Characterization of the filamentous hemagglutinin-like protein FhaS in Bordetella bronchiseptica

Bordetella filamentous hemagglutinin and adenylate cyclase toxin interactions on the bacterial surface are consistent with FhaB-mediated delivery of ACT to phagocytic cells

Bordetella species that cause respiratory infections in mammals include B. pertussis, which causes human whooping cough, and B. bronchiseptica, which infects nearly all mammals. Both bacterial species produce filamentous hemagglutinin (FhaB) and adenylate cyclase toxin (ACT), prominent surface-associated and secreted virulence factors that contribute to persistence in the lower respiratory tract by inhibiting clearance by phagocytic cells. FhaB and ACT proteins interact with themselves, each other, and host cells. Using immunoblot analyses, we showed that ACT binds to FhaB on the bacterial surface before it can be detected in culture supernatants. We determined that SphB1, a surface protease identified based on its requirement for FhaB cleavage, is also required for ACT cleavage, and we determined that the presence of ACT blocks SphB1-dependent and -independent cleavage of FhaB, but the presence of FhaB does not affect SphB1-dependent cleavage of ACT. The primary SphB1-dependent cleavage site on ACT is proximal to ACT's active site, in a region that is critical for ACT activity. We also determined that FhaB-bound ACT on the bacterial surface can intoxicate host cells producing CR3, the receptor for ACT. In addition to increasing our understanding of FhaB, ACT, and FhaB-ACT interactions on the Bordetella surface, our data are consistent with a model in which FhaB functions as a novel toxin delivery system by binding to ACT and allowing its release upon binding of ACT to its receptor, CR3, on phagocytic cells.IMPORTANCEBacteria need to control the variety, abundance, and conformation of proteins on their surface to survive. Members of the Gram-negative bacterial genus Bordetella include B. pertussis, which causes whooping cough in humans, and B. bronchiseptica, which causes respiratory infections in a broad range of mammals. These species produce two prominent virulence factors, the two-partner secretion (TPS) effector FhaB and adenylate cyclase toxin (ACT), that interact with themselves, each other, and host cells. Here, we determined that ACT binds FhaB on the bacterial surface before being detected in culture supernatants and that ACT bound to FhaB can be delivered to eukaryotic cells. Our data are consistent with a model in which FhaB delivers ACT specifically to phagocytic cells. This is the first report of a TPS system facilitating the delivery of a separate polypeptide toxin to target cells and expands our understanding of how TPS systems contribute to bacterial pathogenesis.

Relaxed specificity of BcpB transporters mediates interactions between Burkholderia cepacia complex contact-dependent growth inhibition systems

Belonging to the two-partner secretion family of proteins, contact-dependent growth inhibition (CDI) systems mediate interbacterial antagonism among closely related Gram-negative bacteria. The toxic portion of a large surface protein, BcpA/CdiA, is delivered to the cytoplasm of neighboring cells where it inhibits growth. Translocation of the antibacterial polypeptide out of the producing cell requires an associated outer membrane transporter, BcpB/CdiB. Some bacteria, including many Burkholderia species, encode multiple distinct CDI systems, but whether there is interaction between these systems is largely unknown. Using Burkholderia cepacia complex species as a model, here we show that related BcpB transporters exhibit considerable secretion flexibility and can secrete both cognate and non-cognate BcpA substrates. We also identified an additional unique Burkholderia dolosa CDI system capable of mediating interbacterial competition and demonstrated that its BcpB transporter has similar relaxed substrate specificity. Our results showed that two BcpB transporters (BcpB-2 and BcpB-3) were able to secrete all four of the B. dolosa BcpA toxins, while one transporter (BcpB-1) appeared unable to secrete even its cognate BcpA substrate under the tested conditions. This flexibility provided a competitive advantage, as strains lacking the full repertoire of BcpB proteins had decreased CDI activity. Similar results were obtained in Burkholderia multivorans, suggesting that secretion flexibility may be a conserved feature of Burkholderia CDI systems. Together these findings suggest that the interaction between distinct CDI systems enhances the efficiency of bacterial antagonism. IMPORTANCE The Burkholderia cepacia complex (Bcc) is a group of related opportunistic bacterial pathogens that occupy a diverse range of ecological niches and exacerbate disease in patients with underlying conditions. Contact-dependent growth inhibition (CDI) system proteins, produced by Gram-negative bacteria, contain antagonistic properties that allow for intoxication of closely related neighboring bacteria via a secreted protein, BcpA. Multiple unique CDI systems can be found in the same bacterial strain, and here we show that these distinct systems interact in several Bcc species. Our findings suggest that the interaction between CDI system proteins is important for interbacterial toxicity. Understanding the mechanism of interplay between CDI systems provides further insight into the complexity of bacterial antagonism. Moreover, since many bacterial species are predicted to encode multiple CDI systems, this study suggests that interactions between these distinct systems likely contribute to the overall competitive fitness of these species.

The regulation of bacterial two-partner secretion systems

Two-partner secretion (TPS) systems, also known as Type Vb secretion systems, allow the translocation of effector proteins across the outer membrane of Gram-negative bacteria. By secreting different classes of effectors, including cytolysins and adhesins, TPS systems play important roles in bacterial pathogenesis and host interactions. Here, we review the current knowledge on TPS systems regulation and highlight specific and common regulatory mechanisms across TPS functional classes. We discuss in detail the specific regulatory networks identified in various bacterial species and emphasize the importance of understanding the context-dependent regulation of TPS systems. Several regulatory cues reflecting host environment during infection, such as temperature and iron availability, are common determinants of expression for TPS systems, even across relatively distant species. These common regulatory pathways often affect TPS systems across subfamilies with different effector functions, representing conserved global infection-related regulatory mechanisms.

DegP Initiates Regulated Processing of Filamentous Hemagglutinin in Bordetella bronchiseptica

Filamentous hemagglutinin (FhaB) is a critical virulence factor for both Bordetella pertussis, the causal agent of whooping cough, and the closely related species Bordetella bronchiseptica. FhaB is an adhesin, suppresses inflammatory cytokine production, and protects against phagocytic cell clearance during infection. Regulated degradation of the FhaB C-terminal prodomain is required to establish a persistent infection in mice. Two proteases, CtpA in the periplasm and SphB1 on the bacterial surface, are known to mediate FhaB processing, and we recently determined that CtpA functions before, and controls the FhaB cleavage site of, SphB1. However, the data indicate that another periplasmic protease must initiate degradation of the prodomain by removing a portion of the FhaB C terminus that inhibits CtpA-mediated degradation. Using a candidate approach, we identified DegP as the initiating protease. Deletion of degP or substitution of its predicted catalytic residue resulted in reduced creation of FHA′ (the main product of FhaB processing) and an accumulation of full-length FhaB in whole-cell lysates. Also, FHA′ was no longer released into culture supernatants in degP mutants. Alterations of the FhaB C terminus that relieve inhibition of CtpA abrogate the need for DegP, consistent with DegP functioning prior to CtpA in the processing pathway. DegP is not required for secretion of FhaB through FhaC or for adherence of the bacteria to host cells, indicating that DegP acts primarily as a protease and not a chaperone for FhaB in B. bronchiseptica. Our results highlight a role for HtrA family proteases in activation of virulence factors in pathogenic bacteria.

The Two TpsB-Like Proteins in Anabaena sp. Strain PCC 7120 Are Involved in Secretion of Selected Substrates

The outer membrane of Gram-negative bacteria acts as an initial diffusion barrier that shields the cell from the environment. It contains many membrane-embedded proteins required for functionality of this system. These proteins serve as solute and lipid transporters or as machines for membrane insertion or secretion of proteins. The genome of Anabaena sp. strain PCC 7120 codes for two outer membrane transporters termed TpsB1 and TpsB2. They belong to the family of the two-partner secretion system proteins which are characteristic of pathogenic bacteria. Because pathogenicity of Anabaena sp. strain PCC 7120 has not been reported, the function of these two cyanobacterial TpsB proteins was analyzed. TpsB1 is encoded by alr1659, while TpsB2 is encoded by all5116 The latter is part of a genomic region containing 11 genes encoding TpsA-like proteins. However, tpsB2 is transcribed independently of a tpsA gene cluster. Bioinformatics analysis revealed the presence of at least 22 genes in Anabaena sp. strain PCC 7120 putatively coding for substrates of the TpsB system, suggesting a rather global function of the two TpsB proteins. Insertion of a plasmid into each of the two genes resulted in altered outer membrane integrity and antibiotic resistance. In addition, the expression of genes coding for the Clp and Deg proteases is dysregulated in these mutants. Moreover, for two of the putative substrates, a dependence of the secretion on functional TpsB proteins could be confirmed. We confirm the existence of a two-partner secretion system in Anabaena sp. strain PCC 7120 and predict a large pool of putative substrates.IMPORTANCE Cyanobacteria are important organisms for the ecosystem, considering their contribution to carbon fixation and oxygen production, while at the same time some species produce compounds that are toxic to their environment. As a consequence, cyanobacterial overpopulation might negatively impact the diversity of natural communities. Thus, a detailed understanding of cyanobacterial interaction with the environment, including other organisms, is required to define their impact on ecosystems. While two-partner secretion systems in pathogenic bacteria are well known, we provide a first description of the cyanobacterial two-partner secretion system.

Counter-Selection Method for Markerless Allelic Exchange in Bordetella bronchiseptica Based on sacB Gene From Bacillus subtilis

Bordetella bronchiseptica is a gram-negative bacterium that causes respiratory tract infections. It is a natural pathogen of a wide variety of mammals, including some used as laboratory models. This makes B. bronchiseptica an ideal organism to study pathogen-host interactions in order to unveil molecular mechanisms behind pathogenic processes. Even though genetic engineering is an essential tool in this area, there are just a few reports about genome manipulation techniques in this organism. In this article we describe an allelic exchange protocol based on double crossover recombination facilitated by the Bacillus subtilis sacB gene that can be applied for partial or complete gene knockouts, single-nucleotide mutations, or even introduction of coding sequences for transcriptional fusions. In contrast to previously employed techniques, this protocol renders genetically manipulated chromosomes without foreign DNA and enables the construction of successive genome manipulation using the same vector backbone. The entire procedure has been developed for fast and reliable manipulations with a total duration of 2 weeks. © 2020 Wiley Periodicals LLC. Basic Protocol 1: Setting up strains Basic Protocol 2: Homologous recombination (first crossing-over) Alternate Protocol: B. bronchiseptica electroporation Basic Protocol 3: Screening for sucrose-sensitive clones Basic Protocol 4: Homologous recombination (second crossing-over) Basic Protocol 5: PCR screening of putative marker-exchange mutants Support Protocol: Electrocompetent cell preparation.

BrlA and AbaA Govern Virulence-Required Dimorphic Switch, Conidiation, and Pathogenicity in a Fungal Insect Pathogen

Dimorphic plant and human mycopathogens require a switch from the usual yeast growth to filamentous growth for host tissue penetration, and the switch is controlled by multiple signaling systems other than the central developmental pathway. Unlike these fungi, dimorphic insect mycopathogens usually grow by hyphal extension, infect the host by hyphal penetration through the insect cuticle, and switch to unicellular blastospores from the penetrating hyphae only after entry into the host hemocoel, where blastospore propagation by yeast-like budding accelerates host mummification. Here, we report a dependence of the virulence-required dimorphic transition on the central pathway activators BrlA and AbaA in Beauveria bassiana Deletion of brlA or abaA abolished both aerial conidiation and submerged blastospore formation in vitro despite no negative impact on hyphal growth in various media, including a broth mimic of insect hemolymph. The hyphae of either deletion mutant lost insect pathogenicity through normal cuticle penetration, contrasting with a high infectivity of wild-type hyphae. The mutant hyphae injected into the host hemocoel failed to form blastospores, resulting in slower lethal action. Uncovered by transcriptomic analysis, several genes involved in host adhesion and cuticle degradation were sharply repressed in both deletion mutants versus wild type. However, almost all signaling genes homologous to those acting in the dimorphic switch of other fungi were not differentially expressed at a significant level and hence unlikely to be involved in shutting down the dimorphic switch of each deletion mutant. Therefore, like aerial conidiation, the submerged dimorphic switch in vitro and in vivo is a process of asexual development governed by the two central pathway activators in B. bassiana IMPORTANCE Dimorphic insect mycopathogens infect the host by hyphal penetration through the host cuticle and switch from the penetrating hyphae to unicellular blastospores after entry into the host hemocoel, where blastospore propagation by yeast-like budding accelerates host mummification to death. The fungal virulence-required dimorphic switch is confirmed as a process of asexual development directly regulated by BrlA and AbaA, two key activators of the central developmental pathway in an insect mycopathogen. This finding unveils a novel mechanism distinct from the control of the dimorphic switch by multiple signaling systems other than the central developmental pathway in dimorphic plant and human mycopathogens, which switch from the usual yeast growth to filamentous growth required for pathogenicity through host tissue penetration.

Regulated, sequential processing by multiple proteases is required for proper maturation and release of Bordetella filamentous hemagglutinin

Filamentous hemagglutinin (FHA) is a critically important virulence factor produced by Bordetella species that cause respiratory infections in humans and other animals. It is also a prototypical member of the widespread two partner secretion (TPS) pathway family of proteins. First synthesized as a ~370 kDa protein called FhaB, its C-terminal ~1,200 amino acid 'prodomain' is removed during translocation to the cell surface via the outer membrane channel FhaC. Here, we identify CtpA as a periplasmic protease that is responsible for the regulated degradation of the prodomain and for creation of an intermediate polypeptide that is cleaved by the autotransporter protease SphB1 to generate FHA. We show that the central prodomain region is required to initiate degradation of the prodomain and that CtpA degrades the prodomain after a third, unidentified protease (P3) first removes the extreme C-terminus of the prodomain. Stepwise proteolysis by P3, CtpA and SphB1 is required for maturation of FhaB, release of FHA into the extracellular milieu, and full function in vivo. These data support a substantially updated model for the mechanism of secretion, maturation and function of this model TPS protein.

Extracellular polymeric substances of biofilms: Suffering from an identity crisis

Microbial biofilms can be both cause and cure to a range of emerging societal problems including antimicrobial tolerance, water sanitation, water scarcity and pollution. The identities of extracellular polymeric substances (EPS) responsible for the establishment and function of biofilms are poorly understood. The lack of information on the chemical and physical identities of EPS limits the potential to rationally engineer biofilm processes, and impedes progress within the water and wastewater sector towards a circular economy and resource recovery. Here, a multidisciplinary roadmap for addressing this EPS identity crisis is proposed. This involves improved EPS extraction and characterization methodologies, cross-referencing between model biofilms and full-scale biofilm systems, and functional description of isolated EPS with in situ techniques (e.g. microscopy) coupled with genomics, proteomics and glycomics. The current extraction and spectrophotometric characterization methods, often based on the principle not to compromise the integrity of the microbial cells, should be critically assessed, and more comprehensive methods for recovery and characterization of EPS need to be developed.

Two-Partner Secretion: Combining Efficiency and Simplicity in the Secretion of Large Proteins for Bacteria-Host and Bacteria-Bacteria Interactions

Initially identified in pathogenic Gram-negative bacteria, the two-partner secretion (TPS) pathway, also known as Type Vb secretion, mediates the translocation across the outer membrane of large effector proteins involved in interactions between these pathogens and their hosts. More recently, distinct TPS systems have been shown to secrete toxic effector domains that participate in inter-bacterial competition or cooperation. The effects of these systems are based on kin vs. non-kin molecular recognition mediated by specific immunity proteins. With these new toxin-antitoxin systems, the range of TPS effector functions has thus been extended from cytolysis, adhesion, and iron acquisition, to genome maintenance, inter-bacterial killing and inter-bacterial signaling. Basically, a TPS system is made up of two proteins, the secreted TpsA effector protein and its TpsB partner transporter, with possible additional factors such as immunity proteins for protection against cognate toxic effectors. Structural studies have indicated that TpsA proteins mainly form elongated β helices that may be followed by specific functional domains. TpsB proteins belong to the Omp85 superfamily. Open questions remain on the mechanism of protein secretion in the absence of ATP or an electrochemical gradient across the outer membrane. The remarkable dynamics of the TpsB transporters and the progressive folding of their TpsA partners at the bacterial surface in the course of translocation are thought to be key elements driving the secretion process.

Characterisation of the Bordetella pertussis secretome under different media

Our understanding of the Bordetella pertussis secretome remains limited including the role of different growth conditions in the secretome. In this study the secretome of L1423, a clinical isolate from the 2008-2012 Australian epidemic, cultured on Stainer-Scholte (SS) and Thalen-IJssel (THIJS) media for 12h was characterised using liquid chromatography-mass spectrometry (LC-MS/MS). In the supernatant, LC-MS/MS identified 260 proteins with 143 bioinformatically predicted to be secreted. Eighty percent of proteins were identified in both media. Proteins secreted were functionally associated with cell surface (41%), pathogenicity (16%) and transport (17%). The most abundant proteins identified were pathogenic proteins including toxins (PtxA and CyaA), adhesins (TcfA) and type III secretion (T3SS) proteins. There were 46 proteins found uniquely in THIJS including 8 virulence associated proteins. These included T3SS proteins, adhesins (FhaL and FhaS) and a putative toxin (BP1251). Nine proteins were found uniquely in SS and these were metabolic and transport-related proteins. None of the unique proteins detected in SS were known to be virulence associated. This study found that THIJS promotes secretion of virulence factors based on the number of unique virulence proteins found and may be a growth media of choice for the study of B. pertussis virulence and vaccine development.

BIOLOGICAL SIGNIFICANCE

Over the past two decades, the number of B. pertussis notifications has risen despite vaccination. There is a greater need to understand the biology behind B. pertussis infections. The secretome of B. pertussis in two different media was characterised using LC-MS/MS. The results showed that THIJS promotes secretion of importance virulence factors which may be important for the development of vaccines.

Diversity of secretion systems associated with virulence characteristics of the classical bordetellae

Secretion systems are key virulence factors, modulating interactions between pathogens and the host's immune response. Six potential secretion systems (types 1-6; T1SS-T6SS) have been discussed in classical bordetellae, respiratory commensals/pathogens of mammals. The prototypical Bordetella bronchiseptica strain RB50 genome seems to contain all six systems, whilst two human-restricted subspecies, Bordetella parapertussis and Bordetella pertussis, have lost different subsets of these. This implicates secretion systems in the divergent evolutionary histories that have led to their success in different niches. Based on our previous work demonstrating that changes in secretion systems are associated with virulence characteristics, we hypothesized there would be substantial divergence of the loci encoding each amongst sequenced strains. Here, we describe extensive differences in secretion system loci; 10 of the 11 sequenced strains had lost subsets of genes or one entire secretion system locus. These loci contained genes homologous to those present in the respective loci in distantly related organisms, as well as genes unique to bordetellae, suggesting novel and/or auxiliary functions. The high degree of conservation of the T3SS locus, a complex machine with interdependent parts that must be conserved, stands in dramatic contrast to repeated loss of T5aSS 'autotransporters', which function as an autonomous unit. This comparative analysis provided insights into critical aspects of each pathogen's adaptation to its different niche, and the relative contributions of recombination, mutation and horizontal gene transfer. In addition, the relative conservation of various secretion systems is an important consideration in the ongoing search for more highly conserved protective antigens for the next generation of pertussis vaccines.

New Insight into Filamentous Hemagglutinin Secretion Reveals a Role for Full-Length FhaB in Bordetella Virulence

Bordetella filamentous hemagglutinin (FHA), a primary component of acellular pertussis vaccines, contributes to virulence, but how it functions mechanistically is unclear. FHA is first synthesized as an ~370-kDa preproprotein called FhaB. Removal of an N-terminal signal peptide and a large C-terminal prodomain (PD) during secretion results in "mature" ~250-kDa FHA, which has been assumed to be the biologically active form of the protein. Deletion of two C-terminal subdomains of FhaB did not affect production of functional FHA, and the mutant strains were indistinguishable from wild-type bacteria for their ability to adhere to the lower respiratory tract and to suppress inflammation in the lungs of mice. However, the mutant strains, which produced altered FhaB molecules, were eliminated from the lower respiratory tract much faster than wild-type B. bronchiseptica, suggesting a defect in resistance to early immune-mediated clearance. Our results revealed, unexpectedly, that full-length FhaB plays a critical role in B. bronchiseptica persistence in the lower respiratory tract.

IMPORTANCE

The Bordetella filamentous hemagglutinin (FHA) is a primary component of the acellular pertussis vaccine and an important virulence factor. FHA is initially produced as a large protein that is processed during secretion to the bacterial surface. As with most processed proteins, the mature form of FHA has been assumed to be the functional form of the protein. However, our results indicate that the full-length form plays an essential role in virulence in vivo. Furthermore, we have found that FHA contains intramolecular regulators of processing and that this control of processing is integral to its virulence activities. This report highlights the advantage of studying protein maturation and function simultaneously, as a role for the full-length form of FHA was evident only from in vivo infection studies and not from in vitro studies on the production or maturation of FHA or even from in vitro virulence-associated activity assays.

Structure-function analysis reveals that the Pseudomonas aeruginosa Tps4 two-partner secretion system is involved in CupB5 translocation

Pseudomonas aeruginosa is a Gram-negative opportunistic bacterium, synonymous with cystic fibrosis patients, which can cause chronic infection of the lungs. This pathogen is a model organism to study biofilms: a bacterial population embedded in an extracellular matrix that provide protection from environmental pressures and lead to persistence. A number of Chaperone-Usher Pathways, namely CupA-CupE, play key roles in these processes by assembling adhesive pili on the bacterial surface. One of these, encoded by the cupB operon, is unique as it contains a nonchaperone-usher gene product, CupB5. Two-partner secretion (TPS) systems are comprised of a C-terminal integral membrane β-barrel pore with tandem N-terminal POTRA (POlypeptide TRansport Associated) domains located in the periplasm (TpsB) and a secreted substrate (TpsA). Using NMR we show that TpsB4 (LepB) interacts with CupB5 and its predicted cognate partner TpsA4 (LepA), an extracellular protease. Moreover, using cellular studies we confirm that TpsB4 can translocate CupB5 across the P. aeruginosa outer membrane, which contrasts a previous observation that suggested the CupB3 P-usher secretes CupB5. In support of our findings we also demonstrate that tps4/cupB operons are coregulated by the RocS1 sensor suggesting P. aeruginosa has developed synergy between these systems. Furthermore, we have determined the solution-structure of the TpsB4-POTRA1 domain and together with restraints from NMR chemical shift mapping and in vivo mutational analysis we have calculated models for the entire TpsB4 periplasmic region in complex with both TpsA4 and CupB5 secretion motifs. The data highlight specific residues for TpsA4/CupB5 recognition by TpsB4 in the periplasm and suggest distinct roles for each POTRA domain.

Two-partner secretion: as simple as it sounds?

The two-partner secretion (TPS) pathway is a branch of type V secretion. TPS systems are dedicated to the secretion across the outer membrane of long proteins that form extended β-helices. They are composed of a 'TpsA' cargo protein and a 'TpsB' transporter, which belongs to the Omp85 superfamily. This basic design can be supplemented by additional components in some TPS systems. X-ray structures are available for the conserved TPS domain of several TpsA proteins and for one TpsB transporter. However, the molecular mechanisms of two-partner secretion remain to be deciphered, and in particular, the specific role(s) of the TPS domain and the conformational dynamics of the TpsB transporter. Deciphering the TPS pathway may reveal functional features of other transporters of the Omp85 superfamily.

The Burkholderia bcpAIOB genes define unique classes of two-partner secretion and contact dependent growth inhibition systems

Microbes have evolved many strategies to adapt to changes in environmental conditions and population structures, including cooperation and competition. One apparently competitive mechanism is contact dependent growth inhibition (CDI). Identified in Escherichia coli, CDI is mediated by Two-Partner Secretion (TPS) pathway proteins, CdiA and CdiB. Upon cell contact, the toxic C-terminus of the TpsA family member CdiA, called the CdiA-CT, inhibits the growth of CDI(-) bacteria. CDI(+) bacteria are protected from autoinhibition by an immunity protein, CdiI. Bioinformatic analyses indicate that CDI systems are widespread amongst α, β, and γ proteobacteria and that the CdiA-CTs and CdiI proteins are highly variable. CdiI proteins protect against CDI in an allele-specific manner. Here we identify predicted CDI system-encoding loci in species of Burkholderia, Ralstonia and Cupriavidus, named bcpAIOB, that are distinguished from previously-described CDI systems by gene order and the presence of a small ORF, bcpO, located 5' to the gene encoding the TpsB family member. A requirement for bcpO in function of BcpA (the TpsA family member) was demonstrated, indicating that bcpAIOB define a novel class of TPS system. Using fluorescence microscopy and flow cytometry, we show that these genes are expressed in a probabilistic manner during culture of Burkholderia thailandensis in liquid medium. The bcpAIOB genes and extracellular DNA were required for autoaggregation and adherence to an abiotic surface, suggesting that CDI is required for biofilm formation, an activity not previously attributed to CDI. By contrast to what has been observed in E. coli, the B. thailandensis bcpAIOB genes only mediated interbacterial competition on a solid surface. Competition occurred in a defined spatiotemporal manner and was abrogated by allele-specific immunity. Our data indicate that the bcpAIOB genes encode distinct classes of CDI and TPS systems that appear to function in sociomicrobiological community development.

Type V secretion: mechanism(s) of autotransport through the bacterial outer membrane

Autotransport in Gram-negative bacteria denotes the ability of surface-localized proteins to cross the outer membrane (OM) autonomously. Autotransporters perform this task with the help of a β-barrel transmembrane domain localized in the OM. Different classes of autotransporters have been investigated in detail in recent years; classical monomeric but also trimeric autotransporters comprise many important bacterial virulence factors. So do the two-partner secretion systems, which are a special case as the transported protein resides on a different polypeptide chain than the transporter. Despite the great interest in these proteins, the exact mechanism of the transport process remains elusive. Moreover, different periplasmic and OM factors have been identified that play a role in the translocation, making the term ‘autotransport’ debatable. In this review, we compile the wealth of details known on the mechanism of single autotransporters from different classes and organisms, and put them into a bigger perspective. We also discuss recently discovered or rediscovered classes of autotransporters.

A novel sensor kinase is required for Bordetella bronchiseptica to colonize the lower respiratory tract

Bacterial virulence is influenced by the activity of two-component regulator systems (TCSs), which consist of membrane-bound sensor kinases that allow bacteria to sense the external environment and cytoplasmic, DNA-binding response regulator proteins that control appropriate gene expression. Respiratory pathogens of the Bordetella genus require the well-studied TCS BvgAS to control the expression of many genes required for colonization of the mammalian respiratory tract. Here we describe the identification of a novel gene in Bordetella bronchiseptica, plrS, the product of which shares sequence homology to several NtrY-family sensor kinases and is required for B. bronchiseptica to colonize and persist in the lower, but not upper, respiratory tract in rats and mice. The plrS gene is located immediately 5' to and presumably cotranscribed with a gene encoding a putative response regulator, supporting the idea that PlrS and the product of the downstream gene may compose a TCS. Consistent with this hypothesis, the PlrS-dependent colonization phenotype requires a conserved histidine that serves as the site of autophosphorylation in other sensor kinases, and in strains lacking plrS, the production and/or cellular localization of several immune-recognized proteins is altered in comparison to that in the wild-type strain. Because plrS is required for colonization and persistence only in the lower respiratory tract, a site where innate and adaptive immune mechanisms actively target infectious agents, we hypothesize that its role may be to allow Bordetella to resist the host immune response.

Virulence determinants involved in differential host niche adaptation of Neisseria meningitidis and Neisseria gonorrhoeae

Neisseria meningitidis and Neisseria gonorrhoeae are the only pathogenic species of the genus Neisseria. Although these two species are closely related, they specialized on survival in completely different environments within the human host-the nasopharynx in the case of N. meningitidis versus the urogenital tract in the case of N. gonorrhoeae. The genetic background of these differences has not yet been determined. Here, we present a comparison of all characterized transcriptional regulators in these species, delineating analogous functions and disclosing differential functional developments of these DNA-binding proteins with a special focus on the recently characterized regulator FarR and its contribution to divergent host niche adaptation in the two Neisseria spp. Furthermore, we summarize the present knowledge on two-partner secretion systems in meningococci, highlighting their overall expression among meningococcal strains in contrast to the complete absence in gonococci. Concluding, the decisive role of these two entirely different factors in host niche adaptation of the two human pathogenic Neisseria species is depicted, illuminating another piece of the puzzle to locate the molecular basis of their differences in preferred colonization sites and pathogenicity.

Genome dynamics of Bartonella grahamii in micro-populations of woodland rodents

Background

Rodents represent a high-risk reservoir for the emergence of new human pathogens. The recent completion of the 2.3 Mb genome of Bartonella grahamii, one of the most prevalent blood-borne bacteria in wild rodents, revealed a higher abundance of genes for host-cell interaction systems than in the genomes of closely related human pathogens. The sequence variability within the global B. grahamii population was recently investigated by multi locus sequence typing, but no study on the variability of putative host-cell interaction systems has been performed.

Results

To study the population dynamics of B. grahamii, we analyzed the genomic diversity on a whole-genome scale of 27 B. grahamii strains isolated from four different species of wild rodents in three geographic locations separated by less than 30 km. Even using highly variable spacer regions, only 3 sequence types were identified. This low sequence diversity contrasted with a high variability in genome content. Microarray comparative genome hybridizations identified genes for outer surface proteins, including a repeated region containing the fha gene for filamentous hemaggluttinin and a plasmid that encodes a type IV secretion system, as the most variable. The estimated generation times in liquid culture medium for a subset of strains ranged from 5 to 22 hours, but did not correlate with sequence type or presence/absence patterns of the fha gene or the plasmid.

Conclusion

Our study has revealed a geographic microstructure of B. grahamii in wild rodents. Despite near-identity in nucleotide sequence, major differences were observed in gene presence/absence patterns that did not segregate with host species. This suggests that genetically similar strains can infect a range of different hosts.

Role of the type III secretion system in a hypervirulent lineage of Bordetella bronchiseptica

Despite the fact that closely related bacteria can cause different levels of disease, the genetic changes that cause some isolates to be more pathogenic than others are generally not well understood. We use a combination of approaches to determine which factors contribute to the increased virulence of a Bordetella bronchiseptica lineage. A strain isolated from a host with B. bronchiseptica-induced disease, strain 1289, was 60-fold more virulent in mice than one isolated from an asymptomatically infected host, strain RB50. Transcriptome analysis and quantitative reverse transcription-PCR showed that the type III secretion system (TTSS) genes were more highly expressed by strain 1289 than strain RB50. Compared to strain RB50, strain 1289 exhibited greater TTSS-mediated cytotoxicity of a mammalian cell line. Additionally, we show that the increase in virulence of strain 1289 compared to that of RB50 was partially attributable to the TTSS. Using multilocus sequence typing, we identified another strain from the same lineage as strain 1289. Similar to strain 1289, we implicate the TTSS in the increased virulence of this strain. Together, our data suggest that the TTSS is involved in the increased virulence of a B. bronchiseptica lineage which appears to be disproportionately associated with disease. These data are consistent with the view that B. bronchiseptica lineages can have different levels of virulence, which may contribute to this species' ability to cause different severities of respiratory disease.

Natural-host animal models indicate functional interchangeability between the filamentous haemagglutinins of Bordetella pertussis and Bordetella bronchiseptica and reveal a role for the mature C-terminal domain, but not the RGD motif, during infection

Bacteria of the Bordetella genus cause respiratory tract infections. Both broad host range (e.g. Bordetella bronchiseptica) and human-adapted (e.g. Bordetella pertussis) strains produce a surface-exposed and secreted protein called filamentous haemagglutinin (FHA) that functions in adherence and immunomodulation. Previous studies using B. pertussis and cultured mammalian cells identified several FHA domains with potential roles in host cell interactions, including an Arg-Gly-Asp (RGD) triplet that was reported to bind integrins on epithelial cells and monocytes to activate host signalling pathways. We show here that, in contrast to our previous report, the fhaB genes of B. pertussis and B. bronchiseptica are functionally interchangeable, at least with regard to the various in vitro and in vivo assays investigated. This result is significant because it indicates that information obtained studying FHA using B. bronchiseptica and natural-host animal models should apply to B. pertussis FHA as well. We also show that the C-terminus of mature FHA, which we name the MCD, mediates adherence to epithelial and macrophage-like cells and is required for colonization of the rat respiratory tract and modulation of the inflammatory response in mouse lungs. We could not, however, detect a role for the RGD in any of these processes.

Serendipitous discovery of an immunoglobulin-binding autotransporter in Bordetella species

We describe the serendipitous discovery of BatB, a classical-type Bordetella autotransporter (AT) protein with an approximately 180-kDa passenger domain that remains noncovalently associated with the outer membrane. Like genes encoding all characterized protein virulence factors in Bordetella species, batB transcription is positively regulated by the master virulence regulatory system BvgAS. BatB is predicted to share similarity with immunoglobulin A (IgA) proteases, and we showed that BatB binds Ig in vitro. In vivo, a Bordetella bronchiseptica DeltabatB mutant was unable to overcome innate immune defenses and was cleared from the lower respiratory tracts of mice more rapidly than wild-type B. bronchiseptica. This defect was abrogated in SCID mice, suggesting that BatB functions to resist clearance during the first week postinoculation in a manner dependent on B- and T-cell-mediated activities. Taken together with the previous demonstration that polymorphonuclear neutrophils (PMN) are critical for the control of B. bronchiseptica in mice, our data support the hypothesis that BatB prevents nonspecific antibodies from facilitating PMN-mediated clearance during the first few days postinoculation. Neither of the strictly human-adapted Bordetella subspecies produces a fully functional BatB protein; nucleotide differences within the putative promoter region prevent batB transcription in Bordetella pertussis, and although expressed, the batB gene of human-derived Bordetella parapertussis (B. parapertussis(hu)) contains a large in-frame deletion relative to batB of B. bronchiseptica. Taken together, our data suggest that BatB played an important role in the evolution of virulence and host specificity among the mammalian-adapted bordetellae.

A functional two-partner secretion system contributes to adhesion of Neisseria meningitidis to epithelial cells

Neisseria meningitidis is a frequent commensal of the human nasopharynx causing severe invasive infections in rare cases. A functional two-partner secretion (TPS) system in N. meningitidis, composed of the secreted effector protein HrpA and its cognate transporter HrpB, is identified and characterized in this study. Although all meningococcal strains harbor at least one TPS system, the hrpA genes display significant C-terminal sequence variation. Meningococcal genes encoding the TPS effector proteins and their transporters are closely associated and transcribed into a single mRNA. HrpA proteins are translocated across the meningococcal outer membrane by their cognate transporters HrpB and mainly released into the environment. During this process, HrpA is proteolytically processed to a mature 180-kDa form. In contrast to other known TPS systems, immature HrpA proteins are stable in the absence of HrpB and accumulate within the bacterial cell. A small percentage of mature HrpA remains associated with the bacteria and contributes to the interaction of meningococci with epithelial cells.

Construction and validation of a first-generation Bordetella bronchiseptica long-oligonucleotide microarray by transcriptional profiling the Bvg regulon

Background

Bordetella bronchiseptica is a bacterial respiratory pathogen that infects a broad range of mammals, causing chronic and often subclinical infections. Gene expression in Bordetella is regulated by a two-component sensory transduction system, BvgAS, which controls the expression of a spectrum of phenotypic phases transitioning between a virulent (Bvg⁺) phase and a non-virulent (Bvg^-) phase.

Results

Based on the genomic sequence and using the freely available software ArrayOligoSelector, a long oligonucleotide B. bronchiseptica microarray was designed and assembled. This long-oligonucleotide microarray was subsequently tested and validated by comparing changes in the global expression profiles between B. bronchiseptica RB50 and its Bvg^- phase-locked derivative, RB54. Data from this microarray analysis revealed 1,668 Bvg-regulated genes, which greatly expands the BvgAS regulon defined in previous reports. For previously reported Bvg-regulated transcripts, the gene expression data presented here is congruent with prior findings. Additionally, quantitative real-time PCR data provided an independent verification of the microarray expression values.

Conclusion

The results presented here provide a comprehensive, genome-wide portrait of transcripts encompassing the BvgAS regulon, while also providing data validating the long-oligonucleotide microarray described here for studying gene expression in Bordetella bronchiseptica.

In silico analysis of large microbial surface proteins

We analyzed the longest predicted proteins encoded in complete microbial genomes. They could be separated into two main classes: non-ribosomal peptide synthetases involved in secondary metabolism and surface proteins, many of them with a predicted or experimentally observed role in bacterial adhesion and biofilm formation. Such proteins, generally showing a repetitive structure, are widespread among prokaryotes and can be grouped into several different families based on sequence alignment, characteristics and predicted motifs. This classification may help in the characterization of newly described adhesins. The results of this study indicate that cell-cell interactions and biofilm formation are common events in the microbial world and take place via similar molecular mechanisms.

Autoregulation is essential for precise temporal and steady-state regulation by the Bordetella BvgAS phosphorelay

The Bordetella BvgAS virulence control system is prototypical of phosphorelays that use a polydomain sensor and a response regulator to control gene expression in response to environmental cues. BvgAS controls the expression of at least three distinct phenotypic phases (Bvg(-), Bvg(i), and Bvg(+)) by differentially regulating the expression of at least four classes of genes. Among the loci regulated by BvgAS is bvgAS itself. We investigated the role of autoregulation in the ability of BvgAS to control multiple gene expression patterns in a temporal and steady-state manner by constructing Bordetella bronchiseptica strains in which the bvgAS promoter was replaced with constitutively active promoters. Our results show that positive autoregulation of bvgAS transcription is required for the temporal expression of multiple phenotypic phases that occurs in response to a shift from Bvg(-)-phase conditions to Bvg(+)-phase conditions. Autoregulation was also shown to contribute to steady-state regulation; it influences the sensitivity of the system in response to subtle differences in signal intensity. In addition, considered in relation to BvgA and BvgS activities demonstrated in vitro, our results provide insight into how BvgA and BvgS function mechanistically.

Topology and maturation of filamentous haemagglutinin suggest a new model for two‐partner secretion

Two-partner secretion (TPS) is the most widely distributed secretion pathway known. These systems export large exoproteins through highly conserved channel-forming beta-barrel proteins. Filamentous haemagglutinin (FHA), expressed by Bordetella species, is the prototypical TPS family member. Here we show that the C-terminus of mature FHA, as opposed to the N-terminus as previously proposed, is exposed on the cell surface and is required for mediating adherence to cultured epithelial cells. We show that the C-terminus of the FHA pro-protein (FhaB) is required for FHA function in vitro and in vivo and we show that cleavage of FhaB to form FHA is not the mechanism by which FHA is released from the cell. Based on these data, we propose a new model for TPS. This model provides an explanation for the energetics of export of globular protein domains across membranes in the absence of ATP and it suggests a new mechanism for the control of protein folding.

Bordetella filamentous hemagglutinin plays a critical role in immunomodulation, suggesting a mechanism for host specificity

Bordetella pertussis, the causative agent of the acute childhood respiratory disease whooping cough, is a human-adapted variant of Bordetella bronchiseptica, which displays a broad host range and typically causes chronic, asymptomatic infections. These pathogens express a similar but not identical surface-exposed and secreted protein called filamentous hemagglutinin (FHA) that has been proposed to function as both a primary adhesin and an immunomodulator. To test the hypothesis that FHA plays an important role in determining host specificity and/or the propensity to cause acute versus chronic disease, we constructed a B. bronchiseptica strain expressing FHA from B. pertussis (FHA(Bp)) and compared it with wild-type B. bronchiseptica in several natural-host infection models. FHA(Bp) was able to substitute for FHA from B. bronchiseptica (FHA(Bb)) with regard to its ability to mediate adherence to several epithelial and macrophage-like cell lines in vitro, but it was unable to substitute for FHA(Bb) in vivo. Specifically, FHA(Bb), but not FHA(Bp), allowed B. bronchiseptica to colonize the lower respiratory tracts of rats, to modulate the inflammatory response in the lungs of immunocompetent mice, resulting in decreased lung damage and increased bacterial persistence, to induce a robust anti-Bordetella antibody response in these immunocompetent mice, and to overcome innate immunity and cause a lethal infection in immunodeficient mice. These results indicate a critical role for FHA in B. bronchiseptica-mediated immunomodulation, and they suggest a role for FHA in host specificity.