Functional characterization of SsaE, a novel chaperone protein of the type III secretion system encoded by Salmonella pathogenicity island 2

Salmonella Typhimurium exploits host polyamines for assembly of the type 3 secretion machinery

Bacterial pathogens utilize the factors of their hosts to infect them, but which factors they exploit remain poorly defined. Here, we show that a pathogenic Salmonella enterica serovar Typhimurium (STm) exploits host polyamines for the functional expression of virulence factors. An STm mutant strain lacking principal genes required for polyamine synthesis and transport exhibited impaired infectivity in mice. A polyamine uptake-impaired strain of STm was unable to inject effectors of the type 3 secretion system into host cells due to a failure of needle assembly. STm infection stimulated host polyamine production by increasing arginase expression. The decline in polyamine levels caused by difluoromethylornithine, which inhibits host polyamine production, attenuated STm colonization, whereas polyamine supplementation augmented STm pathogenesis. Our work reveals that host polyamines are a key factor promoting STm infection, and therefore a promising therapeutic target for bacterial infection.

Pathogenic bacteria experience pervasive RNA polymerase backtracking during infection

Pathogenic bacteria and their eukaryotic hosts are in a constant arms race. Hosts have numerous defense mechanisms at their disposal that not only challenge the bacterial invaders, but have the potential to disrupt molecular transactions along the bacterial chromosome. However, it is unclear how the host impacts association of proteins with the bacterial chromosome at the molecular level during infection. This is partially due to the lack of a method that could detect these events in pathogens while they are within host cells. We developed and optimized a system capable of mapping and measuring levels of bacterial proteins associated with the chromosome while they are actively infecting the host (referred to as PIC-seq). Here, we focused on the dynamics of RNA polymerase (RNAP) movement and association with the chromosome in the pathogenic bacterium Salmonella enterica as a model system during infection. Using PIC-seq, we found that RNAP association patterns with the chromosome change during infection genome-wide, including at regions that encode for key virulence genes. Importantly, we found that infection of a host significantly increases RNAP backtracking on the bacterial chromosome. RNAP backtracking is the most common form of disruption to RNAP progress on the chromosome. Interestingly, we found that the resolution of backtracked RNAPs via the anti-backtracking factors GreA and GreB is critical for pathogenesis, revealing a new class of virulence genes. Altogether, our results strongly suggest that infection of a host significantly impacts transcription by disrupting RNAP movement on the chromosome within the bacterial pathogen. The increased backtracking events have important implications not only for efficient transcription, but also for mutation rates as stalled RNAPs increase the levels of mutagenesis.

Secreted in a Type III Secretion System-Dependent Manner, EsaH and EscE Are the Cochaperones of the T3SS Needle Protein EsaG of Edwardsiella piscicida

The intracellular EscE protein tightly controls the secretion of the type III secretion system (T3SS) middle and late substrates in Edwardsiella piscicida. However, the regulation of secretion by EscE is incompletely understood. In this work, we reveal that EscE interacts with EsaH and EsaG. The crystal structures of the EscE-EsaH complex and EscE-EsaG-EsaH complex were resolved at resolutions of 1.4 Å and 1.8 Å, respectively. EscE and EsaH form a hydrophobic groove to engulf the C-terminal region of EsaG (56 to 73 amino acids [aa]), serving as the cochaperones of T3SS needle protein EsaG in E. piscicida. V61, K62, M64, and M65 of EsaG play a pivotal role in maintaining the conformation of the ternary complex of EscE-EsaG-EsaH, thereby maintaining the stability of EsaG. An in vivo experiment revealed that EscE and EsaH stabilize each other, and both of them stabilize EsaG. Meanwhile, either EscE or EsaH can be secreted through the T3SS. The secondary structure of EsaH lacks the fourth and fifth α helices presented in its homologs PscG, YscG, and AscG. Insertion of the α4 and α5 helices of PscG or swapping the N-terminal 25 aa of PscG with those of EsaH starkly decreases the protein level of the chimeric EsaH, resulting in instability of EsaG and deactivation of the T3SS. To the best of our knowledge, these data represent the first reported structure of the T3SS needle complex of pathogens from Enterobacteriaceae and the first evidence for the secretion of T3SS needle chaperones. IMPORTANCE Edwardsiella piscicida causes severe hemorrhagic septicemia in fish. Inactivation of the type III secretion system (T3SS) increases its 50% lethal dose (LD₅₀) by ~10 times. The secretion of T3SS middle and late substrates in E. piscicida is tightly controlled by the intracellular steady-state protein level of EscE, but the mechanism is incompletely understood. In this study, EscE was found to interact with and stabilize EsaH in E. piscicida. The EscE-EsaH complex is structurally analogous to T3SS needle chaperones. Further study revealed that EscE and EsaH form a hydrophobic groove to engulf the C-terminal region of EsaG, serving as the cochaperones stabilizing the T3SS needle protein EsaG. Interestingly, both EscE and EsaH are secreted. Our study reveals that the EscE-EsaH complex controls T3SS protein secretion by stabilizing EsaG, whose secretion in turn leads to the secretion of the middle and late T3SS substrates.

Transmembrane domains of type III-secreted proteins affect bacterial-host interactions in enteropathogenic E. coli

Many bacterial pathogens utilize a specialized secretion system, termed type III secretion system (T3SS), to translocate effector proteins into host cells and establish bacterial infection. The T3SS is anchored within the bacterial membranes and contains a long needle/filament that extends toward the host-cell and forms, at its distal end, a pore complex within the host membrane. The T3SS pore complex consists of two bacterial proteins, termed SctB and SctE, which have conflicting targeting indications; a signal sequence that targets to secretion to the extracellular environment via the T3SS, and transmembrane domains (TMDs) that target to membrane localization. In this study, we investigate whether the TMD sequences of SctB and SctE have special features that differentiate them from classical TMDs and allow them to escape bacterial membrane integration. For this purpose, we exchanged the SctB and SctE native TMDs for alternative hydrophobic sequences and found that the TMD sequences of SctB and SctE dictate membrane destination (bacterial versus host membrane). Moreover, we examined the role of the SctB TMD sequence in the activity of the full-length protein, post secretion, and found that the TMD does not serve only as a hydrophobic segment, but is also involved in the ability of the protein to translocate itself and other proteins into and across the host cell membrane.

Role of OB-Fold Protein YdeI in Stress Response and Virulence of Salmonella enterica Serovar Enteritidis

An essential feature of the pathogenesis of the Salmonella enterica serovar Enteritidis wild type (WT) is its ability to survive under diverse microenvironmental stress conditions, such as encountering antimicrobial peptides (AMPs) or glucose and micronutrient starvation. These stress factors trigger virulence genes carried on Salmonella pathogenicity islands (SPIs) and determine the efficiency of enteric infection. Although the oligosaccharide/oligonucleotide binding-fold (OB-fold) family of proteins has been identified as an important stress response and virulence determinant, functional information on members of this family is currently limited. In this study, we decipher the role of YdeI, which belongs to OB-fold family of proteins, in stress response and virulence of S Enteritidis. When ydeI was deleted, the ΔydeI mutant showed reduced survival during exposure to AMPs or glucose and Mg²⁺ starvation stress compared to the WT. Green fluorescent protein (GFP) reporter and quantitative real-time PCR (qRT-PCR) assays showed ydeI was transcriptionally regulated by PhoP, which is a major regulator of stress and virulence. Furthermore, the ΔydeI mutant displayed ∼89% reduced invasion into HCT116 cells, ∼15-fold-reduced intramacrophage survival, and downregulation of several SPI-1 and SPI-2 genes encoding the type 3 secretion system apparatus and effector proteins. The mutant showed attenuated virulence compared to the WT, confirmed by its reduced bacterial counts in feces, mesenteric lymph node (mLN), spleen, and liver of C57BL/6 mice. qRT-PCR analyses of the ΔydeI mutant displayed differential expression of 45 PhoP-regulated genes, which were majorly involved in metabolism, transport, membrane remodeling, and drug resistance under different stress conditions. YdeI is, therefore, an important protein that modulates S Enteritidis virulence and adaptation to stress during infection.IMPORTANCE S Enteritidis during its life cycle encounters diverse stress factors inside the host. These intracellular conditions allow activation of specialized secretion systems to cause infection. We report a conserved membrane protein, YdeI, and elucidate its role in protection against various intracellular stress conditions. A key aspect of the study of a pathogen's stress response mechanism is its clinical relevance during host-pathogen interaction. Bacterial adaptation to stress plays a vital role in evolution of a pathogen's resistance to therapeutic agents. Therefore, investigation of the role of YdeI is vital for understanding the molecular basis of regulation of Salmonella pathogenesis. In conclusion, our findings may contribute to finding potential targets to develop new intervention strategies for treatment and prevention of enteric diseases.

Stress response, amino acid biosynthesis and pathogenesis genes expressed in Salmonella enterica colonizing tomato shoot and root surfaces

Salmonella enterica can colonize all parts of the tomato plant. Tomatoes have been frequently implicated in salmonellosis outbreaks. In agricultural settings, Salmonella must overcome stress, nutritional and competition barriers to become established on plant surfaces. Knowledge of the genetic mechanisms underlying Salmonella-plant associations is limited, especially when growing epiphytically. A genome-wide transcriptomic analysis of Salmonella Typhimurium (SeT) was conducted with RNA-Seq to elucidate strategies for epiphytic growth on live, intact tomato shoot and root surfaces. Six plasmid-encoded and 123 chromosomal genes were significantly (using Benjamini-Hochberg adjusted p-values) up-regulated; 54 and 110 detected in SeT on shoots and roots, respectively, with 35 common to both. Key signals included NsrR regulon genes needed to mitigate nitrosative stress, oxidative stress genes and host adaptation genes, including environmental stress, heat shock and acid-inducible genes. Several amino acid biosynthesis genes and genes indicative of sulphur metabolism and anaerobic respiration were up-regulated. Some Type III secretion system (T3SS) effector protein genes and their chaperones from pathogenicity island-2 were expressed mostly in SeT on roots. Gene expression in SeT was validated against SeT and also the tomato outbreak strain Salmonella Newport with a high correlation (R ² = 0.813 and 0.874, respectively; both p < 0.001). Oxidative and nitrosative stress response genes, T3SS2 genes and amino acid biosynthesis may be needed for Salmonella to successfully colonize tomato shoot and root surfaces.

Salmonella enterica Effectors SifA, SpvB, SseF, SseJ, and SteA Contribute to Type III Secretion System 1-Independent Inflammation in a Streptomycin-Pretreated Mouse Model of Colitis

Salmonella enterica serovar Typhimurium (S. Typhimurium) induces inflammatory changes in the ceca of streptomycin-pretreated mice. In this mouse model of colitis, the type III secretion system 1 (T3SS-1) has been shown to induce rapid inflammatory change in the cecum at early points, 10 to 24 h after infection. Five proteins, SipA, SopA, SopB, SopD, and SopE2, have been identified as effectors involved in eliciting intestinal inflammation within this time range. In contrast, a T3SS-1-deficient strain was shown to exhibit inflammatory changes in the cecum at 72 to 120 h postinfection. However, the effectors eliciting T3SS-1-independent inflammation remain to be clarified. In this study, we focused on two T3SS-2 phenotypes, macrophage proliferation and cytotoxicity, to identify the T3SS-2 effectors involved in T3SS-1-independent inflammation. We identified a mutant strain that could not induce cytotoxicity in a macrophage-like cell line and that reduced intestinal inflammation in streptomycin-pretreated mice. We also identified five T3SS-2 effectors, SifA, SpvB, SseF, SseJ, and SteA, associated with T3SS-1-independent macrophage cytotoxicity. We then constructed a strain lacking T3SS-1 and all the five T3SS-2 effectors, termed T1S5. The S. Typhimurium T1S5 strain significantly reduced cytotoxicity in macrophages in the same manner as a mutant invA spiB strain (T1T2). Finally, the T1S5 strain elicited no inflammatory changes in the ceca of streptomycin-pretreated mice. We conclude that these five T3SS-2 effectors contribute to T3SS-1-independent inflammation.

Chaperone-mediated secretion switching from early to middle substrates in the type III secretion system encoded by Salmonella pathogenicity island 2

The bacterial type III secretion system (T3SS) delivers virulence proteins, called effectors, into eukaryotic cells. T3SS comprises a transmembrane secretion apparatus and a complex network of specialized chaperones that target protein substrates to this secretion apparatus. However, the regulation of secretion switching from early (needle and inner rod) to middle (tip/filament and translocators) substrates is incompletely understood. Here, we investigated chaperone-mediated secretion switching from early to middle substrates in the T3SS encoded by Salmonella pathogenicity island 2 (SPI2), essential for systemic infection. Our findings revealed that the protein encoded by ssaH regulates the secretion of an inner rod and early substrate, SsaI. Structural modeling revealed that SsaH is structurally similar to class III chaperones, known to associate with proteins in various pathogenic bacteria. The SPI2 protein SsaE was identified as a class V chaperone homolog and partner of SsaH. A pulldown analysis disclosed that SsaH and SsaE form a heterodimer, which interacted with another early substrate, the needle protein SsaG. Moreover, SsaE also helped stabilize SsaH and a middle substrate, SseB. We also found that SsaE regulates cellular SsaH levels to translocate the early substrates SsaG and SsaI and then promotes the translocation of SseB by stabilizing it. In summary, our results indicate that the class III chaperone SsaH facilitates SsaI secretion, and a heterodimer of SsaH and the type V chaperone SsaE then switches secretion to SsaG. This is the first report of a chaperone system that regulates both early and middle substrates during substrate switching for T3SS assembly.

Tat-exported peptidoglycan amidase-dependent cell division contributes to Salmonella Typhimurium fitness in the inflamed gut

Salmonella enterica serovar Typhimurium (S. Tm) is a cause of food poisoning accompanied with gut inflammation. Although mucosal inflammation is generally thought to be protective against bacterial infection, S. Tm exploits the inflammation to compete with commensal microbiota, thereby growing up to high densities in the gut lumen and colonizing the gut continuously at high levels. However, the molecular mechanisms underlying the beneficial effect of gut inflammation on S. Tm competitive growth are poorly understood. Notably, the twin-arginine translocation (Tat) system, which enables the transport of folded proteins outside bacterial cytoplasm, is well conserved among many bacterial pathogens, with Tat substrates including virulence factors and virulence-associated proteins. Here, we show that Tat and Tat-exported peptidoglycan amidase, AmiA- and AmiC-dependent cell division contributes to S. Tm competitive fitness advantage in the inflamed gut. S. Tm tatC or amiA amiC mutants feature a gut colonization defect, wherein they display a chain form of cells. The chains are attributable to a cell division defect of these mutants and occur in inflamed but not in normal gut. We demonstrate that attenuated resistance to bile acids confers the colonization defect on the S. Tm amiA amiC mutant. In particular, S. Tm cell chains are highly sensitive to bile acids as compared to single or paired cells. Furthermore, we show that growth media containing high concentrations of NaCl and sublethal concentrations of antimicrobial peptides induce the S. Tm amiA amiC mutant chain form, suggesting that gut luminal conditions such as high osmolarity and the presence of antimicrobial peptides impose AmiA- and AmiC-dependent cell division on S. Tm. Together, our data indicate that Tat and the Tat-exported amidases, AmiA and AmiC, are required for S. Tm luminal fitness in the inflamed gut, suggesting that these proteins might comprise effective targets for novel antibacterial agents against infectious diarrhea.

For proteins residing outside the bacterial cytoplasm, transport is an essential step for adequate function. The twin-arginine translocation (Tat) system enables the transport of folded proteins across the cytoplasmic membrane in prokaryotes. It has recently become clear that this system plays a pivotal role in the detrimental effects of many bacterial pathogens, suggesting Tat as a novel therapeutic target against their infection. In particular, the bacterial enteropathogen Salmonella Typhimurium causes foodborne diarrhea by colonizing the gut interior space. Here, we describe that the S. Typhimurium Tat system contributes to intestinal infection by facilitating colonization of the gut by this pathogen. We also identify that two Tat-exported enzymes, peptidoglycan amidase AmiA and AmiC, are responsible for the Tat-dependent colonization. S. Typhimurium strains having nonfunctional Tat systems or lacking these enzymes undergo filamentous growth in the gut interior owing to defective cell division. Notably, this chain form of S. Typhimurium cells is highly sensitive to bile acids, rendering it less competitive with native bacteria in the gut. The data presented here suggest that the Tat system and associated amidases may comprise promising therapeutic targets for Salmonella diarrhea, and that controlling bacterial shape might be new strategy for regulating intestinal enteropathogen infection.

Novel T3SS effector EseK in Edwardsiella piscicida is chaperoned by EscH and EscS to express virulence

Bacterium usually utilises type III secretion systems (T3SS) to deliver effectors directly into host cells with the aids of chaperones. Hence, it is very important to identify bacterial T3SS effectors and chaperones for better understanding of host-pathogen interactions. Edwardsiella piscicida is an invasive enteric bacterium, which infects a wide range of hosts from fish to human. Given E. piscicida encodes a functional T3SS to promote infection, very few T3SS effectors and chaperones have been identified in this bacterium so far. Here, we reported that EseK is a new T3SS effector protein translocated by E. piscicida. Bioinformatic analysis indicated that escH and escS encode two putative class I T3SS chaperones. Further investigation indicated that EscH and EscS can enhance the secretion and translocation of EseK. EscH directly binds EseK through undetermined binding domains, whereas EscS binds EseK via its N-terminal α-helix. We also found that EseK has an N-terminal chaperone-binding domain, which binds EscH and EscS to form a ternary complex. Zebrafish infection experiments showed that EseK and its chaperones EscH and EscS are necessary for bacterial colonisation in zebrafish. This work identified a new T3SS effector, EseK, and its two T3SS chaperones, EscH and EscS, in E. piscicida, which enriches our knowledge of bacterial T3SS effector-chaperone interaction and contributes to our understanding of bacterial pathogenesis.

Bordetella effector BopN is translocated into host cells via its N-terminal residues

Bordetella bronchiseptica infects a wide variety of mammals, the type III secretion system (T3SS) being involved in long-term colonization by Bordetella of the trachea and lung. T3SS translocates virulence factors (commonly referred to as effectors) into host cells, leading to alterations in the host's physiological function. The Bordetella effectors BopN and BteA are known to have roles in up-regulation of IL-10 and cytotoxicity, respectively. Nevertheless, the mechanism by which BopN is translocated into host cells has not been examined in sufficient detail. Therefore, to determine the precise mechanisms of translocation of BopN into host cells, truncated derivatives of BopN were built and the derivatives' ability to translocate into host cells evaluated by adenylate cyclase-mediated translocation assay. It was found that N-terminal amino acid (aa) residues 1-200 of BopN are sufficient for its translocation into host cells. Interestingly, BopN translocation was completely blocked by deletion of the N-terminal aa residues 6-50, indicating that the N-terminal region is critical for BopN translocation. Furthermore, BopN appears to play an auxiliary role in BteA-mediated cytotoxicity. Thus, BopN can apparently translocate into host cells and may facilitate activity of BteA.

HpaB-Dependent Secretion of Type III Effectors in the Plant Pathogens Ralstonia solanacearum and Xanthomonas campestris pv. vesicatoria

Plant pathogenic bacteria exerts their pathogenicity through the injection of large repertoires of type III effectors (T3Es) into plant cells, a mechanism controlled in part by type III chaperones (T3Cs). In Ralstonia solanacearum, the causal agent of bacterial wilt, little is known about the control of type III secretion at the post-translational level. Here, we provide evidence that the HpaB and HpaD proteins do act as bona fide R. solanacearum class IB chaperones that associate with several T3Es. Both proteins can dimerize but do not interact with each other. After screening 38 T3Es for direct interactions, we highlighted specific and common interacting partners, thus revealing the first picture of the R. solanacearum T3C-T3E network. We demonstrated that the function of HpaB is conserved in two phytopathogenic bacteria, R. solanacearum and Xanthomonas campestris pv. vesicatoria (Xcv). HpaB from Xcv is able to functionally complement a R. solanacearum hpaB mutant for hypersensitive response elicitation on tobacco plants. Likewise, Xcv is able to translocate a heterologous T3E from R. solanacearum in an HpaB-dependent manner. This study underlines the central role of the HpaB class IB chaperone family and its potential contribution to the bacterial plasticity to acquire and deliver new virulence factors.

The Bordetella Secreted Regulator BspR Is Translocated into the Nucleus of Host Cells via Its N-Terminal Moiety: Evaluation of Bacterial Effector Translocation by the Escherichia coli Type III Secretion System

Bordetella bronchiseptica is genetically related to B. pertussis and B. parapertussis, which cause respiratory tract infections in humans. These pathogens possess a large number of virulence factors, including the type III secretion system (T3SS), which is required for the delivery of effectors into the host cells. In a previous study, we identified a transcriptional regulator, BspR, that is involved in the regulation of the T3SS-related genes in response to iron-starved conditions. A unique feature of BspR is that this regulator is secreted into the extracellular milieu via the T3SS. To further characterize the role of BspR in extracellular localization, we constructed various truncated derivatives of BspR and investigated their translocation into the host cells using conventional translocation assays. In this study, the effector translocation was evaluated by the T3SS of enteropathogenic E. coli (EPEC), since the exogenous expression of BspR triggers severe repression of the Bordetella T3SS expression. The results of the translocation assays using the EPEC T3SS showed that the N-terminal 150 amino acid (aa) residues of BspR are sufficient for translocation into the host cells in a T3SS-dependent manner. In addition, exogenous expression of BspR in HeLa cells demonstrated that the N-terminal 100 aa residues are involved in the nuclear localization. In contrast, the N-terminal 54 aa residues are sufficient for the extracellular secretion into the bacterial culture supernatant via the EPEC T3SS. Thus, BspR is not only a transcriptional regulator in bacteria cytosol, but also functions as an effector that translocates into the nuclei of infected host cells.

Identification of the docking site between a type III secretion system ATPase and a chaperone for effector cargo

A number of Gram-negative pathogens utilize type III secretion systems (T3SSs) to inject bacterial effector proteins into the host. An important component of T3SSs is a conserved ATPase that captures chaperone-effector complexes and energizes their dissociation to facilitate effector translocation. To date, there has been limited work characterizing the chaperone-T3SS ATPase interaction despite it being a fundamental aspect of T3SS function. In this study, we present the 2.1 Å resolution crystal structure of the Salmonella enterica SPI-2-encoded ATPase, SsaN. Our structure revealed a local and functionally important novel feature in helix 10 that we used to define the interaction domain relevant to chaperone binding. We modeled the interaction between the multicargo chaperone, SrcA, and SsaN and validated this model using mutagenesis to identify the residues on both the chaperone and ATPase that mediate the interaction. Finally, we quantified the benefit of this molecular interaction on bacterial fitness in vivo using chromosomal exchange of wild-type ssaN with mutants that retain ATPase activity but no longer capture the chaperone. Our findings provide insight into chaperone recognition by T3SS ATPases and demonstrate the importance of the chaperone-T3SS ATPase interaction for the pathogenesis of Salmonella.

Functional characterization of the type III secretion ATPase SsaN encoded by Salmonella pathogenicity island 2

A type III secretion system (T3SS) is utilized by a large number of gram-negative bacteria to deliver effectors directly into the cytosol of eukaryotic host cells. One essential component of a T3SS is an ATPase that catalyzes the unfolding of proteins, which is followed by the translocation of effectors through an injectisome. Here we demonstrate a functional role of the ATPase SsaN, a component of Salmonella pathogenicity island 2 T3SS (T3SS-2) in Salmonella enterica serovar Typhimurium. SsaN hydrolyzed ATP in vitro and was essential for T3SS function and Salmonella virulence in vivo. Protein-protein interaction analyses revealed that SsaN interacted with SsaK and SsaQ to form the C ring complex. SsaN and its complex co-localized to the membrane fraction under T3SS-2 inducing conditions. In addition, SsaN bound to Salmonella pathogenicity island 2 (SPI-2) specific chaperones, including SsaE, SseA, SscA, and SscB that facilitated translocator/effector secretion. Using an in vitro chaperone release assay, we demonstrated that SsaN dissociated a chaperone-effector complex, SsaE and SseB, in an ATP-dependent manner. Effector release was dependent on a conserved arginine residue at position 192 of SsaN, and this was essential for its enzymatic activity. These results strongly suggest that the T3SS-2-associated ATPase SsaN contributes to T3SS-2 effector translocation efficiency.

[Virulence determinant of Chromobacterium violaceum]

Chromobacterium violaceum is a Gram-negative bacterium that infects humans and animals with fatal sepsis. The infection with C. violaceum is rare in case of those who are healthy, but once established, C. violaceum causes sever disease accompanied by abscess formation in the lungs, liver and spleen. Furthermore, C. violaceum is resistant to a broad range of antibiotics, which in some cases renders the antimicrobial therapy for this infection difficult. Thus, the infection with C. violaceum displays high mortality rates unless initial proper antimicrobial therapy. In contrast, the infection mechanism had completely remained unknown. To this end, we have tried to identify virulence factors-associated with C. violaceum infection. Two distinct type III secretion systems (TTSSs) were thought to be one of the most important virulence factors, which are encoded by Chromobacterium pathogenicity island 1/1a and 2 (Cpi-1/-1a and -2) respectively. Our results have shown that Cpi-1/-1a-encoded TTSS, but not Cpi-2, is indispensable for the virulence in a mouse infection model. C. violaceum caused fulminant hepatitis in a Cpi-1/-1a-encoded TTSS-dependent manner. We next have identified 16 novel effectors secreted from Cpi-1/-1a-encoded TTS machinery. From these effectors, we found that CopE (Chromobacterium outer protein E) has similarities to a guanine nucleotide exchange factor (GEF) for Rho GTPases. CopE acts as GEF for Rac1 and Cdc42, leading to induction of actin cytoskeletal rearrangement. Interestingly, C. violaceum invades cultured human epithelial cells in a CopE-dependent manner. Finally, an inactivation of CopE by disruption of copE gene or amino acid point mutation leading to loss of GEF activity attenuates significantly the mouse virulence of C. violaceum. These results suggest that Cpi-1/-1a-encoded TTSS is a major virulence determinant for C. violaceum infection, and that CopE contributes to the virulence in part of this pathogen.

Membrane and chaperone recognition by the major translocator protein PopB of the type III secretion system of Pseudomonas aeruginosa

The type III secretion system is a widespread apparatus used by pathogenic bacteria to inject effectors directly into the cytoplasm of eukaryotic cells. A key component of this highly conserved system is the translocon, a pore formed in the host membrane that is essential for toxins to bypass this last physical barrier. In Pseudomonas aeruginosa the translocon is composed of PopB and PopD, both of which before secretion are stabilized within the bacterial cytoplasm by a common chaperone, PcrH. In this work we characterize PopB, the major translocator, in both membrane-associated and PcrH-bound forms. By combining sucrose gradient centrifugation experiments, limited proteolysis, one-dimensional NMR, and β-lactamase reporter assays on eukaryotic cells, we show that PopB is stably inserted into bilayers with its flexible N-terminal domain and C-terminal tail exposed to the outside. In addition, we also report the crystal structure of the complex between PcrH and an N-terminal region of PopB (residues 51-59), which reveals that PopB lies within the concave face of PcrH, employing mostly backbone residues for contact. PcrH is thus the first chaperone whose structure has been solved in complex with both type III secretion systems translocators, revealing that both molecules employ the same surface for binding and excluding the possibility of formation of a ternary complex. The characterization of the major type III secretion system translocon component in both membrane-bound and chaperone-bound forms is a key step for the eventual development of antibacterials that block translocon assembly.

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

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

Protein export according to schedule: architecture, assembly, and regulation of type III secretion systems from plant- and animal-pathogenic bacteria

Flagellar and translocation-associated type III secretion (T3S) systems are present in most gram-negative plant- and animal-pathogenic bacteria and are often essential for bacterial motility or pathogenicity. The architectures of the complex membrane-spanning secretion apparatuses of both systems are similar, but they are associated with different extracellular appendages, including the flagellar hook and filament or the needle/pilus structures of translocation-associated T3S systems. The needle/pilus is connected to a bacterial translocon that is inserted into the host plasma membrane and mediates the transkingdom transport of bacterial effector proteins into eukaryotic cells. During the last 3 to 5 years, significant progress has been made in the characterization of membrane-associated core components and extracellular structures of T3S systems. Furthermore, transcriptional and posttranscriptional regulators that control T3S gene expression and substrate specificity have been described. Given the architecture of the T3S system, it is assumed that extracellular components of the secretion apparatus are secreted prior to effector proteins, suggesting that there is a hierarchy in T3S. The aim of this review is to summarize our current knowledge of T3S system components and associated control proteins from both plant- and animal-pathogenic bacteria.

Gap1 functions as a molecular chaperone to stabilize its interactive partner Gap3 during biogenesis of serine-rich repeat bacterial adhesin

Serine-rich repeat glycoproteins (SRRPs) are important bacterial adhesins that are conserved in streptococci and staphylococci. Fimbriae-associated protein (Fap1) from Streptococcus parasanguinis, was the first SRRP identified; it plays an important role in bacterial biofilm formation. A gene cluster encoding glycosyltransferases and accessory secretion components is required for Fap1 biogenesis. Two glycosylation-associated proteins, Gap1 and Gap3 within the cluster, interact with each other and function in concert in Fap1 biogenesis. Here we report the new molecular events underlying contribution of the interaction to Fap1 biogenesis. The Gap1-deficient mutant rendered Gap3 unstable and degraded in vitro and in vivo. Inactivation of a gene encoding protease ClpP reversed the phenotype of the gap1 mutant, suggesting that ClpP is responsible for degradation of Gap3. Molecular chaperone GroEL was co-purified with Gap3 only when Gap1 was absent and also reacted with Gap1 monoclonal antibody, suggesting that Gap1 functions as a specific chaperone for Gap3. The N-terminal interacting domains of Gap1 mediated the Gap3 stability and Fap1 biogenesis. Gap1 homologues from Streptococcus agalactiae and Staphylococcus aureus also interacted with and stabilized corresponding Gap3 homologues, suggesting that the chaperone activity of the Gap1 homologues is common in biogenesis of SRRPs.

Salmonella type III effector SpvC, a phosphothreonine lyase, contributes to reduction in inflammatory response during intestinal phase of infection

Salmonella phosphothreonine lyase SpvC inactivates the dual-phosphorylated host mitogen-activated protein kinases (MAPK) through β-elimination. While SpvC can be secreted in vitro by both Salmonella pathogenicity island (SPI)-1 and SPI-2 type III secretion systems (T3SSs), translocation of this protein into the host cell cytosol has only been demonstrated by SPI-2 T3SS. In this study, we show that SpvC can be delivered into the host cell cytoplasm by both SPI-1 and SPI-2 T3SSs. Dephosphorylation of the extracellular signal-regulated protein kinases (ERK) was detected in an SPI-1 T3SS-dependent manner 2 h post infection. Using a mouse model for Salmonella enterocolitis, which was treated with streptomycin prior to infection, we observed that mice infected with Salmonella enterica serovar Typhimurium strains lacking the spvC gene showed pronounced colitis when compared with mice infected with the wild-type strain 1 day after infection. The effect of SpvC on the development of colitis was characterized by reduced mRNA levels of the pro-inflammatory cytokines and chemokines, and reduced inflammation with less infiltration of neutrophils. Furthermore, the reduction in inflammation by SpvC resulted in increased bacterial dissemination in spleen of mice infected with Salmonella. Collectively, our findings suggest that SpvC exerts as an anti-inflammatory effector and the attenuation of intestinal inflammatory response by SpvC is involved in systemic infection of Salmonella.

Impact of the N-terminal secretor domain on YopD translocator function in Yersinia pseudotuberculosis type III secretion

Type III secretion systems (T3SSs) secrete needle components, pore-forming translocators, and the translocated effectors. In part, effector recognition by a T3SS involves their N-terminal amino acids and their 5' mRNA. To investigate whether similar molecular constraints influence translocator secretion, we scrutinized this region within YopD from Yersinia pseudotuberculosis. Mutations in the 5' end of yopD that resulted in specific disruption of the mRNA sequence did not affect YopD secretion. On the other hand, a few mutations affecting the protein sequence reduced secretion. Translational reporter fusions identified the first five codons as a minimal N-terminal secretion signal and also indicated that the YopD N terminus might be important for yopD translation control. Hybrid proteins in which the N terminus of YopD was exchanged with the equivalent region of the YopE effector or the YopB translocator were also constructed. While the in vitro secretion profile was unaltered, these modified bacteria were all compromised with respect to T3SS activity in the presence of immune cells. Thus, the YopD N terminus does harbor a secretion signal that may also incorporate mechanisms of yopD translation control. This signal tolerates a high degree of variation while still maintaining secretion competence suggestive of inherent structural peculiarities that make it distinct from secretion signals of other T3SS substrates.

Killing of Candida albicans filaments by Salmonella enterica serovar Typhimurium is mediated by sopB effectors, parts of a type III secretion system

Although bacterial-fungal interactions shape microbial virulence during polymicrobial infections, only a limited number of studies have evaluated this interaction on a genetic level. We report here that one interaction is mediated by sopB, an effector of a type III secretion system (TTSS) of Salmonella enterica serovar Typhimurium. In these studies, we screened 10 TTSS effector-related mutants and determined their role in the killing of C. albicans filaments in vitro during coinfection in planktonic environments. We found that deleting the sopB gene (which encodes inositol phosphatase) was associated with a significant decrease in C. albicans killing at 25°C after 5 days, similar to that caused by the deletion of sipB (which encodes TTSS translocation machinery components). The sopB deletion dramatically influenced the killing of C. albicans filaments. It was associated with repressed filamentation in the Caenorhabditis elegans model of C. albicans-S. Typhimurium coinfection, as well as with biofilm formation by C. albicans. We confirmed that SopB translocated to fungal filaments through SipB during coinfection. Using quantitative real-time PCR assays, we found that the Candida supernatant upregulated the S. Typhimurium genes associated with C. albicans killing (sopB and sipB). Interestingly, the sopB effector negatively regulated the transcription of CDC42, which is involved in fungal viability. Taken together, these results indicate that specific TTSS effectors, including SopB, play a critical role in bacterial-fungal interactions and are important to S. Typhimurium in order to selectively compete with fungal pathogens. These findings highlight a new role for TTSS of S. Typhimurium in the intestinal tract and may further explain the evolution and maintenance of these traits.

The Chromobacterium violaceum type III effector CopE, a guanine nucleotide exchange factor for Rac1 and Cdc42, is involved in bacterial invasion of epithelial cells and pathogenesis

The type III secretion system (T3SS) encoded by Chromobacterium pathogenicity islands 1 and 1a (Cpi-1/-1a) is critical for Chromobacterium violaceum pathogenesis. T3SS-dependent virulence is commonly characterized by type III effector virulence function, but the full repertoire of the effector proteins of Cpi-1/-1a T3SS is unknown. In this study, we showed that expression of Cpi-1/-1a T3SS is controlled by the master regulator CilA. We used transcriptional profiling with DNA microarrays to define CilA regulon and identified genes encoding T3SS effectors whose translocation into host cells was dependent on Cpi-1/-1a T3SS. From these effectors, we found that CopE (CV0296) has similarities to a guanine nucleotide exchange factor (GEF) for Rho GTPases in its C-terminal portion. The N-terminal portions (1-81 amino acids) of CopE and a CivB as a putative chaperone were required for its translocation. CopE specifically activates Rac1 and Cdc42 followed by the induction of actin cytoskeletal rearrangement. Interestingly, C. violaceum invades human epithelial HeLa cells in a Cpi-1/-1a-encoded T3SS- and CopE-dependent manner. Finally, C. violaceum strains lacking copE and expressing a CopE-G168V deficient in GEF activity were attenuated for virulence in mice, suggesting that CopE contributes to the virulence of this pathogen.

Phenotypic and molecular characterization of Salmonella enterica serovar Sofia, an avirulent species in Australian poultry

Salmonella enterica serovar Sofia (S. Sofia) is often isolated from chickens in Australia. However, despite its high frequency of isolation from chicken and chicken meat products, S. Sofia is rarely associated with animal or human salmonellosis, presumably because this serovar is avirulent in nature. The objective of this work was to investigate the phenotypic and molecular properties of S. Sofia in order to assess its pathogenic potential. Our in vivo studies support the observation that this serovar can colonize tissues, but does not cause disease in chickens. This was further confirmed with tissue culture assays, which showed that the ability of S. Sofia to adhere, invade and survive intracellularly is significantly diminished compared with the pathogenic Salmonella enterica serovar Typhimurium (S. Typhimurium) 82/6915. Molecular analysis of Salmonella pathogenicity islands (SPIs) showed that most of the differences observed in SPI1 to SPI5 of S. Sofia could be attributed to minor changes in the sequences, as indicated by a loss or gain of restriction cleavage sites within these regions. Sequence analysis demonstrated that the majority of virulence genes identified were predicted to encode proteins sharing a high identity (75–100 %) with corresponding proteins from S. Typhimurium. However, a number of virulence genes in S. Sofia have accumulated mutations predicted to affect transcription and/or translation. The avirulence of this serovar is probably not the result of a single genetic change but rather of a series of alterations in a large number of virulence-associated genes. The acquisition of any single virulence gene will almost certainly not be sufficient to restore S. Sofia virulence.

Identification of Vibrio cholerae type III secretion system effector proteins

AM-19226 is a pathogenic O39 serogroup Vibrio cholerae strain that lacks the typical virulence factors for colonization (toxin-coregulated pilus [TCP]) and toxin production (cholera toxin [CT]) and instead encodes a type III secretion system (T3SS). The mechanism of pathogenesis is unknown, and few effector proteins have been identified. We therefore undertook a survey of the open reading frames (ORFs) within the ∼49.7-kb T3SS genomic island to identify potential effector proteins. We identified 15 ORFs for their ability to inhibit growth when expressed in yeast and then used a β-lactamase (TEM1) fusion reporter system to demonstrate that 11 proteins were bona fide effectors translocated into HeLa cells in vitro in a T3SS-dependent manner. One effector, which we named VopX (A33_1663), is conserved only in V. cholerae and Vibrio parahaemolyticus T3SS-positive strains and has not been previously studied. A vopX deletion reduces the ability of strain AM-19226 to colonize in vivo, and the bile-induced expression of a vopX-lacZ transcriptional fusion in vitro is regulated by the T3SS-encoded transcriptional regulators VttR(A) and VttR(B). An RLM1 yeast deletion strain rescued the growth inhibition induced by VopX expression, suggesting that VopX interacts with components of the cell wall integrity mitogen-activated protein kinase (MAPK) pathway. The collective results show that the V. cholerae T3SS encodes multiple effector proteins, one of which likely has novel activities that contribute to disease via interference with eukaryotic signaling pathways.

Membrane targeting and pore formation by the type III secretion system translocon

The type III secretion system (T3SS) is a complex macromolecular machinery employed by a number of Gram-negative species to initiate infection. Toxins secreted through the system are synthesized in the bacterial cytoplasm and utilize the T3SS to pass through both bacterial membranes and the periplasm, thus being introduced directly into the eukaryotic cytoplasm. A key element of the T3SS of all bacterial pathogens is the translocon, which comprises a pore that is inserted into the membrane of the target cell, allowing toxin injection. Three macromolecular partners associate to form the translocon: two are hydrophobic and one is hydrophilic, and the latter also associates with the T3SS needle. In this review, we discuss recent advances on the biochemical and structural characterization of the proteins involved in translocon formation, as well as their participation in the modification of intracellular signalling pathways upon infection. Models of translocon assembly and regulation are also discussed.

Chromobacterium pathogenicity island 1 type III secretion system is a major virulence determinant for Chromobacterium violaceum-induced cell death in hepatocytes

Chromobacterium violaceum is a Gram-negative bacterium that causes fatal septicaemia in humans and animals. C. violaceum ATCC 12472 possesses genes associated with two distinct type III secretion systems (T3SSs). One of these systems is encoded by Chromobacterium pathogenicity islands 1 and 1a (Cpi-1/-1a), another is encoded by Chromobacterium pathogenicity island 2 (Cpi-2). Here we show that C. violaceum causes fulminant hepatitis in a mouse infection model, and Cpi-1/-1a-encoded T3SS is required for its virulence. In addition, using C. violaceum strains with defined mutations in the genes that encode the Cpi-1/-1a or Cpi-2 locus in combination with cultured mammalian cell lines, we found that C. violaceum is able to induce cytotoxicity in a Cpi-1/-1a-dependent manner. Characterization of Chromobacterium-induced cytotoxicity revealed that cell lysis by C. violaceum infection involves the formation of pore structures on the host cell membrane, as demonstrated by protection by cytotoxicity in the presence of osmoprotectants. Finally, we demonstrated that CipB, a Cpi-1/-1a effector, is implicated in translocator-mediated pore formation and the ability of CipB to form a pore is essential for Chromobacterium-induced cytotoxicity. These results strongly suggest that Cpi-1/-1a-encoded T3SS is a virulence determinant that causes fatal infection by the induction of cell death in hepatocytes.