O-Polysaccharide epitopic heterogeneity at the surface of Brucella spp. studied by enzyme-linked immunosorbent assay and flow cytometry

Development of a Multiplex Bead Assay to Detect Serological Responses to Brucella Species in Domestic Pigs and Wild Boar with the Potential to Overcome Cross-Reactivity with Yersinia enterocolitica O:9

The aim of this study was to develop a multiplex bead assay using a Brucella rLPS antigen, a Brucella suis smooth antigen, and a Yersinia enterocolitica O:9 antigen that not only discriminates Brucella-infected from Brucella-uninfected pigs and wild boar, but also overcomes the cross reactivity with Y. enterocolitica O:9. Sera from 126 domestic pigs were tested: 29 pigs were Brucella infected, 80 were non-infected and 17 were confirmed to be false positive serological reactors (FPSR). Sera from 49 wild boar were tested: 18 were positive and 31 were negative. Using the rLPS antigen, 26/29 Brucella-infected domestic pigs and 15/18 seropositive wild boar were positive, while 75/80 non-Brucella infected domestic pigs, all FPSR, and all seronegative wild boar were negative. Using the smooth B. suis 1330 antigen, all Brucella-infected domestic pigs, 9/17 FPSR and all seropositive wild boar were positive, while all non-infected pigs and 30/31 seronegative wild boar were negative. The ratio of the readouts from the smooth B. suis antigen and Y. enterocolitica O:9 antigen enabled discriminating all Brucella infected individuals from the FPSR domestic pigs. These results demonstrate the potential of this assay for use in the surveillance of brucellosis, overcoming the cross-reactivity with Y. enterocolitica.

When the Going Gets Rough: The Significance of Brucella Lipopolysaccharide Phenotype in Host-Pathogen Interactions

Brucella is a facultatively intracellular bacterial pathogen and the cause of worldwide zoonotic infections, infamous for its ability to evade the immune system and persist chronically within host cells. Despite the frequent association with attenuation in other Gram-negative bacteria, a rough lipopolysaccharide phenotype is retained by Brucella canis and Brucella ovis, which remain fully virulent in their natural canine and ovine hosts, respectively. While these natural rough strains lack the O-polysaccharide they, like their smooth counterparts, are able to evade and manipulate the host immune system by exhibiting low endotoxic activity, resisting destruction by complement and antimicrobial peptides, entering and trafficking within host cells along a similar pathway, and interfering with MHC-II antigen presentation. B. canis and B. ovis appear to have compensated for their roughness by alterations to their outer membrane, especially in regards to outer membrane proteins. B. canis, in particular, also shows evidence of being less proinflammatory in vivo, suggesting that the rough phenotype may be associated with an enhanced level of stealth that could allow these pathogens to persist for longer periods of time undetected. Nevertheless, much additional work is required to understand the correlates of immune protection against the natural rough Brucella spp., a critical step toward development of much-needed vaccines. This review will highlight the significance of rough lipopolysaccharide in the context of both natural disease and host–pathogen interactions with an emphasis on natural rough Brucella spp. and the implications for vaccine development.

Comparative proteomic analysis of outer membrane vesicles from Brucella suis, Brucella ovis, Brucella canis and Brucella neotomae

Gram-negative bacteria release nanovesicles, called outer membrane vesicles (OMVs), from their outer membrane. Proteomics has been used to determine their composition. OMVs contain proteins able to elicit an immune response, so they have been proposed as a model to develop acellular vaccines. In this study, OMVs of Brucella suis, B. ovis, B. canis, and B. neotomae were purified and analyzed by SDS-PAGE, transmission electron microscopy and liquid chromatography coupled to mass spectrometry to determine the pan-proteome of these vesicles. In addition, antigenic proteins were detected by western blot with anti-Brucella sera. The in silico analysis of the pan-proteome revealed many homologous proteins, such as Omp16, Omp25, Omp31, SodC, Omp2a, and BhuA. Proteins contained in the vesicles from different Brucella species were detected by anti-Brucella sera. The occurrence of previously described immunogenic proteins derived from OMVs supports the use of these vesicles as candidates to be evaluated as an acellular brucellosis vaccine.

Loci Associated With Antibody Response in Feral Swine (Sus scrofa) Infected With Brucella suis

Feral swine (Sus scrofa) are a destructive invasive species widespread throughout the United States that disrupt ecosystems, damage crops, and carry pathogens of concern for the health of domestic stock and humans including Brucella suis-the causative organism for swine brucellosis. In domestic swine, brucellosis results in reproductive failure due to abortions and infertility. Contact with infected feral swine poses spillover risks to domestic pigs as well as humans, companion animals, wildlife, and other livestock. Genetic factors influence the outcome of infectious diseases; therefore, genome wide association studies (GWAS) of differential immune responses among feral swine can provide an understanding of disease dynamics and inform management to prevent the spillover of brucellosis from feral swine to domestic pigs. We sought to identify loci associated with differential antibody responses among feral swine naturally infected with B. suis using a case-control GWAS. Tissue, serum, and genotype data (68,516 bi-allelic single nucleotide polymorphisms) collected from 47 feral swine were analyzed in this study. The 47 feral swine were culture positive for Brucella spp. Of these 47, 16 were antibody positive (cases) whereas 31 were antibody negative (controls). Single-locus GWAS were performed using efficient mixed-model association eXpedited (EMMAX) methodology with three genetic models: additive, dominant, and recessive. Eight loci associated with seroconversion were identified on chromosome 4, 8, 9, 10, 12, and 18. Subsequent bioinformatic analyses revealed nine putative candidate genes related to immune function, most notably phagocytosis and induction of an inflammatory response. Identified loci and putative candidate genes may play an important role in host immune responses to B. suis infection, characterized by a detectable bacterial presence yet a differential antibody response. Given that antibody tests are used to evaluate brucellosis infection in domestic pigs and for disease surveillance in invasive feral swine, additional studies are needed to fully understand the genetic component of the response to B. suis infection and to more effectively translate estimates of Brucella spp. antibody prevalence among feral swine to disease control management action.

Overexpression of wbkF gene in Brucella abortus RB51WboA leads to increased O-polysaccharide expression and enhanced vaccine efficacy against B. abortus 2308, B. melitensis 16M, and B. suis 1330 in a murine brucellosis model

Brucella abortus RB51 is an attenuated, stable, spontaneous rough mutant derived in the laboratory from the virulent strain B. abortus 2308. Previous studies discovered that the wboA gene, which encodes a glycosyltransferase required for synthesis of the O-polysaccharide, is disrupted in strain RB51 by an IS711 element. However, complementation of strain RB51 with a functional wboA gene (strain RB51WboA) does not confer it a smooth phenotype but results in low levels of cytoplasmic O-polysaccharide synthesis. In this study, we asked if increasing the potential availability of bactoprenol priming precursors in strain RB51WboA would increase the levels of O-polysaccharide synthesis and enhance the protective efficacy against virulent Brucella challenge. To achieve this, we overexpressed the wbkF gene, which encodes a putative undecaprenyl-glycosyltransferase involved in bactoprenol priming for O-polysaccharide polymerization, in strain RB51WboA to generate strain RB51WboAKF. In comparison to strain RB51WboA, strain RB51WboAKF expressed higher levels of O-polysaccharide, but was still attenuated and remained phenotypically rough. Mice immunized with strain RB51WboAKF developed increased levels of smooth LPS-specific serum antibodies, primarily of IgG2a and IgG3 isotype. Splenocytes from mice vaccinated with strain RB51WboAKF secreted higher levels of antigen-specific IFN-γ and TNF-α and contained more numbers of antigen-specific IFN-γ secreting CD4⁺ and CD8⁺ T lymphocytes when compared to those of the RB51 or RB51WboA vaccinated groups. Immunization with strain RB51WboAKF conferred enhanced protection against virulent B. abortus 2308, B. melitensis 16M and B. suis 1330 challenge when compared to the currently used vaccine strains. Our results suggest that strain RB51WboAKF has the potential to be a more efficacious vaccine than its parent strain in natural hosts.

WadD, a New Brucella Lipopolysaccharide Core Glycosyltransferase Identified by Genomic Search and Phenotypic Characterization

Brucellosis, an infectious disease caused by Brucella, is one of the most extended bacterial zoonosis in the world and an important cause of economic losses and human suffering. The lipopolysaccharide (LPS) of Brucella plays a major role in virulence as it impairs normal recognition by the innate immune system and delays the immune response. The LPS core is a branched structure involved in resistance to complement and polycationic peptides, and mutants in glycosyltransferases required for the synthesis of the lateral branch not linked to the O-polysaccharide (O-PS) are attenuated and have been proposed as vaccine candidates. For this reason, the complete understanding of the genes involved in the synthesis of this LPS section is of particular interest. The chemical structure of the Brucella LPS core suggests that, in addition to the already identified WadB and WadC glycosyltransferases, others could be implicated in the synthesis of this lateral branch. To clarify this point, we identified and constructed mutants in 11 ORFs encoding putative glycosyltransferases in B. abortus. Four of these ORFs, regulated by the virulence regulator MucR (involved in LPS synthesis) or the BvrR/BvrS system (implicated in the synthesis of surface components), were not required for the synthesis of a complete LPS neither for virulence or interaction with polycationic peptides and/or complement. Among the other seven ORFs, six seemed not to be required for the synthesis of the core LPS since the corresponding mutants kept the O-PS and reacted as the wild type with polyclonal sera. Interestingly, mutant in ORF BAB1_0953 (renamed wadD) lost reactivity against antibodies that recognize the core section while kept the O-PS. This suggests that WadD is a new glycosyltransferase adding one or more sugars to the core lateral branch. WadD mutants were more sensitive than the parental strain to components of the innate immune system and played a role in chronic stages of infection. These results corroborate and extend previous work indicating that the Brucella LPS core is a branched structure that constitutes a steric impairment preventing the elements of the innate immune system to fight against Brucella.

Synthetic glycoconjugates characterize the fine specificity of Brucella A and M monoclonal antibodies

The dominant cell wall antigen of Brucella bacteria is the O-polysaccharide component of the smooth lipopolysaccharide. Infection by various Brucella biovars causes abortions and infertility in a wide range of domestic and wild animals and debilitating disease in humans. Diagnosis relies on the detection of antibodies to the A and M antigens expressed in the O-polysaccharide. This molecule is a homopolymer of the rare monosaccharide, 4-formamido-4,6-dideoxy-d-mannopyranose (Rha4NFo). The A epitope is created by a uniform α1,2 linked internal polymeric sequence capped by a distinct tetrasaccharide sequence defining the M antigen. Unique oligosaccharides only available by chemical synthesis and conjugated via reducing and non-reducing residues to bovine serum albumin have revealed the structural basis of the fine specificity that allows the discrimination of these closely related A and M epitopes. All three M specific monoclonal antibodies (mAbs) are inferred to possess groove type binding sites open at each end, and recognize an α1,3 linked Rha4NFo disaccharide as a part of a trisaccharide epitope, which in two mAbs includes the terminal Rha4NFo residue. The binding site of one of these antibodies is sufficiently large to engage up to six Rha4NFo residues and involves weak recognition of α1,2 linked Rha4NFo residues. The third mAb binds an internal trisaccharide epitope of the M tetrasaccharide. Two A specific mAbs also possess groove type binding sites that accommodate six and four α1,2 linked Rha4NFo residues.

Brucellosis: Improved Diagnostics and Vaccine Insights from Synthetic Glycans

Brucellosis is a serious zoonotic bacterial disease that is ranked by the World Health Organization among the top seven “neglected zoonoses” that threaten human health and cause poverty. It is a costly, highly contagious disease that affects ruminants, cattle, sheep, goats, and other productive animals such as pigs. Symptoms include abortions, infertility, decreased milk production, weight loss, and lameness. Brucellosis is also the most common bacterial disease that is transmitted from animals to humans, with approximately 500 000 new human cases each year. Detection and slaughter of infected animals is required to eradicate the disease, as vaccination alone is currently insufficient. However, as the most protective vaccines compromise serodiagnosis, this creates policy dilemmas, and these often result in the failure of eradication and control programs. Detection of antibodies to the Brucella bacterial cell wall O-polysaccharide (OPS) component of smooth lipopolysaccharide is used in diagnosis of this disease, and the same molecule contributes important protective efficacy to currently deployed veterinary whole-cell vaccines. This has set up a long-standing paradox that while Brucella OPS confers protective efficacy to vaccines, its presence results in similar antibody profiles in infected and vaccinated animals. Consequently, differentiation of infected from vaccinated animals (DIVA) is not possible, and this limits efforts to combat the disease. Recent clarification of the chemical structure of Brucella OPS as a block copolymer of two oligosaccharide sequences has provided an opportunity to utilize unique oligosaccharides only available via chemical synthesis in serodiagnostic tests for the disease. These oligosaccharides show excellent sensitivity and specificity compared with the native polymer used in current commercial tests and have the added advantage of assisting discrimination between brucellosis and infections caused by several bacteria with OPS that share some structural features with those of Brucella. During synthesis and immunochemical evaluation of these synthetic antigens, it became apparent that an opportunity existed to create a polysaccharide–protein conjugate vaccine that would not create antibodies that give false positive results in diagnostic tests for infection. This objective was reduced to practice, and immunization of mice showed that antibodies to the Brucella A antigen could be developed without reacting in a diagnostic test based on the M antigen. A conjugate vaccine of this type could readily be developed for use in humans and animals. However, as chemical methods advance and modern methods of bacterial engineering mature, it is expected that the principles elucidated by these studies could be applied to the development of an inexpensive and cost-effective vaccine to combat endemic brucellosis in animals.

New Features in the Lipid A Structure of Brucella suis and Brucella abortus Lipopolysaccharide

Brucellaceae are Gram-negative bacteria that cause brucellosis, one of the most distributed worldwide zoonosis, transmitted to humans by contact with either infected animals or their products. The lipopolysaccharide exposed on the cell surface has been intensively studied and is considered a major virulence factor of Brucella. In the last years, structural studies allowed the determination of new structures in the core oligosaccharide and the O-antigen of this lipopolysaccharide. In this work, we have reinvestigated the lipid A structure isolated from B. suis and B. abortus lipopolysaccharides. A detailed study by MALDI-TOF mass spectrometry in the positive and negative ion modes of the lipid A moieties purified from both species was performed. Interestingly, a new feature was detected: the presence of a pyrophosphorylethanolamine residue substituting the backbone. LID-MS/MS analysis of some of the detected ions allowed assurance that the Lipid A structure composed by the diGlcN3N disaccharide, mainly hexa-acylated and penta-acylated, bearing one phosphate and one pyrophosphorylethanolamine residue. Graphical abstract ᅟ.

Novel Solutions for Vaccines and Diagnostics To Combat Brucellosis

Brucellosis is diagnosed by detection of antibodies in the blood of animals and humans that are specific for two carbohydrate antigens, termed A and M, which are present concurrently in a single cell wall O-polysaccharide. Animal brucellosis vaccines contain these antigenic determinants, and consequently infected and vaccinated animals cannot be differentiated as both groups produce A and M specific antibodies. We hypothesized that chemical synthesis of a pure A vaccine would offer unique identification of infected animals by a synthetic M diagnostic antigen that would not react with antibodies generated by this vaccine. Two forms of the A antigen, a hexasaccharide and a heptasaccharide conjugated to tetanus toxoid via reducing and nonreducing terminal sugars, were synthesized and used as lead vaccine candidates. Mouse antibody profiles to these immunogens showed that to avoid reaction with diagnostic M antigen it was essential to maximize the induction of anti-A antibodies that bind internal oligosaccharide sequences and minimize production of antibodies directed toward the terminal nonreducing monosaccharide. This objective was achieved by conjugation of Brucella O-polysaccharide to tetanus toxoid via its periodate oxidized terminal nonreducing monosaccharide, thereby destroying terminal epitopes and focusing the antibody response on internal A epitopes. This establishes the method to resolve the decades-long challenge of how to create effective brucellosis vaccines without compromising diagnosis of infected animals.

Monoclonal Antibody-Defined Specific C Epitope of Brucella O-Polysaccharide Revisited

The C epitope of Brucella O-polysaccharide (O-PS) has so far lacked definitive structural identity. Revised structures for this antigen revealed a unique capping perosamine tetrasaccharide consisting of a sequence of 1,2:1,3:1,2 interresidue linkages. Here, using synthetic oligosaccharide glycoconjugates, the α-1,3 linkage of the O-PS is shown to be an integral structural requirement of this epitope. Although A-dominant strains possess only one or two copies of the capping tetrasaccharide, this creates a unique pentasaccharide antigenic determinant with the linkage sequence 1,2:1,3:1,2:1,2 that is always present in major pathogenic Brucella species.

Development of an improved competitive ELISA based on a monoclonal antibody against lipopolysaccharide for the detection of bovine brucellosis

Background

Brucellosis is the most common bacterial zoonosis, and serological tests are routinely used in brucellosis control and eradication programs. In order to improve the accuracy of serological diagnostic method used in bovine brucellosis detection, this study developed an improved competitive ELISA with higher specificity and good sensitivity.

Results

This study prepared 12 monoclonal antibodies against smooth Brucella lipopolysaccharide. One monoclonal antibody 3 F9, presented C epitope specificity, was used to develop a competitive ELISA for the serological detection of bovine brucellosis. The competitive ELISA, a commercial competitive ELISA kit, the rose-bengal plate agglutination test, and a microplate agglutination test were all used in the detection of 6 hyperimmune antisera against other commonly cross-reacted bacterial pathogens and 110 clinical bovine serum samples. The results of the test comparisons indicated that the competitive ELISA had higher specificity than the commercial competitive ELISA kit and RBT, and comparable sensitivity with the commercial ELISA kit.

Conclusions

This study provided a valuable detection tool with high specificity and good sensitivity, which prevent the wrong-culling of bovines in the eradication campaigns of bovine brucellosis.

Electronic supplementary material

The online version of this article (doi:10.1186/s12917-015-0436-3) contains supplementary material, which is available to authorized users.

A protein-conjugate approach to develop a monoclonal antibody-based antigen detection test for the diagnosis of human brucellosis

Human brucellosis is most commonly diagnosed by serology based on agglutination of fixed Brucella abortus as antigen. Nucleic acid amplification techniques have not proven capable of reproducibly and sensitively demonstrating the presence of Brucella DNA in clinical specimens. We sought to optimize a monoclonal antibody-based assay to detect Brucella melitensis lipopolysaccharide in blood by conjugating B. melitensis LPS to keyhole limpet hemocyanin, an immunogenic protein carrier to maximize IgG affinity of monoclonal antibodies. A panel of specific of monoclonal antibodies was obtained that recognized both B. melitensis and B. abortus lipopolysaccharide epitopes. An antigen capture assay was developed that detected B. melitensis in the blood of experimentally infected mice and, in a pilot study, in naturally infected Peruvian subjects. As a proof of principle, a majority (7/10) of the patients with positive blood cultures had B. melitensis lipopolysaccharide detected in the initial blood specimen obtained. One of 10 patients with relapsed brucellosis and negative blood culture had a positive serum antigen test. No seronegative/blood culture negative patients had a positive serum antigen test. Analysis of the pair of monoclonal antibodies (2D1, 2E8) used in the capture ELISA for potential cross-reactivity in the detection of lipopolysaccharides of E. coli O157:H7 and Yersinia enterocolitica O9 showed specificity for Brucella lipopolysaccharide. This new approach to develop antigen-detection monoclonal antibodies against a T cell-independent polysaccharide antigen based on immunogenic protein conjugation may lead to the production of improved rapid point-of-care-deployable assays for the diagnosis of brucellosis and other infectious diseases.

Entrance and survival of Brucella pinnipedialis hooded seal strain in human macrophages and epithelial cells

Marine mammal Brucella spp. have been isolated from pinnipeds (B. pinnipedialis) and cetaceans (B. ceti) from around the world. Although the zoonotic potential of marine mammal brucellae is largely unknown, reports of human disease exist. There are few studies of the mechanisms of bacterial intracellular invasion and multiplication involving the marine mammal Brucella spp. We examined the infective capacity of two genetically different B. pinnipedialis strains (reference strain; NTCT 12890 and a hooded seal isolate; B17) by measuring the ability of the bacteria to enter and replicate in cultured phagocytes and epithelial cells. Human macrophage-like cells (THP-1), two murine macrophage cell lines (RAW264.7 and J774A.1), and a human malignant epithelial cell line (HeLa S3) were challenged with bacteria in a gentamicin protection assay. Our results show that B. pinnipedialis is internalized, but is then gradually eliminated during the next 72 – 96 hours. Confocal microscopy revealed that intracellular B. pinnipedialis hooded seal strain colocalized with lysosomal compartments at 1.5 and 24 hours after infection. Intracellular presence of B. pinnipedialis hooded seal strain was verified by transmission electron microscopy. By using a cholesterol-scavenging lipid inhibitor, entrance of B. pinnipedialis hooded seal strain in human macrophages was significantly reduced by 65.8 % (± 17.3), suggesting involvement of lipid-rafts in intracellular entry. Murine macrophages invaded by B. pinnipedialis do not release nitric oxide (NO) and intracellular bacterial presence does not induce cell death. In summary, B. pinnipedialis hooded seal strain can enter human and murine macrophages, as well as human epithelial cells. Intracellular entry of B. pinnipedialis hooded seal strain involves, but seems not to be limited to, lipid-rafts in human macrophages. Brucella pinnipedialis does not multiply or survive for prolonged periods intracellulary.

The epitopic and structural characterization of Brucella suis biovar 2 O-polysaccharide demonstrates the existence of a new M-negative C-negative smooth Brucella serovar

The brucellae are Gram-negative bacteria that cause an important zoonosis. Studies with the main Brucella species have shown that the O-antigens of the Brucella smooth lipopolysaccharide are α-(1 → 2) and α-(1 → 3)-linked N-formyl-perosamine polysaccharides that carry M, A and C (A = M, A>M and A<M) epitopes relevant in serodiagnosis and typing. We report that, in contrast to the B. suis biovar 1 O-antigen used as a reference or to all described Brucella O-antigens, B. suis biovar 2 O-antigen failed to bind monoclonal antibodies of C (A = M), C (M>A) and M specificities. However, the biovar 2 O-antigen bound monoclonal antibodies to the Brucella A epitope, and to the C/Y epitope shared by brucellae and Yersinia enterocolitica O:9, a bacterium that carries an N-formyl-perosamine O-antigen in exclusively α-(1 → 2)-linkages. By (13)C NMR spectroscopy, B. suis biovar 1 but not B. suis biovar 2 or Y. enterocolitica O:9 polysaccharide showed the signal characteristic of α-(1 → 3)-linked N-formyl-perosamine, indicating that biovar 2 may altogether lack this linkage. Taken together, the NMR spectroscopy and monoclonal antibody analyses strongly suggest a role for α-(1 → 3)-linked N-formyl-perosamine in the C (A = M) and C (M>A) epitopes. Moreover, they indicate that B. suis biovar 2 O-antigen lacks some lipopolysaccharide epitopes previously thought to be present in all smooth brucellae, thus representing a new brucella serovar that is M-negative, C-negative. Serologically and structurally this new serovar is more similar to Y. enterocolitica O:9 than to other brucellae.

Lipopolysaccharide heterogeneity in the atypical group of novel emerging Brucella species

Recently, novel Brucella strains with phenotypic characteristics that were atypical for strains belonging to the genus Brucella have been reported. Phenotypically many of these strains were initially misidentified as Ochrobactrum spp. Two novel species have been described so far for these strains, i.e., B. microti and B. inopinata, and other strains genetically related to B. inopinata may constitute other novel species as well. In this study, we analyzed the lipopolysaccharides (LPS) (smooth LPS [S-LPS] and rough LPS [R-LPS]) of these atypical strains using different methods and a panel of monoclonal antibodies (MAbs) directed against several epitopes of the Brucella O-polysaccharide (O-PS) and R-LPS. Among the most striking results, Brucella sp. strain BO2, isolated from a patient with chronic destructive pneumonia, showed a completely distinct S-LPS profile in silver stain gels that looked more similar to that of enterobacterial S-LPS. This strain also failed to react with MAbs against Brucella O-PS epitopes and showed weak reactivity with anti-R-LPS MAbs. B. inopinata reference strain BO1 displayed an M-dominant S-LPS type with some heterogeneity relative to the classical M-dominant Brucella S-LPS type. Australian wild rodent strains belonging also to the B. inopinata group showed a classical A-dominant S-LPS but lacked the O-PS common (C) epitopes, as previously reported for B. suis biovar 2 strains. Interestingly, some strains also failed to react with anti-R-LPS MAbs, such as the B. microti reference strain and B. inopinata BO1, suggesting modifications in the core-lipid A moieties of these strains. These results have several implications for serological typing and serological diagnosis and underline the need for novel tools for detection and correct identification of such novel emerging Brucella spp.

Antigenic, immunologic and genetic characterization of rough strains B. abortus RB51, B. melitensis B115 and B. melitensis B18

The lipopolysaccharide (LPS) is considered the major virulent factor in Brucella spp. Several genes have been identified involved in the synthesis of the three LPS components: lipid A, core and O-PS. Usually, Brucella strains devoid of O-PS (rough mutants) are less virulent than the wild type and do not induce undesirable interfering antibodies. Such of them proved to be protective against brucellosis in mice. Because of these favorable features, rough strains have been considered potential brucellosis vaccines. In this study, we evaluated the antigenic, immunologic and genetic characteristics of rough strains B.abortus RB51, B.melitensis B115 and B.melitensis B18. RB51 derived from B.abortus 2308 virulent strain and B115 is a natural rough strain in which the O-PS is present in the cytoplasm. B18 is a rough rifampin-resistan mutant isolated in our laboratory.

The surface antigenicity of RB51, B115 and B18 was evaluated by testing their ability to bind antibodies induced by rough or smooth Brucella strains. The antibody response induced by each strain was evaluated in rabbits. Twenty-one genes, involved in the LPS-synthesis, were sequenced and compared with the B.melitensis 16M strain.

The results indicated that RB51, B115 and B18 have differences in antigenicity, immunologic and genetic properties. Particularly, in B115 a nonsense mutation was detected in wzm gene, which could explain the intracellular localization of O-PS in this strain.

Complementation studies to evaluate the precise role of each mutation in affecting Brucella morphology and its virulence, could provide useful information for the assessment of new, attenuated vaccines for brucellosis.

Immunization of mice with gamma-irradiated Brucella neotomae and its recombinant strains induces protection against virulent B. abortus, B. melitensis, and B. suis challenge

Human brucellosis, a zoonotic disease of major public health concern in several developing countries, is primarily caused by Brucella abortus, Brucella melitensis, and Brucella suis. No brucellosis vaccine is available for human use. The aim of this study was to determine if Brucella neotomae, a bacterium not known to cause disease in any host, can be used for developing brucellosis vaccines. B. neotomae and its recombinant strains overexpressing superoxide dismutase and a 26 kDa periplasmic protein were rendered non-replicative through exposure to gamma-radiation and used as vaccines in a murine brucellosis model. All three vaccines induced antigen-specific antibody and T cell responses. The vaccinated mice showed significant resistance against challenge with virulent B. abortus 2308, B. melitensis 16 M, and B. suis 1330. These results demonstrate that the avirulent B. neotomae is a promising platform for developing a safe and effective vaccine for human brucellosis.

Evaluation of indirect enzyme-linked immunosorbent assays and IgG avidity assays using a protein A-peroxidase conjugate for serological distinction between Brucella abortus S19-vaccinated and -infected cows

This study aimed to evaluate the use of protein A-peroxidase (horseradish peroxidase [HRPO]) in indirect enzyme-linked immunosorbent assays (iELISAs) and IgG avidity assays for serological distinction between Brucella abortus S19-vaccinated and -infected cows. Four groups were analyzed: GI, 41 nonvaccinated seropositive cows; GII, 79 S19-vaccinated heifers analyzed at 3 months postvaccination; GIII, 105 S19-vaccinated cows analyzed after 24 months of age; and GIV, 278 nonvaccinated seronegative cows. IgG levels and avidity to B. abortus smooth lipopolysaccharide (S-LPS) were determined using anti-bovine IgG-HRPO or protein A-HRPO conjugates. Similar levels of IgG anti-S-LPS were found with GI using both conjugates. Lower IgG levels were detected with GII, GIII, and GIV using protein A-HRPO. Both conjugates showed high performance in discriminating GI from GIII, with high sensitivity (Se; 97.6%) and specificity (Sp; 97.1%). Protein A-HRPO was better in distinguishing GI from GIV (Se, 97.6%; Sp, 94.6%) and GI from GII (Se, 80.5%; Sp, 94.9%). Protein A-HRPO excluded a higher number of positive samples with GII and GIV. IgG avidity showed that protein A-HRPO, but not anti-IgG-HRPO, was able to distinguish nonvaccinated from vaccinated cattle, showing a higher avidity index (AI) with GI than with GII, with 78% of serum samples in GII showing an AI of <50%. Therefore, the iELISA using B. abortus S-LPS antigen and protein A-HRPO conjugate for preferential detection of the IgG2 subclass was shown to be suitable for serological distinction between S19-vaccinated and -infected cows. Also, antibodies generated after vaccination showed lower avidity, suggesting a role for the IgG2 subclass as an antibody of higher-affinity maturation after infection, constituting an additional tool for differentiating vaccinated from infected cattle.

Brucella suis identification and biovar typing by real-time PCR

Fast and accurate identification of Brucella suis at the biovar level is an important issue for public health laboratories because some of the biovars that infect suidae (boars and pigs) are pathogenic for humans while others are not. Since classical biovar typing methods are often time-consuming, hard to standardize and require high-level biosafety containment, methodological improvements are desirable. This article describes new single nucleotide polymorphism (SNP) signatures for the rapid identification and biovar characterization of B. suis. These SNPs were included together with previously described ones in real-time PCR assays applicable to low-biosafety conditions. Allelic profiles unique for each B. suis biovar were defined and the most relevant signatures were determined on a collection of 137 field strains of worldwide origin characterized previously. Biovars assigned with both present and classical methods were globally consistent except for some biovar 3 field strains which matched the allelic profile of biovar 1.

Identification of smooth and rough forms in cultures of Brucella melitensis strains by flow cytometry

Brucella melitensis strains may occur as either smooth or rough variants depending on the expression of O-polysaccharides (OPS) as a component of the bacterial outer membrane lipopolysaccharide (LPS). The wboA gene, which codes for the enzyme glycosyl transferase, is essential for the assembly of O-chain in Brucella. Deletion of wboA in smooth virulent B. melitensis 16M results in a rough mutant designated WRR51. We developed a flow cytometric method to determine the proportion of B. melitensis cells displaying surface O-polysaccharide (OPS) in liquid culture. OPS was detected using polyclonal antibodies from rabbits immunized with smooth (S) or rough (R) Brucella LPS. First, we evaluated the binding of these antibodies to 16M (S), WRR51 (R) and complemented WRR51 expressing the wboA gene (S) as well as to their corresponding GFP-expressing derivative strains 16M/GFP, WRR51/GFP and WRR51/GFP+wboA. The rough mutants did not react with anti-S-LPS nor did the smooth strains react with anti-R-LPS. Second, using different ratios of 16M/GFP and WRR51/GFP, we were able to detect the presence of 1% rough bacteria spiked into a sample of smooth organisms. Third, we evaluated the purity of cultures of B. melitensis strains grown in a fermenter. These flow cytometric methods may be useful for quality control of process development for large-scale vaccine production.

Efficacy of several serological tests and antigens for diagnosis of bovine brucellosis in the presence of false-positive serological results due to Yersinia enterocolitica O:9

Yersinia enterocolitica O:9 bears a smooth lipopolysaccharide (S-LPS) of Brucella sp. O-chain A+C/Y epitopic structure and is a cause of false-positive serological reactions (FPSR) in standard tests for cattle brucellosis. Brucella S-LPS, cross-reacting S-LPSs representing several O-chain epitope combinations, Brucella core lipid A epitopes (rough LPS), Brucella abortus S-LPS-derived polysaccharide, native hapten polysaccharide, rough LPS group 3 outer membrane protein complexes, recombinant BP26, and cytosolic proteins were tested in enzyme-linked immunosorbent assays (ELISA) and precipitation tests to detect cattle brucellosis (sensitivity) and to differentiate it from FPSR (specificity). No single serological test and antigen combination showed 100% sensitivity and specificity simultaneously. Immunoprecipitation tests with native hapten polysaccharide, counterimmunoelectrophoresis with cytosolic proteins, and a chaotropic ELISA with Brucella S-LPS were 100% specific but less sensitive than the Rose Bengal test, complement fixation, and indirect ELISA with Brucella S-LPSs and native hapten or S-LPS-derived polysaccharides. A competitive ELISA with Brucella S-LPS and M84 C/Y-specific monoclonal antibody was not 100% specific and was less sensitive than other tests. ELISA with Brucella suis bv. 2 S-LPS (deficient in C epitopes), Escherichia hermannii S-LPSs [lacking the contiguous alpha-(1-2)-linked perosamine residues characteristic of Y. enterocolitica S-LPS], BP26 recombinant protein, and Brucella cytosolic fractions did not provide adequate sensitivity/specificity ratios. Although no serological test and antigen combination fully resolved the diagnosis of bovine brucellosis in the presence of FPSR, some are simple and practical alternatives to the brucellin skin test currently recommended for differential diagnosis.

Development and evaluation as vaccines in mice of Brucella melitensis Rev.1 single and double deletion mutants of the bp26 and omp31 genes coding for antigens of diagnostic significance in ovine brucellosis

The live attenuated Brucella melitensis Rev.1 strain is considered the best vaccine available for the prophylaxis of brucellosis in sheep caused by either B. melitensis or Brucella ovis. However, its application stimulates antibody responses in vaccinated animals indistinguishable by the current conventional serological tests from those observed in infected animals. The periplasmic protein BP26 and the outer membrane protein (OMP) Omp31 are immunodominant antigens in the serological responses of B. melitensis and B. ovis infected sheep, respectively. Accordingly, vaccine strain Rev.1 single and double deletion mutants of the bp26 and omp31 genes were developed, based on the principle that the use of such mutants as vaccines in association with diagnostic tests based on BP26 and Omp31 antigens would allow the serological differentiation between infected and vaccinated animals. The deletion mutants obtained were indistinguishable from the parental Rev.1 strain by conventional bacteriological and typing tests. The expression of their major surface antigens, as determined by reactivity with specific monoclonal antibodies (MAbs), remained unaffected, i.e. smooth-lipopolysaccharide (S-LPS) and OMPs besides in the expression of the antigens whose respective genes were deleted. The bp26 and omp31 deletions did not modify the kinetics of splenic infection nor the residual virulence of Rev.1 in the BALB/c mouse model. Vaccination of BALB/c mice with the deletion mutants conferred significant protective immunity against B. melitensis strain H38 or B. ovis strain PA challenges, to the same extent as that induced by parental Rev.1 strain. Thus, these Rev.1 bp26 or omp31 deletion mutants are promising vaccine candidates against B. melitensis and B. ovis infections and will be further evaluated in sheep.

Lipopolysaccharide heterogeneity in Brucella strains isolated from marine mammals

Smooth lipopolysaccharides (S-LPSs) from Brucella strains isolated from seals, dolphins, porpoises, an otter and a minke whale were characterized by ELISA using monoclonal antibodies (mAbs) directed against seven previously defined O-polysaccharide (O-PS) epitopes and by Western blot after SDS-PAGE. All strains studied were A-dominant as shown by specific polyclonal sera and this was also confirmed by the mAbs. However, binding patterns in ELISA of mAbs to the specific common (C) epitopes were rather heterogeneous, and for some strains, such as those isolated from striped dolphins, binding of these mAbs was much reduced or negative as had previously been shown for Brucella suis biovar 2 strains. Western blot after SDS-PAGE showed the typical A-dominant strain banding pattern for all marine mammal Brucella isolates, but the average S-LPS size was shorter in many of these compared to reference Brucella abortus strain 544. Thus, S-LPSs of the marine mammal isolates show heterogeneity with regard to their O-PS C epitope content and their average size.

Brucella abortus vaccine strain RB51 produces low levels of M-like O-antigen

Brucella abortus RB51 is a rough (R) stable vaccine strain used in cattle and is believed to be devoid of O-side chain. We analyzed by use of a panel of monoclonal antibodies (MAbs) directed against seven previously defined O-polysaccharide (O-PS) epitopes the O-chain expression in strain RB51. Two MAbs specific for the C/Y (A=M) and C (M>A) epitopes showed low bindings in ELISA to strain RB51. O-chain expression was further confirmed by Western blot after SDS-PAGE of strain RB51. In particular, the MAb of C (M>A) specificity, showing preferential binding to M-dominant smooth (S) Brucella strains, revealed in strain RB51 a typical smooth-lipopolysaccharide (S-LPS) pattern which resembled that of M-dominant S-LPS. Thus, the results clearly show that strain RB51 produces low levels of M-like O-antigen.

Genetic organisation of the lipopolysaccharide O-antigen biosynthesis region of brucella melitensis 16M (wbk)

Brucella spp. are Gram-negative, facultative intracellular bacteria that cause a zoonotic world-wide disease. As in other Gram-negative bacteria, its S-LPS (smooth lipopolysaccharide) is a major determinant of virulence. The Brucella melitensis 16M LPS O-antigen is a homopolymer of 4-formamido-4,6, dideoxymannose. In this study, the previously cloned 14-kb wbk gene cluster was sequenced, and seven open reading frames (ORFs) as well as four insertion sequences were identified. Six of the seven ORFs are homologous to LPS biosynthesis genes from other organisms. The gmd, per and wbkC gene products are predicted to be involved in 4-formamido-4,6,dideoxymannose synthesis. By deletion experiments, we demonstrated that the putative formyltransferase WbkC is absolutely required for the O-side-chain production. The wbkA gene product is similar to several mannosyltransferases and is probably involved in the polymerisation of the B. melitensis O-side-chain. We also identified two genes (wzm and wzt) encoding proteins with high similarity to several two-component ABC (ATP-binding cassette) transporters. Their implication in O-antigen translocation across the inner membrane was confirmed by gene replacement. Finally, no function has been assigned to the wbkB gene either by homology search or functionally, because deletion of wbkB did not interfere with the O-antigen structure. The seven ORFs have a low G + C content, indicating that they might have been acquired by lateral transfer from a progenitor with more A + T rich DNA.

Conservation of seven genes involved in the biosynthesis of the lipopolysaccharide O-side chain in Brucella spp

Seven genes of the wb locus of Brucella melitensis 16M involved in the biosynthesis of the lipopolysaccharide O-side chain have been recently identified, i.e. wbkA, gmd, per, wzm, wzt, wbkB, and wbkC, coding, respectively, for proteins homologous to mannosyltransferase, GDP-mannose 4,6 dehydratase, perosamine synthetase, ABC-type transporter (integral membrane protein), ABC-type transporter (ATPase domain), a hypothetical protein of unknown function, and a putative formyl transferase. The seven genes have a G + C content lower (around 48%) than that typical of Brucella spp. (58%) and thus may have been acquired from a species other than Brucella. In the present study, we analyzed by polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) the seven O-chain biosynthetic genes for polymorphism among Brucella spp. PCR-RFLP showed that the seven genes are highly conserved and occur even in the naturally rough species B. ovis and B. canis and also in rough strains of B. abortus and B. melitensis. Nevertheless, the few polymorphisms that were observed consisted of absence of additional restriction sites sometimes allowing differentiation at the species level (e.g. B. ovis) or at the biovar or strain level. There were no apparent deletions or insertions in the PCR-amplified genes in any of the Brucella strains studied. In conclusion, the seven O-chain biosynthetic genes studied appear to be highly conserved among Brucella spp. and thus may have been acquired before species differentiation. Some of the species- or biovar-specific markers detected could be used for molecular typing of brucellae in addition to those previously described.