Secretion of the orgC gene product by Salmonella enterica serovar Typhimurium

Genomic typing and virulence gene profile analysis of Salmonella Derby from different sources

Salmonella enterica serovar Derby (S. Derby) is one of the most common Salmonella serovars which can infect poultry, swine, and humans. With the reduction of the sequencing cost and the improvement of sequencing technology, whole genome sequencing (WGS) has become an important method for bacterial determination, molecular investigation, and pathogenic tracing analysis. In this study, we investigated S. Derby isolates from different sources in China using in-silico multilocus sequence typing (MLST), core genome MLST (cgMLST) and whole genome MLST (wgMLST) analysis based on WGS. The results showed that 21 S. Derby strains were divided into 3 STs using MLST analysis, including ST40 (n = 19, accounting for 90.48%), ST71 (n = 1, accounting for 4.76%) and ST8016 (n = 1, accounting for 4.76%). cgMLST and wgMLST analysis categorized the tested strains into 13 cgSTs and 21 wgSTs, respectively. The minimum spanning trees of cgMLST and wgMLST both divided these strains into 3 clusters and 4 singletons. In addition, virulence gene profiles of S. Derby isolates were also analyzed, and a total of 174 virulence genes belonged to 8 categories were identified. In summary, we studied genomic typing, phylogenetic relationship and virulence gene profiles of S. Derby strains from different sources in China. These findings were beneficial for the epidemiology and pathogenesis of Salmonella.

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.

A protein secreted by the Salmonella type III secretion system controls needle filament assembly

Type III protein secretion systems (T3SS) are encoded by several pathogenic or symbiotic bacteria. The central component of this nanomachine is the needle complex. Here we show in a Salmonella Typhimurium T3SS that assembly of the needle filament of this structure requires OrgC, a protein encoded within the T3SS gene cluster. Absence of OrgC results in significantly reduced number of needle substructures but does not affect needle length. We show that OrgC is secreted by the T3SS and that exogenous addition of OrgC can complement a ∆orgC mutation. We also show that OrgC interacts with the needle filament subunit PrgI and accelerates its polymerization into filaments in vitro. The structure of OrgC shows a novel fold with a shared topology with a domain from flagellar capping proteins. These findings identify a novel component of T3SS and provide new insight into the assembly of the type III secretion machine.

HilD and PhoP independently regulate the expression of grhD1, a novel gene required for Salmonella Typhimurium invasion of host cells

When Salmonella is grown in the nutrient-rich lysogeny broth (LB), the AraC-like transcriptional regulator HilD positively controls the expression of genes required for Salmonella invasion of host cells, such as the Salmonella pathogenicity island 1 (SPI-1) genes. However, in minimal media, the two-component system PhoP/Q activates the expression of genes necessary for Salmonella replication inside host cells, such as the SPI-2 genes. Recently, we found that the SL1344_1872 hypothetical gene, located in a S. Typhimurium genomic island, is co-expressed with the SPI-1 genes. In this study we demonstrate that HilD induces indirectly the expression of SL1344_1872 when S. Typhimurium is grown in LB; therefore, we named SL1344_1872 as grhD1 for gene regulated by HilD. Furthermore, we found that PhoP positively controls the expression of grhD1, independently of HilD, when S. Typhimurium is grown in LB or N-minimal medium. Moreover, we demonstrate that the grhD1 gene is required for the invasion of S. Typhimurium into epithelial cells, macrophages and fibroblasts, as well as for the intestinal inflammatory response caused by S. Typhimurium in mice. Thus, our results reveal a novel virulence factor of Salmonella, whose expression is positively and independently controlled by the HilD and PhoP transcriptional regulators.

Measurement of Effector Protein Translocation Using Phosphorylatable Epitope Tags and Phospho-Specific Antibodies

Numerous bacterial pathogens employ specialized protein secretion machineries to directly inject anti-host proteins, termed effector proteins, into eukaryotic cells. Effector proteins carrying small phosphorylatable tags can be used to detect and quantify effector protein injection. Here, we describe the use of the ELK- and GSK-tags to detect the translocation of the Y. pestis YopE effector protein into RAW 264.7 macrophage-like cells using immunoblot analysis with phospho-specific antibodies.

Development of a cell culture method to isolate and enrich Salmonella enterica serotype enteritidis from shell eggs for subsequent detection by real-time PCR

Salmonella enterica serotype Enteritidis is a major cause of nontyphoidal salmonellosis from ingestion of contaminated raw or undercooked shell eggs. Current techniques used to identify Salmonella serotype Enteritidis in eggs are extremely laborious and time-consuming. In this study, a novel eukaryotic cell culture system was combined with real-time PCR analysis to rapidly identify Salmonella serotype Enteritidis in raw shell eggs. The system was compared to the standard microbiological method of the International Organization for Standardization (Anonymous, Microbiology of food and animal feeding stuffs-horizontal method for the detection of Salmonella, 2002). The novel technique utilizes a mouse macrophage cell line (RAW 264.7) as the host for the isolation and intracellular replication of Salmonella serotype Enteritidis. Exposure of macrophages to Salmonella serotype Enteritidis-contaminated eggs results in uptake and intracellular replication of the bacterium, which can subsequently be detected by real-time PCR analysis of the DNA released after disruption of infected macrophages. Macrophage monolayers were exposed to eggs contaminated with various quantities of Salmonella serotype Enteritidis. As few as 10 CFU/ml was detected in cell lysates from infected macrophages after 10 h by real-time PCR using primer and probe sets specific for DNA segments located on the Salmonella serotype Enteritidis genes sefA and orgC. Salmonella serotype Enteritidis could also be distinguished from other non-serogroup D Salmonella serotypes by using the sefA- and orgC-specific primer and probe sets. Confirmatory identification of Salmonella serotype Enteritidis in eggs was also achieved by isolation of intracellular bacteria from lysates of infected macrophages on xylose lysine deoxycholate medium. This method identifies Salmonella serotype Enteritidis from eggs in less than 10 h compared to the more than 5 days required for the standard reference microbiological method of the International Organization for Standardization (Microbiology of food and animal feeding stuffs-horizontal method for the detection of Salmonella, 2002).

Salicylidene acylhydrazides that affect type III protein secretion in Salmonella enterica serovar typhimurium

A collection of nine salicylidene acylhydrazide compounds were tested for their ability to inhibit the activity of virulence-associated type III secretion systems (T3SSs) in Salmonella enterica serovar Typhimurium. The compounds strongly affected Salmonella pathogenicity island 1 (SPI1) T3SS-mediated invasion of epithelial cells and in vitro secretion of SPI1 invasion-associated effector proteins. The use of a SPI1 effector beta-lactamase fusion protein implicated intracellular entrapment of the protein construct upon application of a salicylidene acylhydrazide, whereas the use of chromosomal transcriptional gene fusions revealed a compound-mediated transcriptional silencing of SPI1. Salicylidene acylhydrazides also affected intracellular bacterial replication in murine macrophage-like cells and blocked the transport of an epitope-tagged SPI2 effector protein. Two of the compounds significantly inhibited bacterial motility and expression of extracellular flagellin. We conclude that salicylidene acylhydrazides affect bacterial T3SS activity in S. enterica and hence could be used as lead substances when designing specific inhibitors of bacterial T3SSs in order to pharmaceutically intervene with bacterial virulence.

Measurement of effector protein injection by type III and type IV secretion systems by using a 13-residue phosphorylatable glycogen synthase kinase tag

Numerous bacterial pathogens use type III secretion systems (T3SSs) or T4SSs to inject or translocate virulence proteins into eukaryotic cells. Several different reporter systems have been developed to measure the translocation of these proteins. In this study, a peptide tag-based reporter system was developed and used to monitor the injection of T3S and T4S substrates. The glycogen synthase kinase (GSK) tag is a 13-residue phosphorylatable peptide tag derived from the human GSK-3beta kinase. Translocation of a GSK-tagged protein into a eukaryotic cell results in host cell protein kinase-dependent phosphorylation of the tag, which can be detected with phosphospecific GSK-3beta antibodies. A series of expression plasmids encoding Yop-GSK fusion proteins were constructed to evaluate the ability of the GSK tag to measure the injection of Yops by the Yersinia pestis T3SS. GSK-tagged YopE, YopH, LcrQ, YopK, YopN, and YopJ were efficiently phosphorylated when translocated into HeLa cells. Similarly, the injection of GSK-CagA by the Helicobacter pylori T4SS into different cell types was measured via phosphorylation of the GSK tag. The GSK tag provides a simple method to monitor the translocation of T3S and T4S substrates.

PhoP-induced genes within Salmonella pathogenicity island 1

The invasive pathogen Salmonella enterica has evolved a sophisticated device that allows it to enter nonphagocytic host cells. This process requires the expression of Salmonella pathogenicity island 1 (SPI-1), which encodes a specialized type III protein secretion system (TTSS). This TTSS delivers a set of effectors that produce a marked rearrangement of the host cytoskeleton, generating a profuse membrane ruffling at the site of interaction, driving bacterial entry. It has been shown that the PhoP/PhoQ two-component system represses the expression of the SPI-1 machinery by down-regulating the transcription of its master regulator, HilA. In this work, we reveal the presence of a PhoP-activated operon within SPI-1. This operon is composed of the orgB and orgC genes, which encode a protein that interacts with the InvC ATPase and a putative effector protein of the TTSS, respectively. Under PhoP-inducing conditions, expression of this operon is directly activated by the phosphorylated form of the response regulator, which recognizes a PhoP box located at the -35 region relative to the transcription start site. Additionally, under invasion-inducing conditions, orgBC expression is driven both by the prgH promoter, induced by the SPI-1 master regulator HilA, and by the directly controlled PhoP/PhoQ promoter. Together, these results indicate that in contrast to the rest of the genes encompassed in the SPI-1 locus, orgBC is expressed during and after Salmonella entry into its host cell, and they suggest a role for the products of this operon after host cell internalization.

Microarray analysis and motif detection reveal new targets of the Salmonella enterica serovar Typhimurium HilA regulatory protein, including hilA itself

DNA regulatory motifs reflect the direct transcriptional interactions between regulators and their target genes and contain important information regarding transcriptional networks. In silico motif detection strategies search for DNA patterns that are present more frequently in a set of related sequences than in a set of unrelated sequences. Related sequences could be genes that are coexpressed and are therefore expected to share similar conserved regulatory motifs. We identified coexpressed genes by carrying out microarray-based transcript profiling of Salmonella enterica serovar Typhimurium in response to the spent culture supernatant of the probiotic strain Lactobacillus rhamnosus GG. Probiotics are live microorganisms which, when administered in adequate amounts, confer a health benefit on the host. They are known to antagonize intestinal pathogens in vivo, including salmonellae. S. enterica serovar Typhimurium causes human gastroenteritis. Infection is initiated by entry of salmonellae into intestinal epithelial cells. The expression of invasion genes is tightly regulated by environmental conditions, as well as by many bacterial factors including the key regulator HilA. One mechanism by which probiotics may antagonize intestinal pathogens is by influencing invasion gene expression. Our microarray experiment yielded a cluster of coexpressed Salmonella genes that are predicted to be down-regulated by spent culture supernatant. This cluster was enriched for genes known to be HilA dependent. In silico motif detection revealed a motif that overlaps the previously described HilA box in the promoter region of three of these genes, spi4_H, sicA, and hilA. Site-directed mutagenesis, beta-galactosidase reporter assays, and gel mobility shift experiments indicated that sicA expression requires HilA and that hilA is negatively autoregulated.

Immunization of mice with YscF provides protection from Yersinia pestis infections

BACKGROUND

Yersinia pestis, the causative agent of plague, is a pathogen with a tremendous ability to cause harm and panic in populations. Due to the severity of plague and its potential for use as a bioweapon, better preventatives and therapeutics for plague are desirable. Subunit vaccines directed against the F1 capsular antigen and the V antigen (also known as LcrV) of Y. pestis are under development. However, these new vaccine formulations have some possible limitations. The F1 antigen is not required for full virulence of Y. pestis and LcrV has a demonstrated immunosuppressive effect. These limitations could damper the ability of F1/LcrV based vaccines to protect against F1-minus Y. pestis strains and could lead to a high rate of undesired side effects in vaccinated populations. For these reasons, the use of other antigens in a plague vaccine formulation may be advantageous.

RESULTS

Desired features in vaccine candidates would be antigens that are conserved, essential for virulence and accessible to circulating antibody. Several of the proteins required for the construction or function of the type III secretion system (TTSS) complex could be ideal contenders to meet the desired features of a vaccine candidate. Accordingly, the TTSS needle complex protein, YscF, was selected to investigate its potential as a protective antigen. In this study we describe the overexpression, purification and use of YscF as a protective antigen. YscF immunization triggers a robust antibody response to YscF and that antibody response is able to afford significant protection to immunized mice following challenge with Y. pestis. Additionally, evidence is presented that suggests antibody to YscF is likely not protective by blocking the activity of the TTSS.

CONCLUSION

In this study we investigated YscF, a surface-expressed protein of the Yersinia pestis type III secretion complex, as a protective antigen against experimental plague infection. Immunization of mice with YscF resulted in a high anti-YscF titer and provided protection against i.v. challenge with Y. pestis. This is the first report to our knowledge utilizing a conserved protein from the type III secretion complex of a gram-negative pathogen as a candidate for vaccine development.

An analysis of type-III secretion gene clusters in Chromobacterium violaceum

Chromobacterium violaceum is an environmental Gram-negative bacterium that is common in soil and water in tropical and sub-tropical regions. It is also a model organism for studying quorum-sensing and is a rare but deadly human pathogen. Recent completion of the genome sequence of C. violaceum strain ATCC 12472 revealed the presence of genes associated with type-III secretion systems (TTSSs). One of these systems resembles the Spi-1 system found in Salmonella enterica, whereas another is similar to the Spi-2 system from the same organism. Here, we present a detailed analysis and a fresh annotation of the two gene clusters. Moreover, we highlight the presence of several genes encoding putative type-III effector proteins that lead us to predict that this organism can manipulate vesicular trafficking, the actin cytoskeleton and apoptotic pathways within mammalian cells to its own advantage.

Expression and secretion of Salmonella pathogenicity island-2 virulence genes in response to acidification exhibit differential requirements of a functional type III secretion apparatus and SsaL

Salmonella pathogenicity island (SPI)-2 is pivotal to the intracellular survival of Salmonella and for virulence in mammals. SPI-2 encodes virulence factors (called effectors) that are translocated into the host cell, a type III secretion apparatus and a two-component regulatory system that regulates intracellular expression of SPI-2. Salmonella SPI-2 secretion activity appears to be induced in response to acidification of the vacuole in which it replicates. Here we show that the expression of the SPI-2 proteins, SseB and SseD (filament and pore forming components of the secretion apparatus, respectively) in response to acidification requires an intact secretion system and SsaL, a Salmonella homologue of SepL, a regulator required for type III-dependent secretion of translocators but not effectors in attaching and effacing gastrointestinal pathogens. We show that the expression of SPI-2-encoded effectors is acid-regulated but can be uncoupled from the expression of filament and translocon components, thus showing a differential requirement of SsaL for expression. The secretion and translocation of SPI-2-encoded effectors requires SsaL, but SsaL is dispensable for the secretion of SPI-2 effectors encoded in other pathogenicity loci, suggesting a secretion regulation function for SsaL. Further, we demonstrate that the differential expression of adjacent genes within the sseA operon (sseD and sseE) occurs at the transcriptional level. These data indicate that a Salmonella SPI-2 activation state is achieved by an acidregulated response that requires SsaL. These data also suggest the existence of a previously unrecognized regulatory element within SPI-2 for the "effector operon" region downstream of sseD that might demarcate the expression of translocators and effectors.