The type III secreted protein BopD in Bordetella bronchiseptica is complexed with BopB for pore formation on the host plasma membrane

The Bordetella effector protein BteA induces host cell death by disruption of calcium homeostasis

Bordetella pertussis is the causative agent of whooping cough in humans, a disease that has recently experienced a resurgence. In contrast, Bordetella bronchiseptica infects the respiratory tract of various mammalian species, causing a range of symptoms from asymptomatic chronic carriage to acute illness. Both pathogens utilize type III secretion system (T3SS) to deliver the effector protein BteA into host cells. Once injected, BteA triggers a cascade of events leading to caspase 1-independent necrosis through a mechanism that remains incompletely understood. We demonstrate that BteA-induced cell death is characterized by the fragmentation of the cellular endoplasmic reticulum and mitochondria, the formation of necrotic balloon-like protrusions, and plasma membrane permeabilization. Importantly, genome-wide CRISPR-Cas9 screen targeting 19,050 genes failed to identify any host factors required for BteA cytotoxicity, suggesting that BteA does not require a single nonessential host factor for its cytotoxicity. We further reveal that BteA triggers a rapid and sustained influx of calcium ions, which is associated with organelle fragmentation and plasma membrane permeabilization. The sustained elevation of cytosolic Ca2+ levels results in mitochondrial calcium overload, mitochondrial swelling, cristolysis, and loss of mitochondrial membrane potential. Inhibition of calcium channels with 2-APB delays both the Ca2+ influx and BteA-induced cell death. Our findings indicate that BteA exploits essential host processes and/or redundant pathways to disrupt calcium homeostasis and mitochondrial function, ultimately leading to host cell death.IMPORTANCEThe respiratory pathogens Bordetella pertussis and Bordetella bronchiseptica exhibit cytotoxicity toward a variety of mammalian cells, which depends on the type III secretion effector BteA. Moreover, the increased virulence of B. bronchiseptica is associated with enhanced expression of T3SS and BteA. However, the molecular mechanism underlying BteA cytotoxicity is elusive. In this study, we performed a CRISPR-Cas9 screen, revealing that BteA-induced cell death depends on essential or redundant host processes. Additionally, we demonstrate that BteA disrupts calcium homeostasis, which leads to mitochondrial dysfunction and cell death. These findings contribute to closing the gap in our understanding of the signaling cascades targeted by BteA.

Assessment of the Safety and Probiotic Properties of Enterococcus faecium B13 Isolated from Fermented Chili

Enterococcus faecium B13, selected from fermentation chili, has been proven to promote animal growth by previous studies, but it belongs to opportunistic pathogens, so a comprehensive evaluation of its probiotic properties and safety is necessary. In this study, the probiotic properties and safety of B13 were evaluated at the genetic and phenotype levels in vitro and then confirmed in vivo. The genome of B13 contains one chromosome and two plasmids. The average nucleotide identity indicated that B13 was most closely related to the fermentation-plant-derived strain. The strain does not carry the major virulence genes of the clinical E. faecium strains but contains aac(6')-Ii, ant (6)-Ia, msrC genes. The strain had a higher tolerance to acid at pH 3.0, 4.0, and 0.3% bile salt and a 32.83% free radical DPPH clearance rate. It can adhere to Caco-2 cells and reduce the adhesion of E. coli to Caco-2 cells. The safety assessment revealed that the strain showed no hemolysis and did not exhibit gelatinase, ornithine decarboxylase, lysine decarboxylase, or tryptophanase activity. It was sensitive to twelve antibiotics but was resistant to erythromycin, rifampicin, tetracycline, doxycycline, and minocycline. Experiments in vivo have shown that B13 can be located in the ileum and colon and has no adverse effects on experiment animals. After 28 days of feeding, B13 did not remarkable change the α-diversity of the gut flora or increase the virulence genes. Our study demonstrated that E. faecium B13 may be used as a probiotic candidate.

BopN is a Gatekeeper of the Bordetella Type III Secretion System

The classical Bordetella species infect the respiratory tract of mammals. While B. bronchiseptica causes rather chronic respiratory infections in a variety of mammals, the human-adapted species B. pertussis and B. parapertussisHU cause an acute respiratory disease known as whooping cough or pertussis. The virulence factors include a type III secretion system (T3SS) that translocates effectors BteA and BopN into host cells. However, the regulatory mechanisms underlying the secretion and translocation activity of T3SS in bordetellae are largely unknown. We have solved the crystal structure of BopN of B. pertussis and show that it is similar to the structures of gatekeepers that control access to the T3SS channel from the bacterial cytoplasm. We further found that BopN accumulates at the cell periphery at physiological concentrations of calcium ions (2 mM) that inhibit the secretion of BteA and BopN. Deletion of the bopN gene in B. bronchiseptica increased secretion of the BteA effector into calcium-rich medium but had no effect on secretion of the T3SS translocon components BopD and BopB. Moreover, the ΔbopN mutant secreted approximately 10-fold higher amounts of BteA into the medium of infected cells than the wild-type bacteria, but it translocated lower amounts of BteA into the host cell cytoplasm. These data demonstrate that BopN is a Bordetella T3SS gatekeeper required for regulated and targeted translocation of the BteA effector through the T3SS injectisome into host cells. IMPORTANCE The T3SS is utilized by many Gram-negative bacteria to deliver effector proteins from bacterial cytosol directly into infected host cell cytoplasm in a regulated and targeted manner. Pathogenic bordetellae use the T3SS to inject the BteA and BopN proteins into infected cells and upregulate the production of the anti-inflammatory cytokine interleukin-10 (IL-10) to evade host immunity. Previous studies proposed that BopN acted as an effector in host cells. In this study, we report that BopN is a T3SS gatekeeper that regulates the secretion and translocation activity of Bordetella T3SS.

Bcr4 Is a Chaperone for the Inner Rod Protein in the Bordetella Type III Secretion System

Bordetella bronchiseptica injects virulence proteins called effectors into host cells via a type III secretion system (T3SS) conserved among many Gram-negative bacteria. Small proteins called chaperones are required to stabilize some T3SS components or localize them to the T3SS machinery. In a previous study, we identified a chaperone-like protein named Bcr4 that regulates T3SS activity in B. bronchiseptica. Bcr4 does not show strong sequence similarity to well-studied T3SS proteins of other bacteria, and its function remains to be elucidated. Here, we investigated the mechanism by which Bcr4 controls T3SS activity. A pulldown assay revealed that Bcr4 interacts with BscI, based on its homology to other bacterial proteins, to be an inner rod protein of the T3SS machinery. An additional pulldown assay using truncated Bcr4 derivatives and secretion profiles of B. bronchiseptica producing truncated Bcr4 derivatives showed that the Bcr4 C-terminal region is necessary for the interaction with BscI and activation of the T3SS. Moreover, the deletion of BscI abolished the secretion of type III secreted proteins from B. bronchiseptica and the translocation of a cytotoxic effector into cultured mammalian cells. Finally, we show that BscI is unstable in the absence of Bcr4. These results suggest that Bcr4 supports the construction of the T3SS machinery by stabilizing BscI. This is the first demonstration of a chaperone for the T3SS inner rod protein among the virulence bacteria possessing the T3SS. IMPORTANCE The type III secretion system (T3SS) is a needle-like complex that projects outward from bacterial cells. Bordetella bronchiseptica uses the T3SS to inject virulence proteins into host cells. Our previous study reported that a protein named Bcr4 is essential for the secretion of virulence proteins from B. bronchiseptica bacterial cells and delivery through the T3SS. Because other bacteria lack a Bcr4 homologue, the function of Bcr4 has not been elucidated. In this study, we discovered that Bcr4 interacts with BscI, a component of the T3SS machinery. We show that a B. bronchiseptica BscI-deficient strain was unable to secrete type III secreted proteins. Furthermore, in a B. bronchiseptica strain that overproduces T3SS component proteins, Bcr4 is required to maintain BscI in bacterial cells. These results suggest that Bcr4 stabilizes BscI to allow construction of the T3SS in B. bronchiseptica.

High Prevalence and Overexpression of Fosfomycin-Resistant Gene fosX in Enterococcus faecium From China

Enterococci are one of the main causes of gastrointestinal tract infections in the healthcare system and can develop resistance to fosfomycin through plasmid or chromosomally encoded fosfomycin resistance genes. To investigate the mechanisms of fosfomycin resistance, a total of 4,414 clinical isolates of non-replicated clinical enterococci collected from 62 hospitals in 26 provinces or cities in China were tested. Antibiotic susceptibility testing, detection of fosfomycin resistance genes, and cloning of the fosX gene were done. The PFGE, MLST, qRT-PCR, and next genome sequencing were carried out. The results revealed that the fosfomycin-resistant rate of enterococci was 3.5% (153/4,414), and the major resistance mechanism was fosX (101/153) and fosB (52/153) genes. The fosX gene could increase 4- fold fosfomycin MIC in Enterococcus faecium BM4105RF transformants, and the results of PFGE showed the 101 E. faecium carrying fosX were grouped into 48 pulse types. The multilocus sequence typing identified ST555 as the vast majority of STs, mostly distributed in Shanghai, China. Furthermore, the fosX gene expression was strongly related to the fosfomycin-resistant levels of enterococci. The present study was the first to describe the high prevalence presence of the fosX gene in E. faecium from China.

Comparative Phosphoproteomics of Classical Bordetellae Elucidates the Potential Role of Serine, Threonine and Tyrosine Phosphorylation in Bordetella Biology and Virulence

The Bordetella genus is divided into two groups: classical and non-classical. Bordetella pertussis, Bordetella bronchiseptica and Bordetella parapertussis are known as classical bordetellae, a group of important human pathogens causing whooping cough or whooping cough-like disease and hypothesized to have evolved from environmental non-classical bordetellae. Bordetella infections have increased globally driving the need to better understand these pathogens for the development of new treatments and vaccines. One unexplored component in Bordetella is the role of serine, threonine and tyrosine phosphorylation. Therefore, this study characterized the phosphoproteome of classical bordetellae and examined its potential role in Bordetella biology and virulence. Applying strict identification of localization criteria, this study identified 70 unique phosphorylated proteins in the classical bordetellae group with a high degree of conservation. Phosphorylation was a key regulator of Bordetella metabolism with proteins involved in gluconeogenesis, TCA cycle, amino acid and nucleotide synthesis significantly enriched. Three key virulence pathways were also phosphorylated including type III secretion system, alcaligin synthesis and the BvgAS master transcriptional regulatory system for virulence genes in Bordetella. Seven new phosphosites were identified in BvgA with 6 located in the DNA binding domain. Of the 7, 4 were not present in non-classical bordetellae. This suggests that serine/threonine phosphorylation may play an important role in stabilizing/destabilizing BvgA binding to DNA for fine-tuning of virulence gene expression and that BvgA phosphorylation may be an important factor separating classical from non-classical bordetellae. This study provides the first insight into the phosphoproteome of classical Bordetella species and the role that Ser/Thr/Tyr phosphorylation may play in Bordetella biology and virulence.

Transcriptional Downregulation of a Type III Secretion System under Reducing Conditions in Bordetella pertussis

Bordetella pertussis uses a type III secretion system (T3SS) to inject virulence proteins into host cells. Although the B. pertussis T3SS was presumed to be involved in host colonization, efficient secretion of type III secreted proteins from B. pertussis has not been observed. To investigate the roles of type III secreted proteins during infection, we attempted to optimize culture conditions for the production and secretion of a type III secreted protein, BteA, in B. pertussis We observed that B. pertussis efficiently secretes BteA in ascorbic acid-depleted (AsA^-) medium. When L2 cells, a rat lung epithelial cell line, were infected with B. pertussis cultured in the AsA^- medium, BteA-dependent cytotoxicity was observed. We also performed an immunofluorescence assay of L2 cells infected with B. pertussis Clear fluorescence signals of Bsp22, a needle structure of T3SS, were detected on the bacterial surface of B. pertussis cultured in the AsA^- medium. Since ascorbic acid is known as a reducing agent, we cultured B. pertussis in liquid medium containing other reducing agents such as 2-mercaptoethanol and dithioerythritol. Under these reducing conditions, the production of type III secreted proteins was repressed. These results suggest that in B. pertussis, the production and secretion of type III secreted proteins are downregulated under reducing conditions.IMPORTANCE The type III secretion system (T3SS) of Bordetella pertussis forms a needlelike structure that protrudes from the bacterial cell surface. B. pertussis uses a T3SS to translocate virulence proteins called effectors into host cells. The culture conditions for effector production in B. pertussis have not been investigated. We attempted to optimize culture medium compositions for producing and secreting type III secreted proteins. We found that B. pertussis secretes type III secreted proteins in reducing agent-deprived liquid medium and that BteA-secreting B. pertussis provokes cytotoxicity against cultured mammalian cells. These results suggest that redox signaling is involved in the regulation of B. pertussis T3SS.

Bordetella Type III Secretion Injectosome and Effector Proteins

Pertussis, also known as whooping cough, is a resurging acute respiratory disease of humans primarily caused by the Gram-negative coccobacilli Bordetella pertussis, and less commonly by the human-adapted lineage of B. parapertussis _HU. The ovine-adapted lineage of B. parapertussis _OV infects only sheep, while B. bronchiseptica causes chronic and often asymptomatic respiratory infections in a broad range of mammals but rarely in humans. A largely overlapping set of virulence factors inflicts the pathogenicity of these bordetellae. Their genomes also harbor a pathogenicity island, named bsc locus, that encodes components of the type III secretion injectosome, and adjacent btr locus with the type III regulatory proteins. The Bsc injectosome of bordetellae translocates the cytotoxic BteA effector protein, also referred to as BopC, into the cells of the mammalian hosts. While the role of type III secretion activity in the persistent colonization of the lower respiratory tract by B. bronchiseptica is well recognized, the functionality of the type III secretion injectosome in B. pertussis was overlooked for many years due to the adaptation of laboratory-passaged B. pertussis strains. This review highlights the current knowledge of the type III secretion system in the so-called classical Bordetella species, comprising B. pertussis, B. parapertussis, and B. bronchiseptica, and discusses its functional divergence. Comparison with other well-studied bacterial injectosomes, regulation of the type III secretion on the transcriptional and post-transcriptional level, and activities of BteA effector protein and BopN protein, homologous to the type III secretion gatekeepers, are addressed.

Bordetella bronchiseptica Bcr4 antagonizes the negative regulatory function of BspR via its role in type III secretion

Bordetella species, including B. pertussis, have a type III secretion system that is highly conserved among gram-negative pathogenic bacteria. Genes encoding the component proteins of the type III secretion system are localized at the bsc locus in the Bordetella genome. Here, the function of a hypothetical protein Bcr4 encoded at the bsc locus in the B. bronchiseptica genome was investigated. A Bcr4-deficient mutant was created and the amounts of type III secreted proteins (e.g., BopB, BopN and Bsp22) in both the supernatant fraction and whole-cell lysates of the Bcr4-deficient mutant were determined. It was found that the amounts of these proteins were significantly lower than in the wild-type strain. The amounts of type III secreted proteins in the supernatant fraction and whole-cell lysates were much greater in a Bcr4-overproducing strain than in the wild-type strain. The type III secreted protein BspR reportedly negatively regulates the type III secretion system. Here, it was observed that a Bcr4 + BspR double-knockout mutant did not secrete type III secreted proteins, whereas the amounts of these proteins in whole-cell lysates of this mutant were nearly equal to those in whole-cell lysates of the BspR-deficient mutant. Bcr4 thus appears to play an essential role in the extracellular secretion of type III secreted proteins. Our data also suggest that Bcr4 antagonizes the negative regulatory function of BspR.

On Burkholderiales order microorganisms and cystic fibrosis in Russia

BACKGROUND

Microbes infecting cystic fibrosis patients' respiratory tract are important in determining patients' functional status. Representatives of Burkholderiales order are the most dangerous. The goal of our investigation was to reveal the diversity of Burkholderiales, define of their proportion in the microbiome of various parts of respiratory tract and determine the pathogenicity of the main representatives.

RESULTS

In more than 500 cystic fibrosis patients, representing all Federal Regions of Russia, 34.0% were infected by Burkholderia cepacia complex (Bcc), 21.0% by Achromobacter spp. and 12.0% by Lautropia mirabilis. B. cenocepacia was the most numerous species among the Bcc (93.0%), and A. ruhlandii was the most numerous among Achromobacter spp. (58.0%). The most abundant genotype in Bcc was sequence type (ST) 709, and in Achromobacter spp. it was ST36. These STs constitute Russian epidemic strains. Whole genome sequencing of strains A. ruhlandii SCCH3:Ach33-1365 ST36 and B. cenocepacia GIMC4560:Bcn122 ST709 revealed huge resistomes and many virulence factors, which may explain the difficulties in eradicating these strains. An experience of less dangerous B. cenocepcia ST710 elimination was described. Massively parallel sequencing of 16S rDNA amplicons, including V1-V4 hypervariable regions, was used to definite "healthy" microbiome characteristics. Analysis of maxillary sinus lavage of 7 patients revealed infection with Proteobacteria of the same ST as pathogens from sputum, suggesting that the maxillary sinus is a source of infection in cystic fibrosis patients.

CONCLUSIONS

Characterization of the Russian epidemic bacterial strains in the sputum and sinuses of cystic fibrosis patients have better defined the importance of Burkholderiales bacteria. This information may aid in the development of effective approaches for treatment of this disease.

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.

BteA Secreted from the Bordetella bronchiseptica Type III Secetion System Induces Necrosis through an Actin Cytoskeleton Signaling Pathway and Inhibits Phagocytosis by Macrophages

BteA is one of the effectors secreted from the Bordetella bronchiseptica type III secretion system. It has been reported that BteA induces necrosis in mammalian cells; however, the roles of BteA during the infection process are largely unknown. In order to investigate the BteA functions, morphological changes of the cells infected with the wild-type B. bronchiseptica were examined by time-lapse microscopy. L2 cells, a rat lung epithelial cell line, spread at 1.6 hours after B. bronchiseptica infection. Membrane ruffles were observed at peripheral parts of infected cells during the cell spreading. BteA-dependent cytotoxicity and cell detachment were inhibited by addition of cytochalasin D, an actin polymerization inhibitor. Domain analyses of BteA suggested that two separate amino acid regions, 200–312 and 400–658, were required for the necrosis induction. In order to examine the intra/intermolecular interactions of BteA, the amino- and the carboxyl-terminal moieties were purified as recombinant proteins from Escherichia coli. The amino-terminal moiety of BteA appeared to interact with the carboxyl-terminal moiety in the pull-down assay in vitro. When we measured the amounts of bacteria phagocytosed by J774A.1, a macrophage-like cell line, the phagocytosed amounts of B. bronchiseptica strains that deliver BteA into the host cell cytoplasm were significantly lower than those of strains that lost the ability to translocate BteA into the host cell cytoplasm. These results suggest that B. bronchiseptica induce necrosis by exploiting the actin polymerization signaling pathway and inhibit macrophage phagocytosis.

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.

Interaction of Bordetella bronchiseptica and Its Lipopolysaccharide with In Vitro Culture of Respiratory Nasal Epithelium

The nasal septa of fetal rabbits at 26 days of gestation were harvested by cesarean section of the does while under anesthesia and then exposed to Bordetella bronchiseptica or its lipopolysaccharide (LPS) for periods of 2 and 4 hours. A total of 240 explants were used. The tissues were examined using the Hematoxylin & Eosin technique. Then, semithin sections (0.5 μm) were stained with toluidine blue and examined with indirect immunoperoxidase (IPI) and lectin histochemistry. The most frequent and statistically significant findings were as follows: (1) cell death and increased goblet cell activity when exposed to bacteria and (2) cell death, cytoplasmic vacuolation and infiltration of polymorphonuclear leukocytes when exposed to LPS. The lesions induced by the bacterium were more severe than with LPS alone, except for the cytoplasmic vacuolation in epithelial cells. IPI stained the ciliated border of the epithelium with the bacterium more intensely, while LPS lectin histochemistry preferentially labeled the cytoplasm of goblet cell. These data indicate that B. bronchiseptica and its LPS may have an affinity for specific glycoproteins that would act as adhesion receptors in both locations.

Iron starvation regulates the type III secretion system in Bordetella bronchiseptica

The type III secretion system (T3SS) plays a key role in the exertion of full virulence by Bordetella bronchiseptica. However, little is known about the environmental stimuli that induce expression of T3SS genes. Here, it is reported that iron starvation is a signal for T3SS gene expression in B. bronchiseptica. It was found that, when B. bronchiseptica is cultured under iron-depleted conditions, secretion of type III secreted proteins is greater than that in bacteria grown under iron-replete conditions. Furthermore, it was confirmed that induction of T3SS-dependent host cell cytotoxicity and hemolytic activity is greatly enhanced by infection with iron-depleted Bordetella. In contrast, production of filamentous hemagglutinin is reduced in iron-depleted Bordetella. Thus, B. bronchiseptica controls the expression of virulence genes in response to iron starvation.

Phenotypic and genomic analysis of hypervirulent human-associated Bordetella bronchiseptica

Background

B. bronchiseptica infections are usually associated with wild or domesticated animals, but infrequently with humans. A recent phylogenetic analysis distinguished two distinct B. bronchiseptica subpopulations, designated complexes I and IV. Complex IV isolates appear to have a bias for infecting humans; however, little is known regarding their epidemiology, virulence properties, or comparative genomics.

Results

Here we report a characterization of the virulence of human-associated complex IV B. bronchiseptica strains. In in vitro cytotoxicity assays, complex IV strains showed increased cytotoxicity in comparison to a panel of complex I strains. Some complex IV isolates were remarkably cytotoxic, resulting in LDH release levels in A549 cells that were 10- to 20-fold greater than complex I strains. In vivo, a subset of complex IV strains was found to be hypervirulent, with an increased ability to cause lethal pulmonary infections in mice. Hypercytotoxicity in vitro and hypervirulence in vivo were both dependent on the activity of the bsc T3SS and the BteA effector. To clarify differences between lineages, representative complex IV isolates were sequenced and their genomes were compared to complex I isolates. Although our analysis showed there were no genomic sequences that can be considered unique to complex IV strains, there were several loci that were predominantly found in complex IV isolates.

Conclusion

Our observations reveal a T3SS-dependent hypervirulence phenotype in human-associated complex IV isolates, highlighting the need for further studies on the epidemiology and evolutionary dynamics of this B. bronchiseptica lineage.

The type III secreted protein BspR regulates the virulence genes in Bordetella bronchiseptica

Bordetella bronchiseptica is closely related with B. pertussis and B. parapertussis, the causative agents of whooping cough. These pathogenic species share a number of virulence genes, including the gene locus for the type III secretion system (T3SS) that delivers effector proteins. To identify unknown type III effectors in Bordetella, secreted proteins in the bacterial culture supernatants of wild-type B. bronchiseptica and an isogenic T3SS-deficient mutant were compared with iTRAQ-based, quantitative proteomic analysis method. BB1639, annotated as a hypothetical protein, was identified as a novel type III secreted protein and was designated BspR (Bordetellasecreted protein regulator). The virulence of a BspR mutant (ΔbspR) in B. bronchiseptica was significantly attenuated in a mouse infection model. BspR was also highly conserved in B. pertussis and B. parapertussis, suggesting that BspR is an essential virulence factor in these three Bordetella species. Interestingly, the BspR-deficient strain showed hyper-secretion of T3SS-related proteins. Furthermore, T3SS-dependent host cell cytotoxicity and hemolytic activity were also enhanced in the absence of BspR. By contrast, the expression of filamentous hemagglutinin, pertactin, and adenylate cyclase toxin was completely abolished in the BspR-deficient strain. Finally, we demonstrated that BspR is involved in the iron-responsive regulation of T3SS. Thus, Bordetella virulence factors are coordinately but inversely controlled by BspR, which functions as a regulator in response to iron starvation.

Btc22 chaperone is required for secretion and stability of the type III secreted protein Bsp22 in Bordetella bronchiseptica

The type III secretion system (T3SS) is a sophisticated protein secretion machinery that delivers bacterial virulence proteins into host cells. A needle-tip protein, Bsp22 , is one of the secreted substrates of the T3SS and plays an essential role in the full function of the T3SS in Bordetella bronchiseptica. In this study, we found that BB1618 functions as a chaperone for Bsp22 . The deletion of BB1618 resulted in a dramatic impairment of Bsp22 secretion into the culture supernatants and Bsp22 stability in the bacterial cytosol. In contrast, the secretion of other type III secreted proteins was not affected by the BB1618 mutation. Furthermore, the BB1618 mutant strain could not induce cytotoxicity and displayed the same phenotypes as the Bsp22 mutant strain. An immunoprecipitation assay demonstrated that BB1618 interacts with Bsp22 , but not with BopB and BopD . Thus, we identified BB1618 as a specific type III chaperone for Bsp22 . Therefore, we propose that BB1618 be renamed Btc22 for the Bordetella type III chaperone for Bsp22 .

Transcriptional profiling of the iron starvation response in Bordetella pertussis provides new insights into siderophore utilization and virulence gene expression

Serological studies of patients with pertussis and the identification of antigenic Bordetella pertussis proteins support the hypothesis that B. pertussis perceives an iron starvation cue and expresses multiple iron source utilization systems in its natural human host environment. Furthermore, previous studies using a murine respiratory tract infection model showed that several of these B. pertussis iron systems are required for colonization and persistence and are differentially expressed over the course of infection. The present study examined genome-wide changes in B. pertussis gene transcript abundance in response to iron starvation in vitro. In addition to known iron source utilization genes, we identified a previously uncharacterized iron-repressed cytoplasmic membrane transporter system, fbpABC, that is required for the utilization of multiple structurally distinct siderophores including alcaligin, enterobactin, ferrichrome, and desferrioxamine B. Expression of type III secretion system genes was also found to be upregulated during iron starvation in both B. pertussis strain Tohama I and Bordetella bronchiseptica strain RB50. In a survey of type III secretion system protein production by an assortment of B. pertussis laboratory-adapted and low-passage clinical isolate strains, iron limitation increased the production and secretion of the type III secretion system-specific translocation apparatus tip protein Bsp22 in all Bvg-proficient strains. These results indicate that iron starvation in the infected host is an important environmental cue influencing not only Bordetella iron transport gene expression but also the expression of other important virulence-associated genes.

Differential expression of type III effector BteA protein due to IS481 insertion in Bordetella pertussis

BACKGROUND

Bordetella pertussis is the primary etiologic agent of the disease pertussis. Universal immunization programs have contributed to a significant reduction in morbidity and mortality of pertussis; however, incidence of the disease, especially in adolescents and adults, has increased in several countries despite high vaccination coverage. During the last three decades, strains of Bordetella pertussis in circulation have shifted from the vaccine-type to the nonvaccine-type in many countries. A comparative proteomic analysis of the strains was performed to identify protein(s) involved in the type shift.

METHODOLOGY/PRINCIPAL FINDING

Proteomic analysis identified one differentially expressed protein in the B. pertussis strains: the type III cytotoxic effector protein BteA, which is responsible for host cell death in Bordetella bronchiseptica infections. Immunoblot analysis confirmed the prominent expression of BteA protein in the nonvaccine-type strains but not in the vaccine-type strains. Sequence analysis of the vaccine-type strains revealed an IS481 insertion in the 5' untranslated region of bteA, -136 bp upstream of the bteA start codon. A high level of bteA transcripts from the IS481 promoter was detected in the vaccine-type strains, indicating that the transcript might be an untranslatable form. Furthermore, BteA mutant studies demonstrated that BteA expression in the vaccine-type strains is down-regulated by the IS481 insertion.

CONCLUSION/SIGNIFICANCE

The cytotoxic effector BteA protein is expressed at higher levels in B. pertussis nonvaccine-type strains than in vaccine-type strains. This type-dependent expression is due to an insertion of IS481 in B. pertussis clinical strains, suggesting that augmented expression of BteA protein might play a key role in the type shift of B. pertussis.

Bordetella evades the host immune system by inducing IL-10 through a type III effector, BopN

The inflammatory response is one of several host alert mechanisms that recruit neutrophils from the circulation to the area of infection. We demonstrate that Bordetella, a bacterial pathogen, exploits an antiinflammatory cytokine, interleukin-10 (IL-10), to evade the host immune system. We identified a Bordetella effector, BopN, that is translocated into the host cell via the type III secretion system, where it induces enhanced production of IL-10. Interestingly, the BopN effector translocates itself into the nucleus and is involved in the down-regulation of mitogen-activated protein kinases. Using pharmacological blockade, we demonstrated that BopN-induced IL-10 production is mediated, at least in part, by its ability to block the extracellular signal-regulated kinase pathway. We also showed that BopN blocks nuclear translocation of nuclear factor κB p65 (NF-κBp65) but, in contrast, promotes nuclear translocation of NF-κBp50. A BopN-deficient strain was unable to induce IL-10 production in mice, resulting in the elimination of bacteria via neutrophil infiltration into the pulmonary alveoli. Furthermore, IL-10–deficient mice effectively eliminated wild-type as well as BopN mutant bacteria. Thus, Bordetella exploits BopN as a stealth strategy to shut off the host inflammatory reaction. These results explain the ability of Bordetella species to avoid induction of the inflammatory response.

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.

N-terminal residues of SipB are required for its surface localization on Salmonella enterica serovar Typhimurium

SipB, one of the invasion proteins encoded in Salmonella pathogenicity island 1 (SPI-1), is known to be secreted outside the cell, where it functions as a translocon by assembling into a host-cell plasma membrane-integral structure. Here, we confirmed that wild-type SipB could be localized to the bacterial outer membrane, and further showed that its localization was dependent on extracellular secretion, and was independent of the presence of the SipD protein. Proteinase K susceptibility and immunofluorescence assays indicated that SipB was not incorporated into the outer membrane, but rather was displayed on the bacterial surface. Finally, mutation studies revealed that the N-terminal 100-140 aa (especially amino acids 135-138) of SipB were required for its localization on the bacterial outer membrane.

The morphological changes in cultured cells caused by Bordetella pertussis adenylate cyclase toxin

Bordetella pertussis is the causative agent for human whooping cough. It was found that Bordetella pertussis infection caused a change in shape from flat to round in L2 cells, which are derived from rat type 2 alveolar cells. This phenomenon was reproduced using the culture supernatant of B. pertussis, and bacterium-free adenylate cyclase toxin (CyaA) was identified as the factor responsible. A purified preparation of wild-type CyaA but not an enzyme-dead mutant caused the cell rounding. It was examined whether CyaA causes similar morphological changes in various cultured cell lines. L2, EBL, HEK293T, MC3T3-E1, NIH 3T3, and Vero cells were rounded by the toxin whereas Caco-2, Eph4, and MDCK cells were not, although all these cells showed a significant elevation of the intracellular cAMP level in response to CyaA treatment, which indicates that there is no quantitative correlation between the rounding phenotype and the intracellular cAMP level. CyaA has been believed to target various immunocompetent cells and support the establishment of the bacterial infection by subverting the host immune responses. The possibility that CyaA may also affect tissue cells such as respiratory epithelial cells and may be involved in the pathogenesis of the bacterial infection is also indicated.

BopC is a novel type III effector secreted by Bordetella bronchiseptica and has a critical role in type III-dependent necrotic cell death

In Bordetella bronchiseptica, the functional type III secretion system (TTSS) is required for the induction of necrotic cell death in infected mammalian cells. To identify the factor(s) involved in necrotic cell death, type III-secreted proteins from B. bronchiseptica were analyzed using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry and electrospray ionization tandem mass spectrometry. We identified a 69-kDa secreted protein designated BopC. The gene encoding BopC is located outside of the TTSS locus and is also highly conserved in both Bordetella parapertussis and Bordetella pertussis. The results of a lactate dehydrogenase release assay and terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end-labeling assay demonstrated that BopC is required for necrotic cell death. It has been reported that tyrosine-phosphorylated proteins (PY) of host cells are dephosphorylated during B. bronchiseptica infection in a TTSS-dependent manner. We found that BopC is also involved in PY dephosphorylation in infected host cells. It appears that the necrotic cell death triggered by BopC occurs prior to the PY reduction in host cells, because Bordetella-induced cell death was not affected even in the presence of a dephosphorylation inhibitor. Furthermore, a translocation assay showed that the signal sequence for both secretion into culture supernatant and translocation into the host cell is located in 48 amino acid residues of the BopC N terminus. This report reveals for the first time that a novel type III effector, BopC, is required for the induction of necrotic cell death during Bordetella infection.

Type III secretion: more systems than you think

The type III secretion (T3S) pathway allows bacteria to inject effector proteins into the cytosol of target animal or plant cells. T3S systems evolved into seven families that were distributed among Gram-negative bacteria by horizontal gene transfer. There are probably a few hundred effectors interfering with control and signaling in eukaryotic cells and offering a wealth of new tools to cell biologists.

Downregulation of mitogen-activated protein kinases by the Bordetella bronchiseptica Type III secretion system leads to attenuated nonclassical macrophage activation

Bordetella bronchiseptica utilizes a type III secretion system (TTSS) to establish a persistent infection of the murine respiratory tract. Previous studies have shown that the Bordetella TTSS mediated cytotoxicity in different cell types, inhibition of NF-kappaB in epithelial cells, and differentiation of dendritic cells into a semimature state. Here we demonstrate modulation of mitogen-activated protein kinase (MAPK) signaling pathways and altered cytokine production in macrophages and dendritic cells by the Bordetella TTSS. In macrophages, the MAPKs ERK and p38 were downregulated. This resulted in attenuated production of interleukin- (IL-)6 and IL-10. In contrast, the Th-1-polarizing cytokine IL-12 was produced at very low levels and remained unmodulated by the Bordetella TTSS. In dendritic cells, ERK was transiently activated, but this failed to alter cytokine profiles. These results suggest that the Bordetella TTSS modulates antigen-presenting cells in a cell type-specific manner and the secretion of high levels of IL-6 and IL-10 by macrophages might be important for pathogen clearance.