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

A Reporter System for Fast Quantitative Monitoring of Type 3 Protein Secretion in Enteropathogenic E. coli

The type 3 secretion system is essential for pathogenesis of several human and animal Gram-negative bacterial pathogens. The T3SS comprises a transmembrane injectisome, providing a conduit from the bacterial cytoplasm to the host cell cytoplasm for the direct delivery of effectors (including toxins). Functional studies of T3SS commonly monitor the extracellular secretion of proteins by SDS-PAGE and western blot analysis, which are slow and semi-quantitative in nature. Here, we describe an enzymatic reporter-based quantitative and rapid in vivo assay for T3SS secretion studies in enteropathogenic E. coli (EPEC). The assay monitors the secretion of the fusion protein SctA-PhoA through the injectisome based on a colorimetric assay that quantifies the activity of alkaline phosphatase. We validated the usage of this reporter system by following the secretion in the absence of various injectisome components, including domains of the gatekeeper essential for T3SS function. This platform can now be used for the isolation of mutations, functional analysis and anti-virulence compound screening.

Deciphering the Mechanical Properties of Type III Secretion System EspA Protein by Single Molecule Force Spectroscopy

Bacterial pathogens inject virulence factors into host cells during bacterial infections using type III secretion systems. In enteropathogenic Escherichia coli, this system contains an external filament, formed by a self-oligomerizing protein called E. coli secreted protein A (EspA). The EspA filament penetrates the thick viscous mucus layer to facilitate the attachment of the bacteria to the gut-epithelium. To do that, the EspA filament requires noteworthy mechanical endurance considering the mechanical shear stresses found within the intestinal tract. To date, the mechanical properties of the EspA filament and the structural and biophysical knowledge of monomeric EspA are very limited, mostly due to the strong tendency of the protein to self-oligomerize. To overcome this limitation, we employed a single molecule force spectroscopy (SMFS) technique and studied the mechanical properties of EspA. Force extension dynamic of (I91)₄-EspA-(I91)₄ chimera revealed two structural unfolding events occurring at low forces during EspA unfolding, thus indicating no unique mechanical stability of the monomeric protein. SMFS examination of purified monomeric EspA protein, treated by a gradually refolding protocol, exhibited similar mechanical properties as the EspA protein within the (I91)₄-EspA-(I91)₄ chimera. Overall, our results suggest that the mechanical integrity of the EspA filament likely originates from the interactions between EspA monomers and not from the strength of an individual monomer.

Cell-associated hemolysis activity in the clinical strain of Pseudomonas fluorescens MFN1032

BACKGROUND

MFN1032 is a clinical Pseudomonas fluorescens strain able to grow at 37 degrees C. MFN1032 cells induce necrosis and apoptosis in rat glial cells at this temperature. This strain displays secretion-mediated hemolytic activity involving phospholipase C and cyclolipopeptides. Under laboratory conditions, this activity is not expressed at 37 degrees C. This activity is tightly regulated and is subject to phase variation.

RESULTS

We found that MFN1032 displays a cell-associated hemolytic activity distinct from the secreted hemolytic activity. Cell-associated hemolysis was expressed at 37 degrees C and was only detected in vitro in mid log growth phase in the presence of erythrocytes. We studied the regulation of this activity in the wild-type strain and in a mutant defective in the Gac two-component pathway. GacS/GacA is a negative regulator of this activity. In contrast to the Pseudomonas fluorescens strains PfO-1 and Pf5, whose genomes have been sequenced, the MFN1032 strain has the type III secretion-like genes hrcRST belonging to the hrpU operon. We showed that disruption of this operon abolished cell-associated hemolytic activity. This activity was not detected in P.fluorescens strains carrying similar hrc genes, as for the P. fluorescens psychrotrophic strain MF37.

CONCLUSIONS

To our knowledge this the first demonstration of cell-associated hemolytic activity of a clinical strain of Pseudomonas fluorescens. Moreover, this activity seems to be related to a functional hrpU operon and is independent of biosurfactant production. Precise link between a functional hrpU operon and cell-associated hemolytic activity remains to be elucidated.

Bordetella Bsp22 forms a filamentous type III secretion system tip complex and is immunoprotective in vitro and in vivo

Type III secretion system (T3SS) tip complexes serve as adaptors that bridge the T3SS needle and the pore-forming translocation apparatus. In this report we demonstrate that Bsp22, the most abundantly secreted substrate of the Bordetella T3SS, self-polymerizes to form the Bordetella bronchiseptica tip complex. Bsp22 is required for both T3SS-mediated cytotoxicity against eukaryotic cells and haemoglobin release from erythrocytes. Bacterial two-hybrid analysis and protein pull-down assays demonstrated the ability of Bsp22 to associate with itself and to bind BopD, a component of the Bordetella translocation pore. Immunoblot and cross-linking analysis of secreted proteins or purified Bsp22 showed extensive multimerization which was shown by transmission electron microscopy to lead to the formation of variable length flexible filaments. Immunoelectron microscopy revealed Bsp22 filaments on the surface of bacterial cells. Given its required role in secretion and cell-surface exposure, we tested the protective effects of antibodies against Bsp22 in vitro and in vivo. Polyclonal antisera against Bsp22 fully protected epithelial cells from T3SS-dependent killing and immunization with Bsp22 protected mice against Bordetella infection. Of the approximately 30 genes which encode the Bordetella T3SS apparatus, bsp22 is the only one without characterized orthologues in other well-characterized T3SS loci. A maximum likelihood phylogenetic analysis indicated that Bsp22 defines a new subfamily of T3SS tip complex proteins. Given its immunogenic and immunoprotective properties and high degree of conservation among Bordetella species, Bsp22 and its homologues may prove useful for diagnostics and next-generation subunit vaccines.