Rapid and efficient transposon mutagenesis of Bartonella henselae by transposome technology

Adhesion of Bartonella henselae to Fibronectin Is Mediated via Repetitive Motifs Present in the Stalk of Bartonella Adhesin A

Adhesion to host cells is the first and most crucial step in infections with pathogenic Gram-negative bacteria and is often mediated by trimeric autotransporter adhesins (TAAs). Bartonella henselae targets the extracellular matrix glycoprotein fibronectin (Fn) via the Bartonella adhesin A (BadA) attaching the bacteria to the host cell. The TAA BadA is characterized by a highly repetitive passenger domain consisting of 30 neck/stalk domains with various degrees of similarity. To elucidate the motif sequences mediating Fn binding, we generated 10 modified BadA constructs and verified their expression via Western blotting, confocal laser scanning, and electron microscopy. We analyzed their ability to bind human plasma Fn using quantitative whole-cell enzyme-linked immunosorbent assays (ELISAs) and fluorescence microscopy. Polyclonal antibodies targeting a 15-mer amino acid motif sequence proved to reduce Fn binding. We suggest that BadA adheres to Fn in a cumulative effort with quick saturation primarily via unpaired β-strands appearing in motifs repeatedly present throughout the neck/stalk region. In addition, we demonstrated that the length of truncated BadA constructs correlates with the immunoreactivity of human patient sera. The identification of BadA-Fn binding regions will support the development of new "antiadhesive" compounds inhibiting the initial adherence of B. henselae and other TAA-expressing pathogens to host cells. IMPORTANCE Trimeric autotransporter adhesins (TAAs) are important virulence factors and are widely present in various pathogenic Gram-negative bacteria. TAA-expressing bacteria cause a wide spectrum of human diseases, such as cat scratch disease (Bartonella henselae), enterocolitis (Yersinia enterocolitica), meningitis (Neisseria meningitis), and bloodstream infections (multidrug-resistant Acinetobacter baumannii). TAA-targeted antiadhesive strategies (against, e.g., Bartonella adhesin A [BadA], Yersinia adhesin A [YadA], Neisseria adhesin A [NadA], and Acinetobacter trimeric autotransporter [Ata]) might represent a universal strategy to counteract such bacterial infections. BadA is one of the best characterized TAAs, and because of its high number of (sub)domains, it serves as an attractive adhesin to study the domain-function relationship of TAAs in the infection process. The identification of common binding motifs between TAAs (here, BadA) and their major binding partner (here, fibronectin) provides a basis toward the design of novel "antiadhesive" compounds preventing the initial adherence of Gram-negative bacteria in infections.

Long-Read Sequencing Reveals Genetic Adaptation of Bartonella Adhesin A Among Different Bartonella henselae Isolates

Bartonella henselae is the causative agent of cat scratch disease and other clinical entities such as endocarditis and bacillary angiomatosis. The life cycle of this pathogen, with alternating host conditions, drives evolutionary and host-specific adaptations. Human, feline, and laboratory adapted B. henselae isolates often display genomic and phenotypic differences that are related to the expression of outer membrane proteins, for example the Bartonella adhesin A (BadA). This modularly-structured trimeric autotransporter adhesin is a major virulence factor of B. henselae and is crucial for the initial binding to the host via the extracellular matrix proteins fibronectin and collagen. By using next-generation long-read sequencing we demonstrate a conserved genome among eight B. henselae isolates and identify a variable genomic badA island with a diversified and highly repetitive badA gene flanked by badA pseudogenes. Two of the eight tested B. henselae strains lack BadA expression because of frameshift mutations. We suggest that active recombination mechanisms, possibly via phase variation (i.e., slipped-strand mispairing and site-specific recombination) within the repetitive badA island facilitate reshuffling of homologous domain arrays. The resulting variations among the different BadA proteins might contribute to host immune evasion and enhance long-term and efficient colonisation in the differing host environments. Considering the role of BadA as a key virulence factor, it remains important to check consistently and regularly for BadA surface expression during experimental infection procedures.

Progress towards an inducible, replication-proficient transposon delivery vector for Chlamydia trachomatis

Background Genetic systems have been developed for Chlamydia but the extremely low transformation frequency remains a significant bottleneck.  Our goal is to develop a self-replicating transposon delivery vector for C. trachomatis which can be expanded prior to transposase induction. Methods We made E. coli/ C. trachomatis shuttle vectors bearing the Himar1 C9  transposase under control of the tet promoter and a novel rearrangement of the Himar1 transposon with the β-lactamase gene.  Activity of the transposase was monitored by immunoblot and by DNA sequencing. Results We constructed pSW2-mCh-C9, a C. trachomatis plasmid designed to act as a self-replicating vector carrying both the Himar1 C9  transposase under tet promoter control and its transposon.  However, we were unable to recover this plasmid in C. trachomatis following multiple attempts at transformation. Therefore, we assembled two new deletion plasmids pSW2-mCh-C9-ΔTpon carrying only the Himar1 C9  transposase (under tet promoter control) and a sister vector (same sequence backbone) pSW2-mCh-C9-ΔTpase carrying its cognate transposon.  We demonstrated that the biological components that make up both pSW2-mCh-C9-ΔTpon and pSW2-mCh-C9-ΔTpase are active in E. coli.  Both these plasmids could be independently recovered in C. trachomatis. We attempted to perform lateral gene transfer by transformation and mixed infection with C. trachomatis strains bearing pSW2-mCh-C9-ΔTpon and pSW2-RSGFP-Tpon (a green fluorescent version of pSW2-mCh-C9-ΔTpase).  Despite success in achieving mixed infections, it was not possible to recover progeny bearing both versions of these plasmids. Conclusions We have designed a self-replicating plasmid vector pSW2-mCh-C9 for C. trachomatis carrying the Himar1 C9  transposase under tet promoter control.  Whilst this can be transformed into E. coli it cannot be recovered in C. trachomatis.  Based on selected deletions and phenotypic analyses we conclude that low level expression from the tet inducible promoter is responsible for premature transposition and hence plasmid loss early on in the transformation process.

The Bartonella henselae SitABCD transporter is required for confronting oxidative stress during cell and flea invasion

Bartonella henselae is a zoonotic pathogen that possesses a flea-cat-flea transmission cycle and causes cat scratch disease in humans via cat scratches and bites. In order to establish infection, B. henselae must overcome oxidative stress damage produced by the mammalian host and arthropod vector. B. henselae encodes for putative Fe²⁺ and Mn²⁺ transporter SitABCD. In B. henselae, SitAB knockdown increases sensitivity to hydrogen peroxide. We consistently show that SitAB knockdown decreases the ability of B. henselae to survive in both human endothelial cells and cat fleas, thus demonstrating that the SitABCD transporter plays an important role during the B. henselae infection cycle.

A gene transfer agent and a dynamic repertoire of secretion systems hold the keys to the explosive radiation of the emerging pathogen Bartonella

Gene transfer agents (GTAs) randomly transfer short fragments of a bacterial genome. A novel putative GTA was recently discovered in the mouse-infecting bacterium Bartonella grahamii. Although GTAs are widespread in phylogenetically diverse bacteria, their role in evolution is largely unknown. Here, we present a comparative analysis of 16 Bartonella genomes ranging from 1.4 to 2.6 Mb in size, including six novel genomes from Bartonella isolated from a cow, two moose, two dogs, and a kangaroo. A phylogenetic tree inferred from 428 orthologous core genes indicates that the deadly human pathogen B. bacilliformis is related to the ruminant-adapted clade, rather than being the earliest diverging species in the genus as previously thought. A gene flux analysis identified 12 genes for a GTA and a phage-derived origin of replication as the most conserved innovations. These are located in a region of a few hundred kb that also contains 8 insertions of gene clusters for type III, IV, and V secretion systems, and genes for putatively secreted molecules such as cholera-like toxins. The phylogenies indicate a recent transfer of seven genes in the virB gene cluster for a type IV secretion system from a cat-adapted B. henselae to a dog-adapted B. vinsonii strain. We show that the B. henselae GTA is functional and can transfer genes in vitro. We suggest that the maintenance of the GTA is driven by selection to increase the likelihood of horizontal gene transfer and argue that this process is beneficial at the population level, by facilitating adaptive evolution of the host-adaptation systems and thereby expansion of the host range size. The process counters gene loss and forces all cells to contribute to the production of the GTA and the secreted molecules. The results advance our understanding of the role that GTAs play for the evolution of bacterial genomes.

Viruses are selfish genetic elements that replicate and transfer their own DNA, often killing the host cell in the process. Unlike viruses, gene transfer agents (GTAs) transfer random pieces of the bacterial genome rather than their own DNA. GTAs are widespread in bacterial genomes, but it is not known whether they are beneficial to the bacterium. In this study, we have used the emerging pathogen Bartonella as our model to study the evolution of GTAs. We sequenced the genomes of six isolates of Bartonella, including two new strains isolated from wild moose in Sweden. Using a comparative genomics approach, we searched for innovations in the last common ancestor that could help explain the explosive radiation of the genus. Surprisingly, we found that a gene cluster for a GTA and a phage-derived origin of replication was the most conserved innovation, indicative of strong selective constraints. We argue that the reason for the remarkable stability of the GTA is that it provides a mechanism to duplicate and recombine genes for secretion systems. This leads to adaptability to a broad range of hosts.

Heme binding proteins of Bartonella henselae are required when undergoing oxidative stress during cell and flea invasion

Bartonella are hemotropic bacteria responsible for emerging zoonoses. These heme auxotroph alphaproteobacteria must import heme for their growth, since they cannot synthesize it. To import exogenous heme, Bartonella genomes encode for a complete heme uptake system enabling transportation of this compound into the cytoplasm and degrading it to release iron. In addition, these bacteria encode for four or five outer membrane heme binding proteins (Hbps). The structural genes of these highly homologous proteins are expressed differently depending on oxygen, temperature and heme concentrations. These proteins were hypothesized as being involved in various cellular processes according to their ability to bind heme and their regulation profile. In this report, we investigated the roles of the four Hbps of Bartonella henselae, responsible for cat scratch disease. We show that Hbps can bind heme in vitro. They are able to enhance the efficiency of heme uptake when co-expressed with a heme transporter in Escherichia coli. Using B. henselae Hbp knockdown mutants, we show that these proteins are involved in defense against the oxidative stress, colonization of human endothelial cell and survival in the flea.

Heme degrading protein HemS is involved in oxidative stress response of Bartonella henselae

Bartonellae are hemotropic bacteria, agents of emerging zoonoses. These bacteria are heme auxotroph Alphaproteobacteria which must import heme for supporting their growth, as they cannot synthesize it. Therefore, Bartonella genome encodes for a complete heme uptake system allowing the transportation of this compound across the outer membrane, the periplasm and the inner membranes. Heme has been proposed to be used as an iron source for Bartonella since these bacteria do not synthesize a complete system required for iron Fe³⁺uptake. Similarly to other bacteria which use heme as an iron source, Bartonellae must transport this compound into the cytoplasm and degrade it to allow the release of iron from the tetrapyrrole ring. For Bartonella, the gene cluster devoted to the synthesis of the complete heme uptake system also contains a gene encoding for a polypeptide that shares homologies with heme trafficking or degrading enzymes. Using complementation of an E. coli mutant strain impaired in heme degradation, we demonstrated that HemS from Bartonella henselae expressed in E. coli allows the release of iron from heme. Purified HemS from B. henselae binds heme and can degrade it in the presence of a suitable electron donor, ascorbate or NADPH-cytochrome P450 reductase. Knocking down the expression of HemS in B. henselae reduces its ability to face H₂O₂ induced oxidative stress.

Identification of a novel nanoRNase in Bartonella

In Escherichia coli, only one essential oligoribonuclease (Orn) can degrade oligoribonucleotides of five residues and shorter in length (nanoRNA). In Bacillus subtilis, NrnA and NrnB, which do not show any sequence similarity to Orn, have been identified as functional analogues of Orn. Sequence comparisons did not identify orn, nrnA or nrnB homologues in the genomes of the Chlamydia/Cyanobacteria and Alphaproteobacteria family members. Screening a genomic library from Bartonella birtlesii, a member of the Alphaproteobacteria, for genes that can complement a conditional orn mutant in E. coli, we identified BA0969 (NrnC) as a functional analogue of Orn. NrnC is highly conserved (more than 80 % identity) in the Bartonella genomes sequenced to date. Biochemical characterization showed that this protein exhibits oligo RNA degradation activity (nanoRNase activity). Like Orn from E. coli, NrnC is inhibited by micromolar amounts of 3'-phosphoadenosine 5'-phosphate in vitro. NrnC homologues are widely present in genomes of Alphaproteobacteria. Knock down of nrnC decreases the growth ability of Bartonella henselae, demonstrating the importance of nanoRNase activity in this bacterium.

Hemin binding protein C is found in outer membrane vesicles and protects Bartonella henselae against toxic concentrations of hemin

Bartonella species are gram-negative, emerging bacterial pathogens found in two distinct environments. In the gut of the obligately hematophagous arthropod vector, bartonellae are exposed to concentrations of heme that are toxic to other bacteria. In the bloodstream of the mammalian host, access to heme and iron is severely restricted. Bartonellae have unusually high requirements for heme, which is their only utilizable source of iron. Although heme is essential for Bartonella survival, little is known about genes involved in heme acquisition and detoxification. We developed a strategy for high-efficiency transposon mutagenesis to screen for genes in B. henselae heme binding and uptake pathways. We identified a B. henselae transposon mutant that constitutively expresses the hemin binding protein C (hbpC) gene. In the wild-type strain, transcription of B. henselae hbpC was upregulated at arthropod temperature (28°C), compared to mammalian temperature (37°C). In the mutant strain, temperature-dependent regulation was absent. We demonstrated that HbpC binds hemin and localizes to the B. henselae outer membrane and outer membrane vesicles. Overexpression of hbpC in B. henselae increased resistance to heme toxicity, implicating HbpC in protection of B. henselae from the toxic levels of heme present in the gut of the arthropod vector. Experimental inoculation of cats with B. henselae strains demonstrated that both constitutive expression and deletion of hbpC affect the ability of B. henselae to infect the cat host. Modulation of hbpC expression appears to be a strategy employed by B. henselae to survive in the arthropod vector and the mammalian host.

The Afipia toolbox and its use to isolate flagellar mutants

Afipia felis, a Gram-negative alphaproteobacterium, has been implicated as one of the causative agents of cat scratch disease. To identify and begin to examine the virulence traits of this organism, we developed and tested a highly efficient transposon delivery system and a stable plasmid vector expressing green fluorescent protein. The transposome system is based on a Tn5-derived transposon and a phage restriction endonuclease type I inhibitor. Electroporation of this construct produced a library of >2600 mutants, which were screened for flagella biosynthesis mutants using a monoclonal antibody to Afipia flagellin. Insertion loci for two selected mutants were located in the genes for flagellin and flagellin biosynthesis FlhA, confirming the validity of the approach.

Screening of genes involved in chitinase production in Aeromonas caviae CB101 via transposon mutagenesis

AIMS

To find genes involved in chitinase production in chitinolytic bacterium Aeromonas caviae CB101.

METHODS AND RESULTS

By transposome mutagenesis, a high-quality mutant library containing around 20,000 insertion mutants was constructed in A. caviae CB101. Mutants with higher, lower and delayed chitinase-producing abilities were identified and analysed further. Genomic sequences flanking the insertion sites of these mutants were amplified by thermal asymmetric interlaced-PCR, cloned and sequenced. The mutated genes involved in chitinase production and/or secretion in CB101 include (i) nagA and nagB gene homologues that are related to the metabolism of the chitin digestion product GlcNAc; (ii) ftsX and exeL gene homologues that are related to transport or secretion systems; (iii) varA and rpoH gene homologues that are related to transcriptional regulator sequences; (iv) other genes with unknown functions.

CONCLUSIONS

Transposome mutagenesis is an efficient method to identify genes involved in the chitinase production in CB101. Chitinase production in CB101 is a complex system, and genes with various functions were identified in this study.

SIGNIFICANCE AND IMPACT OF THE STUDY

Understanding regulation of chitinase production in CB101 would make molecular engineering of the bacterium for higher enzyme production possible.

Analysis of fluorescent protein expression in transformants of Rickettsia monacensis, an obligate intracellular tick symbiont

We developed and applied transposon-based transformation vectors for molecular manipulation and analysis of spotted fever group rickettsiae, which are obligate intracellular bacteria that infect ticks and, in some cases, mammals. Using the Epicentre EZ::TN transposon system, we designed transposons for simultaneous expression of a reporter gene and a chloramphenicol acetyltransferase (CAT) resistance marker. Transposomes (transposon-transposase complexes) were electroporated into Rickettsia monacensis, a rickettsial symbiont isolated from the tick Ixodes ricinus. Each transposon contained an expression cassette consisting of the rickettsial ompA promoter and a green fluorescent protein (GFP) reporter gene (GFPuv) or the ompB promoter and a red fluorescent protein reporter gene (DsRed2), followed by the ompA transcription terminator and a second ompA promoter CAT gene cassette. Selection with chloramphenicol gave rise to rickettsial populations with chromosomally integrated single-copy transposons as determined by PCR, Southern blotting, and sequence analysis. Reverse transcription-PCR and Northern blots demonstrated transcription of all three genes. GFPuv transformant rickettsiae exhibited strong fluorescence in individual cells, but DsRed2 transformants did not. Western blots confirmed expression of GFPuv in R. monacensis and in Escherichia coli, but DsRed2 was expressed only in E. coli. The DsRed2 gene, but not the GFPuv gene, contains many GC-rich amino acid codons that are rare in the preferred codon suite of rickettsiae, possibly explaining the failure to express DsRed2 protein in R. monacensis. We demonstrated that our vectors provide a means to study rickettsia-host cell interactions by visualizing GFPuv-fluorescent R. monacensis associated with actin tails in tick host cells.

Unusual trafficking pattern of Bartonella henselae -containing vacuoles in macrophages and endothelial cells

Bartonella henselae, the agent of cat-scratch disease and vasculoproliferative disorders in humans, is a fastidious facultative intracellular pathogen, whose interaction with macrophages and endothelial cells (ECs) is crucial in the pathogenesis of these diseases. However, little is known about the subcellular compartment in which B. henselae resides. Two hours after infection of murine macrophages and human ECs, the majority of B. henselae-containing vacuoles (BCVs) lack typical endocytic marker proteins, fail to acidify, and do not fuse with lysosomes, suggesting that B. henselae resides in a non-endocytic compartment. In contrast to human umbilical vein endothelial cells, bacterial death and lysosomal fusion with BCVs is apparent in J774A.1 macrophages at 24 h. This phenomenon of delayed lysosomal fusion requires bacterial viability, and is confined to the BCV itself. Using magnetic selection, we enriched for transposon-mutagenized B. henselae trapped in lysosomes of macrophages 2 h after infection. Genes affected appear to be relevant to the intracellular lifestyle in macrophages and ECs and include some previously implicated in Bartonella pathogenicity. We conclude that B. henselae has a specific capacity to actively avoid the host endocytic pathway after entry of macrophages and ECs, from within a specialized non-endocytic membrane-bound vacuole.

Infection of human CD34+ progenitor cells with Bartonella henselae results in intraerythrocytic presence of B. henselae

Although there is evidence that endothelial cells are important targets for human pathogenic Bartonella species, the primary niche of infection is unknown. Here we elucidated whether human CD34+ hematopoietic progenitor cells (HPCs) internalize B. henselae and may serve as a potential niche of the pathogen. We showed that B. henselae does not adhere to or invade human erythrocytes. In contrast, B. henselae invades and persists in HPCs as shown by gentamicin protection assays, confocal laser scanning microscopy (CLSM), and electron microscopy (EM). Fluorescence-activated cell sorting (FACS) analysis of glycophorin A expression revealed that erythroid differentiation of HPCs was unaffected following infection with B. henselae. The number of intracellular B. henselae continuously increased over a 13-day period. When HPCs were infected with B. henselae immediately after isolation, intracellular bacteria were subsequently detectable in differentiated erythroid cells on day 9 and day 13 after infection, as shown by CLSM, EM, and FACS analysis. Our data provide, for the first time, evidence that a bacterial pathogen is able to infect and persist in differentiating HPCs, and suggest that HPCs might serve as a potential primary niche in Bartonella infections.

Bartonella adhesin a mediates a proangiogenic host cell response

Bartonella henselae causes vasculoproliferative disorders in humans. We identified a nonfimbrial adhesin of B. henselae designated as Bartonella adhesin A (BadA). BadA is a 340-kD outer membrane protein encoded by the 9.3-kb badA gene. It has a modular structure and contains domains homologous to the Yersinia enterocolitica nonfimbrial adhesin (Yersinia adhesin A). Expression of BadA was restored in a BadA-deficient transposon mutant by complementation in trans. BadA mediates the binding of B. henselae to extracellular matrix proteins and to endothelial cells, possibly via beta1 integrins, but prevents phagocytosis. Expression of BadA is crucial for activation of hypoxia-inducible factor 1 in host cells by B. henselae and secretion of proangiogenic cytokines (e.g., vascular endothelial growth factor). BadA is immunodominant in B. henselae-infected patients and rodents, indicating that it is expressed during Bartonella infections. Our results suggest that BadA, the largest characterized bacterial protein thus far, is a major pathogenicity factor of B. henselae with a potential role in the induction of vasculoproliferative disorders.

Molecular and Cellular Basis ofBartonellaPathogenesis

The genus Bartonella comprises several important human pathogens that cause a wide range of clinical manifestations: cat-scratch disease, trench fever, Carrion's disease, bacteremia with fever, bacillary angiomatosis and peliosis, endocarditis, and neuroretinitis. Common features of bartonellae include transmission by blood-sucking arthropods and the specific interaction with endothelial cells and erythrocytes of their mammalian hosts. For each Bartonella species, the invasion and persistent intracellular colonization of erythrocytes are limited to a specific human or animal reservoir host. In contrast, endothelial cells are target host cells in probably all mammals, including humans. Bartonellae subvert multiple cellular functions of human endothelial cells, resulting in cell invasion, proinflammatory activation, suppression of apoptosis, and stimulation of proliferation, which may cumulate in vasoproliferative tumor growth. This review summarizes our understanding of Bartonella-host cell interactions and the molecular mechanisms of bacterial virulence and persistence. In addition, current controversies and unanswered questions in this area are highlighted.

Bartonella henselae inhibits apoptosis in Mono Mac 6 cells

Bartonella henselae causes the vasculoproliferative disorders bacillary angiomatosis and peliosis probably resulting from the release of vasculoendothelial growth factor (VEGF) from infected epithelial or monocytic host cells. Here we demonstrate that B. henselae in addition to VEGF induction was also capable of inhibiting the endogenous sucide programme of monocytic host cells. Our results show that B. henselae inhibits pyrrolidine dithiocarbamate (PDTC)-induced apoptosis in Mono Mac 6 cells. B. henselae was observed to be present in a vacuolic compartment of Mono Mac 6 cells. Direct contact of B. henselae with Mono Mac 6 cells was crucial for inhibition of apoptosis as shown by the use of a two-chamber model. Inhibition of apoptosis was paralleled by diminished caspase-3 activity which was significantly reduced in PDTC-stimulated and B. henselae-infected cells. The anti-apoptotic effect of B. henselae was accompanied by (i) the activation of the transcription factor NF-kappaB and (ii) the induction of cellular inhibitor of apoptosis proteins-1 and -2 (cIAP-1, -2). Our results suggest a new synergistic mechanism in B. henselae pathogenicity by (i) inhibition of host cell apoptosis via activation of NF-kappaB and (ii) induction of host cell VEGF secretion.