation of filamentous tubules that radiate outwards from the SCV, termed Salmonella-induced filaments (SIFs). SIF formation is concomitant with the onset of replication within host epithelial cells. SIF biogenesis, formation and maintenance of the SCV, and the intracellular positioning of the SCV within the host cell requires translocation of bacterial proteins (effectors) into the host cell. Effectors secreted by the type III secretion system encoded on Salmonella pathogenicity island 2 (T3SS2) function to interfere with host cellular processes and promote both intracellular survival and replication of S. Typhimurium. Seven T3SS2-secreted effectors, SifA, SopD2, PipB2, SteA, SseJ, SseF, and SseG have previously been implicated to play complementary, redundant, and/or antagonistic roles with respect to SIF biogenesis, intracellular positioning of the SCV, and SCV membrane dynamics modulation during infection. We undertook a systematic study to delineate the contribution of each effector to these processes by (i) deleting all seven of these effectors in a single S. Typhimurium strain; and (ii) deleting combinations of multiple effectors based on putative effector function. Using this deletion mutant library, we show that each of SIF biogenesis, intracellular SCV localization, intramacrophage replication, colonization, and virulence depends on the activities of multiple effectors. Together, our data demonstrates the complex interplay between these seven effectors and highlights the necessity to study T3SS2-secreted effectors as groups, rather than studies of individual effectors.

The motility regulator flhDC drives intracellular accumulation and tumor colonization of Salmonella

Background

Salmonella have potential as anticancer therapeutic because of their innate tumor specificity. In clinical studies, this specificity has been hampered by heterogeneous responses. Understanding the mechanisms that control tumor colonization would enable the design of more robust therapeutic strains. Two mechanisms that could affect tumor colonization are intracellular accumulation and intratumoral motility. Both of these mechanisms have elements that are controlled by the master motility regulator flhDC. We hypothesized that 1) overexpressing flhDC in Salmonella increases intracellular bacterial accumulation in tumor cell masses, and 2) intracellular accumulation of Salmonella drives tumor colonization in vitro.

Methods

To test these hypotheses, we transformed Salmonella with genetic circuits that induce flhDC and express green fluorescent protein after intracellular invasion. The genetically modified Salmonella was perfused into an in vitro tumor-on-a-chip device. Time-lapse fluorescence microscopy was used to quantify intracellular and colonization dynamics within tumor masses. A mathematical model was used to determine how these mechanisms are related to each other.

Results

Overexpression of flhDC increased intracellular accumulation and tumor colonization 2.5 and 5 times more than control Salmonella, respectively (P < 0.05). Non-motile Salmonella accumulated in cancer cells 26 times less than controls (P < 0.001). Minimally invasive, ΔsipB, Salmonella colonized tumor masses 2.5 times less than controls (P < 0.05). When flhDC was selectively induced after penetration into tumor masses, Salmonella both accumulated intracellularly and colonized tumor masses 2 times more than controls (P < 0.05). Mathematical modeling of tumor colonization dynamics demonstrated that intracellular accumulation increased retention of Salmonella in tumors by effectively causing the bacteria to bind to cancer cells and preventing leakage out of the tumors. These results demonstrated that increasing intracellular bacterial density increased overall tumor colonization and that flhDC could be used to control both.

Conclusions

This study demonstrates a mechanistic link between motility, intracellular accumulation and tumor colonization. Based on our results, we envision that therapeutic strains of Salmonella could use inducible flhDC to drive tumor colonization. More intratumoral bacteria would enable delivery of higher therapeutic payloads into tumors and would improve treatment efficacy.

Electronic supplementary material

The online version of this article (10.1186/s40425-018-0490-z) contains supplementary material, which is available to authorized users.

How to do business with lysosomes: Salmonella leads the way

Pathogens have devised various strategies to alter the host endomembrane system towards building their replicative niche. This is aptly illustrated by Salmonella Typhimurium, whereby it remodels the host endolysosomal system to form a unique niche, also known as Salmonella-containing vacuole (SCV). Decades of research using in vitro cell-based infection studies have revealed intricate details of how Salmonella effectors target endocytic trafficking machinery of the host cell to acquire membrane and nutrients for bacterial replication. Unexpectedly, Salmonella requires host factors involved in endosome-lysosome fusion for its intravacuolar replication. Understanding how Salmonella obtains selective content from lysosomes, that is nutrients, but not active hydrolases, needs further exploration. Recent studies have described heterogeneity in the composition and pH of lysosomes, which will be highly relevant to explore, not only in the context of Salmonella infection, but also for other intracellular pathogens that interact with the endolysosomal pathway.

Methods to Illuminate the Role of Salmonella Effector Proteins during Infection: A Review

Intracellular bacterial pathogens like Salmonella enterica use secretion systems, such as the Type III Secretion System, to deliver virulence factors into host cells in order to invade and colonize these cells. Salmonella virulence factors include a suite of effector proteins that remodel the host cell to facilitate bacterial internalization, replication, and evasion of host immune surveillance. A number of diverse and innovative approaches have been used to identify and characterize the role of effector proteins during infection. Recent techniques for studying infection using single cell and animal models have illuminated the contribution of individual effector proteins in infection. This review will highlight the techniques applied to study Salmonella effector proteins during infection. It will describe how different approaches have revealed mechanistic details for effectors in manipulating host cellular processes including: the dynamics of effector translocation into host cells, cytoskeleton reorganization, membrane trafficking, gene regulation, and autophagy.

What the SIF Is Happening-The Role of Intracellular Salmonella-Induced Filaments

A common strategy among intracellular bacterial pathogens is to enter into a vacuolar environment upon host cell invasion. One such pathogen, Salmonella enterica, resides within the Salmonella-containing vacuole (SCV) inside epithelial cells and macrophages. Salmonella hijacks the host endosomal system to establish this unique intracellular replicative niche, forming a highly complex and dynamic network of Salmonella-induced filaments (SIFs). SIFs radiate outwards from the SCV upon onset of bacterial replication. SIF biogenesis is dependent on the activity of bacterial effector proteins secreted by the Salmonella-pathogenicity island-2 (SPI-2) encoded type III secretion system. While the presence of SIFs has been known for almost 25 years, their precise role during infection remains elusive. This review summarizes our current knowledge of SCV maturation and SIF biogenesis, and recent advances in our understanding of the role of SIFs inside cells.

The Salmonella effector SseJ disrupts microtubule dynamics when ectopically expressed in normal rat kidney cells

Salmonella effector protein SseJ is secreted by Salmonella into the host cell cytoplasm where it can then modify host cell processes. Whilst host cell small GTPase RhoA has previously been shown to activate the acyl-transferase activity of SseJ we show here an un-described effect of SseJ protein production upon microtubule dynamism. SseJ prevents microtubule collapse and this is independent of SseJ's acyl-transferase activity. We speculate that the effects of SseJ on microtubules would be mediated via its known interactions with the small GTPases of the Rho family.

A Genome-Wide siRNA Screen Implicates Spire1/2 in SipA-Driven Salmonella Typhimurium Host Cell Invasion

Salmonella Typhimurium (S. Tm) is a leading cause of diarrhea. The disease is triggered by pathogen invasion into the gut epithelium. Invasion is attributed to the SPI-1 type 3 secretion system (T1). T1 injects effector proteins into epithelial cells and thereby elicits rearrangements of the host cellular actin cytoskeleton and pathogen invasion. The T1 effector proteins SopE, SopB, SopE2 and SipA are contributing to this. However, the host cell factors contributing to invasion are still not completely understood. To address this question comprehensively, we used Hela tissue culture cells, a genome-wide siRNA library, a modified gentamicin protection assay and S. TmSipA, a sopBsopE2sopE mutant which strongly relies on the T1 effector protein SipA to invade host cells. We found that S. TmSipA invasion does not elicit membrane ruffles, nor promote the entry of non-invasive bacteria "in trans". However, SipA-mediated infection involved the SPIRE family of actin nucleators, besides well-established host cell factors (WRC, ARP2/3, RhoGTPases, COPI). Stage-specific follow-up assays and knockout fibroblasts indicated that SPIRE1 and SPIRE2 are involved in different steps of the S. Tm infection process. Whereas SPIRE1 interferes with bacterial binding, SPIRE2 influences intracellular replication of S. Tm. Hence, these two proteins might fulfill non-redundant functions in the pathogen-host interaction. The lack of co-localization hints to a short, direct interaction between S. Tm and SPIRE proteins or to an indirect effect.

The Making and Taking of Lipids: The Role of Bacterial Lipid Synthesis and the Harnessing of Host Lipids in Bacterial Pathogenesis

In order to survive environmental stressors, including those induced by growth in the human host, bacterial pathogens will adjust their membrane physiology accordingly. These physiological changes also include the use of host-derived lipids to alter their own membranes and feed central metabolic pathways. Within the host, the pathogen is exposed to many stressful stimuli. A resulting adaptation is for pathogens to scavenge the host environment for readily available lipid sources. The pathogen takes advantage of these host-derived lipids to increase or decrease the rigidity of their own membranes, to provide themselves with valuable precursors to feed central metabolic pathways, or to impact host signalling and processes. Within, we review the diverse mechanisms that both extracellular and intracellular pathogens employ to alter their own membranes as well as their use of host-derived lipids in membrane synthesis and modification, in order to increase survival and perpetuate disease within the human host. Furthermore, we discuss how pathogen employed mechanistic utilization of host-derived lipids allows for their persistence, survival and potentiation of disease. A more thorough understanding of all of these mechanisms will have direct consequences for the development of new therapeutics, and specifically, therapeutics that target pathogens, while preserving normal flora.

How Bacteria Subvert Animal Cell Structure and Function

Bacterial pathogens encode a wide variety of effectors and toxins that hijack host cell structure and function. Of particular importance are virulence factors that target actin cytoskeleton dynamics critical for cell shape, stability, motility, phagocytosis, and division. In addition, many bacteria target organelles of the general secretory pathway (e.g., the endoplasmic reticulum and the Golgi complex) and recycling pathways (e.g., the endolysosomal system) to establish and maintain an intracellular replicative niche. Recent research on the biochemistry and structural biology of bacterial effector proteins and toxins has begun to shed light on the molecular underpinnings of these host-pathogen interactions. This exciting work is revealing how pathogens gain control of the complex and dynamic host cellular environments, which impacts our understanding of microbial infectious disease, immunology, and human cell biology.

Salmonellae interactions with host processes

Salmonellae invasion and intracellular replication within host cells result in a range of diseases, including gastroenteritis, bacteraemia, enteric fever and focal infections. In recent years, considerable progress has been made in our understanding of the molecular mechanisms that salmonellae use to alter host cell physiology; through the delivery of effector proteins with specific activities and through the modulation of defence and stress response pathways. In this Review, we summarize our current knowledge of the complex interplay between bacterial and host factors that leads to inflammation, disease and, in most cases, control of the infection by its animal hosts, with a particular focus on Salmonella enterica subsp. enterica serovar Typhimurium. We also highlight gaps in our knowledge of the contributions of salmonellae and the host to disease pathogenesis, and we suggest future avenues for further study.

Salmonella modulation of the phagosome membrane, role of SseJ

Salmonellae have the ability to invade, persist and replicate within an intracellular phagosome termed the Salmonella-containing vacuole (SCV). Salmonellae alter lipid and protein content of the SCV membrane and manipulate cytoskeletal elements in contact with the SCV using the Salmonella pathogenicity island 1 (SPI-2) type III secretion system effectors. These modifications result in microtubular-based movement and morphological changes, which include endosomal tubulation of the SCV membrane. SseJ is a SPI-2 effector that localizes to the cytoplasmic face of the SCV and esterifies cholesterol through its glycerophospholipid : cholesterol acyltransferase activity. SseJ enzymatic activity as well as localization to the SCV are determined by binding to the small mammalian GTPase, RhoA. This review will focus on current knowledge about the role of SseJ in SCV membrane modification and will discuss how the hypothesis that a major role of SPI-2 effectors is to modify SCV protein and lipid content to promote bacterial intracellular survival.

Autophagic clearance of bacterial pathogens: molecular recognition of intracellular microorganisms

Autophagy is involved in several physiological and pathological processes. One of the key roles of the autophagic pathway is to participate in the first line of defense against the invasion of pathogens, as part of the innate immune response. Targeting of intracellular bacteria by the autophagic machinery, either in the cytoplasm or within vacuolar compartments, helps to control bacterial proliferation in the host cell, controlling also the spreading of the infection. In this review we will describe the means used by diverse bacterial pathogens to survive intracellularly and how they are recognized by the autophagic molecular machinery, as well as the mechanisms used to avoid autophagic clearance.

Salmonella enterica serovar Typhimurium skills to succeed in the host: virulence and regulation

Salmonella enterica serovar Typhimurium is a primary enteric pathogen infecting both humans and animals. Infection begins with the ingestion of contaminated food or water so that salmonellae reach the intestinal epithelium and trigger gastrointestinal disease. In some patients the infection spreads upon invasion of the intestinal epithelium, internalization within phagocytes, and subsequent dissemination. In that case, antimicrobial therapy, based on fluoroquinolones and expanded-spectrum cephalosporins as the current drugs of choice, is indicated. To accomplish the pathogenic process, the Salmonella chromosome comprises several virulence mechanisms. The most important virulence genes are those located within the so-called Salmonella pathogenicity islands (SPIs). Thus far, five SPIs have been reported to have a major contribution to pathogenesis. Nonetheless, further virulence traits, such as the pSLT virulence plasmid, adhesins, flagella, and biofilm-related proteins, also contribute to success within the host. Several regulatory mechanisms which synchronize all these elements in order to guarantee bacterial survival have been described. These mechanisms govern the transitions from the different pathogenic stages and drive the pathogen to achieve maximal efficiency inside the host. This review focuses primarily on the virulence armamentarium of this pathogen and the extremely complicated regulatory network controlling its success.

An amoeba phagocytosis model reveals a novel developmental switch in the insect pathogen Bacillus thuringiensis

The Bacillus cereus group bacteria contain pathogens of economic and medical importance. From security and health perspectives, the lethal mammalian pathogen Bacillus anthracis remains a serious threat. In addition the potent insect pathogen Bacillus thuringiensis is extensively used as a biological control agent for insect pests. This relies upon the industrial scale induction of bacterial spore formation with the associated production of orally toxic Cry-toxins. Understanding the ecology and potential alternative developmental fates of these bacteria is therefore important. Here we describe the use of an amoeba host model to investigate the influence of environmental bactivorous protists on both spores and vegetative cells of these pathogens. We demonstrate that the bacteria can respond to different densities of amoeba by adopting different behaviours and developmental fates. We show that spores will germinate in response to factors excreted by the amoeba, and that the bacteria can grow and reproduce on these factors. We show that in low densities of amoeba, that the bacteria will seek to colonise the surface of the amoeba as micro-colonies, resisting phagocytosis. At high amoeba densities, the bacteria change morphology into long filaments and macroscopic rope-like structures which cannot be ingested due to size exclusion. We suggest these developmental fates are likely to be important both in the ecology of these bacteria and also during animal host colonisation and immune evasion.

Impact of Salmonella enterica Type III Secretion System Effectors on the Eukaryotic Host Cell

Type III secretion systems are molecular machines used by many Gram-negative bacterial pathogens to inject proteins, known as effectors, directly into eukaryotic host cells. These proteins manipulate host signal transduction pathways and cellular processes to the pathogen’s advantage. Salmonella enterica possesses two virulence-related type III secretion systems that deliver more than forty effectors. This paper reviews our current knowledge about the functions, biochemical activities, host targets, and impact on host cells of these effectors. First, the concerted action of effectors at the cellular level in relevant aspects of the interaction between Salmonella and its hosts is analyzed. Then, particular issues that will drive research in the field in the near future are discussed. Finally, detailed information about each individual effector is provided.

Reduced invasion to human epithelial cell lines of Salmonella enterica serovar Typhi carrying S. Typhimurium sopD2

Salmonella enterica serovar Typhi and Typhimurium are closely related serovars. However, S. Typhi, a human-specific pathogen, has 5% of genes as pseudogenes, far more than S. Typhimurium, which only has 1%. One of these pseudogenes corresponds to sopD2, which in S. Typhimurium encodes an effector protein involved in Salmonella-containing vacuole biogenesis in human epithelial cell lines, which is needed for full virulence of the pathogen. We investigated whether S. Typhi trans-complemented with the functional sopD2 gene from S. Typhimurium (sopD2(STM) ) would reduce the invasion of human epithelial cell lines. Our results showed that the presence of sopD2(STM) in S. Typhi significantly modified the bacterial ability to alter cellular permeability and decrease the CFUs recovered after cell invasion of human epithelial cell line. These results add to mounting evidence that pseudogenes contribute to S. Typhi adaptation to humans.

Salmonella-induced tubular networks

Salmonella virulence relies on its capacity to replicate inside various cell types in a membrane-bound compartment, the Salmonella-containing vacuole (SCV). A unique feature of Salmonella-infected cells is the presence of tubular structures originating from and connected to the SCV, which often extend throughout the cell cytoplasm. These tubules include the well-studied Salmonella-induced filaments (SIFs), enriched in lysosomal membrane proteins. However, recent studies revealed that the Salmonella-induced tubular network is more extensive than previously thought and includes three types of tubules distinct from SIFs: sorting nexin tubules, Salmonella-induced secretory carrier membrane protein 3 (SCAMP3) tubules and lysosome-associated membrane protein 1 (LAMP1)-negative tubules. In this review, we examine the molecular mechanisms involved in the formation of Salmonella-induced tubular networks and discuss the importance of the tubules for Salmonella virulence and establishment of a Salmonella intracellular replicative niche.

The virulence protein SopD2 regulates membrane dynamics of Salmonella-containing vacuoles

Salmonella enterica serovar Typhimurium is a Gram-negative bacterial pathogen causing gastroenteritis in humans and a systemic typhoid-like illness in mice. The capacity of Salmonella to cause diseases relies on the establishment of its intracellular replication niche, a membrane-bound compartment named the Salmonella-containing vacuole (SCV). This requires the translocation of bacterial effector proteins into the host cell by type three secretion systems. Among these effectors, SifA is required for the SCV stability, the formation of Salmonella-induced filaments (SIFs) and plays an important role in the virulence of Salmonella. Here we show that the effector SopD2 is responsible for the SCV instability that triggers the cytoplasmic release of a sifA⁻ mutant. Deletion of sopD2 also rescued intra-macrophagic replication and increased virulence of sifA⁻ mutants in mice. Membrane tubular structures that extend from the SCV are the hallmark of Salmonella-infected cells. Until now, these unique structures have not been observed in the absence of SifA. The deletion of sopD2 in a sifA⁻ mutant strain re-established membrane trafficking from the SCV and led to the formation of new membrane tubular structures, the formation of which is dependent on other Salmonella effector(s). Taken together, our data demonstrate that SopD2 inhibits the vesicular transport and the formation of tubules that extend outward from the SCV and thereby contributes to the sifA⁻ associated phenotypes. These results also highlight the antagonistic roles played by SopD2 and SifA in the membrane dynamics of the vacuole, and the complex actions of SopD2, SifA, PipB2 and other unidentified effector(s) in the biogenesis and maintenance of the Salmonella replicative niche.

Salmonella typhimurium is a bacterial pathogen that causes diseases ranging from gastroenteritis to typhoid fever. This bacterium survives inside eukaryotic cells within a membrane-bound compartment, namely the Salmonella-containing vacuole. Salmonella injects proteins, named effectors, into the infected cell. These effectors change the biology of the infected cell and collectively support Salmonella replication and virulence. The effector SifA plays a key role in the bacterial vacuole stability and in the formation of membrane tubules that extend from the vacuole. Absence of SifA leads to the disruption of the vacuolar membrane and, therefore to the release of bacteria in the cytosolic compartment. Consequently, this mutant presents significant replication and virulence defects. Here, we show that an additional Salmonella effector, SopD2, is responsible for the membrane instability of the sifA⁻ vacuole. In addition, we demonstrate that SopD2 acts as an inhibitor of vesicle transport from the vacuole and that it down-modulates the formation of tubular structures. These findings describe a role for SopD2 as an antagonist of SifA in terms of vacuolar membrane dynamics.

Autophagy in immunity against intracellular bacteria

Autophagy is an innate immune defense mechanism against various intracellular bacterial pathogens, such as Salmonella enterica serovar Typhimurium (S. typhimurium), Listeria monocytogenes and Shigella flexneri. S. typhimurium uses type three secretion systems (T3SSs) to invade mammalian cells and replicate in Salmonella-containing vacuoles (SCVs). A small population of intracellular S. typhimurium is targeted by autophagy shortly after infection. Evidence suggests that these bacteria are present within SCVs that have been damaged by high levels of T3SS activity. Autophagy limits the growth of S. typhimurium in host cells. Therefore, autophagy can be considered to protect the cytosol of eukaryotic cells from bacterial colonization. L. monocytogenes secretes the pore-forming cytolysin listeriolysin O (LLO) to disrupt the phagosome and escape into the cytosol, where it acquires actin-based motility. Autophagy can target L. monocytogenes in the cytosol under specific experimental conditions. However, L. monocytogenes utilizes several virulence factors to evade being killed by the autophagy system. A newly appreciated population of L. monocytogenes undergoes slow growth in specialized vacuoles termed spacious Listeria-containing phagosomes (SLAPs), the formation of which requires bacterial LLO and host autophagy. In the cytosol, S. flexneri can also be a target for autophagy in the absence of a T3SS effector, IcsB, that normally impairs the interaction between Atg5 and wild-type bacteria. Therefore, autophagy can recognize intracellular bacteria in a variety of ways, leading to different fates for these bacteria in host cells. The inefficient autophagy of enteric bacteria in genetically compromised individuals may contribute to the pathogenesis of Crohn's disease.

Salmonella-containing vacuoles display centrifugal movement associated with cell-to-cell transfer in epithelial cells

Intracellular Salmonella enterica serovar Typhimurium (serovar Typhimurium) occupies a Salmonella-containing vacuole (SCV) where bacterial effector proteins are secreted into the host cell using type III secretion systems (T3SS). Cytoskeletal motor proteins and T3SS-delivered effector proteins facilitate SCV positioning to juxtanuclear positions where bacterial replication occurs. Here, we show that this characteristic SCV positioning is not maintained by all SCVs during infection of HeLa cells. Notably, juxtanuclear SCV localization that occurs by 8 to 14 h postinfection is followed by significant centrifugal displacement of a subset of SCVs toward the host cell periphery by 24 h postinfection. This novel phenotype requires bacterial protein synthesis, a functional Salmonella pathogenicity island 2 (SPI-2)-encoded T3SS, intact microtubules, and kinesin-1 motor protein. Bacteria lacking PipB2, a kinesin-recruiting T3SS effector, did not exhibit centrifugal displacement and remained at juxtanuclear positions throughout 24 h of infection. While levels of the SPI-2 effectors PipB2 and SifA increased during 24 h postinfection, a corresponding decrease in levels of the SPI-1 T3SS effectors SipA and SopB, both known to mediate juxtanuclear SCV positioning, was observed. A fluorescence-based assay indicated that wild-type serovar Typhimurium transferred from infected to uninfected epithelial cells while strains deficient in SPI-2 T3SS secretion or PipB2 did not. Our results reveal a novel SCV phenotype implicated in the cell-to-cell spread of serovar Typhimurium during infection.

Host factor reveals the intrinsic enzymatic activity of a bacterial effector

Evaluation of: Lossi NS, Rolhion N, Magee AI, Boyle C, Holden DW: The Salmonella SPI-2 effector SseJ exhibits eukaryotic activator-dependent phospholipase A and glycerophospholipid:cholesterol acyltransferase activity. Microbiology 154, 2680–2688 (2008). The integral role that pathogenic bacterial effectors play during infection is well accepted; however, the biochemical activities of many pathogenic bacterial effectors remain unknown. Complicating the matter is the interplay between the bacterial effector and the host-cell environment. By way of example Lossi et al. demonstrate that recombinant SseJ is catalytically inactive in vitro but can be activated by a eukaryotic proteinaceous factor that remains to be identified and characterized.

Structure and function of Salmonella SifA indicate that its interactions with SKIP, SseJ, and RhoA family GTPases induce endosomal tubulation

The Salmonella typhimurium type III secretion effector protein SifA is essential for inducing tubulation of the Salmonella phagosome and binds the mammalian kinesin-binding protein SKIP. Coexpression of SifA with the effector SseJ induced tubulation of mammalian cell endosomes, similar to that induced by Salmonella infection. Interestingly, GTP-bound RhoA, RhoB, and RhoC also induced endosomal tubulation when coexpressed with SseJ, indicating that SifA likely mimics or activates a RhoA family GTPase. The structure of SifA in complex with the PH domain of SKIP revealed that SifA has two distinct domains; the amino terminus binds SKIP, and the carboxyl terminus has a fold similar to SopE, a Salmonella effector with Rho GTPase guanine nucleotide exchange factor activity (GEF). Similar to GEFs, SifA interacted with GDP-bound RhoA, and purified SseJ and RhoA formed a protein complex, suggesting that SifA, SKIP, SseJ, and RhoA family GTPases cooperatively promote host membrane tubulation.

Dynamic behavior of Salmonella-induced membrane tubules in epithelial cells

Salmonella Typhimurium is a facultative intracellular pathogen that causes acute gastroenteritis in man. Intracellular Salmonella survive and replicate within a modified phagosome known as the Salmonella-containing vacuole (SCV). The onset of intracellular replication is accompanied by the appearance of membrane tubules, called Salmonella-induced filaments (Sifs), extending from the SCV. Sifs are enriched in late endosomal/lysosomal membrane proteins such as lysosome-associated membrane protein 1, but their formation and ability to interact with endosomal compartments are not characterized. In this study, we use live cell imaging techniques to define the dynamics of Sif formation in infected epithelial cells. At early time-points, Sifs are simple tubules extending from the surface of SCVs. These tubules are highly dynamic and exhibit bidirectional, microtubule-dependent movement. At the distal ends of individual Sif tubules, furthest from the SCV, a distinct ‘leader’ domain was often observed. At later times, Sifs develop into highly complex tubular networks that extend throughout the cell and appear less dynamic than nascent Sifs; however, individual tubules continue to display bidirectional dynamics. Sifs can acquire endocytic content by fusion, indicating a sustained interaction with the endocytic pathway. Together, these results show that these Salmonella-induced tubules form a highly dynamic network that involves both microtubule-dependent motility and interactions with endosomal compartments.

The Salmonella SPI-2 effector SseJ exhibits eukaryotic activator-dependent phospholipase A and glycerophospholipid : cholesterol acyltransferase activity

Intracellular replication of Salmonella enterica serovar Typhimurium within membrane-bound compartments, called Salmonella-containing vacuoles, depends on the activities of several effector proteins translocated by the Salmonella pathogenicity island 2 (SPI-2)-encoded type III secretion system. The SPI-2 effector protein SseJ shows similarity at the amino acid level to several GDSL lipases with glycerophospholipid : cholesterol acyltransferase (GCAT) activity. In this study, we show that catalytic serine-dependent phospholipase A (PLA) and GCAT activity of recombinant SseJ is potentiated by factor(s) present in HeLa cells, RAW macrophages and Saccharomyces cerevisiae. SseJ activity was enhanced with increasing amounts of, or preincubation with, eukaryotic cell extracts. Analysis of the activating factor(s) shows that it is soluble and heat- and protease-sensitive. We conclude that PLA and GCAT activities of SseJ are potentiated by proteinaceous eukaryotic factor(s).

YspM, a newly identified Ysa type III secreted protein of Yersinia enterocolitica

Yersinia enterocolitica has three type three secretion systems, the flagellar, the plasmid Ysc type III secretion system (T3SS), and the chromosomal Ysa T3SS. The Ysc T3SS, through the proteins it secretes (Yops), prevents phagocytosis of Y. enterocolitica and is required for disease processes in the mouse host. Recent data demonstrate a role for the Ysa T3SS during initial colonization of the mouse via secretion of Ysps (Yersinia secreted proteins). This work characterizes the discovery of a newly identified Ysa type III secreted protein, YspM. Expression of yspM is regulated by temperature, NaCl concentration, and other known regulators of the ysa system. In addition, YspM is translocated into host cells via the Ysa T3SS. YspM is homologous to proteins classified as GDSL bacterial lipases, which possess a catalytic triad of amino acids (Ser, Asp, and His) located in three of five blocks of amino acid identity. Sequence analysis of the JB580v strain of Y. enterocolitica shows that, due to a premature stop codon, it no longer encodes the fifth block of amino acid identity containing the predicted catalytic histidine. However, seven other biotype 1B strains sequenced did possess the domain. A functional difference between the forms was revealed when YspM was expressed in Saccharomyces cerevisiae. Yeast growth was uninhibited when YspM from JB580v was expressed but greatly inhibited when YspM from Y295 (YspM(Y295)) was expressed. Site-directed mutagenesis of the histidine of YspM(Y295) ablated the toxic effects. These results indicate that YspM is secreted by the Ysa T3SS and that, possibly due to lipase activity, it targets eukaryotic cellular component(s).

Manipulation of rab GTPase function by intracellular bacterial pathogens

Intracellular bacterial pathogens have evolved highly specialized mechanisms to enter and survive within their eukaryotic hosts. In order to do this, bacterial pathogens need to avoid host cell degradation and obtain nutrients and biosynthetic precursors, as well as evade detection by the host immune system. To create an intracellular niche that is favorable for replication, some intracellular pathogens inhibit the maturation of the phagosome or exit the endocytic pathway by modifying the identity of their phagosome through the exploitation of host cell trafficking pathways. In eukaryotic cells, organelle identity is determined, in part, by the composition of active Rab GTPases on the membranes of each organelle. This review describes our current understanding of how selected bacterial pathogens regulate host trafficking pathways by the selective inclusion or retention of Rab GTPases on membranes of the vacuoles that they occupy in host cells during infection.

The Salmonella-containing vacuole: moving with the times

Salmonella pathogenesis is dependent on its ability to invade and replicate within host cells. Following invasion the bacteria remain within a modified phagosome known as the Salmonella-containing vacuole (SCV), within which they will survive and replicate. Invasion and SCV biogenesis are dependent on two Type III secretion systems, T3SS1 and T3SS2, which are used to translocate distinct cohorts of bacterial effector proteins into the host cell. Elucidating the roles of individual effector proteins in SCV biogenesis has proven difficult but several distinct themes are now emerging and it is apparent that SCV biogenesis is an extremely dynamic process involving; extensive membrane remodeling, interactions with the endolysosomal pathway, actin rearrangements and microtubule-based movement and tubule extension.

Salmonellae interplay with host cells

Salmonellae are important causes of enteric diseases in all vertebrates. Characterization of the molecular mechanisms that underpin the interactions of salmonellae with their animal hosts has advanced greatly over the past decade, mainly through the study of Salmonella enterica serovar Typhimurium in tissue culture and animal models of infection. Knowledge of these bacterial processes and host responses has painted a dynamic and complex picture of the interaction between salmonellae and animal cells. This Review focuses on the molecular mechanisms of these host-pathogen interactions, in terms of their context, significance and future perspectives.

During infection of epithelial cells Salmonella enterica serovar Typhimurium undergoes a time-dependent transcriptional adaptation that results in simultaneous expression of three type 3 secretion systems

The biogenesis of the Salmonella-containing vacuole within mammalian cells has been intensively studied over recent years. However, the ability of Salmonella to sense and adapt to the intracellular environment of different types of host cells has received much less attention. To address this issue, we report the transcriptome of Salmonella enterica serovar Typhimurium SL1344 within epithelial cells and show comparisons with Salmonella gene expression inside macrophages. We report that S. Typhimurium expresses a characteristic intracellular transcriptomic signature in response to the environments it encounters within different cell types. The signature involves the upregulation of the mgtBC, pstACS and iro genes for magnesium, phosphate and iron uptake, and Salmonella pathogenicity island 2 (SPI2). Surprisingly, in addition to SPI2, the invasion-associated SPI1 pathogenicity island and the genes involved in flagellar biosynthesis were expressed inside epithelial cells at later stages of the infection, while they were constantly downregulated in macrophage-like cells. To our knowledge, this is the first report of the simultaneous transcription of all three Type Three Secretion Systems (T3SS) within an intracellular Salmonella population. We discovered that S. Typhimurium strain SL1344 was strongly cytotoxic to epithelial cells after 6 h of infection and hypothesize that the time-dependent changes in Salmonella gene expression within epithelial cells reflects the bacterial response to host cells that have been injured by the infection process.

Membrane dynamics and spatial distribution of Salmonella-containing vacuoles

Salmonella enterica are facultative intracellular bacteria that cause intestinal and systemic diseases, and replicate within host cells in a membrane-bound compartment, the Salmonella-containing vacuole. Intravacuolar bacterial replication depends on spatiotemporal regulated interactions with host cell vesicular compartments. Recent studies have shown that type III secretion effector proteins control both the vacuolar membrane dynamics and intracellular positioning of bacterial vacuoles. The functions of these effectors, which are beginning to be understood, disclose a complex hijacking of host cell microtubule motors--kinesins and dynein--and regulators of their function, and suggest interactions with the Golgi complex. Here, we discuss current models describing the mode of action of Salmonella type III secretion effector proteins involved in these processes.

SopD acts cooperatively with SopB during Salmonella enterica serovar Typhimurium invasion

The intracellular bacterial pathogen, Salmonella enterica serovar Typhimurium (S. typhimurium), causes disease in a variety of hosts. To invade and replicate in host cells, these bacteria subvert host molecular machinery using bacterial proteins, called effectors, which they translocate into host cells using specialized protein delivery systems. One of these effectors, SopD, contributes to gastroenteritis, systemic virulence and persistence of S. typhimurium in animal models of infection. Recently, SopD has been implicated in invasion of polarized epithelial cells and here we investigate the features of SopD-mediated invasion. We show that SopD plays a role in membrane fission and macropinosome formation during S. typhimurium invasion, events previously shown to be mediated by the SopB effector. We further demonstrate that SopD acts cooperatively with SopB to promote these events during invasion. Using live cell imaging we show that a SopD-GFP fusion does not localize to HeLa cell cytosol as previously described, but instead is membrane associated. Upon S. typhimurium infection of these cells, SopD-GFP is recruited to the invasion site, and this recruitment required the phosphatase activity of SopB. Our findings demonstrate a role for SopD in manipulation of host-cell membrane during S. typhimurium invasion and reveal the nature of its cooperative action with SopB.

A network of Rab GTPases controls phagosome maturation and is modulated by Salmonella enterica serovar Typhimurium

Members of the Rab guanosine triphosphatase (GTPase) family are key regulators of membrane traffic. Here we examined the association of 48 Rabs with model phagosomes containing a non-invasive mutant of Salmonella enterica serovar Typhimurium (S. Typhimurium). This mutant traffics to lysosomes and allowed us to determine which Rabs localize to a maturing phagosome. In total, 18 Rabs associated with maturing phagosomes, each with its own kinetics of association. Dominant-negative mutants of Rab23 and 35 inhibited phagosome-lysosome fusion. A large number of Rab GTPases localized to wild-type Salmonella-containing vacuoles (SCVs), which do not fuse with lysosomes. However, some Rabs (8B, 13, 23, 32, and 35) were excluded from wild-type SCVs whereas others (5A, 5B, 5C, 7A, 11A, and 11B) were enriched on this compartment. Our studies demonstrate that a complex network of Rab GTPases controls endocytic progression to lysosomes and that this is modulated by S. Typhimurium to allow its intracellular growth.

Mutational analysis of Salmonella translocated effector members SifA and SopD2 reveals domains implicated in translocation, subcellular localization and function

Salmonella enterica serovar Typhimurium is a facultative intracellular pathogen causing disease in several hosts. These bacteria use two distinct type III secretion systems that inject effector proteins into the host cell for invasion and to alter maturation of the Salmonella-containing vacuole. Members of the Salmonella translocated effector (STE) family contain a conserved N-terminal translocation signal of approximately 140 aa. In this study, the STE family member SifA was examined using deletion strategies. Small deletions (approx. 20 residues long) throughout SifA were sufficient to block its secretion and/or translocation into host cells. Transfection of HeLa cells with a GFP-SifA fusion was previously shown to be sufficient to induce formation of Sif-like tubules resembling structures present in Salmonella-infected cells. The present study showed that both N- and C-terminal domains of SifA are required for this phenotype. Furthermore, both domains could induce aggregation of Lamp1-positive compartments, provided they were coupled to the minimal C-terminal membrane-anchoring motif of SifA. Mutation or deletion of the conserved STE N-terminal WEK(I/M)xxFF translocation motif of SopD2 disrupted its association with Lamp1-positive compartments, implicating these residues in both effector translocation and subcellular localization. Interestingly, one GFP-SifA deletion mutant lacking residues 42-101, but retaining the WEK(I/M)xxFF motif, targeted the Golgi apparatus. In addition, short peptides containing the signature WEK(I/M)xxFF motif derived from the N-termini of Salmonella effectors SopD2, SseJ and SspH2 were sufficient to localize GFP to the Golgi. These studies suggest that Salmonella effectors contain multifunctional motifs or domains that regulate several effector traits, including protein secretion/translocation, localization and subversion of host cell systems. Conditions that perturb the tertiary structure of effectors can influence their localization in host cells by liberating cryptic intracellular targeting motifs.

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.

Molecular motors hijacking by intracellular pathogens

Cargoes are transported intracellularly along cytoskeletal tracks composed of actin or tubulin. Their movement involves the action of molecular motor proteins that generate directed movement along microtubules or actin filaments. The three classes of molecular motors--kinesins, dyneins and myosins--are involved in a multiplicity of biological movements such as mitosis, positioning of organelles, intracellular transports and also vesicular sorting through membrane tubulation and fission and delivery to their target compartment. Intracellular pathogens use this molecular machinery to reach their site of replication, to leave their host or to control the dynamics of membrane exchanges with their replication compartment.

SseJ deacylase activity by Salmonella enterica serovar Typhimurium promotes virulence in mice

Salmonella enterica serovar Typhimurium utilizes a type III secretion system (TTSS) encoded on Salmonella pathogenicity island-2 (SPI2) to promote intracellular replication during infection, but little is known about the molecular function of SPI2-translocated effectors and how they contribute to this process. SseJ is a SPI2 TTSS effector protein that is homologous to enzymes called glycerophospholipid-cholesterol acyltransferases and, following translocation, localizes to the Salmonella-containing vacuole and Salmonella-induced filaments. Full virulence requires SseJ, as sseJ null mutants exhibit decreased replication in cultured cells and host tissues. This work demonstrates that SseJ is an enzyme with deacylase activity in vitro and identifies three active-site residues. Catalytic SseJ mutants display wild-type translocation and subcellular localization but fail to complement the virulence defect of an sseJ null mutant. In contrast to the wild type, SseJ catalytic mutants fail to down regulate Salmonella-induced filament formation and fail to restore the sifA null mutant phenotype of loss of phagosomal membrane to sifA sseJ null double mutants, suggesting that wild-type SseJ modifies the vacuolar membrane. This is the first demonstration of an enzymatic activity for a SPI2 effector protein and provides support for the hypothesis that the deacylation of lipids on the Salmonella-containing vacuole membrane is important to bacterial pathogenesis.

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.