DNA sequence and expression of the 36-kilodalton outer membrane protein gene of Brucella abortus

Brucellosis detection and the role of Brucella spp. cell wall proteins

Brucellosis remains an endemic zoonotic disease in many developing countries, causing great harm to public health and devastating losses to livestock. One of the main reasons for the low effectiveness of anti-brucellosis measures is the lack of reliable methods for diagnosing infected animals throughout their lifespan. Classical serological tests, such as the tube agglutination test, rose Bengal plate test, and complement fixation test, as well as commercial enzyme-linked immunosorbent assay kits, are based on the detection of antibodies to the cell wall polysaccharide antigens of Brucella spp. smooth strains. As a result, they do not exclude cross-reactions with related bacteria and fail to differentiate between infected and vaccinated animals. Over the past decades, many attempts have been made to identify immunoreactive and pathogen-specific protein antigens. To date, several studies have investigated Brucella spp. recombinant proteins, including cell wall proteins, as the best antigens for diagnosing brucellosis in animals and humans. However, the available results on the specificity and sensitivity of serological tests based on cell wall proteins are ambiguous and sometimes contradictory. This review aims to provide an overview of the current state of knowledge of the diagnostic value of outer membrane and/or periplasmic proteins of Brucella spp. The goal is to identify future developments that may lead to reliable antigens for serological tests.

Omp2b Porin Alteration in the Course of Evolution of Brucella spp

The genus Brucella comprises major pathogenic species causing disease in livestock and humans, e.g. B. melitensis. In the past few years, the genus has been significantly expanded by the discovery of phylogenetically more distant lineages comprising strains from diverse wildlife animal species, including amphibians and fish. The strains represent several potential new species, with B. inopinata as solely named representative. Being genetically more distant between each other, relative to the “classical” Brucella species, they present distinct atypical phenotypes and surface antigens. Among surface protein antigens, the Omp2a and Omp2b porins display the highest diversity in the classical Brucella species. The genes coding for these proteins are closely linked in the Brucella genome and oriented in opposite directions. They share between 85 and 100% sequence identity depending on the Brucella species, biovar, or genotype. Only the omp2b gene copy has been shown to be expressed and genetic variation is extensively generated by gene conversion between the two copies. In this study, we analyzed the omp2 loci of the non-classical Brucella spp. Starting from two distinct ancestral genes, represented by Australian rodent strains and B. inopinata, a stepwise nucleotide reduction was observed in the omp2b gene copy. It consisted of a first reduction affecting the region encoding the surface L5 loop of the porin, previously shown to be critical in sugar permeability, followed by a nucleotide reduction in the surface L8 loop-encoding region. It resulted in a final omp2b gene size shared between two distinct clades of non-classical Brucella spp. (African bullfrog isolates) and the group of classical Brucella species. Further evolution led to complete homogenization of both omp2 gene copies in some Brucella species such as B. vulpis or B. papionis. The stepwise omp2b deletions seemed to be generated through recombination with the respective omp2a gene copy, presenting a conserved size among Brucella spp., and may involve short direct DNA repeats. Successive Omp2b porin alteration correlated with increasing porin permeability in the course of evolution of Brucella spp. They possibly have adapted their porin to survive environmental conditions encountered and to reach their final status as intracellular pathogen.

Evaluation of the efficacy of outer membrane protein 31 vaccine formulations for protection against Brucella canis in BALB/c mice

Canine brucellosis is an infectious disease caused by the Gram-negative bacterium Brucella canis. Unlike conventional control programs for other species of the genus Brucella, currently there is no vaccine available against canine brucellosis, and preventive measures are simply diagnosis and isolation of infected dogs. New approaches are therefore needed to develop an effective and safe immunization strategy against this zoonotic pathogen. In this study, BALB/c mice were subcutaneously immunized with the following: (i) the recombinant Brucella Omp31 antigen formulated in different adjuvants (incomplete Freund adjuvant, aluminum hydroxide, Quil A, and Montanide IMS 3012 VGPR), (ii) plasmid pCIOmp31, or (iii) pCIOmp31 plasmid followed by boosting with recombinant Omp31 (rOmp31). The immune response and the protective efficacy against B. canis infection were characterized. The different strategies induced a strong immunoglobulin G (IgG) response. Furthermore, spleen cells from rOmp31-immunized mice produced gamma interferon and interleukin-4 (IL-4) after in vitro stimulation with rOmp31, indicating the induction of a mixed Th1-Th2 response. Recombinant Omp31 administered with different adjuvants as well as the prime-boost strategy conferred protection against B. canis. In conclusion, our results suggest that Omp31 could be a useful candidate for the development of a subcellular vaccine against B. canis infection.

The Sinorhizobium meliloti essential porin RopA1 is a target for numerous bacteriophages

The symbiotic nitrogen-fixing bacterium Sinorhizobium meliloti harbors a gene, SMc02396, which encodes a predicted outer membrane porin that is conserved in many symbiotic and pathogenic bacteria in the order Rhizobiales. Here, this gene (renamed ropA1) is shown to be required for infection by two commonly utilized transducing bacteriophages (ΦM12 and N3). Mapping of S. meliloti mutations conferring resistance to ΦM12, N3, or both phages simultaneously revealed diverse mutations mapping within the ropA1 open reading frame. Subsequent tests determined that RopA1, lipopolysaccharide, or both are required for infection by all of a larger collection of Sinorhizobium-specific phages. Failed attempts to disrupt or delete ropA1 suggest that this gene is essential for viability. Phylogenetic analysis reveals that ropA1 homologs in many Rhizobiales species are often found as two genetically linked copies and that the intraspecies duplicates are always more closely related to each other than to homologs in other species, suggesting multiple independent duplication events.

Invasive Escherichia coli vaccines expressing Brucella melitensis outer membrane proteins 31 or 16 or periplasmic protein BP26 confer protection in mice challenged with B. melitensis

Because of the serious economic and medical consequences of brucellosis, efforts are to prevent infection of domestic animals through vaccines. Many disadvantages are associated with the current Brucella melitensis Rev.1 vaccine prompting development of alternative vaccines and delivery. Escherichia coli (DH5α) was engineered to express a plasmid containing the inv gene from Yersinia pseudotuberculosis and the hly gene from Listeria monocytogenes. These recombinant invasive E. coli expressing B. melitensis outer membrane proteins (Omp31 or 16) or the periplasmic protein BP26 were evaluated for protection of mice against virulent B. melitensis. Importantly, these invasive E. coli vaccines induced significant protection against B. melitensis challenged mice. Invasive E. coli may be an ideal vaccine platform with natural adjuvant properties for application against B. melitensis since the E. coli delivery system is non-pathogenic and can deliver antigens to antigen-presenting cells promoting cellular immune responses.

A review of Brucella infection in marine mammals, with special emphasis on Brucella pinnipedialis in the hooded seal (Cystophora cristata)

Brucella spp. were isolated from marine mammals for the first time in 1994. Two novel species were later included in the genus; Brucella ceti and Brucella pinnipedialis, with cetaceans and seals as their preferred hosts, respectively. Brucella spp. have since been isolated from a variety of marine mammals. Pathological changes, including lesions of the reproductive organs and associated abortions, have only been registered in cetaceans. The zoonotic potential differs among the marine mammal Brucella strains. Many techniques, both classical typing and molecular microbiology, have been utilised for characterisation of the marine mammal Brucella spp. and the change from the band-based approaches to the sequence-based approaches has greatly increased our knowledge about these strains. Several clusters have been identified within the B. ceti and B. pinnipedialis species, and multiple studies have shown that the hooded seal isolates differ from other pinniped isolates. We describe how different molecular methods have contributed to species identification and differentiation of B. ceti and B. pinnipedialis, with special emphasis on the hooded seal isolates. We further discuss the potential role of B. pinnipedialis for the declining Northwest Atlantic hooded seal population.

Multiple genus-specific markers in PCR assays improve the specificity and sensitivity of diagnosis of brucellosis in field animals

Brucella-specific nucleotide sequences encoding the BCSP 31 kDa protein, Omp2 and the 16S rRNA were employed in three independent diagnostic PCR assays. Results of the three PCR assays on six reference strains of Brucella were in complete agreement. The results of PCR assays based on bcsp and omp2 on 19 Indian field isolates (human, bovine and murine tissues) also agreed completely. However, when the 16S rRNA gene was employed as the diagnostic target in the PCR, only 14 out of these 19 isolates and 2 out of 7 bovine milk isolates were identified as the genus Brucella. The bovine blood samples were insensitive to 16S rRNA PCR. The antibody-detecting ELISA results of field samples (n=87) from a serologically positive herd in India were compared separately with omp2 and bcsp PCRs of blood (n=62). While the bcsp PCR was the most sensitive, the degree of association of ELISA with omp2 blood PCR (kappa=0.37 at P <0.05) was similar to that with the bcsp blood PCR (kappa =0.34 at P <0.05). An improvement in the correlation between ELISA and blood PCR was noticed (kappa =0.5 at P <0.05) when a consensus result of omp2 and bcsp blood PCR was considered for comparison with ELISA. The use of more than one marker-based PCR gave increased sensitivity and higher specificity and appears to be a more reliable molecular diagnostic approach for screening of field animals.

Diagnostic characterization of a feral swine herd enzootically infected with Brucella

Eighty feral swine were trapped from a herd that had been documented to be seropositive for Brucella and which had been used for Brucella abortus RB51 vaccine trials on a 7,100-hectare tract of land in South Carolina. The animals were euthanized and complete necropsies were performed. Samples were taken for histopathology, Brucella culture, and Brucella serology. Brucella was cultured from 62 (77.5%) animals. Brucella suis was isolated from 55 animals (68.8%), and all isolates were biovar 1. Brucella abortus was isolated from 28 animals (35.0%), and isolates included field strain biovar 1 (21 animals; 26.3%), vaccine strain Brucella abortus S19 (8 animals, 10.0%), and vaccine strain Brucella abortus RB51 (6 animals, 7.5%). Males were significantly more likely to be culture positive than females (92.9% vs. 60.6%). Thirty-nine animals (48.8%) were seropositive. Males also had a significantly higher seropositivity rate than females (61.9% vs. 34.2%). The relative sensitivity rates were significantly higher for the standard tube test (44.6%) and fluorescence polarization assay (42.6%) than the card agglutination test (13.1%). Lesions consistent with Brucella infection were commonly found in the animals surveyed and included inflammatory lesions of the lymph nodes, liver, kidney, and male reproductive organs, which ranged from lymphoplasmacytic to pyogranulomatous with necrosis. This is the first report of an apparent enzootic Brucella abortus infection in a feral swine herd suggesting that feral swine may serve as a reservoir of infection for Brucella abortus as well as Brucella suis for domestic livestock.

Parenteral vaccination of domestic pigs withBrucella abortusstrain RB51

OBJECTIVE

To determine the immunogenicity and efficacy of Brucella abortus strain RB51 (SRB51) as a vaccine in domestic pigs.

ANIMALS

Sixty-eight 6-week-old crossbred domestic pigs and twenty-four 4-month-old gilts.

PROCEDURES

In experiment 1, pigs were vaccinated IM (n = 51) with 2 x 10(10) CFUs of SRB51 or sham inoculated (17). Periodic blood samples were obtained to perform blood cultures, serologic evaluations, and cell-mediated immunity assays. Necropsies were performed at selected times between weeks 1 and 23 after vaccination to determine vaccine clearance. In experiment 2, gilts were similarly vaccinated (n = 18) or sham inoculated (8) and similar samples were obtained after vaccination. Gilts were bred and challenged conjunctivally with 5.0 x 10(7) CFUs of virulent Brucella suis strain 3B. Necropsies were performed on gilts and on fetuses or neonates after abortion or parturition, respectively. Bacterial cultures and serologic evaluations were performed on samples obtained at necropsy to determine vaccine efficacy.

RESULTS

Humoral and cell-mediated immune responses did not differ between vaccinates and controls. After vaccination, SRB51 was not isolated from blood cultures of either group and was isolated from lymphoid tissues of 3 pigs at 2 weeks (n = 2) and 4 weeks (1) after vaccination. No differences were found in isolation of B suis or in seroconversion between vaccinated and control gilts and between their neonates or aborted fetuses.

CONCLUSIONS AND CLINICAL RELEVANCE

Parenteral vaccination with SRB51 does not induce humoral or cell-mediated immune responses. Vaccination with SRB51 did not protect gilts or their neonates and fetuses from virulent challenge with B suis.

Identification of brucella species and biotypes using polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP)

Brucellosis is a worldwide zoonosis causing reproductive failures in livestock and a severe multi-organ disease in humans. The genus Brucella is divided into seven species and various biotypes differing in pathogenicity and host specificity. Although Brucella spp. represent a highly homogenous group of bacteria, RFLPs of selected genes display sufficient polymorphism to distinguish Brucella species and biovars. PCR-RFLP analysis shows excellent typeability, reproducibility, stability, and epidemiological concordance. Consequently, PCR-RFLP assays of specific gene loci can serve as tools for diagnostic, epidemiological, taxonomic, and evolutionary studies. Various PCR-RFLPs used for the identification of Brucella species and biotypes are reviewed.

Chimeric structure of omp2 of Brucella from Pacific common minke whales (Balaenoptera acutorostrata)

In the Pacific common minke whale (Balaenoptera acutorostrata ), a new variant of Brucella has been detected using the polymerase chain reaction. Detailed analysis of the porin protein genes, omp2a and omp2b from the whale Brucella showed that these two genes have some motifs in common with Atlantic marine strains in the 5'-terminal one-third region. On the other hand, the nucleotide sequences in the 3'-terminal two-thirds region of the two genes were almost identical to the respective genes of terrestrial strains. Thus, Pacific whale Brucella omp2 genes are chimeras between marine and terrestrial strains.

Cloning, expression, and purification of Brucella suis outer membrane proteins

Brucella, an aerobic, nonsporeforming, nonmotile Gram-negative coccobacillus, is a NIH/CDC category B bioterror threat agent that causes incapacitating human illness. Medical defense against the bioterror threat posed by Brucella would be strengthened by development of a human vaccine and improved diagnostic tests. Central to advancement of these goals is discovery of bacterial constituents that are immunogenic or antigenic for humans. Outer membrane proteins (OMPs) are particularly attractive for this purpose. In this study, we cloned, expressed, and purified seven predicted OMPs of Brucella suis. The recombinant proteins were fused with 6-His and V5 epitope tags at their C termini to facilitate detection and purification. The B. suis surface genes were PCR synthesized based on their ORF sequences and directly cloned into an entry vector. The recombinant entry constructs were propagated in TOP 10 cells, recombined into a destination vector, pET-DEST42, then transformed into Escherichia coli BL21 cells for IPTG-induced protein expression. The expressed recombinant proteins were confirmed with Western blot analysis using anti-6-His antibody conjugated with alkaline phosphatase. These B. suis OMPs were captured and purified using a HisGrab plate. The purified recombinant proteins were examined for their binding activity with antiserum. Serum derived from a rabbit immunized intramuscularly with dialyzed cell lysate of Brucella rough mutant WRR51. The OMPs were screened using the rabbit antiserum and purified IgG. The results suggested that recombinant B. suis OMPs were successfully cloned, expressed and purified. Some of the expressed OMPs showed high binding activity with immunized rabbit antiserum.

Control of small ruminant brucellosis by use of Brucella melitensis Rev.1 vaccine: laboratory aspects and field observations

Brucellosis vaccines are essential elements in control programs. Since first developed in the mid-1950s, the Brucella melitensis vaccine strain Rev.1 has been used worldwide and its significant value in protecting sheep and goats in endemic areas recognized. This review provides historical background on the development of the vaccine, its use and field complications arising in Israel following changes in the strain's pathogenicity. The urgent need for resolving cases of vaccine strain excretion in the milk, horizontal transfer and a unique case of human infection has led to identification of an atypical B. melitensis biovar 1 strain that resembles strain Rev.1 in susceptibility to penicillin and dyes. An omp2 based PCR method has been developed that traced the lineage of Israeli B. melitensis biovar 1 strains. This locus serves as an epidemiological tag for the Rev.1 vaccine strain. Despite the rapid development of new approaches in the field of vaccination, it is anticipated that in the near future the Rev.1 vaccine would remain the only accepted vaccine in national control programs.

Major outer membrane proteins of Brucella spp.: past, present and future

The major outer membrane proteins (OMPs) of Brucella spp. were initially identified in the early 1980s and characterised as potential immunogenic and protective antigens. They were classified according to their apparent molecular mass as 36-38 kDa OMPs or group 2 porin proteins and 31-34 and 25-27 kDa OMPs which belong to the group 3 proteins. The genes encoding the group 2 porin proteins were identified in the late 1980s and consist of two genes, omp2a and omp2b, which are closely linked in the Brucella genome, and which share a great degree of identity (>85%). In the 1990s, two genes were identified coding for the group 3 proteins and were named omp25 and omp31. The predicted amino acid sequences of omp25 and omp31 share 34% identity. The recent release of the genome sequence of B. melitensis 16 M has revealed the presence of five additional gene products homologous to Omp25 and Omp31. The use of recombinant protein technology and monoclonal antibodies (MAbs) has shown that the major OMPs appear to be of little relevance as antigens in smooth (S) B. abortus or B. melitensis infections i.e. low or no protective activity in the mouse model of infection and low or no immunogenicity during host infection. However, group 3 proteins, in particular Omp31, appear as immunodominant antigen in the course of rough (R) B. ovis infection in rams and as important protective antigen in the B. ovis mouse model of infection. The major OMP genes display diversity and specific markers have been identified for Brucella species, biovars, and strains, including the recent marine mammal Brucella isolates for which new species names have been proposed. Recently, Omp25 has been shown to be involved in virulence of B. melitensis, B. abortus and B. ovis. Mutants lacking Omp25 are indeed attenuated in animal models of infection, and moreover provide levels of protection similar or better than currently used attenuated vaccine strain B. melitensis Rev.1. Therefore, these mutant strains appear interesting vaccine candidates for the future. The other group 3 proteins identified in the genome merit also further investigation related to the development of new vaccines.

PCR as a diagnostic tool for brucellosis

Numerous PCR-based assays have been developed for the identification of Brucella to improve diagnostic capabilities. Collectively, the repertoire of assays addresses several aspects of the diagnostic process. For some purposes, the simple identification of Brucella is adequate (e.g. diagnosis of human brucellosis or contamination of food products). In these cases, a genus-specific PCR assay is sufficient. Genus-specific assays tend to be simple, robust, and somewhat permissive of environmental influences. The main genetic targets utilized for these applications are the Brucella BCSP31 gene and the 16S-23S rRNA operon. Other instances require identification of the Brucella species involved. For example, most government-sponsored brucellosis eradication programs include regulations that stipulate a species-specific response. For epidemiological trace back, strain-specific identification is helpful. Typically, differential PCR-based assays tend to be more complex and consequently more difficult to perform. Several strategies have been explored to differentiate among Brucella species and strains, including locus specific multiplexing (e.g. AMOS-PCR based on IS711), PCR-RFLP (e.g. the omp2 locus), arbitrary-primed PCR, and ERIC-PCR to name a few. This paper reviews some of the major advancements in molecular diagnostics for Brucella including the development of procedures designed for the direct analysis of a variety of clinical samples. While the progress to date is impressive, there is still room for improvement.

Effect of omp10 or omp19 deletion on Brucella abortus outer membrane properties and virulence in mice

The distinctive properties of Brucella outer membrane have been considered to be critical for Brucella sp. virulence. Among the outer membrane molecules possibly related to these properties, Omp10 and Omp19 are immunoreactive outer membrane lipoproteins. Moreover, these proteins of Brucella could constitute a new family of outer membrane proteins specifically encountered in the family RHIZOBIACEAE: We evaluated the impact of omp10 or omp19 deletion on Brucella abortus outer membrane properties and virulence in mice. The omp10 mutant was dramatically attenuated for survival in mice and was defective for growth in minimal medium but was not impaired in intracellular growth in vitro, nor does it display clear modification of the outer membrane properties. Significantly fewer brucellae were recovered from the spleens of mice infected with the omp19 mutant than from those of mice infected with the parent strain at 4 and 8 weeks postinfection. The omp19 mutant exhibited an increase in sensitivity to the polycation polymyxin B and to sodium deoxycholate. These results indicate that inactivation of the omp19 gene alters the outer membrane properties of B. abortus.

Identification of the Brucella melitensis vaccine strain Rev.1 in animals and humans in Israel by PCR analysis of the PstI site polymorphism of its omp2 gene

Adverse effects of strain persistence and secretion in milk have been encountered with the Brucella melitensis vaccine strain Rev.1. Field isolates obtained from vaccinated animals and from a human resembled the vaccine strain Rev.1 by conventional bacteriological tests. The lack of a specific molecular marker that could specifically characterize the commercial vaccine strain prevented confirmation of the homology of the Rev.1-like field isolates to the vaccine strain. The composition of the omp2 locus from two gene copies with differences in their PstI restriction endonuclease sites was used to establish an epidemiologic fingerprint for the omp2 gene in the Rev.1 vaccine strain. Primers designed to amplify DNA sequences that overlap the PstI site revealed a single 282-bp DNA band common to all Brucella spp. Agarose gel electrophoresis of the PstI digests of the PCR products from strains 16M and the vaccine strain Rev.1 revealed a distinctive profile that included three bands: one band for the intact 282-bp fragment amplified from omp2a and two bands resulting from the digestion of the amplified omp2b gene fragment, 238- and 44-bp DNA fragments, respectively. Amplified fragments of 37 Rev.1-like isolates, including 2 human isolates, also exhibited this pattern. In contrast, DNA digests of all other Israeli field isolates, including atypical B. melitensis biotype 1 and representatives of the biotype 2 and 3 isolates, produced two bands of 238 and 44 bp, respectively, corresponding with the digestion of both omp2a and omp2b genes. This method facilitates identification of the Rev.1 vaccine strain in both animals and humans in Israel.

Identification of the Brucella melitensis vaccine strain Rev.1 in animals and humans in Israel by PCR analysis of the PstI site polymorphism of its omp2 gene

Adverse effects of strain persistence and secretion in milk have been encountered with the Brucella melitensis vaccine strain Rev.1. Field isolates obtained from vaccinated animals and from a human resembled the vaccine strain Rev.1 by conventional bacteriological tests. The lack of a specific molecular marker that could specifically characterize the commercial vaccine strain prevented confirmation of the homology of the Rev.1-like field isolates to the vaccine strain. The composition of the omp2 locus from two gene copies with differences in their PstI restriction endonuclease sites was used to establish an epidemiologic fingerprint for the omp2 gene in the Rev.1 vaccine strain. Primers designed to amplify DNA sequences that overlap the PstI site revealed a single 282-bp DNA band common to all Brucella spp. Agarose gel electrophoresis of the PstI digests of the PCR products from strains 16M and the vaccine strain Rev.1 revealed a distinctive profile that included three bands: one band for the intact 282-bp fragment amplified from omp2a and two bands resulting from the digestion of the amplified omp2b gene fragment, 238- and 44-bp DNA fragments, respectively. Amplified fragments of 37 Rev.1-like isolates, including 2 human isolates, also exhibited this pattern. In contrast, DNA digests of all other Israeli field isolates, including atypical B. melitensis biotype 1 and representatives of the biotype 2 and 3 isolates, produced two bands of 238 and 44 bp, respectively, corresponding with the digestion of both omp2a and omp2b genes. This method facilitates identification of the Rev.1 vaccine strain in both animals and humans in Israel.

Characterization of a Brucella species 25-kilobase DNA fragment deleted from Brucella abortus reveals a large gene cluster related to the synthesis of a polysaccharide

In the present study we completed the nucleotide sequence of a Brucella melitensis 16M DNA fragment deleted from B. abortus that accounts for 25,064 bp and show that the other Brucella spp. contain the entire 25-kb DNA fragment. Two short direct repeats of four nucleotides, detected in the B. melitensis 16M DNA flanking both sides of the fragment deleted from B. abortus, might have been involved in the deletion formation by a strand slippage mechanism during replication. In addition to omp31, coding for an immunogenic protein located in the Brucella outer membrane, 22 hypothetical genes were identified. Most of the proteins that would be encoded by these genes show significant homology with proteins involved in the biosynthesis of polysaccharides from other bacteria, suggesting that they might be involved in the synthesis of a Brucella polysaccharide that would be a heteropolymer synthesized by a Wzy-dependent pathway. This polysaccharide would not be synthesized in B. abortus and would be a polysaccharide not identified until present in the genus Brucella, since all of the known polysaccharides are synthesized in all smooth Brucella species. Discovery of a novel polysaccharide not synthesized in B. abortus might be interesting for a better understanding of the pathogenicity and host preference differences observed between the Brucella species. However, the possibility that the genes detected in the DNA fragment deleted in B. abortus no longer lead to the synthesis of a polysaccharide must not be excluded. They might be a remnant of the common ancestor of the alpha-2 subdivision of the class Proteobacteria, with some of its members synthesizing extracellular polysaccharides and, as Brucella spp., living in association with eukaryotic cells.

Effects of exogenous recombinant interleukin-12 on immune responses and protection against Brucella abortus in a murine model

This study determined if murine interleukin-12 (IL-12) would influence immunity in mice vaccinated with live or killed Brucella abortus strain RB51 (SRB51). Mice received live or gamma-irradiated SRB51 bacteria alone, or with IL-12 (0.5 or 1.0 microg, 2x or 3x), whereas other mice received saline or IL-12 alone. Post-vaccination antibody responses to live or killed SRB51 and clearance of live SRB51 from splenic tissue were not influenced by IL-12 treatments. Mice were challenged at 12 weeks with 4 x 10(4) cfu of B. abortus strain 2308 (S2308) and were euthanized 2 weeks later. The highest IL-12 treatment increased (P < 0.05) post-challenge antibody responses when co-administered with killed SRB51. Co-administration of 1.0 microg of IL-12 with live SRB51, but not killed SRB51, reduced (P < 0.05) S2308 colonization of splenic tissues. Our data suggest that although IL-12 may augment protective immunity induced by live SRB51, it does not influence protection induced by vaccination with killed SRB51.

Molecular, antigenic, and functional analyses of Omp2b porin size variants of Brucella spp

Omp2a and Omp2b are highly homologous porins present in the outer membrane of the bacteria from the genus Brucella, a facultative intracellular pathogen. The genes coding for these proteins are closely linked in the Brucella genome and oriented in opposite directions. In this work, we present the cloning, purification, and characterization of four Omp2b size variants found in various Brucella species, and we compare their antigenic and functional properties to the Omp2a and Omp2b porins of Brucella melitensis reference strain 16M. The variation of the Omp2a and Omp2b porin sequences among the various strains of the genus Brucella seems to result mostly from multiple gene conversions between the two highly homologous genes. As shown in this study, this phenomenon has led to the creation of natural Omp2a and Omp2b chimeric proteins in Omp2b porin size variants. The comparison by liposome swelling assay of the porins sugar permeability suggested a possible functional differences between Omp2a and Omp2b, with Omp2a showing a more efficient pore in sugar diffusion. The sequence variability in the Omp2b size variants was located in the predicted external loops of the porin. Several epitopes recognized by anti-Omp2b monoclonal antibodies were mapped by comparison of the Omp2b size variants antigenicity, and two of them were located in the most exposed surface loops. However, since variations are mostly driven by simple exchanges of conserved motifs between the two genes (except for an Omp2b version from an atypical strain of Brucella suis biovar 3), the porin variability does not result in major antigenic variability of the Brucella surface that could help the bacteria during the reinfection of a host. Porin variation in Brucella seems to result mainly in porin conductivity modifications.

Classification of Brucella spp. isolated from marine mammals by DNA polymorphism at the omp2 locus

A number of recent reports have described the isolation and characterization of Brucella strains from a wide variety of marine mammals such as seals, porpoises, dolphins and a minke whale. These strains were identified as brucellae by conventional typing tests. However, their overall characteristics were not assimilable to those of any of the six currently recognized Brucella species and it was suggested that they comprise a new nomen species to be called Brucella maris. In the present study we analysed DNA polymorphism at the omp2 locus of 33 marine mammal Brucella strains isolated from seals, dolphins, porpoises and an otter. The omp2 locus contains two gene copies (named omp2a and omp2b) coding for porin proteins and has been found particularly useful for molecular typing and identification of Brucella at the species, biovar, or strain level. PCR-restriction fragment length polymorphism (RFLP) and DNA sequencing showed that strains isolated from dolphins and porpoises carry two omp2b gene copies instead of one omp2a and one omp2b gene copy or two similar omp2a gene copies reported in the currently recognized species. This observation was also recently made for a minke whale Brucella isolate. The otter and all seal isolates except one were shown to carry one omp2a and one omp2b gene copy as encountered in isolates from terrestrial mammals. By PCR-RFLP of the omp2b gene, a specific marker was detected grouping the marine mammal Brucella isolates. Although marine mammal Brucella isolates may represent a separate group from terrestrial mammal isolates based on omp2b sequence constructed phylogenetic trees, the divergence found between their omp2b and also between their omp2a nucleotide sequences indicates that they form a more heterogeneous group than isolates from terrestrial mammals. Therefore, grouping the marine mammal Brucella isolates into one species Brucella maris seems inappropriate unless the currently recognized Brucella species are grouped. With respect to the current classification of brucellae according to the preferential host, brucellae isolated from such diverse marine mammal species as seals and dolphins could actually comprise more than one species, and at least two new species, B. pinnipediae and B. cetaceae, could be compatible with the classical criteria of host preferentialism and DNA polymorphism at their omp2 locus.

Interruption of the cydB locus in Brucella abortus attenuates intracellular survival and virulence in the mouse model of infection

Brucellosis is characterized by abortion in ruminants and a protracted undulant fever in humans, which often results in severe pathological manifestations. Scant information exists about the molecular mechanisms employed by Brucella abortus to combat host defenses or to persist and replicate within host cells. Transposon (Tn5) mutagenesis of B. abortus and the subsequent screening of mutants for sensitivity to killing in murine macrophages and in the mouse model led to the identification of mutants which were severely attenuated for intracellular survival. One group of mutants was interrupted in cydB, a gene that is part of the cydAB operon encoding cytochrome bd oxidase, which catalyzes an alternate terminal electron transport step in bacterial respiration. The elevated affinity for molecular oxygen of this enzyme in Escherichia coli has suggested that it is involved in the protection of sensitive enzymatic activities such as those of hydrogenases and nitrogenases from damage. B. abortus cydB::Tn5 strains exhibited heightened sensitivity to the respiratory inhibitors zinc and azide, highly reactive oxygen species such as hydrogen peroxide, low pH, and attenuated virulence in the mouse model of infection. Virulence was restored by an intact copy of cydAB or by B. abortus genes encoding the oxidative radical-scavenging enzyme Cu/Zn superoxide dismutase or catalase. These results suggest a bifunctional role for the products of the cydAB operon, both in preventing the buildup of oxidative free radicals and in detoxifying the intracellular compartment, thus indicating the importance of these products in preventing intracellular destruction. Intracellular conditions that favor expression of the cydAB operon are under investigation and may be linked to the acid sensitivity also observed in this strain.

Isolation, sequencing and expression of Bartonella henselae omp43 and predicted membrane topology of the deduced protein

The infection of and interaction of human endothelial cells with Bartonella henselae is one of the most interesting aspects of Bartonella -associated disease. The gene encoding the 43 kDa B. henselae outer membrane protein (Omp43) that binds endothelial cells was cloned and sequenced. Sequence analysis revealed an open reading frame of 1206 nucleotides coding for a protein of 402 amino acids. Analysis of the deduced amino acid sequence shows 38% identity over the entire sequence to the Brucella spp. In addition to this Omp2b porin also shows a signal sequence and peptidase cleavage site. Cleavage of the signal peptide results in a mature 380 amino acid polypeptide with a predicted molecular weight of 42 kDa. Omp43 was expressed in Escherichia coli as a fusion protein. Purified recombinant Omp43 at concentrations of 11 and 2.75 microg/ml bound to intact human umbilical vein endothelial cells. Membrane topology analysis predicts that Omp43 exists as a 16 stranded beta barrel protein, similar to that predicted for the Omp2b Brucella abortus porin. Characterization and expression of the gene encoding Omp43 should provide a tool for further investigation of the role of adherence to endothelial cells in the pathogenesis of B. henselae.

DNA polymorphism in the genus Brucella

The genus Brucella has been described as consisting of six species, three of them including several biovars, which display a high degree of DNA homology by DNA-DNA hybridization. However, DNA polymorphism able to differentiate the six Brucella species and some of their biovars has been shown to exist. This work reviews the DNA variability in the genus Brucella and discusses the relationships between its members according to this genetic variability and a proposal for their evolution based on genetic diversity of the omp2 locus.

Topology prediction of Brucella abortus Omp2b and Omp2a porins after critical assessment of transmembrane beta strands prediction by several secondary structure prediction methods

In order to propose a reliable model for Brucella porin topology, several structure prediction methods were evaluated in their ability to predict porin topology. Four porins of known structure were selected as test-cases and their secondary structure delineated. The specificity and sensitivity of 11 methods were separately evaluated. Our critical assessment shows that some secondary structure prediction methods (PHD, Dsc, Sopma) originally designed to predict globular protein structure are useful on porin topology prediction. The overall best prediction is obtained by combining these three "generalist" methods with a transmembrane beta strand prediction technique. This "consensus" method was applied to Brucella porins Omp2b and Omp2a, sharing no sequence homology with any other porin. The predicted topology is a 16-stranded antiparallel beta barrel with Omp2a showing a higher number of negatively charged residue in the exposed loops than Omp2b. Experiments are in progress to validate the proposed topology and the functional hypotheses. The ability of the proposed consensus method to predict topology of complex outer membrane protein is briefly discussed.

Outer membrane proteins Omp10, Omp16, and Omp19 of Brucella spp. are lipoproteins

The deduced sequences of the Omp10, Omp16, and Omp19 outer membrane proteins of Brucella spp. contain a potential bacterial lipoprotein processing sequence. After extraction with Triton X-114, these three proteins partitioned into the detergent phase. Processing of the three proteins is inhibited by globomycin, a specific inhibitor of lipoprotein signal peptidase. The three proteins were radioimmunoprecipitated from [(3)H]palmitic acid-labeled Brucella abortus lysates with monoclonal antibodies. These results demonstrate that Omp10, Omp16, and Omp19 are lipoproteins.

Molecular characterization of a Brucella species large DNA fragment deleted in Brucella abortus strains: evidence for a locus involved in the synthesis of a polysaccharide

A Brucella melitensis 16M DNA fragment of 17,119 bp, which contains a large region deleted in B. abortus strains and DNA flanking one side of the deletion, has been characterized. In addition to the previously identified omp31 gene, 14 hypothetical genes have been identified in the B. melitensis fragment, most of them showing homology to genes involved in the synthesis of a polysaccharide. Considering that 10 of the 15 genes are missing in B. abortus and that all the polysaccharides described in the Brucella genus (lipopolysaccharide, native hapten, and polysaccharide B) have been detected in all the species, it seems likely that the genes described here might be part of a cluster for the synthesis of a novel Brucella polysaccharide. Several polysaccharides have been identified as important virulence factors, and the discovery of a novel polysaccharide in the brucellae which is probably not synthesized in B. abortus might be interesting for a better understanding of the pathogenicity and host preference differences observed between the Brucella species. However, the possibility that the genes described in this paper no longer encode the synthesis of a polysaccharide cannot be excluded. Brucellae belong to the alpha-2 subdivision of the class Proteobacteria, which includes other microorganisms living in association with eucaryotic cells, some of them synthesizing extracellular polysaccharides involved in the interaction with the host cell. The genes described in this paper might be a remnant of the common ancestor of the alpha-2 subdivision of the class Proteobacteria, and the brucellae might have lost such extracellular polysaccharide during evolution if it was not necessary for survival or for establishment of the infectious process. Nevertheless, further studies are necessary to identify the entire DNA fragment missing in B. abortus strains and to elucidate the mechanism responsible for such deletion, since only 9,948 bp of the deletion was present in the sequenced B. melitensis DNA fragment.

Phenotypic and molecular characterization of a Brucella strain isolated from a minke whale (Balaenoptera acutorostrata)

Isolation of Brucella spp. in marine mammals has been reported during the past several years. A Brucella strain from the spleen and liver of a minke whale (Balaenoptera acutorostrata) was isolated. Conventional typing methods indicated that this isolate was related to the genus Brucella but did not match the profiles of any known Brucella species or biovar. Successful PCR amplification of the Brucella rrs-rrl spacer sequence and of the insertion sequence IS6501 also indicated that the minke whale strain was related to the genus Brucella. In addition, the rrs gene of this strain shared a very high degree of nucleotide identity (>98%) with published Brucella spp. rrs sequences. However, RFLP studies using an IS6501-specific probe showed a unique profile for this strain in comparison with the profiles of the six known Brucella species. Moreover, analysis of the omp2 locus by PCR-RFLP, by Southern hybridization using omp2a- and omp2b-specific probes, and by DNA sequencing showed that the minke whale isolate possesses two copies of the omp2b gene instead of one omp2a and one omp2b gene copy or two copies of the omp2a gene described in the six known Brucella species. Thus, molecular typing methods showed that this isolate is clearly distinct from all other known Brucella species and strains. The specific molecular features of this minke whale Brucella isolate raise questions about the lineage between the Brucella strains isolated from marine mammals and the Brucella species isolated from terrestrial mammals.

Detection and differentiation of the six Brucella species by polymerase chain reaction

BACKGROUND

Brucelosis is a severe acute febrile disease caused by bacteria of the genus Brucella. Its current diagnosis is based on clinical observations that may be complemented by serology and microbiological culture tests; however, the former is limited in sensitivity and specificity, the latter is time consuming. To improve brucelosis diagnosis we developed a test which is specific and sensitive and is capable of differentiating the six species of Brucella.

MATERIALS AND METHODS

Four primers were designed from B. abortus sequences at the well-conserved Omp2 locus that are able to amplify the DNAs of all six species of Brucella.

RESULTS

Our test detected all six species of Brucella. Their differentiation resulted directly from differences in the amplification patterns or was achieved indirectly using a RFLP present in one of the PCR products. The sensitivity and specificity of the new test were then determined; it was applied successfully in confirming the diagnosis of a patient whose clinical history and serology indicated infection with Brucella.

CONCLUSIONS

The results make possible the use of a PCR test for Brucella detection and differentiation without relying on the measurement of the antibodies or microorganism culture. Our first results showed that the PCR test can confirm the presence of Brucella in blood samples of infected patients.

Brucella Omp2a and Omp2b porins: single channel measurements and topology prediction

Brucella usually carry two highly homologous genes (omp2a and omp2b) for porin-like proteins. In several B. abortus biovars the omp2a gene has a large deletion compared to other Brucella omp2's. In this study we have measured Omp2 pore activity in planar bilayers. Omp2b exhibits well-defined trimeric channel activity whilst Omp2a forms monomeric pores of variable size which are smaller than Omp2b. No sequence homology exists between Omp2 and porins of known structure, so hydrophobic moment analysis has been used to model their membrane topology. From this it appears likely that the deletion removes the crucial L3 internal loop.

Humoral immune responses of Brucella-infected cattle, sheep, and goats to eight purified recombinant Brucella proteins in an indirect enzyme-linked immunosorbent assay

Brucellosis research is currently focused on the identification of nonlipopolysaccharide (LPS) antigens which could potentially be useful for the specific serologic diagnosis of brucellosis as well as for vaccinal prophylaxis. On the basis of previous reports, we selected eight Brucella proteins (OMP36, OMP25, OMP19, OMP16, OMP10, p17, p15, and p39) as candidate antigens to be further evaluated. The genes encoding these proteins were cloned, sequenced, and overexpressed in Escherichia coli. The recombinant proteins were purified with a polyhistidine tag and metal chelate affinity chromatography and evaluated in an indirect enzyme-linked immunosorbent assay (iELISA). The specificity of the iELISA was determined with sera from healthy cattle, sheep, and goats and ranged from 95 to 99%, depending on the recombinant antigen and the species tested. Sera from experimentally infected, and from naturally infected, animals were used to evaluate the sensitivity of the iELISA. The antiprotein antibody response was often delayed when compared to the anti-smooth LPS (S-LPS) response and was limited to animals which developed an active brucellosis infection (experimentally infected pregnant animals and sheep and goats from areas where brucellosis is still endemic). Among the recombinant antigens, the three cytoplasmic proteins (p17, p15, and p39) gave the most useful results. More than 80% of the animals positive in S-LPS serology were also positive with one of these cytoplasmic proteins alone or a combination of two of them. None of the recombinant antigens detected experimentally infected nonpregnant cows and sheep or naturally infected cattle. This study is a first step towards the development of a multiprotein diagnostic reagent for brucellosis.

DNA polymorphism at the omp-31 locus of Brucella spp.: evidence for a large deletion in Brucella abortus, and other species-specific markers

The omp-31 gene, encoding a major outer-membrane protein in Brucella melitensis, was PCR-amplified from Brucella strains representing all species and known biovars by using primers selected according to the B. melitensis 16M omp-31 published sequence. Amplification of omp-31 was achieved from DNA of all Brucella species with the exception of Brucella abortus, the only Brucella species where expression of omp-31 was not detected by reactivity with an mAb specific for an epitope located in Omp-31. Southern blot hybridization of plasmid probes, bearing inserts (4.4-17 kb) containing B. melitensis 16M omp-31 and adjacent DNA of different sizes, with HindIII-digested total DNA showed that a large fragment, comprising the entire omp-31 gene and flanking DNA, was actually absent in B. abortus strains. The size of this DNA fragment has been determined to be about 10 kb. Southern blot hybridization with the different plasmid probes identified species-specific markers for B. abortus and B. melitensis. At the biovar level, a specific marker for B. melitensis bv. 1 was also identified. Additionally, PCR-RFLP studies of omp-31 revealed specific markers for Brucella ovis, Brucella canis and Brucella suis bv. 2. Using a combination of omp-31 PCR-RFLP patterns and Southern blot hybridization profiles Brucella species were differentiated with the sole exception of Brucella neotomae which was not differentiated from B. suis bv. 1, 3, 4 and 5. Results presented in this paper demonstrate the potential of omp-31 for differentiating the brucellae and show that B. abortus lacks a large DNA fragment of about 10 kb containing omp-31 and flanking DNA. In such a large deletion, other genes in addition to omp-31 are probably involved. Sequencing of this DNA fragment will help to identify the missing genes in B. abortus which could possibly be involved in the differences of pathogenicity and host preference seen in Brucella species.

Characterization, occurrence, and molecular cloning of a 39-kilodalton Brucella abortus cytoplasmic protein immunodominant in cattle

Monoclonal antibodies and polyclonal antisera recognizing a 39-kDa protein (P39) of brucellin, a cytoplasmic extract from Brucella melitensis rough strain B115, were produced. The P39 was purified by anion-exchange chromatography. Eleven of fourteen Brucella-infected cows whose infections had been detected by the delayed-type hypersensitivity (DTH) test with brucellergen also developed a DTH reaction when purified P39 was used as the trigger. The T-cell proliferative responses to P39 of peripheral blood lymphocytes from Brucella-infected cows were also positive. None of the animals infected with other bacterial species that are presumed to induce immunological cross-reactions with Brucella spp. reacted to P39, either in DTH tests or in lymphocyte proliferation assays. A lambda gt11 genomic library of Brucella abortus was screened with a monoclonal antibody specific for P39, and the gene coding for this protein was subsequently isolated. The nucleotide sequence of the P39 gene was determined, and the deduced amino acid sequence is in accordance with the sequence of an internal peptide isolated from P39.

Brucellosis: an overview

Brucellosis remains a major zoonosis worldwide. Although many countries have eradicated Brucella abortus from cattle, in some areas Brucella melitensis has emerged as a cause of infection in this species as well as in sheep and goats. Despite vaccination campaigns with the Rev 1 strain, B. melitensis remains the principal cause of human brucellosis. Brucella suis is also emerging as an agent of infection in cattle, thus extending its opportunities to infect humans. The recent isolation of distinctive strains of Brucella from marine mammals has extended its ecologic range. Molecular genetic studies have demonstrated phylogenetic affiliation to Agrobacterium, Phyllobacterium, Ochrobactrum, and Rhizobium. Polymerase chain reaction and gene probe development may provide more effective typing methods. Pathogenicity is related to production of lipopolysaccharides containing a poly N-formyl perosamine O chain, CuZn superoxide dismutase, erythrlose phosphate dehydrogenase, stress-induced proteins related to intracellular survival, and adenine and guanine monophosphate inhibitors of phagocyte functions. Protective immunity is conferred by antibody to lipopolysaccharide and T-cell-mediated macrophage activation triggered by protein antigens. Diagnosis still centers on isolation of the organism and serologic test results, especially enzyme immunoassay, which is replacing other methods. Polymerase chain reaction is also under evaluation. Therapy is based on tetracyclines with or without rifampicin, aminoglycosides, or quinolones. No satisfactory vaccines against human brucellosis are available, although attenuated purE mutants appear promising.

Molecular and immunological characterization of the major outer membrane proteins of Brucella

The major outer membrane proteins (OMPs) of Brucella spp. were initially identified in the early 1980s by selective extraction techniques and classified according to their apparent molecular mass as 36-38 kDa OMPs or group 2 porin proteins and 31-34 kDa and 25-27 kDa OMPs which belong to the group 3 proteins. Variation in apparent molecular mass is essentially due to association with peptidoglycan subunits of different sizes. Two genes, omp2a and omp2b, which are closely linked in the Brucella genome, and which share a great degree of homology (> 85%), encode the 36 kDa porin proteins, now named Omp2a and Omp2b proteins respectively. Two genes code for the group 3 OMPs and are named omp25 and omp31. The predicted amino acid sequences of omp25 and omp31 share 34% identity. Furthermore, all Brucella major OMPs share amino acid sequence homology with the major OMPs RopA or RopB of Rhizobium leguminosarum, which supports the close genetic relationship of brucellae with members of the alpha-2 subdivision of the class Proteobacteria. Another characteristic common to the major OMPs of R. leguminosarum and Brucella is that they are tightly, probably covalently, associated with the peptidoglycan. The major OMP genes display diversity among Brucella species, biovars and strains allowing their differentiation, and the polymorphic markers identified have brought new insights into the evolutionary development of the genus Brucella, antigenic variability of brucellae, and future prospects in the field of vaccine development.

T lymphocyte mediated protection against facultative intracellular bacteria

Acquired immunity against intracellular bacteria is T cell dependent. T cells play a major role in protection against intracellular bacteria, but bacterial antigens recognized by T cells have been studied less extensively than bacterial antigens recognized by B cells. Using T lymphocytes from animals immunized against Brucella abortus, we have screened a bacterial genomic library for genes encoding antigens recognized by T cells. Lymphocytes that proliferated to B. abortus proteins were characterized for phenotype and cytokine activity. Bovine and murine lymphocytes recognized common bacterial antigens and possessed similar cytokine profiles, suggesting an analogous immune response in these two animal species. In vivo protection afforded by a particular cell type is dependent on the bacterial antigens presented and mechanisms of antigen presentation. MHC class I and class II gene knockout animals infected with B. abortus have demonstrated that protection to B. abortus is especially dependent on CD8+ T cells. Knowing the cells required for protection, vaccines can be designed to elicit the protective subset of lymphocytes. Currently, we are testing several recombinant B. abortus proteins using different immunization strategies. Finally, bacterial genes activated following intracellular phagocytosis are being examined using a novel, reporter system adapted to B. abortus.

Immunological response to the Brucella abortus GroEL homolog

Western blot (immunoblot) analysis of sera from cattle vaccinated with Brucella abortus S19 exhibit an elevated serologic response to Hsp62, the GroEL homolog (BaGroEL). Serologic screening of individual cows vaccinated with B. abortus S19 revealed no correlation between the immune response to BaGroEL and protection against a challenge with virulent organisms. The humoral immune response to BaGroEL was restricted to a region of the mature protein which mapped to amino acids 317 to 355 and may represent a useful diagnostic tool for monitoring exposure to B. abortus. Immunity to a challenge with virulent B. abortus S2308 was not observed in the BaGroEL vaccinated mouse model.

Cloning, nucleotide sequence, and expression of the Brucella melitensis omp31 gene coding for an immunogenic major outer membrane protein

The gene coding for the major outer membrane protein (OMP) of 31 to 34 kDa, now designated Omp31, of Brucella melitensis 16M was cloned and sequenced. A B. melitensis 16M genomic library was constructed in lambda GEM-12 XhoI half-site arms, and recombinant phages expressing omp31 were identified by using the anti-Omp31 monoclonal antibody (MAb) A59/10F09/G10. Subcloning of insert DNA from a positive phage into pGEM-7Zf allowed the selection of a plasmid bearing a 4.4-kb EcoRI fragment that seemed to contain the entire omp31 gene under control of its own promoter. omp31 was localized within a region of the EcoRI insert of approximately 1.1 kb. Sequencing of this region revealed an open reading frame of 720 bp encoding a protein of 240 amino acids and a predicted molecular mass of 25,307 Da. Cleavage of the first 19 amino acids, showing typical features of signal peptides for protein export, leaves a mature protein of 221 amino acids with a predicted molecular mass of 23,412 Da. The predicted amino acid sequence of B. melitensis 16M Omp31 showed 35.2% identity with the RopB OMP of Rhizobium leguminosarum bv. viciae 248 and 34.3% identity with Omp25 of B. abortus 544. As in Brucella spp., Omp31 was located in the outer membrane of recombinant Escherichia coli, but its reported peptidoglycan association in Brucella cells was not detected in E. coli. The ability of Omp31 to form oligomers resistant to sodium dodecyl sulfate denaturation at low temperatures, a characteristic described for several bacterial porins, was observed in both B. melitensis and recombinant E. coli. The epitope recognized by the anti-Omp31 MAb A59/10F09/G10, for which a protective activity has been suggested, has been delimited to a region of 36 amino acids of Omp31 covering the most hydrophilic part of the protein. The availability of recombinant Omp31 and the identification of the antigenic determinant recognized by MAb A59/10F09/G10 will allow the evaluation of their potential protective activity and their potential for the development of subcellular vaccines against brucellosis.

Cloning of a Brucella melitensis group 3 antigen gene encoding Omp28, a protein recognized by the humoral immune response during human brucellosis

Brucella group 3 antigens (Ags) are outer membrane proteins (OMPs) with a molecular mass ranging from 25 to 30 kDa. The OMPs are of interest partially because of their potential use as vaccine and diagnostic reagents. We used human convalescent antibody (Ab) to clone a gene that encoded a 28-kDa protein from a lambdagt11 library of Brucella melitensis 16M genomic DNA. DNA sequence analysis revealed a single open reading frame that would encode a protein of 26,552 Da. The 28-kDa protein had a primary amino acid sequence that was 43% similar to a previously described Brucella abortus group 3 Ag, Omp25 (P. de Wergifosse, P. Lintermans, J. N. Limet, and A. Cloeckaert, J. Bacteriol. 177:1911-1914, 1995). The similarity to a known group 3 OMP, immunoreactivity with Ab prepared against B. abortus group Ags, immunolabeling of whole cells, and Southern hybridization led to our conclusion that the B. melitensis 28-kDa protein was a group 3 protein distinct from B. abortus Omp25. We designated the B. melitensis protein Omp28. Human convalescent sera from patients infected with B. abortus and Brucella suis as well as rabbit antisera prepared against killed B. abortus whole cells recognized B. melitensis Omp28 on Western blots (immunoblots). Furthermore, mice and goats infected with smooth strains of B. melitensis produced Abs against Omp28. Our results may begin to explain the variability in molecular weight seen in Brucella group Ags and point toward their possible use in vaccination against infection as well as diagnosis of the disease.

Purification and antigenic analysis of the major 25-kilodalton outer membrane protein of Brucella abortus

The major 25-kDa outer membrane protein (Omp25) of Brucella abortus was purified and antigenically characterized by use of monoclonal antibodies (mAbs). Purification was achieved from the sodium dodecyl sulphate-insoluble (SDS-I) cell wall (CW) fraction of vaccine strain B. abortus B19 which was shown by use of mAbs to contain the two major outer membrane proteins of 25 and 36 kDa linked to peptidoglycan, smooth lipopolysaccharide (S-LPS), and rough LPS (R-LPS). Purity of Omp25 was checked with a number of mAbs directed to the different components of the SDS-I fraction. In ELISA, five anti-Omp25 mAbs, which showed significant binding to B. abortus whole cells and which are probably directed to conformational epitopes well-exposed on the bacterial surface, reacted poorly or not at all with the purified Omp25. Addition of R-LPS to purified Omp25 restored the binding capacity of these mAbs, which suggested that R-LPS may play an important role in reconstitution and exposure of conformational epitopes of Omp25. Immunoelectron microscopy showed that Omp25 was inserted into the R-LPS vesicles. Four of these anti-Omp25 mAbs probably recognize the same or closely located epitopes on Omp25, since one of the mAbs conjugated to peroxidase was inhibited in its binding in ELISA by the three others. Other anti-Omp25 mAbs showed strong binding to purified denatured Omp25 and their binding capacity was not affected by the addition of R-LPS to the purified Omp25. Thus, these results confirmed, as defined by the mAbs, the presence of both sequential and at least one conformational epitope on Omp25.