The SPI2-encoded SseA chaperone has discrete domains required for SseB stabilization and export, and binds within the C-terminus of SseB and SseD

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.

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.

Novel pegylated silver coated carbon nanotubes kill Salmonella but they are non-toxic to eukaryotic cells

BACKGROUND

Resistance of food borne pathogens such as Salmonella to existing antibiotics is of grave concern. Silver coated single walled carbon nanotubes (SWCNTs-Ag) have broad-spectrum antibacterial activity and may be a good treatment alternative. However, toxicity to human cells due to their physico-chemical properties is a serious public health concern. Although pegylation is commonly used to reduce metal nanoparticle toxicity, SWCNTs-Ag have not been pegylated as yet, and the effect of pegylation of SWCNTs-Ag on their anti-bacterial activity and cell cytotoxicity remains to be studied. Further, there are no molecular studies on the anti-bacterial mechanism of SWCNTs-Ag or their functionalized nanocomposites.

MATERIALS AND METHODS

In this study we created novel pegylated SWCNTS-Ag (pSWCNTs-Ag), and employed 3 eukaryotic cell lines to evaluate their cytotoxicity as compared to plain SWCNTS-Ag. Simultaneously, we evaluated their antibacterial activity on Salmonella enterica serovar Typhimurium (Salmonella Typhimurium) by the MIC and growth curve assays. In order to understand the possible mechanisms of action of both SWCNTs-Ag and pSWCNTs-Ag, we used electron microscopy (EM) and molecular studies (qRT-PCR).

RESULTS

pSWCNTs-Ag inhibited Salmonella Typhimurium at 62.5 μg/mL, while remaining non-toxic to human cells. By comparison, plain SWCNTs-Ag were toxic to human cells at 62.5 μg/mL. EM analysis revealed that bacteria internalized either of these nanocomposites after the outer cell membranes were damaged, resulting in cell lysis or expulsion of cytoplasmic contents, leaving empty ghosts. The expression of genes regulating the membrane associated metabolic transporter system (artP, dppA, and livJ), amino acid biosynthesis (trp and argC) and outer membrane integrity (ompF) protiens, was significantly down regulated in Salmonella treated with both pSWCNTs-Ag and SWCNTs-Ag. Although EM analysis of bacteria treated with either SWCNTs-Ag or pSWCNTs-Ag revealed relatively similar morphological changes, the expression of genes regulating the normal physiological processes of bacteria (ybeF), quorum sensing (sdiA), outer membrane structure (safC), invasion (ychP) and virulence (safC, ychP, sseA and sseG) were exclusively down regulated several fold in pSWCNTs-Ag treated bacteria.

CONCLUSIONS

Altogether, the present data shows that our novel pSWCNTs-Ag are non-toxic to human cells at their bactericidal concentration, as compared to plain SWCNTS-Ag. Therefore, pSWCNTs-Ag may be safe alternative antimicrobials to treat foodborne pathogens.

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.

Leptospiral outer membrane protein LipL41 is not essential for acute leptospirosis but requires a small chaperone protein, lep, for stable expression

Leptospirosis is a worldwide zoonosis caused by pathogenic Leptospira spp., but knowledge of leptospiral pathogenesis remains limited. However, the development of mutagenesis systems has allowed the investigation of putative virulence factors and their involvement in leptospirosis. LipL41 is the third most abundant lipoprotein found in the outer membranes of pathogenic leptospires and has been considered a putative virulence factor. LipL41 is encoded on the large chromosome 28 bp upstream of a small open reading frame encoding a hypothetical protein of unknown function. This gene was named lep, for LipL41 expression partner. In this study, lipL41 was found to be cotranscribed with lep. Two transposon mutants were characterized: a lipL41 mutant and a lep mutant. In the lep mutant, LipL41 protein levels were reduced by approximately 90%. Lep was shown through cross-linking and coexpression experiments to bind to LipL41. Lep is proposed to be a molecular chaperone essential for the stable expression of LipL41. The roles of LipL41 and Lep in the pathogenesis of Leptospira interrogans were investigated; surprisingly, neither of these two unique proteins was essential for acute leptospirosis.

A screen for over-secretion of proteins by yeast based on a dual component cellular phosphatase and immuno-chromogenic stain for exported bacterial alkaline phosphatase reporter

Background

To isolate over-secretors, we subjected to saturation mutagenesis, a strain of P.pastoris exporting E. coli alkaline phosphatase (EAP) fused to the secretory domain of the yeast α factor pheromone through cellular PHO1/KEX2 secretory processing signals as the α-sec-EAP reporter protein. Direct chromogenic staining for α-sec-EAP activity is non-specific as its NBT/BCIP substrate cross-reacts with cellular phosphatases which can be inhibited with Levulinic acid. However, the parental E(P) strain only exports detectable levels of α-sec-EAP at 69 hours and not within the 36 hour period post-seeding required for effective screening with the consequent absence of a reference for secretion. We substituted the endogenous cellular phosphatase activity as a comparative reference for secretion rate and levels as well as for colony alignment while elevating specificity and sensitivity of detection of the exported protein with other innovative modifications of the immuno-chromogenic staining application for screening protein export mutants.

Results

Raising the specificity and utility of staining for α-sec-EAP activity required 5 modifications including some to published methods. These included, exploitation of endogenous phosphatase activity, reduction of the cell/protein burden, establishment of the direct relation between concentrations of transcriptional inducer and exported membrane immobilized protein and concentrations of protein exported into growth media, amplification of immuno-specificity and sensitivity of detection of α-sec-EAP reporter enzyme signal and restriction of staining to optimal concentrations of antisera and time periods. The resultant immuno-chromogenic screen allows for the detection of early secretion and as little as 1.3 fold over-secretion of α-sec-EAP reporter protein by E(M) mutants in the presence of 10 fold -216 fold higher concentrations of HSA.

Conclusions

The modified immuno-chromogenic screen is sensitive, specific and has led to the isolation of mutants E(M) over-secreting the α-sec-EAP reporter protein by a minimum of 50 fold higher levels than that exported by non-mutagenized E(P) parental strains. Unselected proteins were also over-secreted.

Identification of novel type III secretion chaperone-substrate complexes of Chlamydia trachomatis

Chlamydia trachomatis is an obligate intracellular bacterial pathogen of humans that uses a type III secretion (T3S) system to manipulate host cells through the delivery of effector proteins into their cytosol and membranes. The function of T3S systems depends on small bacterial cytosolic chaperone-like proteins, which bind T3S substrates and ensure their appropriate secretion. To find novel T3S chaperone-substrate complexes of C. trachomatis we first searched its genome for genes encoding proteins with features of T3S chaperones. We then systematically tested for interactions between candidate chaperones and chlamydial T3S substrates by bacterial two-hybrid. This revealed interactions between Slc1 (a known T3S chaperone) or CT584 and several T3S substrates. Co-immunoprecipation after protein expression in Yersinia enterocolitica and protein overlay binding assays indicated that Slc1 interacted with the N-terminal region of the known T3S substrates Tarp (a previously described substrate of Slc1), CT694, and CT695, and that CT584 interacted with a central region of CT082, which we identified as a C. trachomatis T3S substrate using Y. enterocolitica as a heterologous system. Further T3S assays in Yersinia indicated that Slc1 or CT584 increased the amount of secreted Tarp, CT694, and CT695, or CT082, respectively. Expression of CT584 increased the intra-bacterial stability of CT082, while Slc1 did not affect the stability of its substrates. Overall, this indicated that in C. trachomatis Slc1 is a chaperone of multiple T3S substrates and that CT584 is a chaperone of the newly identified T3S substrate CT082.

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.

A molecular chaperone mediates a two-protein enzyme complex and glycosylation of serine-rich streptococcal adhesins

Serine-rich repeat glycoproteins identified from streptococci and staphylococci are important for bacterial adhesion and biofilm formation. Two putative glycosyltransferases, Gtf1 and Gtf2, from Streptococcus parasanguinis form a two-protein enzyme complex that is required for glycosylation of a serine-rich repeat adhesin, Fap1. Gtf1 is a glycosyltransferase; however, the function of Gtf2 is unknown. Here, we demonstrate that Gtf2 enhances the enzymatic activity of Gtf1 by its chaperone-like property. Gtf2 interacted with Gtf1, mediated the subcellular localization of Gtf1, and stabilized Gtf1. Deletion of invariable amino acid residues in a conserved domain of unknown function (DUF1975) at the N terminus of Gtf2 had a greater impact on Fap1 glycosylation than deletion of the C-terminal non-DUF1975 residues. The DUF1975 deletions concurrently reduced the interaction between Gtf1 and Gtf2, altered the subcellular localization of Gtf1, and destabilized Gtf1, suggesting that DUF1975 is crucial for the chaperone activity of Gtf2. Homologous GtfA and GtfB from Streptococcus agalactiae rescued the glycosylation defect in the gtf1gtf2 mutant; like Gtf2, GtfB also possesses chaperone-like activity. Taken together, our studies suggest that Gtf2 and its homologs possess the conserved molecular chaperone activity that mediates protein glycosylation of bacterial adhesins.

Functional dissection of translocon proteins of the Salmonella pathogenicity island 2-encoded type III secretion system

Background

Type III secretion systems (T3SS) are essential virulence factors of most Gram-negative bacterial pathogens. T3SS deliver effector proteins directly into the cytoplasm of eukaryotic target cells and for this function, the insertion of a subset of T3SS proteins into the target cell membrane is important. These proteins form hetero-oligomeric pores acting as translocon for the delivery of effector proteins. Salmonella enterica is a facultative intracellular pathogen that uses the Salmonella Pathogenicity Island 2 (SPI2)-encoded T3SS to manipulate host cells in order to survive and proliferate within the Salmonella-containing vacuole of host cells. Previous work showed that SPI2-encoded SseB, SseC and SseD act to form the translocon of the SPI2-T3SS.

Results

Here we investigated the structural requirements of SseB and SseD to form a functional translocon. Based on bioinformatic predictions, deletional analyses of SseB and SseD were performed and the effect on secretion by the T3SS, formation of a translocon, translocation of effector proteins and intracellular replication was investigated. Our data showed that both SseB and SseD are very sensitive towards alterations of the primary structure of the proteins. Although proteins encoded by mutant alleles were still secreted, we observed that all mutations resulted in a loss of function of the SPI2-T3SS.

Conclusion

These observations indicate that translocon proteins of the SPI2-T3SS are highly evolved towards the formation of multi-subunit complex in the host cell membrane. Structural alterations are not tolerated and abrogate translocon function.

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

The type III secretion system (T3SS) encoded by Salmonella pathogenicity island 2 (SPI-2) is involved in systemic infection and intracellular replication of Salmonella enterica serovar Typhimurium. In this study, we investigated the function of SsaE, a small cytoplasmic protein encoded within the SPI-2 locus, which shows structural similarity to the T3SS class V chaperones. An S. enterica serovar Typhimurium ssaE mutant failed to secrete SPI-2 translocator SseB and SPI-2-dependent effector PipB proteins. Coimmunoprecipitation and mass spectrometry analyses using an SsaE-FLAG fusion protein indicated that SsaE interacts with SseB and a putative T3SS-associated ATPase, SsaN. A series of deleted and point-mutated SsaE-FLAG fusion proteins revealed that the C-terminal coiled-coil domain of SsaE is critical for protein-protein interactions. Although SseA was reported to be a chaperone for SseB and to be required for its secretion and stability in the bacterial cytoplasm, an sseA deletion mutant was able to secrete the SseB in vitro when plasmid-derived SseB was overexpressed. In contrast, ssaE mutant strains could not transport SseB extracellularly under the same assay conditions. In addition, an ssaE(I55G) point-mutated strain that expresses the SsaE derivative lacking the ability to form a C-terminal coiled-coil structure showed attenuated virulence comparable to that of an SPI-2 T3SS null mutant, suggesting that the coiled-coil interaction of SsaE is absolutely essential for the functional SPI-2 T3SS and for Salmonella virulence. Based on these findings, we propose that SsaE recognizes translocator SseB and controls its secretion via SPI-2 type III secretion machinery.

EscC is a chaperone for the Edwardsiella tarda type III secretion system putative translocon components EseB and EseD

Edwardsiella tarda is a Gram-negative enteric pathogen that causes disease in both humans and animals. Recently, a type III secretion system (T3SS) has been found to contribute to Ed. tarda pathogenesis. EseB, EseC and EseD were shown to be secreted by the T3SS and to be the major components of the extracellular proteins (ECPs). Based on sequence similarity, they have been proposed to function as the 'translocon' of the T3SS needle structure. In this study, it was shown that EseB, EseC and EseD formed a protein complex after secretion, which is consistent with their possible roles as translocon components. The secretion of EseB and EseD was dependent on EscC (previously named Orf2). EscC has the characteristics of a chaperone; it is a small protein (13 kDa), located next to the translocators in the T3SS gene cluster, and has a coiled-coil structure at the N-terminal region as predicted by coils. An in-frame deletion of escC abolished the secretion of EseB and EseD, and complementation of DeltaescC restored the export of EseB and EseD into the culture supernatant. Further studies showed that EscC is not a secreted protein and is located on the membrane and in the cytoplasm. Mutation of escC did not affect the transcription of eseB but reduced the amount of EseB as measured by using an EseB-LacZ fusion protein in Ed. tarda. Co-purification studies demonstrated that EscC formed complexes with EseB and EseD. The results suggest that EscC functions as a T3SS chaperone for the putative translocon components EseB and EseD in Ed. tarda.

Structural characterization of a type III secretion system filament protein in complex with its chaperone

The type III secretion system (TTSS) mediates the specific translocation of bacterial proteins into the cytoplasm of eukaryotic cells, a process essential for the virulence of many Gram-negative pathogens. The enteropathogenic Escherichia coli TTSS protein EspA forms a hollow extracellular filament believed to be a molecular conduit for type III protein translocation. Structural analysis of EspA has been hampered by its polymeric nature. We show that EspA alone is sufficient to form filamentous structures in the absence of other pathogenicity island-encoded proteins. CesA is the recently proposed chaperone of EspA, and we demonstrate that CesA traps EspA in a monomeric state and inhibits its polymerization. Crystallographic analysis of the heterodimeric CesA-EspA complex at a resolution of 2.8 A reveals that EspA contains two long a-helices, which are involved in extensive coiled-coil interactions with CesA.