Epigallocatechin-3-gallate inhibits bacterial virulence and invasion of host cells

Increasing antibiotic resistance and beneficial effects of host microbiota has motivated the search for anti-infective agents that attenuate bacterial virulence rather than growth. For example, we discovered that specific flavonoids such as baicalein and quercetin from traditional medicinal plant extracts could attenuate Salmonella enterica serovar Typhimurium type III protein secretion and invasion of host cells. Here, we show epigallocatechin-3-gallate from green tea extracts also inhibits the activity of S. Typhimurium type III protein effectors and significantly reduces bacterial invasion into host cells. These results reveal additional dietary plant metabolites that can attenuate bacterial virulence and infection of host cells.     

Cloning and Transfer of the Salmonella Pathogenicity Island 2 Type III Secretion System for Studies of a Range of Gram-Negative Genera

The engineering of bacterial strains with specific phenotypes frequently requires the use of blocks or “cassettes” of genes that act together to perform a desired function. The potential benefits of utilizing type III secretion systems in this regard are becoming increasingly realized since these systems can be used to direct interactions with host cells for beneficial purposes such as vaccine development, anticancer therapies, and targeted protein delivery. However, convenient methods to clone and transfer type III secretion systems for studies of a range of different types of bacteria are lacking. In addition to functional applications, such methods would also reveal important information about the evolution of a given type III secretion system, such as its ability to be expressed and functional outside of the strain of origin. We describe here the cloning of the Salmonella enterica serovar Typhimurium pathogenicity island 2 (SPI-2) type III secretion system onto a vector that can be easily transferred to a range of gram-negative bacterial genera. We found that expression of the cloned SPI-2 system in different Gammaproteobacteria and Alphaproteobacteria (as monitored by SseB protein levels) is dependent on the bacterial strain and growth medium. We also demonstrate that the cloned system is functional for secretion, can direct interactions with macrophages, and can be used as a novel tool to analyze the predicted interaction of SseB with host cells. This work provides a foundation for future applications where the cloned SPI-2 region (or other cloned type III systems) can provide a desired function to an engineered gram-negative strain.     

A novel proline-rich protein, EspF, is secreted from enteropathogenic Escherichia coli via the type III export pathway

Enteropathogenic Escherichia coli (EPEC) cause a characteristic attaching and effacing (A/E) lesion in intestinal epithelial cells that is associated with the expression and export of specific bacterial proteins via a type III secretion pathway. These effector proteins and components of the type III export apparatus are encoded on a pathogenicity island known as the locus of enterocyte effacement (LEE). In this study, we describe a proline-rich protein, EspF, encoded by the LEE that is secreted by the EPEC type III secretion apparatus. Whereas an espF deletion mutant does not synthesize or secrete EspF, surprisingly it retains the ability to induce host signaling events, perform A/E activities, and invade host epithelial cells. Although these results do not indicate an obvious role for EspF in the formation of A/E lesions nor in the invasion of epithelial cells, they do not preclude a role played by EspF in other aspects of EPEC pathogenesis.     

Inhibition of expression of virulence genes of Yersinia pestis in Escherichia coli by external guide sequences and RNase P

External guide sequences (EGSs) targeting virulence genes from Yersinia pestis were designed and tested in vitro and in vivo in Escherichia coli. Linear EGSs and M1 RNA-linked EGSs were designed for the yscN and yscS genes that are involved in type III secretion in Y. pestis. RNase P from E. coli cleaves the messages of yscN and yscS in vitro with the cognate EGSs, and the expression of the EGSs resulted in the reduction of the levels of these messages of the virulence genes when those genes were expressed in E. coli.     

Taking possession: biogenesis of the Salmonella-containing vacuole

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

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

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

Remodelling of the actin cytoskeleton is essential for replication of intravacuolar Salmonella

Maturation and maintenance of the intracellular vacuole in which Salmonella replicates is controlled by virulence proteins including the type III secretion system encoded by Salmonella pathogenicity island 2 (SPI-2). Here, we show that, several hours after bacterial uptake into different host cell types, Salmonella induces the formation of an F-actin meshwork around bacterial vacuoles. This structure is assembled de novo from the cellular G-actin pool in close proximity to the Salmonella vacuolar membrane. We demonstrate that the phenomenon does not require the Inv/Spa type III secretion system or cognate effector proteins, which induce actin polymerization during bacterial invasion, but does require a functional SPI-2 type III secretion system, which plays an important role in intracellular replication and systemic infection in mice. Treatment with actin-depolymerizing agents significantly inhibited intramacrophage replication of wild-type Salmonella typhimurium . Furthermore, after this treatment, wild-type bacteria were released into the host cell cytoplasm, whereas SPI-2 mutant bacteria remained within vacuoles. We conclude that actin assembly plays an important role in the establishment of an intracellular niche that sustains bacterial growth.     

Bacterial invasion: Force feeding by Salmonella

Recent findings have shed new light on mammalian-cell invasion by Salmonella. Using a type III secretion system, Salmonella deliver virulence factors into the host cell that directly activate signal transduction pathways, initiating cytoskeletal rearrangements and bacterial uptake by a ruffling mechanism.     

Structural and functional properties of chimeric EspA-FliCi filaments of EPEC

Enteropathogenic Escherichia coli utilise a filamentous type III secretion system to translocate effector proteins into host gut epithelial cells. The primary constituent of the extracellular component of the filamentous type III secretion system is EspA. This forms a long flexible helical conduit between the bacterium and host and has a structure almost identical to that of the flagella filament. We have inserted the D3 domain of FliCi (from Salmonella typhimurium) into the outer domain of EspA and have studied the structure and function of modified filaments when expressed in an enteropathogenic E. coli espA mutant. We found that the chimeric protein EspA-FliCi filaments were biologically active as they supported protein secretion and translocation [assessed by their ability to trigger actin polymerisation beneath adherent bacteria (fluorescent actin staining test)]. The expressed filaments were recognised by both EspA and FliCi antisera. Visualisation and analysis of the chimeric filaments by electron microscopy after negative staining showed that, remarkably, EspA filaments are able to tolerate a large protein insertion without a significant effect on their helical architecture.     

Salmonella host cell invasion emerged by acquisition of a mosaic of separate genetic elements, including Salmonella pathogenicity island 1 (SPI1), SPI5, and sopE2

Salmonella spp. possess a conserved type III secretion system encoded within the pathogenicity island 1 (SPI1; centisome 63), which mediates translocation of effector proteins into the host cell cytosol to trigger responses such as bacterial internalization. Several translocated effector proteins are encoded in other regions of the Salmonella chromosome. It remains unclear how this complex chromosomal arrangement of genes for the type III apparatus and the effector proteins emerged and how the different effector proteins cooperate to mediate virulence. By Southern blotting, PCR, and phylogenetic analyses of highly diverse Salmonella spp., we show here that effector protein genes located in the core of SPI1 are present in all Salmonella lineages. Surprisingly, the same holds true for several effector protein genes located in distant regions of the Salmonella chromosome, namely, sopB (SPI5, centisome 20), sopD (centisome 64), and sopE2 (centisomes 40 to 42). Our data demonstrate that sopB, sopD, and sopE2, along with SPI1, were already present in the last common ancestor of all contemporary Salmonella spp. Analysis of Salmonella mutants revealed that host cell invasion is mediated by SopB, SopE2, and, in the case of Salmonella enterica serovar Typhimurium SL1344, by SopE: a sopB sopE sopE2-deficient triple mutant was incapable of inducing membrane ruffling and was >100-fold attenuated in host cell invasion. We conclude that host cell invasion emerged early during evolution by acquisition of a mosaic of genetic elements (SPI1 itself, SPI5 [sopB], and sopE2) and that the last common ancestor of all contemporary Salmonella spp. was probably already invasive.     

Molecular and functional analysis of the type III secretion signal of the Salmonella enterica InvJ protein

Central to the pathogenicity of Salmonella enterica is the function of a type III secretion system (TTSS) encoded within a pathogenicity island at centisome 63 (SPI-1). An essential component of this system is a supramolecular structure termed the needle complex. Proteins to be delivered into host cells possess specific signals that route them to the type III secretion pathway. In addition, some bacterial proteins have signals that deliver them to the secretion complex to either become their structural components or exert their function at that location. One of these proteins is InvJ, which controls the length of the needle substructure of the needle complex. In this study, we have analysed the signal that targets InvJ to the TTSS. We found that amino acid residues 4 to 7 of InvJ are necessary and sufficient to mediate secretion of InvJ or a reporter protein in a TTSS-dependent manner. InvJ secretion was found to be essential for its function in needle length determination, effector protein secretion and bacterial invasion of epithelial cells. Frameshift mutagenesis analysis indicated that the InvJ type III secretion signal sequence tolerates significant alterations in its amino acid sequence without affecting InvJ secretion. Introduction of silent mutations in the secretion signal coding sequence that result in drastically different predicted mRNA folds had no effect on InvJ secretion or expression.     

Effective Identification of Bacterial Type III Secretion Signals Using Joint Element Features

Type III secretion system (T3SS) plays important roles in bacteria and host cell interactions by specifically translocating type III effectors into the cytoplasm of the host cells. The N-terminal amino acid sequences of the bacterial type III effectors determine their specific secretion via type III secretion conduits. It is still unclear as to how the N-terminal sequences guide this specificity. In this work, the amino acid composition, secondary structure, and solvent accessibility in the N-termini of type III and non-type III secreted proteins were compared and contrasted. A high-efficacy mathematical model based on these joint features was developed to distinguish the type III proteins from the non-type III ones. The results indicate that secondary structure and solvent accessibility may make important contribution to the specific recognition of type III secretion signals. Analysis also showed that the joint feature of the N-terminal 6^th–10^th amino acids are especially important for guiding specific type III secretion. Furthermore, a genome-wide screening was performed to predict Salmonella type III secreted proteins, and 8 new candidates were experimentally validated. Interestingly, type III secretion signals were also predicted in gram-positive bacteria and yeasts. Experimental validation showed that two candidates from yeast can indeed be secreted through Salmonella type III secretion conduit. This research provides the first line of direct evidence that secondary structure and solvent accessibility contain important features for guiding specific type III secretion. The new software based on these joint features ensures a high accuracy (general cross-validation sensitivity of ∼96% at a specificity of ∼98%) in silico identification of new type III secreted proteins, which may facilitate our understanding about the specificity of type III secretion and the evolution of type III secreted proteins.     

A common structural motif in the binding of virulence factors to bacterial secretion chaperones

Salmonella invasion protein A (SipA) is translocated into host cells by a type III secretion system (T3SS) and comprises two regions: one domain binds its cognate type III secretion chaperone, InvB, in the bacterium to facilitate translocation, while a second domain functions in the host cell, contributing to bacterial uptake by polymerizing actin. We present here the crystal structures of the SipA chaperone binding domain (CBD) alone and in complex with InvB. The SipA CBD is found to consist of a nonglobular polypeptide as well as a large globular domain, both of which are necessary for binding to InvB. We also identify a structural motif that may direct virulence factors to their cognate chaperones in a diverse range of pathogenic bacteria. Disruption of this structural motif leads to a destabilization of several chaperone-substrate complexes from different species, as well as an impairment of secretion in Salmonella.     

Exploration of Chlamydial Type III Secretion System Reconstitution in Escherichia coli

Background

Type III secretion system is a virulent factor for many pathogens, and is thought to play multiple roles in the development cycle and pathogenesis of chlamydia, an important human pathogen. However, due to the obligate intracellular parasitical nature of chlamydiae and a lack of convenient genetic methodology for the organisms, very limited approaches are available to study the chlamydial type III secretion system. In this study, we explored the reconstitution of a chlamydial type III secretion in Escherichia coli.

Results

We successfully cloned all 6 genomic DNA clusters of the chlamydial type III secretion system into three bacterial plasmids. 5 of the 6 clusters were found to direct mRNA synthesis from their own promoters in Escherichia coli transformed with the three plasmids. Cluster 5 failed to express mRNA using its own promoters. However, fusion of cluster 5 to cluster 6 resulted in the expression of cluster 5 mRNA. Although only two of the type III secretion system proteins were detected transformed E. coli due to limited antibody availability, type III secretion system-like structures were detected in ultrathin sections in a small proportion of transformed E. coli.

Conclusions

We have successfully generated E. coli expressing all genes of the chlamydial type III secretion system. This serves as a foundation for optimal expression and assembly of the recombinant chlamydial type III secretion system, which may be extremely useful for the characterization of the chlamydial type III secretion system and for studying its role in chlamydial pathogenicity.     

The Multivalent Adhesion Molecule SSO1327 plays a key role in Shigella sonnei pathogenesis

Shigella sonnei is a bacterial pathogen and causative agent of bacillary dysentery. It deploys a type III secretion system to inject effector proteins into host epithelial cells and macrophages, an essential step for tissue invasion and immune evasion. Although the arsenal of bacterial effectors and their cellular targets have been studied extensively, little is known about the prerequisites for deployment of type III secreted proteins during infection. Here, we describe a novel S. sonnei adhesin, SSO1327 which is a multivalent adhesion molecule (MAM) required for invasion of epithelial cells and macrophages and for infection in vivo. The S. sonnei MAM mediates intimate attachment to host cells, which is required for efficient translocation of type III effectors into host cells. SSO1327 is non‐redundant to IcsA; its activity is independent of type III secretion. In contrast to the up‐regulation of IcsA‐dependent and independent attachment and invasion by deoxycholate in Shigella flexneri, deoxycholate negatively regulates IcsA and MAM in S. sonnei resulting in reduction in attachment and invasion and virulence attenuation in vivo. A strain deficient for SSO1327 is avirulent in vivo, but still elicits a host immune response.