Characterization of Salmonella-induced filaments (Sifs) reveals a delayed interaction between Salmonella-containing vacuoles and late endocytic compartments

Candida albicans actively modulates intracellular membrane trafficking in mouse macrophage phagosomes

The intracellular trafficking/survival strategies of the opportunistic human pathogen Candida albicans are poorly understood. Here we investigated the infection of RAW264.7 macrophages with a virulent wild-type (WT) filamentous C. albicans strain and a hyphal signalling-defective mutant (efg1Delta/cph1Delta). A comparative analysis of the acquisition by phagosomes of actin, and of early/late endocytic organelles markers of the different fungal strains was performed and related to Candida's survival inside macrophages. Our results show that both fungal strains have evolved a similar mechanism to subvert the 'lysosomal' system, as seen by the inhibition of the phagosome fusion with compartments enriched in the lysobisphosphatidic acid and the vATPase, and thereby the acquisition of a low pH from the outset of infection. Besides, the virulent WT strain displayed additional specific survival strategies to prevent its targeting to compartmentsdisplaying late endosomal/lysosomal features, such as induction of active recycling out of phagosomes of the lysosomal membrane protein LAMP-1, the lysosomal protease cathepsin D and preinternalized colloidal gold. Finally, both virulent and efg1Delta/cph1Delta mutant fungal strains actively suppressed the production of macrophage nitric oxide (NO), although their cell wall extracts were potent inducers of NO.

The Salmonella effector SptP dephosphorylates host AAA+ ATPase VCP to promote development of its intracellular replicative niche

Virulence effectors delivered into intestinal epithelial cells by Salmonella trigger actin remodeling to direct pathogen internalization and intracellular replication in Salmonella-containing vacuoles (SCVs). One such effector, SptP, functions early during pathogen entry to deactivate Rho GTPases and reverse pathogen-induced cytoskeletal changes following uptake. SptP also harbors a C-terminal protein tyrosine phosphatase (PTPase) domain with no clear host substrates. Investigating SptP's longevity in infected cells, we uncover a late function of SptP, showing that it associates with SCVs, and its PTPase activity increases pathogen replication. Direct SptP binding and specific dephosphorylation of the AAA+ ATPase valosin-containing protein (VCP/p97), a facilitator of cellular membrane fusion and protein degradation, enhanced pathogen replication in SCVs. VCP and its adaptors p47 and Ufd1 were necessary for generating Salmonella-induced filaments on SCVs, a membrane fusion event characteristic of the pathogen replicative phase. Thus, Salmonella regulates the biogenesis of an intracellular niche through SptP-mediated dephosphorylation of VCP.

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.

Salmonella SPI1 effector SipA persists after entry and cooperates with a SPI2 effector to regulate phagosome maturation and intracellular replication

Salmonellae employ two type III secretion systems (T3SSs), SPI1 and SPI2, to deliver virulence effectors into mammalian cells. SPI1 effectors, including actin-binding SipA, trigger initial bacterial uptake, whereas SPI2 effectors promote subsequent replication within customized Salmonella-containing vacuoles (SCVs). SCVs sequester actin filaments and subvert microtubule-dependent motors to migrate to the perinuclear region. We demonstrate that SipA delivery continues after Salmonella internalization, with dosage being restricted by host-mediated degradation. SipA is exposed on the cytoplasmic face of the SCV, from where it stimulates bacterial replication in both nonphagocytic cells and macrophages. Although SipA is sufficient to target and redistribute late endosomes, during infection it cooperates with the SPI2 effector SifA to modulate SCV morphology and ensure perinuclear positioning. Our findings define an unexpected additional function for SipA postentry and reveal precise intracellular communication between effectors deployed by distinct T3SSs underlying SCV biogenesis.

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.

Salmonella sensing of anti-microbial mechanisms to promote survival within macrophages

Salmonella enterica is a facultative intracellular pathogen that replicates within macrophages. The interaction of this pathogen with mammalian cells is a complex process involving hundreds of bacterial products that are sensed by and alter mammalian hosts. Numerous bacterial genes and their protein products have been identified that are required for Salmonella to resist killing by host innate immunity and to modify host processes. Many of these genes are regulated by a specific bacterial sensor, the PhoQ protein, which responds to the acidified phagosome environment. PhoQ is a sensor histidine kinase, which when activated in vivo within acidified macrophage phagosomes, regulates cell surface modifications that promote resistance to antimicrobial peptides and oxidative stress, alter the phagosome to promote intracellular survival, and reduce innate immune recognition. In this review, we discuss mechanisms by which Salmonella interacts with macrophages and focus in detail on recent reports describing the role of antimicrobial peptides and pH in PhoQ activation.

Salmonella trafficking is defined by continuous dynamic interactions with the endolysosomal system

Following invasion of non-phagocytic host cells, Salmonella enterica survives and replicates within a phagosome-like compartment known as the Salmonella-containing vacuole (SCV). It is now well established that SCV biogenesis, like phagosome biogenesis, involves sequential interactions with the endocytic pathway. However, Salmonella is believed to limit these interactions and, in particular, to avoid fusion of terminal lysosomes with the SCV. In this study, we reassessed this process using a high-resolution live-cell imaging approach and found an unanticipated level of interaction between the SCV and the endocytic pathway. Direct interactions, in which late endosomal/lysosomal content was transferred to SCVs, were detected within 30 min of invasion and continued for several hours. Mechanistically, these interactions were very similar to phagosome–lysosome fusion because they were accompanied by rapid acidification of the SCV, could be blocked by chemical perturbation of microtubules or vacuolar acidification and involved the small GTPase Rab7. In comparison with vacuoles containing internalized Escherichia coli or heat-killed Salmonella, SCVs did show some delay of fusion and acidification, although, this appeared to be independent of either type III secretion system. These results provide compelling evidence that inhibition of SCV–lysosome fusion is not the major determinant in establishment of the Salmonella replicative niche in epithelial cells.

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.

Applications of cell imaging in Salmonella research

Salmonella enterica is a Gram-negative enteropathogen that can cause localized infections, typically resulting in gastroenteritis, or systemic infection, e.g., typhoid fever, in both humans and warm-blooded animals. Understanding the mechanisms by which Salmonella induce disease has been the focus of intensive research. This has revealed that Salmonella invasion requires dynamic cross-talk between the microbe and host cells, in which bacterial adherence rapidly leads to a complex sequence of cellular responses initiated by proteins translocated into the host cell by a type III secretion system (T3SS). Once these Salmonella-induced responses have resulted in bacterial invasion, proteins translocated by a second T3SS initiate further modulation of cellular activities to enable survival and replication of the invading pathogen. These processes contribute to Salmonella entry into the host and the clinical symptoms of gastrointestinal and systemic infection. Elucidation of the complex and highly dynamic pathogen-host interactions ultimately requires analysis at the level of single cells and single infection events. To achieve this goal, researchers have applied a diverse range of microscopical methods to examine Salmonella infection in models ranging from whole animal to isolated cells and simple eukaryotic organisms. For example, electron microscopy and confocal microscopy can reveal the juxtaposition of Salmonella, its products, and cellular components at high resolution. Simple light microscopy (LM) can also be used to investigate the interaction of bacteria with host cells and has advantages for live cell imaging, which enables detailed analysis of the dynamics of infection and cellular responses. Here we review the use of imaging techniques in Salmonella research and compare the capabilities of different classes of microscope to address specific types of research question. We also provide protocols and notes on several LM techniques routinely used in our own research.

Mechanism-based probe for the analysis of cathepsin cysteine proteases in living cells

Mechanism-based probes are providing new tools to evaluate the enzymatic activities of protein families in complex mixtures and to assign protein function. The application of these chemical probes for the visualization of protein labeling in cells and proteomic analysis is still challenging. As a consequence, imaging and proteomic analysis often require different sets of chemical probes. Here we describe a mechanism-based probe, azido-E-64, that can be used for both imaging and proteomics. Azido-E-64 covalently modifies active Cathepsin (Cat) B in living cells, an abundant cysteine protease involved in microbial infections, apoptosis, and cancer. Furthermore, azido-E-64 contains an azide chemical handle that can be selectively derivatized with phosphine reagents via the Staudinger ligation, which enables the imaging and proteomic analysis of Cat B. We have utilized azido-E-64 to visualize active Cat B during infection of primary macrophages with Salmonella typhimurium , an facultative intracellular bacterial pathogen. These studies demonstrated that active Cat B is specifically excluded from Salmonella -containing vacuoles, which suggests that inhibition of protease activity within bacteria-containing vacuoles may contribute to bacterial virulence.

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.

Salmonella enterica highly expressed genes are disease specific

During in vitro broth culture, bacterial gene expression is typically dominated by highly expressed factors involved in protein biosynthesis, maturation, and folding, but it is unclear if this also applies to conditions in natural environments. Here, we used a promoter trap strategy with an unstable green fluorescent protein reporter that can be detected in infected mouse tissues to identify 21 Salmonella enterica promoters with high levels of activity in a mouse enteritis model. We then measured the activities of these and 31 previously identified Salmonella promoters in both the enteritis and a murine typhoid fever model. Surprisingly, the data reveal that instead of protein biosynthesis genes, disease-specific genes such as Salmonella pathogenicity island 1 (SPI-1)-associated genes and genes involved in anaerobic respiration (enteritis) or SPI-2-associated genes and genes of the PhoP regulon (typhoid fever), respectively, dominate Salmonella in vivo gene expression. The overall functional profile of highly expressed genes suggests a marked shift in major transcriptional activities to nutrient utilization during enteritis or to fighting against the host during typhoid fever. The large proportion of known and novel essential virulence factors among the identified genes suggests that high expression levels during infection may correlate with functional relevance.

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.

The functional interface between Salmonella and its host cell: opportunities for therapeutic intervention

Salmonella is a facultative intracellular pathogen that causes diseases ranging from self-limiting enteritis to typhoid fever. This bacterium uses two type III secretion systems to deliver effector proteins directly into the host cell to promote infection and disease. Recent characterization of these virulence proteins and their host-cell targets is uncovering the molecular mechanisms of Salmonella pathogenesis and is revealing a picture of the atomic interface between this pathogen and its host. This level of analysis provides the possibility of designing novel therapeutics to disrupt infection and disease processes at the molecular level.

Maturation of Rhodococcus equi-containing vacuoles is arrested after completion of the early endosome stage

Rhodococcus equi is a facultative intracellular bacterium that can cause bronchopneumonia in foals and AIDS patients. Here, we have analyzed R. equi-containing vacuoles (RCVs) in murine macrophages by confocal laser scanning microscopy, by transmission electron microscopy and by immunochemistry upon purification. We show that RCVs progress normally through the early stages of phagosome maturation acquiring PI3P, early endosome antigen-1, and Rab5, and loosing all or much of them within minutes. Although mature RCVs possess the normally late endocytic markers, lysosome-associated membrane proteins, lysobisphosphatidic acid and Rab7, they lack other hallmark features of late endocytic organelles such as possession of cathepsin D, acid beta-glucuronidase, proton-pumping ATPase and the ability to fuse with prelabeled lysosomes. Bacterial strains possessing a virulence-associated plasmid maintain a nonacidified compartment for 48 h, whereas isogenic strains lacking such plasmids acidify progressively. In summary, RCVs represent a novel phagosome maturation stage positioned after completion of the early endosome stage and before reaching a fully mature late endosome compartment. In addition, vacuole biogenesis can be influenced by bacterial plasmids.

Interaction of the Salmonella-containing vacuole with the endocytic recycling system

Upon entry of the pathogen Salmonella enterica serovar Typhimurium into host cells, the majority of bacteria reside in a membrane-bound compartment called the Salmonella-containing vacuole (SCV). Previous studies have established that the SCV transiently interacts with early endosomes but only acquires a subset of late endosomal/lysosomal proteins. However, the complete set of interactions between the SCV and the endocytic machinery has yet to be characterized. In this study, we have shown that four characterized regulators of endocytic recycling were present on the SCV after invasion. Interaction kinetics were different for each of the regulators; ARF6 and Rab4 associated immediately, but their presence was diminished 60 min post-infection, whereas syntaxin13 and Rab11 association peaked at 60 min. Using a dominant negative approach, we determined that Rab11 regulates the recycling of CD44 from the vacuole but had no effect on major histocompatibility complex (MHC) class I recycling. In contrast, syntaxin13 regulated the recycling of MHC class I but not of CD44. We also determined that maturation of the SCV, measured by the acquisition of lysosomal associated membrane protein-1, slowed when recycling was impaired. These findings suggest that protein movement through the endocytic recycling system is regulated through at least two concurrent pathways and that efficient interaction with these pathways is necessary for maturation of the Salmonella-containing vacuole. We also demonstrate the utility of using Salmonella invasion as a model of endosomal recycling events.

The Salmonella effector PipB2 affects late endosome/lysosome distribution to mediate Sif extension

After internalization into mammalian cells, the bacterial pathogen Salmonella enterica resides within a membrane-bound compartment, the Salmonella-containing vacuole (SCV). During its maturation process, the SCV interacts extensively with host cell endocytic compartments, especially late endosomes/lysosomes (LE/Lys) at later stages. These interactions are mediated by the activities of multiple bacterial and host cell proteins. Here, we show that the Salmonella type III effector PipB2 reorganizes LE/Lys compartments in mammalian cells. This activity results in the centrifugal extension of lysosomal glycoprotein-rich membrane tubules, known as Salmonella-induced filaments, away from the SCV along microtubules. Salmonella overexpressing pipB2 induce the peripheral accumulation of LE/Lys compartments, reducing the frequency of LE/Lys tubulation. Furthermore, ectopic expression of pipB2 redistributes LE/Lys, but not other cellular organelles, to the cell periphery. In coexpression studies, PipB2 can overcome the effects of dominant-active Rab7 or Rab34 on LE/Lys positioning. Deletion of a C-terminal pentapeptide motif of PipB2, LFNEF, prevents its peripheral targeting and effect on organelle positioning. The PipB2 homologue PipB does not possess this motif or the same biological activity as PipB2. Therefore, it seems that a divergence in the biological functions of these two effectors can be accounted for by sequence divergence in their C termini.

Insertion of the bacterial type III translocon: not your average needle stick

Bacterial type III secretion systems are thought to translocate virulence proteins directly from the bacterial cytoplasm into host cells through a continuous molecular channel. Little is known about how the apparatus itself interacts with membranes and whether insertion of this structure into the host membrane has consequences for the bacteria apart from its beneficial role in delivering virulence proteins. New evidence suggests that membrane insertion of the bacterial type III apparatus might turn on a calcium-dependent signaling pathway resulting in phagolysosomal fusion.

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

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

A Salmonella typhimurium effector protein SifA is modified by host cell prenylation and S-acylation machinery

SifA is a Salmonella effector protein that is required for maintenance of the vacuolar membrane that surrounds replicating bacteria. It associates with the Salmonella-containing vacuole but how it interacts with the membrane is unknown. Here we show by immunofluorescence, S100 fractionation and Triton X-114 partitioning that the membrane association and targeting properties of SifA are influenced by a motif encoded within the C-terminal six amino acids. This sequence shares homology with both CAAX and Rab geranylgeranyl transferase prenylation motifs. We characterized the post-translational processing of SifA and showed that the cysteine residue within the CAAX motif is modified by isoprenoid addition through the action of protein geranylgeranyl transferase I. SifA was additionally modified by S-acylation of an adjacent cysteine residue. Similar modifications to host cell proteins regulate numerous functions including protein targeting, membrane association, protein-protein interaction, and signal transduction. This is the only known example of a bacterial effector protein that is modified both by mammalian cell S-acylation and prenylation machinery.

Cell tropism of Salmonella enterica

Salmonella serotypes are able to actively cross the intestinal epithelium, mainly but not exclusively through M cells in the follicle-associated epithelium of Peyer's patches. Once reaching the basal side of the epithelium, Salmonella serotypes are internalized by macrophages, dendritic cells, and neutrophils but are not found in fibroblasts or other mesenchymal cells. The outcome of the interaction between Salmonella serotypes and dendritic cells or neutrophils is detrimental to the pathogen. In some host species Salmonella serotypes find a safe haven from humoral defenses and neutrophils within macrophages, and replication within this niche appears to be a prerequisite for the development of a systemic infection. In other host species, macrophages can control bacterial growth and the infection remains localized to the intestine and mesenteric lymph nodes. This review summarizes our knowledge on the cellular tropism of Salmonella serotypes and the bacterial and host factors relevant for these interactions.

The related effector proteins SopD and SopD2 from Salmonella enterica serovar Typhimurium contribute to virulence during systemic infection of mice

Salmonella resides within host cells in a vacuole that it modifies through the action of virulence proteins called effectors. Here we examined the role of two related effectors, SopD and SopD2, in Salmonella pathogenesis. Salmonella enterica serovar Typhimurium (S. Typhimurium) mutants lacking either sopD or sopD2 were attenuated for replication in the spleens of infected mice when competed against wild-type bacteria in mixed infection experiments. A double mutant lacking both effector genes did not display an additive attenuation of virulence in these experiments. The double mutant also competed equally with both of the single mutants. Deletion of either effector impaired bacterial replication in mouse macrophages but not human epithelial cells. Deletion of sopD2 impaired Salmonella's ability to form tubular membrane filaments [Salmonella-induced filaments (Sifs)] in infected cells; the number of Sifs decreased, whereas the number of pseudo-Sifs (thought to be a precursor of Sifs) was increased. Transfection of HeLa cells with the effector SifA induced the formation of Sif-like tubules and these were observed in greater size and number after co-transfection of SifA with SopD2. In infected cells, SifA and SopD2 were localized both to Sifs and to pseudo-Sifs. In contrast, deletion of sopD had no effect on Sif formation. Our results indicate that both SopD and SopD2 contribute to virulence in mice and suggest a functional relationship between these two proteins during systemic infection of the host.

Salmonella impairs RILP recruitment to Rab7 during maturation of invasion vacuoles

After invasion of epithelial cells, Salmonella enterica Typhimurium resides within membrane-bound vacuoles where it survives and replicates. Like endocytic vesicles, the Salmonella-containing vacuoles (SCVs) undergo a maturation process that involves sequential acquisition of Rab5 and Rab7 and displacement toward the microtubule-organizing center. However, SCVs fail to merge with lysosomes and instead develop subsequently into a filamentous network that extends toward the cell periphery. We found that the initial centripetal displacement of the SCV is due to recruitment by Rab7 of Rab7-interacting lysosomal protein (RILP), an effector protein that can simultaneously associate with the dynein motor complex. Unlike the early SCVs, the Salmonella-induced filaments (Sifs) formed later are devoid of RILP and dynein, despite the presence of active Rab7 on their membranes. Kinesin seems to be involved in the elongation of Sifs. SifA, a secreted effector of Salmonella, was found to be at least partly responsible for uncoupling Rab7 from RILP in Sifs and in vitro experiments suggest that SifA may exert this effect by interacting with Rab7. We propose that, by disengaging RILP from Rab7, SifA enables the centrifugal extension of tubules from the Salmonella-containing vacuoles, thereby providing additional protected space for bacterial replication.

Microtubule motors control membrane dynamics of Salmonella-containing vacuoles

Infection of host cells by Salmonella enterica serovar Typhimurium (S. typhimurium) leads to the formation of specialised membrane-bound compartments called Salmonella-containing vacuoles (SCVs). Bacteria remain enclosed by the vacuolar membrane as they divide, and by translocating effector proteins across the vacuolar membrane through the SPI-2 type III secretion system, they interfere with host cell processes in ways that promote bacterial growth. One such effector is SifA, which is required to maintain the integrity of the vacuolar membrane and for the formation in epithelial cells of long tubular structures called Sifs that are connected to SCVs. Unknown effector(s) mediate the assembly of a meshwork of F-actin around SCVs. We report that intracellular bacteria also cause a dramatic accumulation of microtubules around S. typhimurium microcolonies in both epithelial cells and macrophages. Although this process appears to be independent of SPI-2-mediated F-actin assembly, it does require bacterial protein synthesis. In epithelial cells, microtubule accumulation is accompanied by the recruitment of both kinesin and dynein. Inhibition of the activity of either motor prevented both Sif formation and the loss of vacuolar membrane from sifA mutant bacteria. It also resulted in morphologically abnormal vacuoles enclosing wild-type bacteria, and impaired their replication. Our experiments indicate that recruitment of dynein to SCVs is dependent on Rab7 activity. We show that the recently described Rab7 effector RILP is also recruited to SCVs in a Rab7-dependent manner. However, overexpression of RILP did not restore dynein recruitment to SCVs in cells expressing dominant negative Rab7, suggesting that RILP requires a functional Rab7 to be activated at the SCV membrane, or that dynein recruitment is mediated by an effector other than RILP. Together, these experiments indicate that microtubule motors play important roles in regulating vacuolar membrane dynamics during intracellular replication of S. typhimurium.

SseG, a virulence protein that targets Salmonella to the Golgi network

Intracellular replication of the bacterial pathogen Salmonella enterica occurs in membrane-bound compartments called Salmonella-containing vacuoles (SCVs). Maturation of the SCV has been shown to occur by selective interactions with the endocytic pathway. We show here that after invasion of epithelial cells and migration to a perinuclear location, the majority of SCVs become surrounded by membranes of the Golgi network. This process is dependent on the Salmonella pathogenicity island 2 type III secretion system effector SseG. In infected cells, SseG was associated with the SCV and peripheral punctate structures. Only bacterial cells closely associated with the Golgi network were able to multiply; furthermore, mutation of sseG or disruption of the Golgi network inhibited intracellular bacterial growth. When expressed in epithelial cells, SseG co-localized extensively with markers of the trans-Golgi network. We identify a Golgi-targeting domain within SseG, and other regions of the protein that are required for localization of bacteria to the Golgi network. Therefore, replication of Salmonella in epithelial cells is dependent on simultaneous and selective interactions with both endocytic and secretory pathways.

Taking possession: biogenesis of the Salmonella-containing vacuole

The Gram-negative pathogen Salmonella enterica can survive and replicate within a variety of mammalian cells. Regardless of the cell type, internalized bacteria survive and replicate within the Salmonella-containing vacuole, the biogenesis of which is dependent on bacterially encoded virulence factors. In particular, Type III secretion systems translocate bacterial effector proteins into the eukaryotic cell where they can specifically interact with a variety of targets. Salmonella has two distinct Type III secretion systems that are believed to have completely different functions. The SPI2 system is induced intracellularly and is required for intracellular survival in macrophages; it plays no role in invasion but is categorized as being required for Salmonella-containing vacuole biogenesis. In contrast, the SPI1 Type III secretion system is induced extracellularly and is essential for invasion of nonphagocytic cells. Its role in post-invasion processes has not been well studied. Recent studies indicate that Salmonella-containing vacuole biogenesis may be more dependent on SPI1 than previously believed. Other non-SPI2 virulence factors and the host cell itself may play critical roles in determining the intracellular environment of this facultative intracellular pathogen. In this review we discuss the recent advances in determining the mechanisms by which Salmonella regulate Salmonella-containing vacuole biogenesis and the implications of these findings.

Functions and effectors of the Salmonella pathogenicity island 2 type III secretion system

Salmonella enterica uses two functionally distinct type III secretion systems encoded on the pathogenicity islands SPI-1 and SPI-2 to transfer effector proteins into host cells. A major function of the SPI-1 secretion system is to enable bacterial invasion of epithelial cells and the principal role of SPI-2 is to facilitate the replication of intracellular bacteria within membrane-bound Salmonella-containing vacuoles (SCVs). Studies of mutant bacteria defective for SPI-2-dependent secretion have revealed a variety of functions that can be attributed to this secretion system. These include an inhibition of various aspects of endocytic trafficking, an avoidance of NADPH oxidase-dependent killing, the induction of a delayed apoptosis-like host cell death, the control of SCV membrane dynamics, the assembly of a meshwork of F-actin around the SCV, an accumulation of cholesterol around the SCV and interference with the localization of inducible nitric oxide synthase to the SCV. Several effector proteins that are translocated across the vacuolar membrane in a SPI-2-dependent manner have now been identified. These are encoded both within and outside SPI-2. The characteristics of these effectors, and their relationship to the physiological functions listed above, are the subject of this review. The emerging picture is of a multifunctional system, whose activities are explained in part by effectors that control interactions between the SCV and intracellular membrane compartments.

Salmonella type III effectors PipB and PipB2 are targeted to detergent-resistant microdomains on internal host cell membranes

The intracellular pathogen, Salmonella enterica, translocates type III effectors across its vacuolar membrane into host cells. Herein we describe a new Salmonella effector, PipB2, which has sequence similarity to another type III effector, PipB. In phagocytic cells, PipB2 localizes to the Salmonella-containing vacuole (SCV) and tubular extensions from the SCV, Salmonella-induced filaments (Sifs). We used the specific targeting of PipB2 in macrophages to characterize Sifs in phagocytic cells for the first time. In epithelial cells, PipB2 has a unique localization pattern, localizing to SCVs and Sifs and additionally to vesicles at the periphery of infected cells. We further show that the N-terminal 225-amino-acid residues of PipB2 are sufficient for type III translocation and association with SCVs and Sifs, but not peripheral vesicles. Subcellular fractionation demonstrated that both PipB and PipB2 associate with host cell membranes and resist extraction by high salt, high pH and to a significant extent, non-ionic detergent. Furthermore, PipB and PipB2 are enriched in detergent-resistant microdomains (DRMs), also known as lipid rafts, present on membranes of SCVs and Sifs. The enrichment of Salmonella effectors in DRMs on these intracellular membranes probably permits specific interactions with host cell molecules that are concentrated in these signalling platforms.

Selection of small-colony variants of Salmonella enterica serovar typhimurium in nonphagocytic eucaryotic cells

Salmonella enterica strains are enteropathogenic bacteria that survive and proliferate within vacuolar compartments of epithelial and phagocytic cells. Recently, it has been reported that fibroblast cells are capable of restricting S. enterica serovar Typhimurium intracellular growth. Here, we show that prolonged residence of bacteria in the intracellular environment of fibroblasts results in the appearance of genetically stable small-colony variants (SCV). A total of 103 SCV isolates, obtained from four independent infections, were subjected to phenotypic analysis. The following phenotypes were observed: (i) delta-aminolevulinic acid auxotrophy; (ii) requirement for acetate or succinate for growth in glucose minimal medium; (iii) auxotrophy for aromatic amino acids; and (iv) reduced growth rate under aerobic conditions not linked to nutrient auxotrophy. The exact mutations responsible for the SCV phenotype in three representative isolates were mapped in the lpd, hemL, and aroD genes, which code for dihydrolipoamide dehydrogenase, glutamate-1-semyaldehyde aminotransferase, and 3-dehydroquinate dehydratase, respectively. The lpd, hemL, and aroD mutants had intracellular persistence rates in fibroblasts that were 3 to 4 logs higher than that of the parental strain and decreased susceptibility to aminoglycoside antibiotics. All three of these SCV isolates were attenuated in the BALB/c murine typhoid model. Complementation with lpd(+), hem(+), and aroD(+) genes restored the levels of intracellular persistence and antibiotic susceptibility to levels of the wild-type strain. However, virulence was not exhibited by any of the complemented strains. Altogether, our data demonstrate that similar to what it has been reported for SCV isolates of other pathogens, S. enterica SCV display enhanced intracellular persistence in eucaryotic cells and are impaired in the ability to cause overt disease. In addition, they also suggest that S. enterica SCV may be favored in vivo.

Role of lipid-mediated signal transduction in bacterial internalization

Receptor-mediated phagocytosis normally represents an important first line of immune defence. Invading microbes are internalized into phagosomes and are typically killed by exposure to a battery of microbicidal agents. To some intracellular pathogens, however, receptor-mediated phagocytosis represents an opportunity to access a protected niche within the host cell. Another type of intracellular pathogen, including Salmonella enterica serovar Typhimurium and Shigella flexneri, invade host cells in a more direct manner. These pathogens deliver effectors into the host cell via a type III secretion apparatus, initiating a ruffling response that leads to their uptake into intracellular vacuoles. Recent studies have demonstrated the importance of lipid signal transduction events in the uptake of pathogenic bacteria by both receptor-mediated phagocytosis and type III secretion-mediated invasion. In this review we highlight some of these discoveries, with a focus on phospholipid-dependent signalling events.

Salmonella typhimurium SifA effector protein requires its membrane-anchoring C-terminal hexapeptide for its biological function

SifA is a Salmonella typhimurium effector protein that is translocated across the membrane of the Salmonella-containing vacuole by the Salmonella pathogenicity island 2-encoded type III secretion system. SifA is necessary for the formation of Salmonella-induced filaments and for the maintenance of the vacuolar membrane enclosing the pathogen. We have investigated the role of the C-terminal hexapeptide of SifA as a potential site for membrane anchoring. An S. typhimurium strain carrying a deletion of the sequence encoding this hexapeptide (sifA Delta 6) was found to be attenuated for systemic virulence in mice. In mouse macrophages, sifA Delta 6 mutant bacteria displayed a reduced association with vacuolar markers, similar to that of sifA null mutant bacteria, and exhibited a dramatic replication defect. Expression of SifA in epithelial cells results in the mobilization of lysosomal glycoproteins in large vesicular structures and Sif-like tubules. This process requires the presence of the C-terminal hexapeptide domain of SifA. Ectopic expression of truncated or mutated versions of SifA affecting the C-terminal hexapeptide revealed a strong correlation between the membrane binding capability and the biological activity of the protein. Finally, the eleven C-terminal residues of SifA are shown to be sufficient to target the Aequorea green fluorescent protein to membranes. Altogether, our results indicate that membrane anchoring of SifA requires its C-terminal hexapeptide domain, which is important for the biological function of this bacterial effector.

SopD2 is a novel type III secreted effector of Salmonella typhimurium that targets late endocytic compartments upon delivery into host cells

Salmonella typhimuriumis a facultative intracellular pathogen that utilizes two type III secretion systems to deliver virulence proteins into host cells. These proteins, termed effectors, alter host cell function to allow invasion into and intracellular survival/replication within a vacuolar compartment. Here we describe SopD2, a novel member of the Salmonella translocated effector (STE) family, which share a conserved N-terminal type III secretion signal. Disruption of the sopD2 gene prolonged the survival of mice infected with a lethal dose of Salmonella typhimurium, demonstrating a significant role for this effector in pathogenesis. Expression of sopD2 was induced inside host cells and was dependent on functional ssrA/B and phoP/Q, two component regulatory systems. HA-tagged SopD2 was delivered into HeLa cells in a SPI-2-dependent manner and associated with both the Salmonella-containing vacuole and with swollen endosomes elsewhere in the cell. Subcellular fractionation confirmed that SopD2 was membrane associated in host cells, while the closely related effector SopD was localized to the cytosol. A SopD2 fusion to GFP associated with small tubular structures and large vesicles containing late endocytic markers, including Rab7. Surprisingly, expression of N-terminal amino acids 1-150 of SopD2 fused to GFP was sufficient to mediate both binding to late endosomes/lysosomes and swelling of these compartments. These findings demonstrate that the N-terminus of SopD2 is a bifunctional domain required for both type III secretion out of Salmonella as well as late endosome/lysosome targeting following translocation into host cells.

The Salmonella enterica serovar typhimurium translocated effectors SseJ and SifB are targeted to the Salmonella-containing vacuole

The Salmonella enterica serovar Typhimurium type III secretion system (TTSS) encoded in Salmonella pathogenicity island 2 (SPI-2) promotes replication within host cells and systemic infection of mice. The SPI-2 TTSS is expressed following Salmonella internalization into host cells and translocates effectors across the membrane of the Salmonella-containing vacuole (SCV). Two effectors with similar amino-terminal domains, SseJ and SifB, localize to the SCV membrane in infected HEp-2 cells and subsequently traffic away from the SCV along Salmonella-induced-filaments (Sifs). Following infection of RAW cells, SseJ and SifB localize to the SCV as well as LAMP-1-positive, vesicular-appearing structures distant from the SCV. Trafficking of SseJ and SifB away from the SCV requires the SPI-2 effector SifA. Deletion of sseJ, but not sifB, leads to attenuation of Salmonella replication in mice following intraperitoneal inoculation. The contribution of SseJ to in vivo replication is identical in wild-type and sifA deletion backgrounds, suggesting that SseJ trafficking away from the SCV along Sifs is unnecessary for its virulence function.

SseF and SseG are translocated effectors of the type III secretion system of Salmonella pathogenicity island 2 that modulate aggregation of endosomal compartments

The type III secretion system encoded by Salmonella pathogenicity island 2 (SPI 2) is important for intracellular proliferation in infected host cells. Intracellular Salmonella use this system to translocate a set of effector proteins into the host cell. We studied the role of SseF and SseG, two SPI 2-encoded proteins. SseF and SseG are not required for translocation of effector proteins such as SseJ, encoded by genes outside of SPI 2. Rather, both proteins are translocated and interact with phagosomal membranes after translocation. In infected epithelial cells the formation of Salmonella-induced filaments, endosomal aggregates rich in lysosomal glycoproteins, is dependent on the function of SPI 2. We observed that, in mutant strains deficient for sseF or sseG, the formation of aggregated endosomes can take place, but the composition of the structures is different from those observed in cells infected with Salmonella wild type. These observations indicate that SseF and SseG modulate the aggregation of host endosomes.

The Salmonella-containing vacuole is a major site of intracellular cholesterol accumulation and recruits the GPI-anchored protein CD55

Intracellular, pathogenic Salmonella typhimurium avoids phago-lysosome fusion, and exists within a unique vacuolar niche that resembles a late endosome. This model has emerged from studying the trafficking of host proteins to the Salmonella-containing vacuole (SCV). Very little is known about the role of major host lipids during infection. Here, we show using biochemical analyses as well as fluorescence microscopy, that intracellular infection perturbs the host sterol biosynthetic pathway and induces cholesterol accumulation in the SCV. Cholesterol accumulation is seen in both macrophages and epithelial cells: at the terminal stages of infection, as much as 30% of the total cellular cholesterol resides in the SCV. We find that accumulation of cholesterol in the SCV is linked to intracellular bacterial replication and may be dependent on Salmonella pathogenicity island 2 (SPI-2). Furthermore, the construction of a three-dimensional space-filling model yields novel insights into the structure of the SCV: bacteria embedded in cholesterol-rich membranes. Finally, we show that the glycosylphosphatidylinositol (GPI)-anchored protein CD55 is recruited to the SCV. These data suggest that, in contrast to prevailing models, the SCV accumulates components of cholesterol-rich early endocytic pathways during intracellular bacterial replication.

Disruption of the Salmonella-containing vacuole leads to increased replication of Salmonella enterica serovar typhimurium in the cytosol of epithelial cells

Salmonella enterica serovar Typhimurium is a facultative intracellular pathogen that inhabits a vacuolar compartment, called the Salmonella-containing vacuole (SCV), in infected host cells. Maintenance of the SCV is accomplished by SifA, and mutants of this Salmonella pathogenicity island 2 type III effector replicate more efficiently in epithelial cells. Here we demonstrate that enhanced replication of sifA mutants occurs in the cytosol of these cells. Increased replication of wild-type bacteria was also observed in cells treated with wortmannin or expressing Rab5 Q79L or Rab7 N125I, all of which caused a loss of SCV integrity. Our findings demonstrate the requirement of the host cell endosomal system for maintenance of the SCV and that loss of this compartment allows increased replication of serovar Typhimurium in the cytosol of epithelial cells.

SifA, a type III secreted effector of Salmonella typhimurium, directs Salmonella-induced filament (Sif) formation along microtubules

A unique feature of Salmonella enterica serovar typhimurium (S. typhimurium) is its ability to enter into (invade) epithelial cells and elongate the vacuole it occupies into tubular structures called Salmonella-induced filaments (Sifs). This phenotype is dependent on SifA, a Salmonella virulence factor that requires the SPI-2-encoded type III secretion system for delivery into host cells. Previous attempts to study SifA and other type III secreted proteins have been limited by a lack of suitable reagents. We examined SifA function by expressing SifA with two internal hemagglutinin epitope tags. By employing subcellular fractionation techniques, we determined that translocated SifA was membrane associated in infected HeLa cells. Confocal microscopy revealed that SifA associated with the Salmonella vacuole and with Sifs. Our analysis also revealed that microtubules serve as a scaffold for Sifs, and that SifA colocalizes with microtubules at sites of interaction between lysosomal glycoprotein-containing vesicles and Sifs. Treatment with the microtubule inhibitor nocodazole blocked Sif formation but did not prevent SifA translocation into the Salmonella vacuole. While polymerized actin has been observed on Sifs, this phenotype was transient and did not play a role in promoting or maintaining Sif formation. Our findings demonstrate the essential role of microtubule dynamics in the formation of Sifs and the utility of this epitope tagging strategy for the study of bacterial type III secreted proteins.

Complementary activities of SseJ and SifA regulate dynamics of the Salmonella typhimurium vacuolar membrane

The Salmonella pathogenicity island 2 (SPI-2) type III secretion system (TTSS) of Salmonella typhimurium is required for bacterial replication within host cells. It acts by translocating effector proteins across the membrane of the Salmonella-containing vacuole (SCV). The SifA effector is required to maintain the integrity of the SCV membrane, and for the formation in epithelial cells of Salmonella-induced filaments (Sifs), which are tubular extensions of SCVs. We have investigated the role in S. typhimurium virulence of the putative SPI-2 effector genes sifB, srfJ, sseJ and sseI. An S. typhimurium strain carrying a mutation in sseJ was mildly attenuated for systemic virulence in mice, but strains carrying mutations in either srfJ, sseI or sifB had very little or no detectable virulence defect after intraperitoneal inoculation. Expression of SseJ in HeLa cells resulted in the formation of globular membranous compartments (GMCs), the composition of which appears to be similar to that of SCV membranes and Sifs. The formation of GMCs was dependent on the serine residue of the predicted acyltransferase/lipase active site of SseJ. Transiently expressed SseJ also inhibited Sif formation by wild-type bacteria, and was found to associate with Sifs, SCV membranes and simultaneously expressed SifA. Intracellular vacuoles containing sseJ mutant bacteria appeared normal but, in contrast to a sifA mutant, a sifA sseJ double mutant strain did not lose its vacuolar membrane, indicating that loss of vacuolar membrane around sifA mutant bacteria requires the action of SseJ. Collectively, these results suggest that the combined action of SseJ and SifA regulate dynamics of the SCV membrane in infected cells.

The Salmonella pathogenicity island 1 secretion system directs cellular cholesterol redistribution during mammalian cell entry and intracellular trafficking

The bacterial pathogen Salmonella triggers its own uptake into non-phagocytic mammalian cells. Entry is induced by the delivery of bacterial effector pro-teins that subvert signalling and promote cytoskeletal rearrangement, although the molecular mechanisms that co-ordinate initial pathogen-host cell recognition remain poorly characterized. Here we show that cholesterol is essential for Salmonella uptake. Depletion and chelation of plasma membrane cholesterol specifically inhibited bacterial internalization but not adherence. Cholesterol accumulated at bacterial entry sites in cultured cells, and was retained by Salmonella-containing vacuoles following pathogen internalization. Cellular cholesterol redistribution required bacterial effector protein delivery mediated by the Salmonella pathogenicity island (SPI) 1 type III secretion system, but was independent of the SPI2-encoded system.

Microbial pathogenesis: new niches for salmonella

Salmonella occupies a vacuolar compartment inside cells of its host. Recent studies have shown that the fate of this vacuole is different in various cell types, and that the outcome of colonization is determined by both the infecting bacterium and defense mechanisms of the host cell.