pH sensing by intracellular Salmonella induces effector translocation

The Tip Complex: From Host Cell Sensing to Translocon Formation

Type III secretion systems are used by some Gram-negative bacteria to inject effector proteins into targeted eukaryotic cells for the benefit of the bacterium. The type III secretion injectisome is a complex nanomachine comprised of four main substructures including a cytoplasmic sorting platform, an envelope-spanning basal body, an extracellular needle and an exposed needle tip complex. Upon contact with a host cell, secretion is induced, resulting in the formation of a translocon pore in the host membrane. Translocon formation completes the conduit needed for effector secretion into the host cell. Control of type III secretion occurs in response to environmental signals, with the final signal being host cell contact. Secretion control occurs primarily at two sites-the cytoplasmic sorting platform, which determines secretion hierarchy, and the needle tip complex, which is critical for sensing and responding to environmental signals. The best-characterized injectisomes are those from Yersinia, Shigella and Salmonella species where there is a wealth of information on the tip complex and the two translocator proteins. Of these systems, the best characterized from a secretion regulation standpoint is Shigella. In the Shigella system, the tip complex and the first secreted translocon both contribute to secretion control and, thus, both are considered components of the tip complex. In this review, all three of these type III secretion systems are described with discussion focused on the structure and formation of the injectisome tip complex and what is known of the transition from nascent tip complex to assembled translocon pore.

Tiny architects: biogenesis of intracellular replicative niches by bacterial pathogens

Co-evolution of bacterial pathogens with their hosts led to the emergence of a stunning variety of strategies aiming at the evasion of host defences, colonisation of host cells and tissues and, ultimately, the establishment of a successful infection. Pathogenic bacteria are typically classified as extracellular and intracellular; however, intracellular lifestyle comes in many different flavours: some microbes rapidly escape to the cytosol whereas other microbes remain within vacuolar compartments and harness membrane trafficking pathways to generate their host-derived, pathogen-specific replicative niche. Here we review the current knowledge on a variety of vacuolar lifestyles, the effector proteins used by bacteria as tools to take control of the host cell and the main membrane trafficking signalling pathways targeted by vacuolar pathogens as source of membranes and nutrients. Finally, we will also discuss how host cells have developed countermeasures to sense the biogenesis of the aberrant organelles harbouring bacteria. Understanding the dialogue between bacterial and eukaryotic proteins is the key to unravel the molecular mechanisms of infection and in turn, this may lead to the identification of new targets for the development of new antimicrobials.

SsaV Interacts with SsaL to Control the Translocon-to-Effector Switch in the Salmonella SPI-2 Type Three Secretion System

Salmonella Typhimurium is an intracellular pathogen that uses the SPI-2 type III secretion system to deliver virulence proteins across the vacuole membrane surrounding intracellular bacteria. This involves a tightly regulated hierarchy of protein secretion controlled by two molecular switches. We found that SPI-2-encoded proteins SsaP and SsaU are involved in the first but not the second secretion switch. We identify key amino acids of the inner membrane protein SsaV that are required to interact with the so-called gatekeeper protein SsaL and show that the dissociation of SsaV-SsaL causes the second switch, leading to delivery of effector proteins. Our results provide insights into the molecular events controlling virulence-associated type III secretion and suggest a broader model describing how the process is regulated.

Nonflagellar type III secretion systems (nf T3SSs) form a cell surface needle-like structure and an associated translocon that deliver bacterial effector proteins into eukaryotic host cells. This involves a tightly regulated hierarchy of protein secretion. A switch involving SctP and SctU stops secretion of the needle protein. The gatekeeper protein SctW is required for secretion of translocon proteins and controls a second switch to start effector secretion. Salmonella enterica serovar Typhimurium encodes two T3SSs in Salmonella pathogenicity island 1 (SPI-1) and SPI-2. The acidic vacuole containing intracellular bacteria stimulates assembly of the SPI-2 T3SS and its translocon. Sensing the nearly neutral host cytosolic pH is required for effector translocation. Here, we investigated the involvement of SPI-2-encoded proteins SsaP (SctP), SsaU (SctU), SsaV (SctV), and SsaL (SctW) in regulation of secretion. We found that SsaP and SsaU are involved in the first but not the second secretion switch. A random-mutagenesis screen identified amino acids of SsaV that regulate translocon and effector secretion. Single substitutions in subdomain 4 of SsaV or InvA (SPI-1-encoded SctV) phenocopied mutations of their corresponding gatekeepers with respect to translocon and effector protein secretion and host cell interactions. SsaL interacted with SsaV in bacteria exposed to low ambient pH but not after the pH was raised to 7.2. We propose that SsaP and SsaU enable the apparatus to become competent for a secretion switch and facilitate the SsaL-SsaV interaction. This mediates secretion of translocon proteins until neutral pH is sensed, which causes their dissociation, resulting in arrest of translocon secretion and derepression of effector translocation.

Bacterial type III secretion systems: a complex device for the delivery of bacterial effector proteins into eukaryotic host cells

Virulence-associated type III secretion systems (T3SS) serve the injection of bacterial effector proteins into eukaryotic host cells. They are able to secrete a great diversity of substrate proteins in order to modulate host cell function, and have evolved to sense host cell contact and to inject their substrates through a translocon pore in the host cell membrane. T3SS substrates contain an N-terminal signal sequence and often a chaperone-binding domain for cognate T3SS chaperones. These signals guide the substrates to the machine where substrates are unfolded and handed over to the secretion channel formed by the transmembrane domains of the export apparatus components and by the needle filament. Secretion itself is driven by the proton motive force across the bacterial inner membrane. The needle filament measures 20-150 nm in length and is crowned by a needle tip that mediates host-cell sensing. Secretion through T3SS is a highly regulated process with early, intermediate and late substrates. A strict secretion hierarchy is required to build an injectisome capable of reaching, sensing and penetrating the host cell membrane, before host cell-acting effector proteins are deployed. Here, we review the recent progress on elucidating the assembly, structure and function of T3SS injectisomes.

Migration of Type III Secretion System Transcriptional Regulators Links Gene Expression to Secretion

Many plant-pathogenic bacteria of considerable economic importance rely on type III secretion systems (T3SSs) of the Hrc-Hrp 1 family to subvert their plant hosts. T3SS gene expression is regulated through the HrpG and HrpV proteins, while secretion is controlled by the gatekeeper HrpJ. A link between the two mechanisms was so far unknown. Here, we show that a mechanistic coupling exists between the expression and secretion cascades through the direct binding of the HrpG/HrpV heterodimer, acting as a T3SS chaperone, to HrpJ. The ternary complex is docked to the cytoplasmic side of the inner bacterial membrane and orchestrates intermediate substrate secretion, without affecting early substrate secretion. The anchoring of the ternary complex to the membranes potentially keeps HrpG/HrpV away from DNA. In their multiple roles as transcriptional regulators and gatekeeper chaperones, HrpV/HrpG provide along with HrpJ potentially attractive targets for antibacterial strategies.

On the basis of scientific/economic importance, Pseudomonas syringae and Erwinia amylovora are considered among the top 10 plant-pathogenic bacteria in molecular plant pathology. Both employ type III secretion systems (T3SSs) of the Hrc-Hrp 1 family to subvert their plant hosts. For Hrc-Hrp 1, no functional link was known between the key processes of T3SS gene expression and secretion. Here, we show that a mechanistic coupling exists between expression and secretion cascades, through formation of a ternary complex involving the T3SS proteins HrpG, HrpV, and HrpJ. Our results highlight the functional and structural properties of a hitherto-unknown complex which orchestrates intermediate T3SS substrate secretion and may lead to better pathogen control through novel targets for antibacterial strategies.

Vibrio parahaemolyticus Senses Intracellular K+ To Translocate Type III Secretion System 2 Effectors Effectively

Many Gram-negative bacterial symbionts and pathogens employ a type III secretion system (T3SS) to live in contact with eukaryotic cells. Because T3SSs inject bacterial proteins (effectors) directly into host cells, the switching of secretory substrates between translocators and effectors in response to host cell attachment is a crucial step for the effective delivery of effectors. Here, we show that the protein secretion switch of Vibrio parahaemolyticus T3SS2, which is a main contributor to the enteropathogenicity of a food poisoning bacterium, is regulated by two gatekeeper proteins, VgpA and VgpB. In the absence of these gatekeepers, effector secretion was activated, but translocator secretion was abolished, causing the loss of virulence. We found that the K⁺ concentration, which is high inside the host cell but low outside, is a key factor for VgpA- and VgpB-mediated secretion switching. Exposure of wild-type bacteria to K⁺ ions provoked both gatekeeper and effector secretions but reduced the level of secretion of translocators. The secretion protein profile of wild-type bacteria cultured with 0.1 M KCl was similar to that of gatekeeper mutants. Furthermore, depletion of K⁺ ions in host cells diminished the efficiency of T3SS2 effector translocation. Thus, T3SS2 senses the high intracellular concentration of K⁺ of the host cell so that T3SS2 effectors can be effectively injected.

The pathogenesis of many Gram-negative bacterial pathogens arises from a type III secretion system (T3SS), whereby bacterial proteins (effectors) are directly injected into host cells. The injected effectors then modify host cell functions. For effective delivery of effector proteins, bacteria need to both recognize host cell attachment and switch the type of secreted proteins. Here, we identified gatekeeper proteins that play important roles in a T3SS2 secretion switch of Vibrio parahaemolyticus, a causative agent of food-borne gastroenteritis. We also found that K⁺, which is present in high concentrations inside the host cell but in low concentrations outside, is a key factor for the secretion switch. Thus, V. parahaemolyticus senses the high intracellular K⁺ concentration, triggering the effective injection of effectors.

Immunization with recombinant Salmonella expressing SspH2-EscI protects mice against wild type Salmonella infection

Background

Enhancing caspase-1 activation in macrophages is helpful for the clearance of intracellular bacteria in mice. Our previous studies have shown that EscI, an inner rod protein of type III system in E. coli can enhance caspase-1 activation. The purpose of this study was to further analyze the prospect of EscI in the vaccine design.

Results

A recombinant Salmonella expressing SspH2-EscI fusion protein using the promotor of Salmonella effector SspH2, X4550(pYA3334-P-SspH2-EscI), was constructed. A control recombinant Salmonella expressing SspH2 only X4550(pYA3334-P-SspH2) was also constructed. In the early stage of in vitro infection of mouse peritoneal macrophages, X4550(pYA3334-P-SspH2-EscI) could significantly (P < 0.05) enhance intracellular caspase-1 activation and pyroptotic cell death of macrophages, when compared with X4550(pYA3334-P-SspH2). Except for the intracellular pH value, the levels of reactive oxygen species, intracellular concentration of calcium ions, nitric oxide and mitochondrial membrane potential in macrophages were not significantly different between the cells infected with X4550(pYA3334-P-SspH2-EscI) and those infected with X4550(pYA3334-P-SspH2). Besides, only lower inflammatory cytokines secretion was induced by X4550(pYA3334-P-SspH2-EscI) than X4550(pYA3334-P-SspH2). After intravenous immunization of mice (1 × 10⁶ cfu/mouse), the colonization of X4550(pYA3334-P-SspH2-EscI) in mice was significantly limited at one week post immunization (wpi), when compared with X4550(pYA3334-P-SspH2) (P < 0.05). The population of activated CD8⁺T lymphocytes in mouse spleens induced by X4550(pYA3334-P-SspH2-EscI) was lower than that induced by X4550(pYA3334-P-SspH2) at 2–3 wpi, and the ratio of CD4⁺T cells to CD8⁺T cells decreased. The blood coagulation assay indicated that no significant difference was found between X4550(pYA3334-P-SspH2-EscI) and uninfected control, while X4550(pYA3334-P-SspH2) could induce the quick coagulation. Notably, immunization of X4550(pYA3334-P-SspH2-EscI) could limit the colonization of challenged Salmonella strains in the early stage of infection and provide more effective protection.

Conclusion

The activation of caspase-1 in macrophages by EscI can be used in the design of live attenuated Salmonella vaccine candidate.

Systematic Identification of Intracellular-Translocated Candidate Effectors in Edwardsiella piscicida

Many bacterial pathogens inject effectors directly into host cells to target a variety of host cellular processes and promote bacterial dissemination and survival. Identifying the bacterial effectors and elucidating their functions are central to understanding the molecular pathogenesis of these pathogens. Edwardsiella piscicida is a pathogen with a wide host range, and very few of its effectors have been identified to date. Here, based on the genes significantly regulated by macrophage infection, we identified 25 intracellular translocation-positive candidate effectors, including all five previously reported effectors, namely EseG, EseJ, EseH, EseK, and EvpP. A subsequent secretion analysis revealed diverse secretion patterns for the 25 effector candidates, suggesting that multiple transport pathways were involved in the internalization of these candidate effectors. Further, we identified two novel type VI secretion system (T6SS) putative effectors and three outer membrane vesicles (OMV)-dependent putative effectors among the candidate effectors described above, and further analyzed their contribution to bacterial virulence in a zebrafish model. This work demonstrates an effective approach for screening bacterial effectors and expands the effectors repertoire in E. piscicida.

Novel insights into the mechanism of SepL-mediated control of effector secretion in enteropathogenic Escherichia coli

Type three secretion systems (T3SSs) are virulence determinants employed by several pathogenic bacteria as molecular syringes to inject effector proteins into host cells. Diarrhea‐producing enteropathogenic Escherichia coli (EPEC) uses a T3SS to colonize the intestinal tract. T3S is a highly coordinated process that ensures hierarchical delivery of three classes of substrates: early (inner rod and needle subunits), middle (translocators), and late (effectors). Translocation of effectors is triggered upon host‐cell contact in response to different environmental cues, such as calcium levels. The T3S substrate specificity switch from middle to late substrates in EPEC is regulated by the SepL and SepD proteins, which interact with each other and form a trimeric complex with the chaperone CesL. In this study, we investigated the link between calcium concentration and secretion regulation by the gatekeeper SepL. We found that calcium depletion promotes late substrate secretion in a translocon‐independent manner. Furthermore, the stability, formation, and subcellular localization of the SepL/SepD/CesL regulatory complex were not affected by the absence of calcium. In addition, we demonstrate that SepL interacts in a calcium‐independent manner with the major export gate component EscV, which in turn interacts with both middle and late secretion substrates, providing a docking site for T3S. These results suggest that EscV serves as a binding platform for both the SepL regulatory protein and secreted substrates during the ordered assembly of the T3SS.

Salmonella exploits the host endolysosomal tethering factor HOPS complex to promote its intravacuolar replication

Salmonella enterica serovar typhimurium extensively remodels the host late endocytic compartments to establish its vacuolar niche within the host cells conducive for its replication, also known as the Salmonella-containing vacuole (SCV). By maintaining a prolonged interaction with late endosomes and lysosomes of the host cells in the form of interconnected network of tubules (Salmonella-induced filaments or SIFs), Salmonella gains access to both membrane and fluid-phase cargo from these compartments. This is essential for maintaining SCV membrane integrity and for bacterial intravacuolar nutrition. Here, we have identified the multisubunit lysosomal tethering factor—HOPS (HOmotypic fusion and Protein Sorting) complex as a crucial host factor facilitating delivery of late endosomal and lysosomal content to SCVs, providing membrane for SIF formation, and nutrients for intravacuolar bacterial replication. Accordingly, depletion of HOPS subunits significantly reduced the bacterial load in non-phagocytic and phagocytic cells as well as in a mouse model of Salmonella infection. We found that Salmonella effector SifA in complex with its binding partner; SKIP, interacts with HOPS subunit Vps39 and mediates recruitment of this tethering factor to SCV compartments. The lysosomal small GTPase Arl8b that binds to, and promotes membrane localization of Vps41 (and other HOPS subunits) was also required for HOPS recruitment to SCVs and SIFs. Our findings suggest that Salmonella recruits the host late endosomal and lysosomal membrane fusion machinery to its vacuolar niche for access to host membrane and nutrients, ensuring its intracellular survival and replication.

Intracellular pathogens have devised various strategies to subvert the host membrane trafficking pathways for their growth and survival inside the host cells. Salmonella is one such successful intracellular pathogen that redirects membrane and nutrients from the host endocytic compartments to its replicative niche known as the Salmonella-containing vacuole (SCV) via establishing an interconnected network of tubules (Salmonella-induced filaments or SIFs) that form a continuum with the SCVs. How Salmonella ensures a constant supply of endocytic cargo required for its survival and growth remained unexplored. Our work uncovers a strategy evolved by Salmonella wherein it secretes a bacterial effector into the host cytosol that recruits component of host vesicle fusion machinery-HOPS complex to SCVs and SIFs. HOPS complex promotes docking of the late endocytic compartments at the SCV membrane, prior to their fusion. Thus, depletion of HOPS subunits both in cultured cell lines as well as a mouse model inhibits Salmonella replication, likely due to reduced access to host membranes and nutrients by the vacuolar bacteria. These findings provide mechanistic insights into how this pathogen reroutes the host’s endocytic transport towards its vacuole, ensuring its own intracellular survival and replication.

Regulation of Type III Secretion of Translocon and Effector Proteins by the EsaB/EsaL/EsaM Complex in Edwardsiella tarda

The type III secretion system (T3SS) plays a crucial role in the pathogenesis of many Gram-negative bacteria, including Edwardsiella tarda, an important fish pathogen. Within the E. tarda T3SS, there are three proteins (EsaB/EsaL/EsaM) that are homologous to proteins present in many other bacteria, including SpiC/SsaL/SsaM in Salmonella, SepD/SepL/CesL in enteropathogenic Escherichia coli (EPEC) and enterohemorrhagic E. coli (EHEC), and YscB/YopN/SycN in Yersinia EsaL was found to interact with both EsaB and EsaM within the bacterial cell, as revealed by a coimmunoprecipitation assay. Moreover, EsaM is required for EsaB stability, and the two proteins interact with each other. EsaB, EsaL, and EsaM are all indispensable for the secretion of the T3SS translocon protein EseC into supernatants under pH 5.5 and pH 7.2 conditions. Unlike EseC, EseG is a T3SS effector whose secretion is suppressed by EsaL at pH 7.2 while it is promoted at pH 5.5 condition. Despite this finding, mutant strains lacking EsaB, EsaL, or EsaM (i.e., the ΔesaB, ΔesaL, or ΔesaM strain, respectively) were all outcompeted by wild-type E. tarda during a coinfection model. These results demonstrate that EsaB/EsaL/EsaM form a ternary complex controlling the secretion of T3SS translocon and effector proteins and contributing to E. tarda pathogenesis.

Recombinant Salmonella expressing SspH2-EscI fusion protein limits its colonization in mice

Background

Activation of inflammasome contributes to the clearance of intracellular bacteria. C-terminus of E. coli EscI protein can activate NLRC4 (NLR family, CARD domain containing-4) inflammasome in macrophages. The purpose of this study was to determine if activation of NLRC4 inflammasome by EscI can reduce the colonization of Salmonella in mice.

Results

A recombinant S. typhimurium strain expressing fusion protein of the N-terminal SspH2 (a Salmonella type III secretion system 2 effector) and C-terminal EscI was constructed and designated as X4550(pYA3334-SspH2-EscI). In vitro assay showed that X4550(pYA3334-SspH2-EscI) significantly enhanced IL-1β and IL-18 secretion (P < 0.05) and pyroptotic cell death of mouse peritoneal macrophages, compared with those infected with control strain, X4550(pYA3334-SspH2). In vivo studies showed that colonization of X4550(pYA3334-SspH2-EscI) in both spleen and liver were significantly lower than that of X4550(pYA3334-SspH2) (P < 0.05). The bacterial counts of X4550(pYA3334-SspH2-EscI) in mice decreased, while those of X4550(pYA3334-SspH2) increased over the time after infection. Additionally, X4550(pYA3334-SspH2-EscI) induced a less pathological alteration in spleen and liver than X4550(pYA3334-SspH2).

Conclusion

Fusion protein SspH2-EscI may be translocated into macrophages and activate NLRC4 inflammasome, which limits Salmonella colonization in spleen and liver of mice.

Assembly, structure, function and regulation of type III secretion systems

Type III secretion systems (T3SSs) are protein transport nanomachines that are found in Gram-negative bacterial pathogens and symbionts. Resembling molecular syringes, T3SSs form channels that cross the bacterial envelope and the host cell membrane, which enable bacteria to inject numerous effector proteins into the host cell cytoplasm and establish trans-kingdom interactions with diverse hosts. Recent advances in cryo-electron microscopy and integrative imaging have provided unprecedented views of the architecture and structure of T3SSs. Furthermore, genetic and molecular analyses have elucidated the functions of many effectors and key regulators of T3SS assembly and secretion hierarchy, which is the sequential order by which the protein substrates are secreted. As essential virulence factors, T3SSs are attractive targets for vaccines and therapeutics. This Review summarizes our current knowledge of the structure and function of this important protein secretion machinery. A greater understanding of T3SSs should aid mechanism-based drug design and facilitate their manipulation for biotechnological applications.

Salmonella Co-opts Host Cell Chaperone-mediated Autophagy for Intracellular Growth

Salmonella enterica are invasive intracellular pathogens that replicate within a membrane-bound compartment inside infected host cells known as the Salmonella-containing vacuole. How Salmonella obtains nutrients for growth within this intracellular niche despite the apparent isolation is currently not known. Recent studies have indicated the importance of glucose and related carbon sources for tissue colonization and intracellular proliferation within host cells during Salmonella infections, although none have been found to be essential. We found that wild-type Salmonella are capable of replicating within infected host cells in the absence of both exogenous sugars and/or amino acids. Furthermore, mutants defective in glucose uptake or dependent upon peptides for growth also showed no significant loss in intracellular replication, suggesting host-derived peptides can supply both carbon units and amino acids. Here, we show that intracellular Salmonella recruit the host proteins LAMP-2A and Hsc73, key components of the host protein turnover pathway known as chaperone-mediated autophagy involved in transport of cytosolic proteins to the lysosome for degradation. Host-derived peptides are shown to provide a significant contribution toward the intracellular growth of Salmonella The results reveal a means whereby intracellular Salmonella gain access to the host cell cytosol from within its membrane-bound compartment to acquire nutrients. Furthermore, this study provides an explanation as to how Salmonella evades activation of autophagy mechanisms as part of the innate immune response.

Steps for Shigella Gatekeeper Protein MxiC Function in Hierarchical Type III Secretion Regulation

Type III secretion systems are complex nanomachines used for injection of proteins from Gram-negative bacteria into eukaryotic cells. Although they are assembled when the environmental conditions are appropriate, they only start secreting upon contact with a host cell. Secretion is hierarchical. First, the pore-forming translocators are released. Second, effector proteins are injected. Hierarchy between these protein classes is mediated by a conserved gatekeeper protein, MxiC, in Shigella. As its molecular mechanism of action is still poorly understood, we used its structure to guide site-directed mutagenesis and to dissect its function. We identified mutants predominantly affecting all known features of MxiC regulation as follows: secretion of translocators, MxiC and/or effectors. Using molecular genetics, we then mapped at which point in the regulatory cascade the mutants were affected. Analysis of some of these mutants led us to a set of electron paramagnetic resonance experiments that provide evidence that MxiC interacts directly with IpaD. We suggest how this interaction regulates a switch in its conformation that is key to its functions.

The Structure and Function of Type III Secretion Systems

Type III secretion systems (T3SSs) afford Gram-negative bacteria an intimate means of altering the biology of their eukaryotic hosts--the direct delivery of effector proteins from the bacterial cytoplasm to that of the eukaryote. This incredible biophysical feat is accomplished by nanosyringe "injectisomes," which form a conduit across the three plasma membranes, peptidoglycan layer, and extracellular space that form a barrier to the direct delivery of proteins from bacterium to host. The focus of this chapter is T3SS function at the structural level; we will summarize the core findings that have shaped our understanding of the structure and function of these systems and highlight recent developments in the field. In turn, we describe the T3SS secretory apparatus, consider its engagement with secretion substrates, and discuss the posttranslational regulation of secretory function. Lastly, we close with a discussion of the future prospects for the interrogation of structure-function relationships in the T3SS.

Type Three Secretion System in Attaching and Effacing Pathogens

Enteropathogenic Escherichia coli and enterohemorrhagic E. coli are diarrheagenic bacterial human pathogens that cause severe gastroenteritis. These enteric pathotypes, together with the mouse pathogen Citrobacter rodentium, belong to the family of attaching and effacing pathogens that form a distinctive histological lesion in the intestinal epithelium. The virulence of these bacteria depends on a type III secretion system (T3SS), which mediates the translocation of effector proteins from the bacterial cytosol into the infected cells. The core architecture of the T3SS consists of a multi-ring basal body embedded in the bacterial membranes, a periplasmic inner rod, a transmembrane export apparatus in the inner membrane, and cytosolic components including an ATPase complex and the C-ring. In addition, two distinct hollow appendages are assembled on the extracellular face of the basal body creating a channel for protein secretion: an approximately 23 nm needle, and a filament that extends up to 600 nm. This filamentous structure allows these pathogens to get through the host cells mucus barrier. Upon contact with the target cell, a translocation pore is assembled in the host membrane through which the effector proteins are injected. Assembly of the T3SS is strictly regulated to ensure proper timing of substrate secretion. The different type III substrates coexist in the bacterial cytoplasm, and their hierarchical secretion is determined by specialized chaperones in coordination with two molecular switches and the so-called sorting platform. In this review, we present recent advances in the understanding of the T3SS in attaching and effacing pathogens.

Preparation of Monoclonal Antibodies Against SpiC Protein Secreted by T3SS-2 of Salmonella spp

SpiC protein, a member of Salmonella spp. type III secretion system (T3SS)-2, is necessary for the survival of Salmonella within macrophages, and it plays a vital role in Salmonella pathogenesis. To develop and test monoclonal antibodies (MAbs) against SpiC protein, two recombinant proteins, rHis-SpiC and rGST-SpiC, were expressed in vitro in the prokaryotic expression vectors pET-30(a) and pGEX-6p-1, respectively, and rHis-SpiC protein used to immunize mice. Hybridomas were generated from the splenocytes of these mice and the monoclonal antibodies produced by these cells were assessed using indirect enzyme-linked immunosorbent assay (ELISA) with rGST-SpiC as the coating antigen. An immunoblotting analysis indicated that all seven of the MAbs developed in this study could specifically recognize the SpiC protein. These MAbs will be very useful in the study of SpiC function and for use in the immunodiagnosis of Salmonella infection.

Autophagy Proteins Promote Repair of Endosomal Membranes Damaged by the Salmonella Type Three Secretion System 1

Salmonella Typhimurium (S.Tm) is an enteropathogen requiring multiple virulence factors, including two type three secretion systems (T1 and T2). T1 triggers epithelium invasion in which the bacteria are taken up into endosomes that mature into Salmonella-containing vacuoles (SCV) and trigger T2 induction upon acidification. Mechanisms controlling endosome membrane integrity or pathogen egress into the cytosol are incompletely understood. We screened for host factors affecting invasion and SCV maturation and identified a role for autophagy in sealing endosomal membranes damaged by T1 during host cell invasion. S.Tm-infected autophagy-deficient (atg5(-/-)) cells exhibit reduced SCV dye retention and lower T2 expression but no effects on steps preceding SCV maturation. However, in the absence of T1, autophagy is dispensable for T2 induction. These findings establish a role of autophagy at early stages of S.Tm infection and suggest that autophagy-mediated membrane repair might be generally important for invasive pathogens and endosomal membrane function.

Acidic pH sensing in the bacterial cytoplasm is required for Salmonella virulence

pH regulates gene expression, biochemical activities and cellular behaviors. A mildly acidic pH activates the master virulence regulatory system PhoP/PhoQ in the facultative intracellular pathogen Salmonella enterica serovar Typhimurium. The sensor PhoQ harbors an extracytoplasmic domain implicated in signal sensing, and a cytoplasmic domain controlling activation of the regulator PhoP. We now report that, surprisingly, a decrease in Salmonella's own cytoplasmic pH induces transcription of PhoP-activated genes even when the extracytoplasmic pH remains neutral. Amino acid substitutions in PhoQ's cytoplasmic domain hindered activation by acidic pH and attenuated virulence in mice, but did not abolish activation by low Mg(2+) or the antimicrobial peptide C18G. Conversely, removal of PhoQ's extracytoplasmic domains prevented the response to the latter PhoQ-activating signals but not to acidic pH. PhoP-dependent genes were minimally induced by acidic pH in the non-pathogenic species Salmonella bongori but were activated by low Mg(2+) and C18G as in pathogenic S. enterica. Our findings indicate that the sensor PhoQ enables S. enterica to respond to both host- and bacterial-derived signals that alter its cytoplasmic pH.

Fluorescent and Photostable Silicon Nanoparticles Sensors for Real-Time and Long-Term Intracellular pH Measurement in Live Cells

Fluorescent sensors suitable for dynamic measurement of intracellular pH (pHi) fluctuations should feature the following properties: feeble cytotoxicity, wide-pH range response, and strong fluorescence coupled with good photostability, which are still remaining scanty to date. Herein, by functionalizing fluorescent silicon nanoparticles (SiNPs) with pH-sensitive dopamine (DA) and pH-insensitive rhodamine B isothiocyanate (RBITC), we present the first demonstration of fluorescent SiNPs-based sensors, simultaneously exhibiting minimal toxicity (cell viability of treated cells remains above 95% during 24-h treatment), sensitive fluorescent response to a broad pH range (∼4-10), and bright fluorescence coupled with robust photostability (∼9% loss of fluorescence intensity after 40 min continuous excitation; in contrast, fluorescence of Lyso-tracker is rapidly quenched in 5 min under the same experiment conditions). Taking advantage of these merits, we further employ the resultant fluorescent SiNPs sensors for the detection of lysosomal pH change mediated by nigericin in live HeLa and MCF-7 cells in long-term (e.g., 30 min) manners. Interestingly, two consecutive stages, i.e., alkalization lag phase and logarithmic growth phase, are observed based on recording the whole process of pH change, offering important information for understanding the dynamic process of pHi fluctuations.

Right on Q: genetics begin to unravel Coxiella burnetii host cell interactions

Invasion of macrophages and replication within an acidic and degradative phagolysosome-like vacuole are essential for disease pathogenesis by Coxiella burnetii, the bacterial agent of human Q fever. Previous experimental constraints imposed by the obligate intracellular nature of Coxiella limited knowledge of pathogen strategies that promote infection. Fortunately, new genetic tools facilitated by axenic culture now allow allelic exchange and transposon mutagenesis approaches for virulence gene discovery. Phenotypic screens have illuminated the critical importance of Coxiella's type 4B secretion system in host cell subversion and discovered genes encoding translocated effector proteins that manipulate critical infection events. Here, we highlight the cellular microbiology and genetics of Coxiella and how recent technical advances now make Coxiella a model organism to study macrophage parasitism.

Salmonella Effectors SseF and SseG Interact with Mammalian Protein ACBD3 (GCP60) To Anchor Salmonella-Containing Vacuoles at the Golgi Network

Following infection of mammalian cells, Salmonella enterica serovar Typhimurium (S. Typhimurium) replicates within membrane-bound compartments known as Salmonella-containing vacuoles (SCVs). The Salmonella pathogenicity island 2 type III secretion system (SPI-2 T3SS) translocates approximately 30 different effectors across the vacuolar membrane. SseF and SseG are two such effectors that are required for SCVs to localize close to the Golgi network in infected epithelial cells. In a yeast two-hybrid assay, SseG and an N-terminal variant of SseF interacted directly with mammalian ACBD3, a multifunctional cytosolic Golgi network-associated protein. Knockdown of ACBD3 by small interfering RNA (siRNA) reduced epithelial cell Golgi network association of wild-type bacteria, phenocopying the effect of null mutations of sseG or sseF. Binding of SseF to ACBD3 in infected cells required the presence of SseG. A single-amino-acid mutant of SseG and a double-amino-acid mutant of SseF were obtained that did not interact with ACBD3 in Saccharomyces cerevisiae. When either of these was produced together with the corresponding wild-type effector by Salmonella in infected cells, they enabled SCV-Golgi network association and interacted with ACBD3. However, these properties were lost and bacteria displayed an intracellular replication defect when cells were infected with Salmonella carrying both mutant genes. Knockdown of ACBD3 resulted in a replication defect of wild-type bacteria but did not further attenuate the growth defect of a ΔsseFG mutant strain. We propose a model in which interaction between SseF and SseG enables both proteins to bind ACBD3, thereby anchoring SCVs at the Golgi network and facilitating bacterial replication.

Upon invasion of epithelial cells, the majority of vacuoles containing Salmonella enterica migrate to the perinuclear region-located Golgi network and remain in this region of the cell during the first few rounds of bacterial replication, forming a clustered microcolony of vacuoles. This process requires the action of SseF and SseG, two effector proteins that are translocated by the Salmonella SPI-2 type III secretion system. However, little is known about how they function. Here, we show that both proteins interact with the mammalian Golgi network-associated protein ACBD3. To our knowledge, the SseF-SseG-ACBD3 interaction is the first example of a tethering complex between a pathogen-containing vacuole and a host cell organelle.

Type III secretion systems: the bacterial flagellum and the injectisome

The flagellum and the injectisome are two of the most complex and fascinating bacterial nanomachines. At their core, they share a type III secretion system (T3SS), a transmembrane export complex that forms the extracellular appendages, the flagellar filament and the injectisome needle. Recent advances, combining structural biology, cryo-electron tomography, molecular genetics, in vivo imaging, bioinformatics and biophysics, have greatly increased our understanding of the T3SS, especially the structure of its transmembrane and cytosolic components, the transcriptional, post-transcriptional and functional regulation and the remarkable adaptivity of the system. This review aims to integrate these new findings into our current knowledge of the evolution, function, regulation and dynamics of the T3SS, and to highlight commonalities and differences between the two systems, as well as their potential applications.

Inhibition of Nuclear Transport of NF-ĸB p65 by the Salmonella Type III Secretion System Effector SpvD

Salmonella enterica replicates in macrophages through the action of effector proteins translocated across the vacuolar membrane by a type III secretion system (T3SS). Here we show that the SPI-2 T3SS effector SpvD suppresses proinflammatory immune responses. SpvD prevented activation of an NF-ĸB-dependent promoter and caused nuclear accumulation of importin-α, which is required for nuclear import of p65. SpvD interacted specifically with the exportin Xpo2, which mediates nuclear-cytoplasmic recycling of importins. We propose that interaction between SpvD and Xpo2 disrupts the normal recycling of importin-α from the nucleus, leading to a defect in nuclear translocation of p65 and inhibition of activation of NF-ĸB regulated promoters. SpvD down-regulated pro-inflammatory responses and contributed to systemic growth of bacteria in mice. This work shows that a bacterial pathogen can manipulate host cell immune responses by interfering with the nuclear transport machinery.

Salmonella Typhimurium replicates in macrophages through the action of effector proteins translocated into host cells by a type III secretion system (T3SS). We show that the T3SS effector SpvD targets the NF-ĸB pathway by interfering with nuclear translocation of p65. SpvD interacts with the exportin Xpo2. Perturbation of Xpo2 disrupts recycling of importin-α from the nucleus, leading to abrogation of p65 nuclear translocation. These data show that a bacterial pathogen manipulates host cell immune responses by interfering with nuclear transport machinery.

Safety and protective efficacy of a spiC and crp deletion mutant of Salmonella gallinarum as a live attenuated vaccine for fowl typhoid

With an aim to develop a safe, immunogenic fowl typhoid (FT) vaccine, the safety and efficacy of 1009ΔspiCΔcrp, a spiC and crp deletion mutant of Salmonella gallinarum, were evaluated in chickens. Three-day-old chickens were intramuscularly immunized with 1009ΔspiCΔcrp (1×10(7)CFU) and boosted 7days later (at 10-days old) with the same dose and via the same route (vaccinated group). The vaccinated group showed no clinical symptoms and no differences in body weight compared to the unvaccinated control group. 1009ΔspiCΔcrp bacteria colonized and persisted in the liver and spleen of vaccinated chickens for >14days, and significant specific humoral and cellular immune responses were induced. Vaccinated chickens were challenged with S. gallinarum strain SG9 at 21days post-immunization (24-day-old chickens), and efficient protection was observed based on the mortality and clinical symptoms, as compared to those in the control group. These results demonstrate that 1009ΔspiCΔcrp can be used as a live attenuated vaccine.

MxiA, MxiC and IpaD Regulate Substrate Selection and Secretion Mode in the T3SS of Shigella flexneri

Type III secretion systems (T3SSs) are central virulence devices for many Gram-negative bacterial pathogens of humans, animals & plants. Upon physical contact with eukaryotic host cells, they translocate virulence-mediating proteins, known as effectors, into them during infection. T3SSs are gated from the outside by host-cell contact and from the inside via two cytoplasmic negative regulators, MxiC and IpaD in Shigella flexneri, which together control the effector secretion hierarchy. Their absence leads to premature and increased secretion of effectors. Here, we investigated where and how these regulators act. We demonstrate that the T3SS inner membrane export apparatus protein MxiA plays a role in substrate selection. Indeed, using a genetic screen, we identified two amino acids located on the surface of MxiA's cytoplasmic region (MxiAC) which, when mutated, upregulate late effector expression and, in the case of MxiAI674V, also secretion. The cytoplasmic region of MxiA, but not MxiAN373D and MxiAI674V, interacts directly with the C-terminus of MxiC in a two-hybrid assay. Efficient T3S requires a cytoplasmic ATPase and the proton motive force (PMF), which is composed of the ΔΨ and the ΔpH. MxiA family proteins and their regulators are implicated in utilization of the PMF for protein export. However, our MxiA point mutants show similar PMF utilisation to wild-type, requiring primarily the ΔΨ. On the other hand, lack of MxiC or IpaD, renders the faster T3S seen increasingly dependent on the ΔpH. Therefore, MxiA, MxiC and IpaD act together to regulate substrate selection and secretion mode in the T3SS of Shigella flexneri.

Identification and Characterization of Putative Translocated Effector Proteins of the Edwardsiella ictaluri Type III Secretion System

The bacterial pathogen Edwardsiella ictaluri causes enteric septicemia of catfish (ESC), an economically significant disease of farm-raised channel catfish. Commercial catfish production accounts for the majority of the total fin fish aquaculture in the United States, with almost 300,000 tons produced annually, and ESC is the leading cause of disease loss in the industry. We have demonstrated the survival and replication of E. ictaluri within channel catfish cells and identified a secretion system that is essential for E. ictaluri intracellular replication and virulence. We have also identified nine proteins encoded in the E. ictaluri genome that we believe are actively transferred from the bacterium to the cytoplasm of the host cell and act to manipulate host cell physiology to the advantage of the bacterium. The data presented here confirm that the proteins are actually transferred during an infection, which will lead to further work on approaches to preventing or controlling ESC.

Edwardsiella ictaluri, a major pathogen in channel catfish aquaculture, encodes a type III secretion system (T3SS) that is essential for intracellular replication and virulence. Previous work identified three putative T3SS effectors in E. ictaluri, and in silico analysis of the E. ictaluri genome identified six additional putative effectors, all located on the chromosome outside the T3SS pathogenicity island. To establish active translocation by the T3SS, we constructed translational fusions of each effector to the amino-terminal adenylate cyclase (AC) domain of the Bordetella pertussis adenylate cyclase toxin CyaA. When translocated through the membrane of the Edwardsiella-containing vacuole (ECV), the cyclic AMP produced by the AC domain in the presence of calmodulin in the host cell cytoplasm can be measured. Results showed that all nine effectors were translocated from E. ictaluri in the ECV to the cytoplasm of the host cells in the wild-type strain but not in a T3SS mutant, indicating that translocation is dependent on the T3SS machinery. This confirms that the E. ictaluri T3SS is similar to the Salmonella pathogenicity island 2 T3SS in that it translocates effectors through the membrane of the bacterial vacuole directly into the host cell cytoplasm. Additional work demonstrated that both initial acidification and subsequent neutralization of the ECV were necessary for effector translocation, except for two of them that did not require neutralization. Single-gene mutants constructed for seven of the individual effectors were all attenuated for replication in CCO cells, but only three were replication deficient in head kidney-derived macrophages (HKDM).

IMPORTANCE The bacterial pathogen Edwardsiella ictaluri causes enteric septicemia of catfish (ESC), an economically significant disease of farm-raised channel catfish. Commercial catfish production accounts for the majority of the total fin fish aquaculture in the United States, with almost 300,000 tons produced annually, and ESC is the leading cause of disease loss in the industry. We have demonstrated the survival and replication of E. ictaluri within channel catfish cells and identified a secretion system that is essential for E. ictaluri intracellular replication and virulence. We have also identified nine proteins encoded in the E. ictaluri genome that we believe are actively transferred from the bacterium to the cytoplasm of the host cell and act to manipulate host cell physiology to the advantage of the bacterium. The data presented here confirm that the proteins are actually transferred during an infection, which will lead to further work on approaches to preventing or controlling ESC.

Intramacrophage Infection Reinforces the Virulence of Edwardsiella tarda

Edwardsiella tarda is an important pathogenic bacterium that can replicate in macrophages. However, how the intramacrophage infection process affects the virulence of this bacterium is essentially unknown. Here, we show that E. tarda replicates and induces a caspase-1-dependent cell pyroptosis in a murine macrophage model. Via pyroptosis, intracellular E. tarda escapes to the extracellular milieu, forming a unique bacterial population. Being different from the bacteria cultured alone, this unique population possesses a reprogrammed transcriptional profile, particularly with upregulated type III secretion system (T3SS)/T6SS cluster genes. Subsequent studies revealed that the macrophage-released population gains enhanced infectivity for host epithelial cells and increases resistance to multiple host defenses and hence displays significantly promoted virulence in vivo Further studies indicated that T3SS is essentially required for the macrophage infection process, while T6SS contributes to infection-induced bacterial virulence. Altogether, this work demonstrates that E. tarda can utilize macrophages as a niche for virulence priming and for spreading infection, suggesting a positive role for intramacrophage infection in bacterial pathogenesis.

IMPORTANCE

Many pathogens can replicate in macrophages, which is crucial for their pathogenesis. To survive in the macrophage cell, pathogens are likely to require fitness genes to counteract multiple host-killing mechanisms. Here, Edwardsiella tarda is proved to exit from macrophages during infection. This macrophage-released population displays a reprogrammed transcriptional profile with significantly upregulated type III secretion system (T3SS)/T6SS-related genes. Furthermore, both enhanced infectivity in epithelial cells and activated resistance to complex host defenses were conferred on this macrophage-primed population, which consequently promoted the full virulence of E. tarda in vivo Our work provides evidence that E. tarda can utilize macrophages as a niche for virulence priming and for spreading infection, highlighting the importance of the intramacrophage infection cycle for the pathogenesis of E. tarda.

The Type III Secretion Translocation Pore Senses Host Cell Contact

Type III secretion systems (T3SS) are nano-syringes used by a wide range of Gram-negative pathogens to promote infection by directly injecting effector proteins into targeted host cells. Translocation of effectors is triggered by host-cell contact and requires assembly of a pore in the host-cell plasma membrane, which consists of two translocator proteins. Our understanding of the translocation pore, how it is assembled in the host cell membrane and its precise role in effector translocation, is extremely limited. Here we use a genetic technique to identify protein-protein contacts between pore-forming translocator proteins, as well as the T3SS needle-tip, that are critical for translocon function. The data help establish the orientation of the translocator proteins in the host cell membrane. Analysis of translocon function in mutants that break these contacts demonstrates that an interaction between the pore-forming translocator PopD and the needle-tip is required for sensing host cell contact. Moreover, tethering PopD at a dimer interface also specifically prevents host-cell sensing, arguing that the translocation pore is actively involved in detecting host cell contact. The work presented here therefore establishes a signal transduction pathway for sensing host cell contact that is initiated by a conformational change in the translocation pore, and is subsequently transmitted to the base of the apparatus via a specific contact between the pore and the T3SS needle-tip.

Type III secretion systems (T3SSs) are molecular syringes used by a wide variety of Gram-negative pathogens to directly deliver proteins (effectors) into host cells, allowing the bacteria to cause disease. Injection of proteins is triggered by host-cell contact, but how the machinery to deliver effectors is assembled (the translocon), or indeed, how cell contact is perceived, is unclear. Here we identify protein-protein contacts that are critical for translocon function. Our analysis sheds light on the organization of the translocon, and reveals that host cell contact is perceived by a change in the structure of the translocation pore. This signal is then transmitted to the tip of the T3SS needle, and down to the base of the apparatus.

Implications of Spatiotemporal Regulation of Shigella flexneri Type Three Secretion Activity on Effector Functions: Think Globally, Act Locally

Shigella spp. are Gram-negative bacterial pathogens that infect human colonic epithelia and cause bacterial dysentery. These bacteria express multiple copies of a syringe-like protein complex, the Type Three Secretion apparatus (T3SA), which is instrumental in the etiology of the disease. The T3SA triggers the plasma membrane (PM) engulfment of the bacteria by host cells during the initial entry process. It then enables bacteria to escape the resulting phagocytic-like vacuole. Freed bacteria form actin comets to move in the cytoplasm, which provokes bacterial collision with the inner leaflet of the PM. This phenomenon culminates in T3SA-dependent secondary uptake and vacuolar rupture in neighboring cells in a process akin to what is observed during entry and named cell-to-cell spread. The activity of the T3SA of Shigella flexneri was recently demonstrated to display an on/off regulation during the infection. While the T3SA is active when bacteria are in contact with PM-derived compartments, it switches to an inactive state when bacteria are released within the cytosol. These observations indicate that effector proteins transiting through the T3SA are therefore translocated in a highly time and space constrained fashion, likely impacting on their cellular distribution. Herein, we present what is currently known about the composition, the assembly and the regulation of the T3SA activity and discuss the consequences of the on/off regulation of T3SA on Shigella effector properties and functions during the infection. Specific examples that will be developed include the role of effectors IcsB and VirA in the escape from LC3/ATG8-positive vacuoles formed during cell-to-cell spread and of IpaJ protease activity against N-miristoylated proteins. The conservation of a similar regulation of T3SA activity in other pathogens such as Salmonella or Enteropathogenic Escherichia coli will also be briefly discussed.

Edwardsiella tarda EscE (Orf13 Protein) Is a Type III Secretion System-Secreted Protein That Is Required for the Injection of Effectors, Secretion of Translocators, and Pathogenesis in Fish

The type III secretion system (T3SS) of Edwardsiella tarda is crucial for its intracellular survival and pathogenesis in fish. The orf13 gene (escE) of E. tarda is located 84 nucleotides (nt) upstream of esrC in the T3SS gene cluster. We found that EscE is secreted and translocated in a T3SS-dependent manner and that amino acids 2 to 15 in the N terminus were required for a completely functional T3SS in E. tarda. Deletion of escE abolished the secretion of T3SS translocators, as well as the secretion and translocation of T3SS effectors, but did not influence their intracellular protein levels in E. tarda. Complementation of the escE mutant with a secretion-incompetent EscE derivative restored the secretion of translocators and effectors. Interestingly, the effectors that were secreted and translocated were positively correlated with the EscE protein level in E. tarda. The escE mutant was attenuated in the blue gourami fish infection model, as its 50% lethal dose (LD50) increased to 4 times that of the wild type. The survival rate of the escE mutant-strain-infected fish was 69%, which was much higher than that of the fish infected with the wild-type bacteria (6%). Overall, EscE represents a secreted T3SS regulator that controls effector injection and translocator secretion, thus contributing to E. tarda pathogenesis in fish. The homology of EscE within the T3SSs of other bacterial species suggests that the mechanism of secretion and translocation control used by E. tarda may be commonly used by other bacterial pathogens.

The Salmonella effector protein SifA plays a dual role in virulence

The virulence of Salmonella relies on the expression of effector proteins that the bacterium injects inside infected cells. Salmonella enters eukaryotic cells and resides in a vacuolar compartment on which a number of effector proteins such as SifA are found. SifA plays an essential role in Salmonella virulence. It is made of two distinct domains. The N-terminal domain of SifA interacts with the host protein SKIP. This interaction regulates vacuolar membrane dynamics. The C-terminal has a fold similar to other bacterial effector domains having a guanine nucleotide exchange factor activity. Although SifA interacts with RhoA, it does not stimulate the dissociation of GDP and the activation of this GTPase. Hence it remains unknown whether the C-terminal domain contributes to the function of SifA in virulence. We used a model of SKIP knockout mice to show that this protein mediates the host susceptibility to salmonellosis and to establish that SifA also contributes to Salmonella virulence independently of its interaction with SKIP. We establish that the C-terminal domain of SifA mediates this SKIP-independent contribution. Moreover, we show that the two domains of SifA are functionally linked and participate to the same signalling cascade that supports Salmonella virulence.

A FRET-based DNA biosensor tracks OmpR-dependent acidification of Salmonella during macrophage infection

In bacteria, one paradigm for signal transduction is the two-component regulatory system, consisting of a sensor kinase (usually a membrane protein) and a response regulator (usually a DNA binding protein). The EnvZ/OmpR two-component system responds to osmotic stress and regulates expression of outer membrane proteins. In Salmonella, EnvZ/OmpR also controls expression of another two-component system SsrA/B, which is located on Salmonella Pathogenicity Island (SPI) 2. SPI-2 encodes a type III secretion system, which functions as a nanomachine to inject bacterial effector proteins into eukaryotic cells. During the intracellular phase of infection, Salmonella switches from assembling type III secretion system structural components to secreting effectors into the macrophage cytoplasm, enabling Salmonella to replicate in the phagocytic vacuole. Major questions remain regarding how bacteria survive the acidified vacuole and how acidification affects bacterial secretion. We previously reported that EnvZ sensed cytoplasmic signals rather than extracellular ones, as intracellular osmolytes altered the dynamics of a 17-amino-acid region flanking the phosphorylated histidine. We reasoned that the Salmonella cytoplasm might acidify in the macrophage vacuole to activate OmpR-dependent transcription of SPI-2 genes. To address these questions, we employed a DNA-based FRET biosensor ("I-switch") to measure bacterial cytoplasmic pH and immunofluorescence to monitor effector secretion during infection. Surprisingly, we observed a rapid drop in bacterial cytoplasmic pH upon phagocytosis that was not predicted by current models. Cytoplasmic acidification was completely dependent on the OmpR response regulator, but did not require known OmpR-regulated genes such as ompC, ompF, or ssaC (SPI-2). Microarray analysis highlighted the cadC/BA operon, and additional experiments confirmed that it was repressed by OmpR. Acidification was blocked in the ompR null background in a Cad-dependent manner. Acid-dependent activation of OmpR stimulated type III secretion; blocking acidification resulted in a neutralized cytoplasm that was defective for SPI-2 secretion. Based upon these findings, we propose that Salmonella infection involves an acid-dependent secretion process in which the translocon SseB moves away from the bacterial cell surface as it associates with the vacuolar membrane, driving the secretion of SPI-2 effectors such as SseJ. New steps in the SPI-2 secretion process are proposed.

SepD/SepL-dependent secretion signals of the type III secretion system translocator proteins in enteropathogenic Escherichia coli

The type III protein secretion system (T3SS) encoded by the locus of enterocyte effacement (LEE) is essential for the pathogenesis of attaching/effacing bacterial pathogens, including enteropathogenic Escherichia coli (EPEC), enterohemorrhagic E. coli (EHEC), and Citrobacter rodentium. These pathogens use the T3SS to sequentially secrete three categories of proteins: the T3SS needle and inner rod protein components; the EspA, EspB, and EspD translocators; and many LEE- and non-LEE-encoded effectors. SepD and SepL are essential for translocator secretion, and mutations in either lead to hypersecretion of effectors. However, how SepD and SepL control translocator secretion and secretion hierarchy between translocators and effectors is poorly understood. In this report, we show that the secreted T3SS components, the translocators, and both LEE- and non-LEE-encoded effectors all carry N-terminal type III secretion and translocation signals. These signals all behave like those of the effectors and are sufficient for mediating type III secretion and translocation by wild-type EPEC and hypersecretion by the sepD and sepL mutants. Our results extended previous observations and suggest that the secretion hierarchy of the different substrates is determined by a signal other than the N-terminal secretion signal. We identified a domain located immediately downstream of the N-terminal secretion signal in the translocator EspB that is required for SepD/SepL-dependent secretion. We further demonstrated that this EspB domain confers SepD/SepL- and CesAB-dependent secretion on the secretion signal of effector EspZ. Our results thus suggest that SepD and SepL control and regulate secretion hierarchy between translocators and effectors by recognizing translocator-specific export signals.

IMPORTANCE

Many bacterial pathogens use a syringe-like protein secretion apparatus, termed the type III protein secretion system (T3SS), to secrete and inject numerous proteins directly into the host cells to cause disease. The secreted proteins perform different functions at various stages during infection and are classified into three substrate categories (T3SS components, translocators, and effectors). They all contain secretion signals at their N termini, but how their secretion hierarchy is determined is poorly understood. Here, we show that the N-terminal secretion signals from different substrate categories all behave the same and do not confer substrate specificity. We further characterize the secretion signals of the translocators and identify a translocator-specific signal, demonstrating that substrate-specific secretion signals are required in regulating T3SS substrate hierarchy.

How bacterial pathogens colonize their hosts and invade deeper tissues

Bacterial pathogens have evolved a wide range of strategies to colonize and invade human organs, despite the presence of multiple host defense mechanisms. In this review, we will describe how pathogenic bacteria can adhere and multiply at the surface of host cells, how some bacteria can enter and proliferate inside these cells, and finally how pathogens may cross epithelial or endothelial host barriers and get access to internal tissues, leading to severe diseases in humans.

Composition, formation, and regulation of the cytosolic c-ring, a dynamic component of the type III secretion injectisome

The injectisome is a membrane complex through which some bacteria can inject effector proteins into host cells. This study reveals that the cytosolic C-ring structure has a dynamic relationship to the rest of the injectisome, with implications for the regulation of secretion.

Many gram-negative pathogens employ a type III secretion injectisome to translocate effector proteins into eukaryotic host cells. While the structure of the distal “needle complex” is well documented, the composition and role of the functionally important cytosolic complex remain less well understood. Using functional fluorescent fusions, we found that the C-ring, an essential and conserved cytosolic component of the system, is composed of ~22 copies of SctQ (YscQ in Yersinia enterocolitica), which require the presence of YscQ_C, the product of an internal translation initiation site in yscQ, for their cooperative assembly. Photoactivated localization microscopy (PALM) reveals that in vivo, YscQ is present in both a free-moving cytosolic and a stable injectisome-bound state. Notably, fluorescence recovery after photobleaching (FRAP) shows that YscQ exchanges between the injectisome and the cytosol, with a t_½ of 68 ± 8 seconds when injectisomes are secreting. In contrast, the secretin SctC (YscC) and the major export apparatus component SctV (YscV) display minimal exchange. Under non-secreting conditions, the exchange rate of YscQ is reduced to t_½ = 134 ± 16 seconds, revealing a correlation between C-ring exchange and injectisome activity, which indicates a possible role for C-ring stability in regulation of type III secretion. The stabilization of the C-ring depends on the presence of the functional ATPase SctN (YscN). These data provide new insights into the formation and composition of the injectisome and present a novel aspect of type III secretion, the exchange of C-ring subunits, which is regulated with respect to secretion.

The type III secretion system, also known as the injectisome, is a key virulence factor in many gram-negative bacteria, and is responsible for the transmission of bacterial proteins directly into host cells. While some elements of the system are well characterized, the cytosolic components involved in substrate recognition and handling are not well understood. One of the major questions is the role of the C-ring, an essential yet enigmatic cytosolic injectisome member. We used fluorescence microscopy to analyze the architecture and behavior of the C-ring in live Y. enterocolitica bacteria, a human pathogen. We found that in vivo, the C-ring assembles cooperatively with the help of additional copies of its own C-terminal fragment and has a highly dynamic structure, with C-ring subunits exchanging between the working injectisomes and a cytosolic pool. The rate of exchange is different between secreting and non-secreting injectisomes and depends on the function of the type III secretion ATPase, indicating that the stability of the complex is altered when functioning. This dynamic behaviour raises the possibility that the C-ring is a regulator of targeted protein delivery by the type III secretion system and makes the C-ring a viable target for the development of novel anti-virulence drugs.

Systemic responses of BALB/c mice to Salmonella typhimurium infection

Salmonella typhimurium is a bacterial pathogen that poses a great threat to humans and animals. In order to discover hosts' responses to S. typhimurium infection, we collected and analyzed biofluids and organ tissues from mice which had ingested S. typhimurium. We employed (1)H NMR spectroscopy coupled with multivariate data analysis and immunological techniques. The results indicate that infection leads to a severe impact on mice spleen and ileum, which are characterized by splenomegaly and edematous villi, respectively. We found that increased levels of itaconic acid were correlated with the presence of splenomegaly during infection and may play an important role in Salmonella-containing vacuole acidification. In addition, metabonomic analyses of urine displayed the development of salmonellosis in mice, which is characterized by dynamic changes in energy metabolism. Furthermore, we found that the presence of S. typhimurium activated an anti-oxidative response in infected mice. We also observed changes in the gut microbial co-metabolites (hippurate, TMAO, TMA, methylamine). This investigation sheds much needed light on the host-pathogen interactions of S. typhimurium, providing further information to deepen our understanding of the long co-evolution process between hosts and infective bacteria.

Salmonella methylglyoxal detoxification by STM3117-encoded lactoylglutathione lyase affects virulence in coordination with Salmonella pathogenicity island 2 and phagosomal acidification

Intracellular pathogens such as Salmonella enterica serovar Typhimurium (S. Typhimurium) manipulate their host cells through the interplay of various virulence factors. A multitude of such virulence factors are encoded on the genome of S. Typhimurium and are usually organized in pathogenicity islands. The virulence-associated genomic stretch of STM3117-3120 has structural features of pathogenicity islands and is present exclusively in non-typhoidal serovars of Salmonella. It encodes metabolic enzymes predicted to be involved in methylglyoxal metabolism. STM3117-encoded lactoylglutathione lyase significantly impacts the proliferation of intracellular Salmonella. The deletion mutant of STM3117 (Δlgl) fails to grow in epithelial cells but hyper-replicates in macrophages. This difference in proliferation outcome was the consequence of failure to detoxify methylglyoxal by Δlgl, which was also reflected in the form of oxidative DNA damage and upregulation of kefB in the mutant. Within macrophages, the toxicity of methylglyoxal adducts elicits the potassium efflux channel (KefB) in the mutant which subsequently modulates the acidification of mutant-containing vacuoles (MCVs). The perturbation in the pH of the MCV milieu and bacterial cytosol enhances the Salmonella pathogenicity island 2 translocation in Δlgl, increasing its net growth within macrophages. In epithelial cells, however, the maturation of Δlgl-containing vacuoles were affected as these non-phagocytic cells maintain less acidic vacuoles compared to those in macrophages. Remarkably, ectopic expression of Toll-like receptors 2 and 4 on epithelial cells partially restored the survival of Δlgl. This study identified a novel metabolic enzyme in S. Typhimurium whose activity during intracellular infection within a given host cell type differentially affected the virulence of the bacteria.

Endocytosis of viruses and bacteria

Of the many pathogens that infect humans and animals, a large number use cells of the host organism as protected sites for replication. To reach the relevant intracellular compartments, they take advantage of the endocytosis machinery and exploit the network of endocytic organelles for penetration into the cytosol or as sites of replication. In this review, we discuss the endocytic entry processes used by viruses and bacteria and compare the strategies used by these dissimilar classes of pathogens.

The Dynamic Interactions between Salmonella and the Microbiota, within the Challenging Niche of the Gastrointestinal Tract

Understanding how Salmonella species establish successful infections remains a foremost research priority. This gastrointestinal pathogen not only faces the hostile defenses of the host's immune system, but also faces fierce competition from the large and diverse community of microbiota for space and nutrients. Salmonella have solved these challenges ingeniously. To jump-start growth, Salmonella steal hydrogen produced by the gastrointestinal microbiota. Type 3 effector proteins are subsequently secreted by Salmonella to trigger potent inflammatory responses, which generate the alternative terminal electron acceptors tetrathionate and nitrate. Salmonella exclusively utilize these electron acceptors for anaerobic respiration, permitting metabolic access to abundant substrates such as ethanolamine to power growth blooms. Chemotaxis and flagella-mediated motility enable the identification of nutritionally beneficial niches. The resulting growth blooms also promote horizontal gene transfer amongst the resident microbes. Within the gastrointestinal tract there are opportunities for chemical signaling between host cells, the microbiota, and Salmonella. Host produced catecholamines and bacterial autoinducers form components of this chemical dialogue leading to dynamic interactions. Thus, Salmonella have developed remarkable strategies to initially shield against host defenses and to transiently compete against the intestinal microbiota leading to successful infections. However, the immunocompetent host is subsequently able to reestablish control and clear the infection.

The central metabolism regulator EIIAGlc switches Salmonella from growth arrest to acute virulence through activation of virulence factor secretion

The ability of Salmonella to cause disease depends on metabolic activities and virulence factors. Here, we show that a key metabolic protein, EIIAGlc, is absolutely essential for acute infection, but not for Salmonella survival, in a mouse typhoid fever model. Surprisingly, phosphorylation-dependent EIIAGlc functions, including carbohydrate transport and activation of adenylate cyclase for global regulation, do not explain this virulence phenotype. Instead, biochemical studies, in vitro secretion and translocation assays, and in vivo genetic epistasis experiments suggest that EIIAGlc binds to the type three secretion system 2 (TTSS-2) involved in systemic virulence, stabilizes its cytoplasmic part including the crucial TTSS-2 ATPase, and activates virulence factor secretion. This unexpected role of EIIAGlc reveals a striking direct link between central Salmonella metabolism and a crucial virulence mechanism.

The SdiA-regulated gene srgE encodes a type III secreted effector

Salmonella enterica serovar Typhimurium is a food-borne pathogen that causes severe gastroenteritis. The ability of Salmonella to cause disease depends on two type III secretion systems (T3SSs) encoded in two distinct Salmonella pathogenicity islands, 1 and 2 (SPI1 and SPI2, respectively). S. Typhimurium encodes a solo LuxR homolog, SdiA, which can detect the acyl-homoserine lactones (AHLs) produced by other bacteria and upregulate the rck operon and the srgE gene. SrgE is predicted to encode a protein of 488 residues with a coiled-coil domain between residues 345 and 382. In silico studies have provided conflicting predictions as to whether SrgE is a T3SS substrate. Therefore, in this work, we tested the hypothesis that SrgE is a T3SS effector by two methods, a β-lactamase activity assay and a split green fluorescent protein (GFP) complementation assay. SrgE with β-lactamase fused to residue 40, 100, 150, or 300 was indeed expressed and translocated into host cells, but SrgE with β-lactamase fused to residue 400 or 488 was not expressed, suggesting interference by the coiled-coil domain. Similarly, SrgE with GFP S11 fused to residue 300, but not to residue 488, was expressed and translocated into host cells. With both systems, translocation into host cells was dependent upon SPI2. A phylogenetic analysis indicated that srgE is found only within Salmonella enterica subspecies. It is found sporadically within both typhoidal and nontyphoidal serovars, although the SrgE protein sequences found within typhoidal serovars tend to cluster separately from those found in nontyphoidal serovars, suggesting functional diversification.

Modulation of vacuolar pH is required for replication of Edwardsiella ictaluri in channel catfish macrophages

Previous in vitro work demonstrated that Edwardsiella ictaluri produces an acid-activated urease that can modulate environmental pH through the production of ammonia from urea. Additional work revealed that expression of the E. ictaluri type III secretion system (T3SS) is upregulated by acidic pH. Both the urease and the T3SS were previously shown to be essential to intracellular replication. In this work, fluorescence microscopy with LysoTracker Red DND-99 (LTR) indicated that E. ictaluri-containing vacuoles (ECV) became acidified following ingestion by head kidney-derived macrophages (HKDM). In vivo ratiometric imaging demonstrated a lowered ECV pH, which fell to as low as pH 4 but subsequently increased to pH 6 or greater. Inhibition of vacuolar H(+)-ATPases by use of the specific inhibitor bafilomycin A1 abrogated both ECV acidification and intracellular replication in HKDM. Failure of an E. ictaluri urease knockout mutant to increase the ECV pH in the in vivo ratiometric assay suggests that ammonia produced by the urease reaction mediates the pH increase. Additionally, when the specific arginase inhibitor l-norvaline was used to treat E. ictaluri-infected HKDM, the ECV failed to neutralize and E. ictaluri was unable to replicate. This indicates that the HKDM-encoded arginase enzyme produces the urea used by the E. ictaluri urease enzyme. Failure of the ECV to acidify would prevent both upregulation of the T3SS and activation of the urease enzyme, either of which would prevent E. ictaluri from replicating in HKDM. Failure of the ECV to neutralize would result in a vacuolar pH too low to support E. ictaluri replication.

Bacterial regulon evolution: distinct responses and roles for the identical OmpR proteins of Salmonella Typhimurium and Escherichia coli in the acid stress response

The evolution of new gene networks is a primary source of genetic innovation that allows bacteria to explore and exploit new niches, including pathogenic interactions with host organisms. For example, the archetypal DNA binding protein, OmpR, is identical between Salmonella Typhimurium serovar Typhimurium and Escherichia coli, but regulatory specialization has resulted in different environmental triggers of OmpR expression and largely divergent OmpR regulons. Specifically, ompR mRNA and OmpR protein levels are elevated by acid pH in S. Typhimurium but not in E. coli. This differential expression pattern is due to differences in the promoter regions of the ompR genes and the E. coli ompR orthologue can be made acid-inducible by introduction of the appropriate sequences from S. Typhimurium. The OmpR regulon in S. Typhimurium overlaps that of E. coli at only 15 genes and includes many horizontally acquired genes (including virulence genes) that E. coli does not have. We found that OmpR binds to its genomic targets in higher abundance when the DNA is relaxed, something that occurs in S. Typhimurium as a result of acid stress and which is a requirement for optimal expression of its virulence genes. The genomic targets of OmpR do not share a strong nucleotide sequence consensus: we propose that the ability of OmpR to recruit additional genes to its regulon arises from its modest requirements for specificity in its DNA targets with its preference for relaxed DNA allowing it to cooperate with DNA-topology-based allostery to modulate transcription in response to acid stress.

A microfluidic-based genetic screen to identify microbial virulence factors that inhibit dendritic cell migration

Microbial pathogens are able to modulate host cells and evade the immune system by multiple mechanisms. For example, Salmonella injects effector proteins into host cells and evades the host immune system in part by inhibiting dendritic cell (DC) migration. The identification of microbial factors that modulate normal host functions should lead to the development of new classes of therapeutics that target these pathways. Current screening methods to identify either host or pathogen genes involved in modulating migration towards a chemical signal are limited because they do not employ stable, precisely controlled chemical gradients. Here, we develop a positive selection microfluidic-based genetic screen that allows us to identify Salmonella virulence factors that manipulate DC migration within stable, linear chemokine gradients. Our screen identified 7 Salmonella effectors (SseF, SifA, SspH2, SlrP, PipB2, SpiC and SseI) that inhibit DC chemotaxis toward CCL19. This method is widely applicable for identifying novel microbial factors that influence normal host cell chemotaxis as well as revealing new mammalian genes involved in directed cell migration.