A bacteriophage-like particle from Bartonella bacilliformis

Co-opting bacterial viruses for DNA exchange: structure and regulation of gene transfer agents

Horizontal gene transfer occurs via a range of mechanisms, including transformation, conjugation and bacteriophage transduction. Gene transfer agents (GTAs) are an alternative, less-studied route for interbacterial DNA exchange. Encoded within bacterial or archaeal genomes, GTAs assemble into phage-like particles that selflessly package and transmit host DNA to recipient bacteria. Several unique features distinguish GTAs from canonical phages such as an inability to self-replicate, thus producing non-infectious particles. GTAs are also deeply integrated into the physiology of the host cell and are maintained under tight host-regulatory control. Recent advances in understanding the structure and regulation of GTAs have provided further insights into a DNA transfer mechanism that is proving increasingly widespread across the bacterial tree of life.

Formal recognition and classification of gene transfer agents as viriforms

Morphological and genetic features strongly suggest that gene transfer agents (GTAs) are caudoviricete-derived entities that have evolved in concert with cellular genomes to such a degree that they should not be considered viruses. Indeed, GTA particles resemble caudoviricete virions, but, in contrast to caudoviricetes (or any viruses), GTAs can encapsidate at best only part of their own genomes, are induced solely in small subpopulations of prokaryotic host cells, and are transmitted vertically as part of cellular genomes during replication and division. Therefore, the lifecycles of GTAs are analogous to virus-derived entities found in the parasitoid wasps, which have recently been recognized as non-virus entities and therefore reclassified as viriforms. We evaluated three distinct, independently exapted GTA groups, for which the genetic basis for GTA particle production has been established. Based on the evidence, we outline a classification scheme for these viriforms.

Applications of bacteriophages against intracellular bacteria

Infectious diseases pose a significant threat to both human and animal populations. Intracellular bacteria are a group of pathogens that invade and survive within the interior of eukaryotic cells, which in turn protect them from antibacterial drugs and the host immune system. Limited penetration of antibacterials into host cells results in insufficient bacterial clearance and treatment failure. Bacteriophages have, over the decades, been proved to play an important role in combating bacterial infections (phage therapy), making them an important alternative to classical antibiotic strategies today. Phages have been found to be effective at killing various species of extracellular bacteria, but little is still known about how phages control intracellular infections. With advances in phage genomics and mechanisms of delivery and cell uptake, the development of phage-based antibacterial strategies to address the treatment of intracellular bacteria has general potential. In this review, we present the current state of knowledge regarding the application of bacteriophages against intracellular bacteria. We cover phage deployment against the most common intracellular pathogens with special attention to therapeutic and preventive strategies.

Femtoplankton: What's New?

Since the discovery of high abundances of virus-like particles in aquatic environment, emergence of new analytical methods in microscopy and molecular biology has allowed significant advances in the characterization of the femtoplankton, i.e., floating entities filterable on a 0.2 µm pore size filter. The successive evidences in the last decade (2010-2020) of high abundances of biomimetic mineral-organic particles, extracellular vesicles, CPR/DPANN (Candidate phyla radiation/Diapherotrites, Parvarchaeota, Aenigmarchaeota, Nanoarchaeota and Nanohaloarchaeota), and very recently of aster-like nanoparticles (ALNs), show that aquatic ecosystems form a huge reservoir of unidentified and overlooked femtoplankton entities. The purpose of this review is to highlight this unsuspected diversity. Herein, we focus on the origin, composition and the ecological potentials of organic femtoplankton entities. Particular emphasis is given to the most recently discovered ALNs. All the entities described are displayed in an evolutionary context along a continuum of complexity, from minerals to cell-like living entities.

Gene Transfer Agents in Symbiotic Microbes

Prokaryotes commonly undergo genome reduction, particularly in the case of symbiotic bacteria. Genome reductions tend toward the energetically favorable removal of unnecessary, redundant, or nonfunctional genes. However, without mechanisms to compensate for these losses, deleterious mutation and genetic drift might otherwise overwhelm a population. Among the mechanisms employed to counter gene loss and share evolutionary success within a population, gene transfer agents (GTAs) are increasingly becoming recognized as important contributors. Although viral in origin, GTA particles package fragments of their "host" genome for distribution within a population of cells, often in a synchronized manner, rather than selfishly packaging genes necessary for their spread. Microbes as diverse as archaea and alpha-proteobacteria have been known to produce GTA particles, which are capable of transferring selective advantages such as virulence factors and antibiotic resistance. In this review, we discuss the various types of GTAs identified thus far, focusing on a defined set of symbiotic alpha-proteobacteria known to carry them. Drawing attention to the predicted presence of these genes, we discuss their potential within the selective marine and terrestrial environments occupied by mutualistic, parasitic, and endosymbiotic microbes.

Small extracellular particles with big potential for horizontal gene transfer: membrane vesicles and gene transfer agents

Bacteria are known to release different types of particles that serve various purposes such as the processing of metabolites, communication, and the transfer of genetic material. One of the most interesting aspects of the production of such particles is the biogenesis and trafficking of complex particles that can carry DNA, RNA, proteins or toxins into the surrounding environment to aid in bacterial survival or lead to gene transfer. Two important bacterial extracellular complexes are membrane vesicles and gene transfer agents. In this review, we will discuss the production, contents and functions of these two types of particles as related to their abilities to facilitate horizontal gene transfer.

Prophage-Driven Genomic Structural Changes Promote Bartonella Vertical Evolution

Bartonella is a genetically diverse group of vector-borne bacteria. Over 40 species have been characterized to date, mainly from mammalian reservoirs and arthropod vectors. Rodent reservoirs harbor one of the largest Bartonella diversity described to date, and novel species and genetic variants are continuously identified from these hosts. Yet, it is still unknown if this significant genetic diversity stems from adaptation to different niches or from intrinsic high mutation rates. Here, we explored the vertical occurrence of spontaneous genomic alterations in 18 lines derived from two rodent-associated Bartonella elizabethae-like strains, evolved in nonselective agar plates under conditions mimicking their vector- and mammalian-associated temperatures, and the transmission cycles between them (i.e., 26 °C, 37 °C, and alterations between the two), using mutation accumulation experiments. After ∼1,000 generations, evolved genomes revealed few point mutations (average of one-point mutation per line), evidencing conserved single-nucleotide mutation rates. Interestingly, three large structural genomic changes (two large deletions and an inversion) were identified over all lines, associated with prophages and surface adhesin genes. Particularly, a prophage, deleted during constant propagation at 37 °C, was associated with an increased autonomous replication at 26 °C (the flea-associated temperature). Complementary molecular analyses of wild strains, isolated from desert rodents and their fleas, further supported the occurrence of structural genomic variations and prophage-associated deletions in nature. Our findings suggest that structural genomic changes represent an effective intrinsic mechanism to generate diversity in slow-growing bacteria and emphasize the role of prophages as promoters of diversity in nature.

The depths of virus exaptation

Viruses are ubiquitous parasites of cellular life forms and the most abundant biological entities on earth. The relationships between viruses and their hosts involve the continuous arms race but are by no account limited to it. Growing evidence shows that, in the course of evolution, viruses and their components are repeatedly recruited (exapted) for host functions. The functions of exapted viruses typically involve either defense from other viruses or cellular competitors or transfer of nucleic acids between cells, or storage functions. Virus exaptation can reach different depths, from recruitment of a fully functional virus to exploitation of defective, partially degraded viruses, to utilization of individual virus proteins.

Bartonella Species, an Emerging Cause of Blood-Culture-Negative Endocarditis

Since the reclassification of the genus Bartonella in 1993, the number of species has grown from 1 to 45 currently designated members. Likewise, the association of different Bartonella species with human disease continues to grow, as does the range of clinical presentations associated with these bacteria. Among these, blood-culture-negative endocarditis stands out as a common, often undiagnosed, clinical presentation of infection with several different Bartonella species. The limitations of laboratory tests resulting in this underdiagnosis of Bartonella endocarditis are discussed. The varied clinical picture of Bartonella infection and a review of clinical aspects of endocarditis caused by Bartonella are presented. We also summarize the current knowledge of the molecular basis of Bartonella pathogenesis, focusing on surface adhesins in the two Bartonella species that most commonly cause endocarditis, B. henselae and B. quintana. We discuss evidence that surface adhesins are important factors for autoaggregation and biofilm formation by Bartonella species. Finally, we propose that biofilm formation is a critical step in the formation of vegetative masses during Bartonella-mediated endocarditis and represents a potential reservoir for persistence by these bacteria.

Beyond the canonical strategies of horizontal gene transfer in prokaryotes

Efforts to identify and characterize strategies for horizontal gene transfer (HGT) in prokaryotes could have overlooked some mechanisms that do not entirely fit in with the canonical ones most often described (conjugation of plasmids, phage transduction and transformation). The difficulty in distinguishing the different HGT strategies could have contributed to underestimate their real extent. Here we review non classical HGT strategies: some that require mobile genetic elements (MGEs) and others independent of MGE. Among those strategies that require MGEs, there is a range of newly reported, hybrid and intermediate MGEs mobilizing only their own DNA, others that mobilize preferentially bacterial DNA, or both. Considering HGT strategies independent of MGE, a few are even not restricted to DNA transfer, but can also mobilize other molecules. This review considers those HGT strategies that are less commonly dealt with in the literature. The real impact of these elements could, in some conditions, be more relevant than previously thought.

One for all or all for one: heterogeneous expression and host cell lysis are key to gene transfer agent activity in Rhodobacter capsulatus

The gene transfer agent (RcGTA) of Rhodobacter capsulatus is the model for a family of novel bacteriophage-related genetic elements that carry out lateral transfer of essentially random host DNA. Genuine and putative gene transfer agents have been discovered in diverse genera and are becoming recognized as potentially an important source of genetic exchange and microbial evolution in the oceans. Despite being discovered over 30 years ago, little is known about many essential aspects of RcGTA biology. Here, we validate the use of direct fluorescence reporter constructs, which express the red fluorescent protein mCherry in R. capsulatus. A construct containing the RcGTA promoter fused to mCherry was used to examine the single-cell expression profiles of wild type and RcGTA overproducer R. capsulatus populations, under different growth conditions and growth phases. The majority of RcGTA production clearly arises from a small, distinct sub-set of the population in the wild type strain and a larger sub-set in the overproducer. The most likely RcGTA release mechanism concomitant with this expression pattern is host cell lysis and we present direct evidence for the release of an intracellular enzyme accompanying RcGTA release. RcGTA ORF s is annotated as a ‘cell wall peptidase’ but we rule out a role in host lysis and propose an alternative function as a key contributor to RcGTA invasion of a target cell during infection.

Recombination, diversity and allele sharing of infectivity proteins between Bartonella species from rodents

The alpha-Proteobacterium Bartonella is a common parasite of voles and mice, giving rise to short-lived (4 weeks to 2 months) infections. Here, we report high sequence diversity in genes of the VirB/VirD type IV secretion system (T4SS), amongst Bartonella from natural rodent populations in NE Poland. The VirB5 protein is predicted to consist of three conserved alpha helices separated by loops of variable length which include numerous indels. The C-terminal domain includes repeat stretches of KEK residues, reflecting underlying homopolymeric stretches of adenine residues. A total of 16 variants of VirB5, associated with host identity, but not bacterial taxon, were identified from 22 Bartonella isolates. One was clearly a recombinant from two others, another included an insertion of two KEK repeats. The virB5 gene appears to evolve via both mutation and recombination, as well as slippage mediated insertion/deletion events. The recombinational units are thought to be relatively short, as there was no evidence of linkage disequilibrium between virB5 and the bepA locus only 5.5 kb distant. The diversity of virB5 is assumed to be related to immunological role of this protein in Bartonella infections; diversity of virB5 may assist persistence of Bartonella in the rodent population, despite the relatively short (3-4 weeks) duration of individual infections. It is clear from the distribution of virB5 and bepA alleles that recombination within and between clades is widespread, and frequently crosses the boundaries of conventionally recognised Bartonella species.

Gene transfer agents: phage-like elements of genetic exchange

Horizontal gene transfer is important in the evolution of bacterial and archaeal genomes. An interesting genetic exchange process is carried out by diverse phage-like gene transfer agents (GTAs) that are found in a wide range of prokaryotes. Although GTAs resemble phages, they lack the hallmark capabilities that define typical phages, and they package random pieces of the producing cell's genome. In this Review, we discuss the defining characteristics of the GTAs that have been identified to date, along with potential functions for these agents and the possible evolutionary forces that act on the genes involved in their production.

Intruders below the radar: molecular pathogenesis of Bartonella spp

Bartonella spp. are facultative intracellular pathogens that employ a unique stealth infection strategy comprising immune evasion and modulation, intimate interaction with nucleated cells, and intraerythrocytic persistence. Infections with Bartonella are ubiquitous among mammals, and many species can infect humans either as their natural host or incidentally as zoonotic pathogens. Upon inoculation into a naive host, the bartonellae first colonize a primary niche that is widely accepted to involve the manipulation of nucleated host cells, e.g., in the microvasculature. Consistently, in vitro research showed that Bartonella harbors an ample arsenal of virulence factors to modulate the response of such cells, gain entrance, and establish an intracellular niche. Subsequently, the bacteria are seeded into the bloodstream where they invade erythrocytes and give rise to a typically asymptomatic intraerythrocytic bacteremia. While this course of infection is characteristic for natural hosts, zoonotic infections or the infection of immunocompromised patients may alter the path of Bartonella and result in considerable morbidity. In this review we compile current knowledge on the molecular processes underlying both the infection strategy and pathogenesis of Bartonella and discuss their connection to the clinical presentation of human patients, which ranges from minor complaints to life-threatening disease.

Molecular prevalence of Bartonella, Babesia, and hemotropic Mycoplasma sp. in dogs with splenic disease

BACKGROUND

Among diseases that cause splenomegaly in dogs, lymphoid nodular hyperplasia (LNH), splenic hemangiosarcoma (HSA), and fibrohistiocytic nodules (FHN) are common diagnoses. The spleen plays an important role in the immunologic control or elimination of vector-transmitted, blood-borne pathogens, including Bartonella sp., Babesia sp., and hemotropic Mycoplasma sp.

OBJECTIVE

To compare the prevalence of Bartonella sp., Babesia sp., and hemotropic Mycoplasma sp. DNA in spleens from dogs with LNH, HSA, and FHN.

MATERIALS AND METHODS

Paraffin-embedded, surgically obtained biopsy tissues from LNH (N = 50), HSA (N = 50), and FHN (N = 37) were collected from the anatomic pathology archives. Spleens from specific pathogen-free (SPF) dogs (N = 8) were used as controls. Bartonella sp., Babesia sp., and Mycoplasma sp. DNA was amplified by PCR, followed by DNA sequencing.

RESULTS

Bartonella sp. DNA was more prevalent in FHN (29.7%) and HSA (26%) as compared to LNH (10%) (P = .019, .0373, respectively) or control spleens (0.0%). The prevalence of Babesia sp. and hemotropic Mycoplasma sp. DNA was significantly lower than Bartonella sp. DNA in HSA (P = .0005, .006, respectively) and FHN (P = .003, .0004, respectively). There was no statistically significant difference in DNA prevalence among the 3 genera in the LNH group.

CONCLUSIONS

The higher prevalence of Bartonella sp. in FHN and HSA warrants future investigations to determine if this bacterium plays a role in the development of these splenic diseases.

Evidence for particle-induced horizontal gene transfer and serial transduction between bacteria

Incubation of the amino acid-deficient strain Escherichia coli AB1157 with particles harvested from an oligotrophic environment revealed evidence of horizontal gene transfer (HGT) with restoration of all deficiencies in revertant cells with frequencies up to 1.94 × 10(-5). None of the markers were preferentially transferred, indicating that the DNA transfer is performed by generalized transduction. The highest gene transfer frequencies were obtained for single markers, with values up to 1.04 × 10(-2). All revertants were able to produce particles of comparable size, appearing at the beginning of the stationary phase. Examination of the revertants using electron microscopy showed bud-like structures with electron-dense bodies. The particles that display the structural features of membrane vesicles were again infectious to E. coli AB1157, producing new infectious particles able to transduce genetic information, a phenomenon termed serial transduction. Thus, the <0.2-μm particle fraction from seawater contains a particle size fraction with high potential for gene transfer. Biased sinusoidal field gel electrophoresis indicated a DNA content for the particles of 370 kbp, which was higher than that of known membrane vesicles. These findings provide evidence of a new method of HGT, in which mobilizable DNA is trafficked from donor to recipient cells via particles.

Run-off replication of host-adaptability genes is associated with gene transfer agents in the genome of mouse-infecting Bartonella grahamii

The genus Bartonella comprises facultative intracellular bacteria adapted to mammals, including previously recognized and emerging human pathogens. We report the 2,341,328 bp genome sequence of Bartonella grahamii, one of the most prevalent Bartonella species in wild rodents. Comparative genomics revealed that rodent-associated Bartonella species have higher copy numbers of genes for putative host-adaptability factors than the related human-specific pathogens. Many of these gene clusters are located in a highly dynamic region of 461 kb. Using hybridization to a microarray designed for the B. grahamii genome, we observed a massive, putatively phage-derived run-off replication of this region. We also identified a novel gene transfer agent, which packages the bacterial genome, with an over-representation of the amplified DNA, in 14 kb pieces. This is the first observation associating the products of run-off replication with a gene transfer agent. Because of the high concentration of gene clusters for host-adaptation proteins in the amplified region, and since the genes encoding the gene transfer agent and the phage origin are well conserved in Bartonella, we hypothesize that these systems are driven by selection. We propose that the coupling of run-off replication with gene transfer agents promotes diversification and rapid spread of host-adaptability factors, facilitating host shifts in Bartonella.

Emerging infectious diseases represent an increasing human health problem with many examples of disease outbreaks caused by transmissions from animals to humans, such as, most recently, the bird flu virus. Genes involved in virulence and antibiotic resistance are often carried by mobile elements like plasmids and viruses, which mediate transfer between cells at an amazing speed. Rodents represent a major carrier of infectious agents, and it is therefore particularly important to study the gene transfer processes in bacteria that use rodents as their natural host reservoir. We have studied the genome of a bacterium that is naturally adapted to mice and identified many more putative host-interaction genes than were observed in previously recognized human pathogens. Furthermore, most of these genes are located in a segment of about 25% of the genome, which was massively amplified and packaged into viral particles. This is the first demonstration of targeted packaging of a portion of the bacterial chromosome into viral particles, and we propose that this is a novel strategy for increased exchange of genes involved in the infectious process.

Epifluorescent microscopy to evaluate bacteriophage capsid integrity

The standard method to evaluate capsid integrity of DNA-based viruses, which depends on access to DNase I, relies on the removal of the capsid by solvent extraction and then the evaluation of the nucleic acid products by electrophoresis. Our method, which is based on the direct detection of capsid-borne DNA directly through the use of epifluorescent microscopy, negates the requirement of DNA extraction.

Prophage-like gene transfer agents-novel mechanisms of gene exchange for Methanococcus, Desulfovibrio, Brachyspira, and Rhodobacter species

Gene transfer agents (GTAs) are novel mechanisms for bacterial gene transfer. They resemble small, tailed bacteriophages in ultrastructure and act like generalized transducing prophages. In contrast to functional prophages, GTAs package random fragments of bacterial genomes and incomplete copies of their own genomes. The packaged DNA content is characteristic of the GTA and ranges in size from 4.4 to 13.6kb. GTAs have been reported in species of Brachyspira, Methanococcus, Desulfovibrio, and Rhodobacter. The best studied GTAs are VSH-1 of the anaerobic, pathogenic spirochete Brachyspira hyodysenteriae and RcGTA of the nonsulfur, purple, photosynthetic bacterium Rhodobacter capsulatus. VSH-1 and RcGTA have likely contributed to the ecology and evolution of these bacteria. The existence of GTAs in phylogenetically diverse bacteria suggests GTAs may be more common in nature than is now appreciated.

Importance of widespread gene transfer agent genes in α-proteobacteria

The gene transfer agent produced by Rhodobacter capsulatus (RcGTA) is a model for several virus-like elements that seem to function solely for mediating gene exchange. Several genes that encode RcGTA are clearly related to bacteriophage genes but the cellular regulatory mechanisms that control RcGTA production indicate that RcGTA is more than just a defective prophage. Genome sequencing projects show that seemingly functional RcGTA-like structural gene clusters are present in many other species of alpha-proteobacteria, which might also produce RcGTA-like particles. Here, we use the genomic sequence data that are currently available to identify candidate GTA-producing species and propose an evolutionary scheme for RcGTA-like elements in the alpha-proteobacteria.

Isolation of Bacteriophages from Bartonella vinsonii subsp. berkhoffii and the Characterization of Pap31 Gene Sequences from Bacterial and Phage DNA

Bacteriophages enhance bacterial survival, facilitate bacterial adaptation to new environmental conditions, assist in the adaptation to a new host species, and enhance bacterial evasion or inactivation of host defense mechanisms. We describe the detection and purification of a novel tailed bacteriophage from Bartonella vinsonii subsp. berkhoffii, which was previously described as a bacteriophage-negative species. We also compare B. vinsonii subsp. berkhoffi Pap31 bacteriophage gene sequences to B. henselae (Houston I), and B. quintana (Fuller) bacteriophage Pap31 sequences. Negative staining electron microscopy of log phase culturesof B. vinsonii subsp. berkhoffii identified bacteriophages, possessing a 50-nm icosahedric head diameter and a 60- to 80-nm contractile tail. Sequence analysis of the bacteriophage Pap31 gene from B. vinsonii subsp. berkhoffii showed three consensus sequences and a 12-bp insertion when compared with Pap31 gene sequences from B. henselae (Houston I) and B. quintana (Fuller) bacteriophages. Isolation of B. vinsonii subsp. berkhoffii bacteriophages containing a Pap31 gene suggests that this heme-binding protein gene might play an important role in bacterial virulence through the genetic exchange of DNA within this subspecies. Defining phage-associated genes may also contribute to the enhanced understanding of the evolutionary relationships among members of the genus Bartonella.

Growth characteristics of Bartonella henselae in a novel liquid medium: primary isolation, growth-phase-dependent phage induction, and metabolic studies

Bartonella henselae is a zoonotic pathogen that usually causes a self-limiting infection in immunocompetent individuals but often causes potentially life-threatening infections, such as bacillary angiomatosis, in immunocompromised patients. Both diagnosis of infection and research into the molecular mechanisms of pathogenesis have been hindered by the absence of a suitable liquid growth medium. It has been difficult to isolate B. henselae directly from the blood of infected humans or animals or to grow the bacteria in liquid culture media under laboratory conditions. Therefore, we have developed a liquid growth medium that supports reproducible in vitro growth (3-h doubling time and a growth yield of approximately 5 x 10(8) CFU/ml) and permits the isolation of B. henselae from the blood of infected cats. During the development of this medium, we observed that B. henselae did not derive carbon and energy from the catabolism of glucose, which is consistent with genome nucleotide sequence data suggesting an incomplete glycolytic pathway. Of interest, B. henselae depleted amino acids from the culture medium and accumulated ammonia in the medium, an indicator of amino acid catabolism. Analysis of the culture medium throughout the growth cycle revealed that oxygen was consumed and carbon dioxide was generated, suggesting that amino acids were catabolized in a tricarboxylic acid (TCA) cycle-dependent mechanism. Additionally, phage particles were detected in the culture supernatants of stationary-phase B. henselae, but not in mid-logarithmic-phase culture supernatants. Enzymatic assays of whole-cell lysates revealed that B. henselae has a complete TCA cycle. Taken together, these data suggest B. henselae may catabolize amino acids but not glucose to derive carbon and energy from its host. Furthermore, the newly developed culture medium should improve isolation of B. henselae and basic research into the pathogenesis of the bacterium.

Mitogenic effect of Bartonella bacilliformis on human vascular endothelial cells and involvement of GroEL

Bartonellae are bacterial pathogens for a wide variety of mammals. In humans, bartonellosis can result in angioproliferative lesions that are potentially life threatening to the patient, including bacillary angiomatosis, bacillary peliosis, and verruga peruana. The results of this study show that Bartonella bacilliformis, the agent of Oroya fever and verruga peruana, produces a proteinaceous mitogen for human vascular endothelial cells (HUVECs) that acts in a dose-dependent fashion in vitro with maximal activity at >or=72 h of exposure and results in a 6- to 20-fold increase in cell numbers relative to controls. The mitogen increases bromodeoxyuridine (BrdU) incorporation into HUVECs by almost twofold relative to controls. The mitogen is sensitive to heat and trypsin but is not affected by the lipopolysaccharide inhibitor polymyxin B. The mitogen does not affect caspase 3 activity in HUVECs undergoing serum starvation-induced apoptosis. The Bartonella mitogen was found in bacterial culture supernatants, the soluble cell lysate fraction, and, to a lesser degree, in insoluble cell fractions of the bacterium. In contrast, soluble cell lysate fractions from closely related B. henselae, although possessing significant mitogenicity for HUVECs, resulted in only about a twofold increase in cell numbers. Biochemical and immunological analyses identified GroEL as a participant in the observed HUVEC mitogenicity. A B. bacilliformis strain containing the intact groES-groEL operon on a multicopy plasmid was generated and used to demonstrate a correlation between HUVEC mitogenicity and GroEL levels in the lysate (r(2) = 0.85). Antiserum to GroEL significantly inhibited mitogenicity of the lysate. Data also show that GroEL is located in the soluble and insoluble fractions (including inner and outer membranes) of the cell and is actively secreted by B. bacilliformis.

Hemin binding, functional expression, and complementation analysis of Pap 31 from Bartonella henselae

Growth of Bartonella henselae is strongly heme dependent, and B. henselae is unable to synthesize heme itself. At least five outer membrane-associated proteins from B. henselae bind hemin, including the 31-kDa protein designated Pap31. The gene of this protein was heterologously expressed in Escherichia coli M15(pREP4) and detected with monoclonal antibodies in the outer membrane fraction. Complementation of the hemA-deficient mutant E. coli K-12 EB53 (aroB tsx malT hemA) with pap31 demonstrated that this protein is involved in heme acquisition and may be an important virulence factor in the pathogenesis of B. henselae.

Establishing a direct role for the Bartonella bacilliformis invasion-associated locus B (IalB) protein in human erythrocyte parasitism

The invasion-associated locus A and B genes (ialAB) of Bartonella bacilliformis were previously shown to confer an erythrocyte-invasive phenotype upon Escherichia coli, indirectly implicating their role in virulence. We report the first direct demonstration of a role for ialB as a virulence factor in B. bacilliformis. The presence of a secretory signal sequence and amino acid sequence similarity to two known outer membrane proteins involved in virulence suggested that IalB was an outer membrane protein. To develop an antiserum for protein localization, the ialB gene was cloned in frame into an expression vector with a six-histidine tag and under control of the lacZ promoter. The IalB fusion protein was purified by nickel affinity chromatography and used to raise polyclonal antibodies. IalB was initially localized to the bacterial membrane fraction. To further localize IalB, B. bacilliformis inner and outer membranes were fractionated by sucrose density gradient centrifugation and identified by appearance, buoyant density (rho), and cytochrome b content. Inner and outer membrane proteins were analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), and IalB was positively identified by Western blot. Contrary to expectations, IalB was localized to the inner membrane of the pathogen. To directly demonstrate a role for IalB in erythrocyte parasitism, the B. bacilliformis ialB gene was disrupted by insertional mutagenesis. The resulting ialB mutant strain was complemented in trans with a replicative plasmid encoding the full-length ialB gene. PCR and high-stringency DNA hybridization confirmed mutagenesis and transcomplementation events. Abrogation and restoration of ialB expression was verified by SDS-PAGE and immunoblotting. In vitro virulence assays showed that mutagenesis of ialB decreased bacterial association and invasion of human erythrocytes by 47 to 53% relative to controls. Transcomplementation of ialB restored erythrocyte association and invasion rates to levels observed in the parental strain. These data provide direct evidence for IalB's role in erythrocyte parasitism and represent the first demonstration of molecular Koch's postulates for a Bartonella species.