the invention comprises a plasmid design and a process for construction of this plasmid expressing a protective antigen against n . meningitidis in an attenuated salmonella strain . the plasmid design includes insertion of the tbpb gene in a pet plasmid to keep this gene under the control of the t7 promoter already present in the pet plasmid and the addition of a metabolic marker ( the e . coli asd gene ) to avoid the use of a plasmid with the ampicillin resistance gene ( a feature not appropriate for a vaccine with potential human use ). also , this plasmid requires a second plasmid inside the vaccine strain to allow the expression of the tbpb antigen . 1 .— cloning of tbpb gene from neisseria meningitidis b : 4 : nt by pcr amplification and gene ligation into the pgem - t plasmid . 3 .— expression analysis of the tbpb gene and testing best bacterial growth conditions to optimize recombinant tbpb protein synthesis . 4 .— insertion of the asd gene into pet21a / tbpb plasmid to replace the use of the antibiotic resistance marker ( ampicillin ) by a metabolic marker to complement chromosomal asd mutation . 5 .— tbpb expression assay in an e . coli strain carrying an asd mutation . 6 .— plasmid transfer to a salmonella typhimurium strain with a mutation in the asd gene that provides dna methylation of plasmids constructions allowing protection to these plasmids when are finally transferred to the vaccine strain which has its restriction / methylation system in operative status 7 .— plasmid transfer to the attenuated vaccine strain carrying the mutated asd gene to be complemented by the wild type asd gene present in the modified pet21a / tbp / asd vector . all the above stages led to a formulation of a pre - clinic assay to evaluate humoral response ( antibodies ) in an oral vaccination procedure according to the protocol already established in mice . then , the experimental approach was performed regarding whether anti - tbpb antibodies induced in vaccinated mice have bactericide activity ( serum ability of killing neisseria meningitidis in an in vitro assay ). in the following examples , each stage of the invention procedure is explained in details . after completing the above stages a plasmid product was obtained . this plasmid derived from pet21a which comprises the tbpb gene of the chilean neisseria meningitidis strain b : 4 : nt , under the control of t7 promoter . it was also possible to establish the tbpb gene sequence of the chilean n . meningitidis b : 4 : nt strain in particular . the specific sequences are illustrated in the fig4 and 5 . therefore , the oral vaccine against meningitis b is formulated in this way . to achieve this , it was required an attenuated salmonella typhimurium strain as a vector , like the one disclosed in the provisional patent pending , 1047 - 2004 , of the same bearer , or the salmonella typhimurium χ4550 which was used to demonstrate the functionality of the afore mentioned plasmid . the invention discloses a procedure to construct a plasmid which allows the expression of the neisseria meningitidis tbpb surface antigen , which gene was cloned and sequenced . this plasmid can be used as a source to synthesize a protective antigen as part of an oral vaccine based on some type of attenuated salmonella strain such as salmonella typhimurium χ4550 . the plasmid required to be modified by insertion of the asd gene in order to be stabilized in the salmonella vaccine strain . at the same time , it was demonstrated that the tbpb antigen , from a chilean strain of group b n . meningitidis , induces an igg serum response and these antibodies showed bactericidal activity against n . meningitidis serogroup b , which confirms that tbpb antigen is a protective antigen when assayed in a murine model . tbpb protein ( b protein which binds human transferrin ) is found in the surface of the neisseria meningitidis bacteria and is part of a complex with tbpa , which is necessary for iron uptake . ( see fig1 , scheme ). free iron is a scarce element in the host and this ion is transported through host body fluids associated to carrier proteins . therefore , this pathogen has developed virulence factors for iron uptake from substances that transport iron in the host such as human transferrin . several pieces of data suggest that tbpb could be a good antigen for vaccine development . with all the above information chromosomal dna from a n . meningitidis b : 4 nt strain obtained from the instituto de salud pública de chile , purified by a standard procedure and used as a model to amplify the tbpb gene in a pcr reaction under conditions described below . the experimental strategy is illustrated in fig2 . the amplification reaction was performed in a final volume of 100 μl in buffer solution of 20 mm tris - hcl , ph 8 . 4 ; 50 mm kcl ; 1 . 5 mm mgcl 2 ; 200 μm of each deoxynucleoside triphosphate ( datp , dgtp , dctp and dttp ) and primers at a final concentration of 0 . 5 μm ( 50 pmol / 100 μl ). primer sequences were as follows : these primers included a nde1 site for the 5 ′ end of the gene and a hindiii site ( both underlined ) for the 3 ′ end of the gene . before the amplification , the n . meningitidis genomic dna of the respective strain ( 0 . 1 - 0 . 5 μg ), already present in the reaction mixture , was denature at 95 ° c . for 5 min . then , 0 . 5 units of pfu dna polymerase were added and tubes were coated with a drop of mineral oil . alternatively , in other assays taq dna polymerase ( 0 . 5 u per tube ) was used . each reaction mixture was subjected to 35 cycles of amplification in a mj research thermocycler . each cycle consisted of 2 min at 95 ° c . ( denaturation step ), 2 min at 52 ° c . for annealing with tbp - 1 and tbp - 1 . 3 primers , and 4 min at 72 ° c . for product elongation . finally , samples were incubated for another 10 min at 72 ° c . and kept at 4 ° c . until analyzed . the amplified fragment was analyzed in a 1 % agarose gel electrophoresis under standard conditions . the size of the fragment obtained was 2 . 1 kb ( fig3 ). fragment separation between 0 . 5 and 10 kb was performed by electrophoresis en horizontal agarose gels ( 0 . 8 - 1 . 0 %) prepared in tae solution ( 40 mm tris - hcl , 2 mm edta , 20 mm sodium acetate adjusted to ph 8 . 0 with glacial acetic acid ). gels were submitted to electrophoresis in tae buffer , at 50 ma for analytical gels , and 30 ma for preparative gels . before placing samples into the gel , these were incubated for 5 min a 65 ° c . with an equal volume of 2 × sample buffer ( 25 % glycerol , 0 . 5 % sds , 0 . 025 % bromophenol blue and 12 mm edta ). dna bands in agarose gels were visualized on a uv transiluminator , previously stained for 10 min in a 1 μg / ml ethidium bromide ( et - br ) solution . gels were photographed under uv light , using polaroid type 667 films . ligation of the tbpb insert into the pet21a expression vector followed by transfer to dh5 α e . coli to obtain plasmid dna necessary for vector modification . the fragment excised from the agarose gel was purified using a qiagen kit . the dna fragment containing the tbpb gene was released by double digestion with ndei and hindiii enzymes under standard conditions and ligated to the pet21a vector ( novagen ) using dna ligase and 1 mm atp ( sambrook et al ., 1989 ). an aliquot of the ligation reaction containing 50 ng of ligated dna was used to transform by electroporation e . coli dh5α cells . this bacterium had the following genotype : f ′ enda1 hsdr17 ( r k − m k + ) gln v44 thi - 1 reca1 gyra ( nal r ) rela1 δ ( laczya - argf ) u169 deor ( φ80 lacaδm15 ). these were prepared according to the method described by miller ( 1994 ) with the following modifications : 100 ml of bacterial culture grown in luria medium until o . d . 600 reached values between 0 . 4 and 0 . 6 , then was transferred into a sterile 150 ml corex tube and centrifuged at 5 , 000 rpm in a gsa rotor ( sorvall ) at 4 ° c . during 15 min . after discarding all culture medium , cells were maintained in ice and carefully suspended in 100 ml of sterile cold distilled water and were centrifuged again under the same conditions . pelleted bacteria were resuspended in 2 ml of distilled cold sterile water and they were distributed into two microfuge sterile tubes . cells were centrifuged at 5 , 000 rpm in the microfuge and finally , the pellet of each tube was resuspended in 100 μl of cold 10 % glycerol solution . the suspension was kept frozen in 40 ul aliquots at − 70 ° c . until used . the average yield of this preparation varied between 10 9 and 10 10 viable cells / ml . this was done according to the method described by chasy et al ., ( 1988 ), with minor modifications of miller ( 1994 ). bio - rad equipment including the gene pulser ™ power supply ( version 2 - 89 ), coupled to a pulse controller was used . in a sterile microfuge tube kept on ice , 40 μl of electrocompetent cells ( 10 8 - 10 9 cells / ml ) were mixed with 0 . 1 to 1 . 0 μg of plasmid dna . the mixture was maintained on ice for 5 min and then transferred to a bio - rad electroporation sterile cuvette of 0 . 2 cm separation between the electrodes . the cuvette maintained on ice was then transferred into a special electroporation chamber to apply the electric pulse . the gene pulser ™ equipment was coupled to the pulse controller unit and the electroporation conditions were fixed at 2 , 500 v , 200ω resistance and 25 uf capacitance , obtaining an electric field of 12 . 5 kv / cm and a time constant between 4 and 5 msec . the actual electroporation conditions were verified by reading the equipment display . the dna and cells mixture was transferred to a sterile tube with 1 ml luria medium . the mixture was incubated at 37 ° c . with constant shaking during 60 min . selection of transformants was done in 1 % luria agar plates containing 100 μg / ml ampicillin . plates were incubated 16 to 24 h at 37 ° c . many clones were obtained and some of them were sequenced in both strands using internal primers . the complete nucleotide sequence of a clone is shown in fig4 and the deduced amino acid sequence obtained from it by a dnastar program is shown in fig5 . modification of the vector by replacement of the ampicillin resistance gene by the asd gene ( aspartate semialdehyde dehydrogenase enzyme ) used as a metabolic marker in order to avoid plasmid loss by asymmetric segregation after replication in the bacterial cells , the plasmid carrying the heterologous antigen gene usually maintained within the attenuated bacteria under selective pressure using an antibiotic resistance gene included into the plasmid . since the use of a vaccine strain with an antibiotic resistant ability is not appropriate for human use , a metabolic gene inserted into the plasmid to complement a disabled cellular function of the host as a feasible alternative for selective pressure . therefore , to maintain the plasmid in the bacteria as long as possible , the ampicillin resistance gene in pet21a vector was replaced by the aspartate dehydrogenase gene ( asd ). the vaccine strain we have selected is an asd mutant that requires diaminopimelic acid ( dap ) to grow . since this metabolite is not present in mammalian extracellular fluids , insertion of the asd gene in the e . coli pet21a vector will replace the requirement of dap for the mutant to grow . in addition , the insertion of the asd gene interrupts the ampicillin resistance gene , causing its inactivation . for this purpose it was necessary to obtain the asd gene from e . coli k - 12 , and to insert it into the pet21a vector containing the tbpb gene , as described in the following paragraph . the amplification reaction was done in 25 μl final volume of a buffer containing 20 mm tris - hcl , ph 8 . 4 ; 50 mm kcl ; 1 . 5 mm mgcl 2 , and 200 μm of each desoxiribonucleoside triphosphate ( datp , dgtp , dctp and dttp ), and 0 . 5 μm each primer ( 50 pmol / 100 ul ). chromosomal dna from e . coli k12 strain was isolated by the grimberg et al . method ( 1989 ) and used as template . primers were designed from the nucleotide sequence of the asd gene ( haziza , 1982 ), included promoter region and the translation stop codon , and restriction sites as indicated : the actact sequence corresponds to the recognition site of the scai enzyme ( underlined ), necessary to include the amplified fragment in the scai site contained in the middle of the ampicillin resistance gene in the vector . the sequence ggatcc ( boldface ) corresponding to a bamhi site was included as an alternative approach . the amplification and purification conditions of the asd gene were similar to that described earlier for the tbpb gene , except that taq polymerase enzyme and the corresponding buffer provided with the polymerase were used during amplification procedure . 3 . 2 .— ligation of the fragment containing the asd gene from e . coli to the pgem - t vector and then subcloned into the pet21a vector . the amplified fragment was purified and ligated to the pgem - t vector and then used to transform dh5α cells by electroporation . transformed bacteria were selected in luria - agar plates containing 100 μg / ml ampicillin . the plasmid was extracted from a positive clone with the following standard procedure here described . the asd gene cloned into the pgem - t vector was obtained by digestion with the scai enzyme and a 1 . 2 kb fragment was purified from an agarose gel with a plasmid dna purification kit . then , the asd gene was ligated into the pet21a vector carrying the tbpb gene and previously linearized with the scai enzyme . the scai site is located within the ampicillin resistance gene , thus this strategy allowed at the same time the inclusion of the asd gene and inactivation of the ampicillin resistance gene . the conditions for this ligation varied slightly from the ones described by sambrook et al ., ( 1989 ) since this fragment had blunt ends . clones containing pet21a plasmid carrying the asd gene from e . coli k12 are shown in fig6 . transformation of the e . coli χ6212 asd mutant strain with the modified vector containing the asd wild type gene after transformation of e . coli χ6212 cells with the modified plasmid vector , they become independent of dap metabolite but , and the plasmid is stably conserved since it carries the asd wild type gene . the e . coli χ6212 strain genotype is : φ80d laczm1 deor δ ( laczya - argf ) u169 supe44 λ − gyra96 ( nal r ) reca1 rela1 enda1 , δasd a4 δzhf - 2 :: tn 10 , hsdr17 ( r − m + ). thus , the pet21a / tbpb / asd prototype vector was transferred to this strain by electroporation as it has been previously described . after colony selection process in absence of dap , clone 10 carrying plasmid pet / tbpb / asd was characterized . in order to assure that the ampicillin resistance gene was indeed inactivated and replaced by asd , parallel cultures were incubated in the presence of 100 μg / ml ampicillin . in addition clone 10 grew in plates that had no dap , demonstrating that this clone carried a functional asd gene and also that ampicillin gene was inactive . transfer of the pet21a / tbpb / asd plasmid from the e . coli χ6212 strain to the salmonella typhimurium χ3730 strain and expression of the tbpb gene in salmonella in order to achieve the expression of the tbpb antigen from neisseria meningitides b : 4 : nt strain into the salmonella vaccine strain it was first necessary to transfer the dual plasmid to the salmonella typhimurium χ3730 strain , for two reasons : i ).— this is a strain that has an altered system of dna modification - restriction because a mutation in the restriction enzyme gene . it methylates foreign dna ( the required plasmids for tbpb antigen expression ) but does not degrade it , facilitating the transfer of plasmids into the vaccine strain which is a wild type restriction enzyme system . a previous passage of plasmid dna through strain χ3730 methylates this dna avoiding degradation when transferred into the vaccine strain , since both strains share the same restriction - modification system but s . typhimurium χ3730 strain only methylates foreign dna increasing transformation efficiency when plasmid dna is introduced into the vaccine strain . ii ) the s . typhimurium χ3730 strain is asd - mutant . thus the incorporation of a plasmid containing a wild type asd gene ensures the independence from dap under determined growth conditions , giving stability to the pet21a / tbpb / asd plasmid construction , since asd gene is required for bacterial survival . that has been previously constructed . the s . typhimurium χ3730 strain genotype is : leu hsdl gale trpd2 rpsl120 ( str r ) δasda1 δ [ zhf - 4 :: tn10 ] mete551 meta22 hsdsa hsdb ilv . in addition , this strain already carried the pgp1 - 2 plasmid . this plasmid contains the rna polymerase gene from t7 phage allowing the expression of the tbpb gene cloned in pet21a because is under the control of the t7 promoter . the s . typhimurium χ3730 / pgp1 - 2 strain was transformed with the pet / tbpb / asd plasmid . the s . typhimurium χ3730 / pgp1 - 2 intermediate strain is a hsd mutant with r − m + phenotype , in contrast to the vaccine strain that is r + m + . the dual system includes the pet - derivative plasmid with the gene to be expressed under the control of the t7 promoter ( p t7 ) and the pgp1 - 2 plasmid ( tabor and richardson , 1985 ) that provides the t7 rna polymerase ( see fig7 ). the expression is triggered by abruptly raise of the incubation temperature from 30 ° c . to 42 ° c ., since the t7 rna polymerase gene , contained in the pgp1 - 2 plasmid is under the left promoter of phage λ ( λ p l ) which depends on temperature raise to function . this promoter is permanently inhibited by the temperature sensitive repressor cl857 , when the temperature is under 30 ° c . thus , cl857 repressor inhibits transcription at the λ p l promoter at 30 ° c . but the repressor becomes active after the culture temperature within these cells raises briefly to 42 ° c . the repressor is inactivated inducing the t7 rna polymerase gene . thus , this enzyme promotes transcription of genes cloned in the pet vector family . because of this , tbpb gene cloned in pet21a is under the control of p t7 , specifically recognized by the t7 rna polymerase . pet21a plasmids ( and its derivatives ) and pgp1 - 2 plasmids are compatible to share the same bacterial cell and are not excluded once inside the bacterium . this is due to their different replication origins ( cole1 and pa15 ) and also they carry different resistance markers , amp and kan , respectively ( ausubel , 1991 ), facilitating the selection pressure . furthermore , pet21a / tbpb construction has already been modified so , instead of presenting ampicillin resistance it carries a wild type asd gene that allows the strain to growth in the absence of dap . transformation has been done by electroporation as previously described and the selection in luria medium without dap and with kanamicyn 50 ug / ml . to verify the presence of both plasmids , 4 colonies were chose from the isolated clones , and the presence of both plasmids was verified by alkaline lysis ( bimboim and doly , 1979 ) with a modification described by sambrook et al ., ( 1989 ), or alternatively , using the appropriate qiagen kit , following provider instructions . two of the selected colonies had both plasmids . 5 . 2 .— expression assays for e . coli ( de3 ) derivatives under iptg inducible promoters . two milliliters of luria broth containing the appropriate selection agent were inoculated with 0 . 1 ml of saturated culture containing the clones of interest and inocula were incubated with shaking at 37 ° c . until od 600 reached 0 . 5 . then , to induce the expression of recombinant proteins , 1 mm iptg was added to the cultures and incubated during 4 hours . cell lysates and the western blot analyses were done as previously mentioned . 5 . 3 .— expression assays using the dual plasmid system for salmonella strains . the dual system consists of the pgp1 - 2 plasmid ( kan r ), containing the t7 rna polymerase under the control of the thermo inducible λ p l phage promoter and of the plasmid derived from pet21a containing the gene of interest under the control of the t7 promoter and the asd gene inserted in this plasmid as a metabolic marker . inoculation of clones carrying the asd gene was done in 2 ml of luria with 50 μg / ml kanamycin ( clones with asd gene ) using 50 μl ( 1 : 40 dilution ) of saturated cultures grown overnight with continuous shaking at 30 ° c ., in the case of e . coli , or at 37 ° c . in the case of s . typhimurium . cultures were grown until a od 600 of 0 . 4 - 0 . 6 was reached — then , iptg was added to a final concentration of 1 mm and cultures were kept in an incubation bath at 42 ° c . for 30 min to induce the expression of genes of interest under the control of t7 promoter in the pet vector . total extracts of induced cultures were prepared and analyzed by western blot as described below . 5 . 4 .— expression of tbpb antigen in s . typhimurium χ3730 assayed by western blot . four colonies carrying the tbpb gene were assayed . all presented a similar level of expression after induction with iptg , as demonstrated by western blot results ( fig8 ). this technique was done as described by towbin et al ., ( 1979 ) and it is based upon the recognition of the antigen with specific antibodies . proteins present in bacterial lysates were previously separated by sds - polyacrylamide gel electrophoresis ( laemmli , 1970 ) and transferred to nitrocellulose membranes . the unstained gel containing the separated proteins was deposited into an electrotransfer system that consists of a sponge over which is set a whatman 3 mm filter paper , followed by the gel with the proteins , the nitrocellulose membrane , another filter paper and finally , another sponge . all this setting was supported between two perforated plastic plates . this system was submerged in a chamber containing the transfer solution ( 25 mm tris - hcl , ph 8 . 4 ; 192 mm glycine and 20 % methanol ), carefully leaving the nitrocellulose towards the anode (+ electrode ) and the gel towards the cathode . the electrotransfer was carried out at 200 ma during 1 h . the nitrocellulose sheet with the electrotransferred proteins , was incubated in a blocking solution of pbs containing 1 % of non - fat milk and incubated at room temperature during 45 min or at 4 ° c . for 10 h with continuous shakingtirring . after blockade of filter free sites , the nitrocellulose filter was incubated during 60 min at room temperature with rabbit polyclonal serum ( 1 : 1000 dilution ) in pbs - 1 % milk solution . the non specific binding of antibodies was eliminated by 3 consecutive 5 min washes with washing solution ( pbs - 0 . 1 % tween 20 ). the specific binding of antibodies to the trpb present in the nitrocellulose , were visualized by incubation during one hour at room temperature with an anti - rabbit igg antibody , conjugated to horse radish peroxidase ( diluted 1 : 1000 ) in the blocking solution ( pbs - 1 % milk ). after washing as previously described , the conjugate was revealed by incubation of the nitrocellulose with 50 ml of 50 mm tris - hcl ph 7 . 4 containing 150 mm nacl , to which 30 mg of 4 - chloro - α - 1 - naphthol previously dissolved in 10 ml of cold methanol and 200 μl of 30 % hydrogen peroxide were added . the reaction was stopped by extensive washing with distilled water . plasmid transfer from s . typhimurium χ3730 strain to the attenuated s . typhimurium χ4550 strain for the expression of the tbpb antigen . from the s . typhimurium χ3730 clones that expressed the tbpb antigen , four containing the pet - tbpb / asd and pgp1 - 2 plasmids were selected . plasmids containing modified methylation pattern were isolated and introduced by electroporation into the s . typhimurium χ4550 vaccine strain . the selection was done by growing with 50 μg / ml kanamycin 50 ug / ml and in the absence of dap . grown colonies of each construction were picked up , and the above described plasmid analysis and expression studies were carried out to these transformants . five pettbpb / asd plus pgp1 - 2 containing colonies expressed tbpb . the expression of this antigen was determined by western blot analysis as described before ( fig9 ). mice immunization with the attenuated oral vaccine ( s . typhimurium χ4550 ) expressing tbpb to evaluate the antigen as inducer of protective humoral response against n . meningitidis . to determine if tbpb will be a suitable antigen to develop an oral live vaccine against the infection by n . meningitidis , the ability to induce specific antibodies after mice immunization through intra gastric pathway with the s . typhimurium χ4550 and the corresponding antigen was evaluated . for this purpose , balb / c mice were used and divided in groups of 8 individuals , which were immunized following the steps described below . 7 . 1 — preparation of the bacterial suspension with the vaccine strain that expresses tbpb . using a saturated inoculum , the culture was grown under continuous shaking ( 250 rpm ) in luria broth containing the selection agent at 37 ° c . until a od 600 of 0 . 4 to 0 . 6 was obtained . cells were recovered from an 1 ml aliquot by centrifugation and were resuspended in 4 parts of pbs and 1 part of 7 . 5 % sodium bicarbonate in a total volume of 200 μl . the bacterial population of the suspension was estimated by extrapolating the od 600 in a growth curve previously established , then it was adjusted to a 1 × 10 7 cfu that was confirmed by counting the number of viable bacteria after seeding appropriate dilutions in luria broth agar plates . 7 . 2 — counting the number of s . typhimurium χ4550 ( vaccine strain ) viable cells carrying recombinant plasmids . clones of interest were grown at different stages in luria broth with the appropriate selection agent , under shaking at 30 ° c . o 37 ° c ., according to the case , until an od 600 of 0 . 2 , 0 . 4 , 0 . 6 , 0 . 8 and 1 . 0 was reached . aliquots in each of these points in the growing culture with convenient dilutions in luria agar plates were seeded . after overnight incubation , colonies were counted and the number of colony forming units ( cfu ) per ml of culture was obtained . these values were an average of at least 2 independent assays . moreover , with these data and the time variable , a growth curve was elaborated . groups of 8 females , pathogen free balb / c mice , aged 8 - 12 weeks were obtained from the animal facility of the facultad de ciencias , pontificia universidad católica de chile . these mice were immunized by oral route , applying the bacterial dose through a gastric probe of approximately 25 gauge × ¾ t . w . ( 0 . 5 × 19 mm ) diameter . the primary immunization consisted of 3 doses administered in a 6 days period , containing 1 × 10 7 cfu / 200 μl ( od 600 = 0 . 4 ) and a booster or secondary immunization as a unique dose ( 1 × 10 7 cfu ) six weeks later . one day before immunization process started ( pre - immune samples ) and 10 days after booster immunization , serum , saliva and feces samples were obtained . before obtaining samples and in order to facilitate animal manipulation , mice were partially anesthetized with cotton containing a few drops of ethylic ether . sera were obtained by centrifugation of 100 to 150 μl blood samples , collected through eye retro - orbital pathway with heparinized microcapillars ( marienfeld , germany ). sera were kept at − 70 ° c . until used . feces of each mouse were collected ( approximately 100 mg ) and pbs with sodium azide 0 . 02 % was added . after strong mixing in a vortex , a suspension was obtained . it was then centrifuged and the supernatant was kept under − 70 ° c . until it was used . antibody detection in saliva was omitted in this study because of lack of reproducibility in the collected volume , and in general , by the small amount of antibodies present in this fluid . to detect antibodies in mice sera and feces it was necessary to prepare a large amount of purified tbpb antigen to immobilize in the elisa plates . protein separation was done according to every and green ( 1982 ) in preparative sds - polyacrylamide gels ( 15 × 13 × 0 . 3 cm ), fifteen ml samples of bacterial cultures of clones expressing tbpb in optimal conditions were used as source of protein . the electrophoresis requires 18 to 20 h at 50 v . to avoid protein staining , bands were visualized with 0 . 1 m kcl . the gel was cut in the band of interest , fractionated into small pieces and put into an electroelution chamber ( eluter , bio - rad ). tbpb was obtained with a moderate yield and in reduced concentration due to its big size ( approximately 80 kda ). to evaluate the obtained protein concentration , markwell method ( 1978 ), a modification of that described by lowry in 1951 was used . alternatively , protein amount in aliquots of the electroeluted protein were compared to bsa dilutions of known concentration , after separation by sds - page electrophoresis . comparison of the intensity of coomasie blue staining obtained for different bsa concentrations allowed to estimate the approximate amount of protein obtained after electroelution . in order to optimize the elisa assays , the binding efficiency of the antigen was increased by the addition of the commercial protein pegotina ™ during this process . an aliquot of 100 μl of pegotina ™ ( 2 ug / ml diluted in pbs , bioschile i . g . s . a .) was added to each well from polystyrene 96 wells plates ( nunc , flat bottom ). plates were left overnight at 37 ° c . on the next day , plates were activated by the addition of 100 μl per well of purified tbpb ( 50 ng per well , diluted in pbs ) and incubation at 37 ° c . during 2 h . then , wells were washed 3 times , 10 min each , with pbs - 0 . 02 % tween 20 . non - specific binding sites were blocked with 200 μl pbs - 1 % bsa , by 1 h incubation at room temperature . plates were washed again with pbs - 0 . 02 % tween 20 and dried over adsorbent towel . double serial dilutions in pbs - 1 % bsa of sera and feces extract samples ( 100 μl ) from immunized mice were added to each well . plates were incubated for 1 hour at 37 ° c . and non - specific antibodies were eliminated by washing 3 times with 300 μl of pbs - 0 . 02 % tween 20 during 5 min each time . specific antibodies bound to the protein used to activate the solid phase were detected with mouse anti - igg or anti - iga antibodies conjugated to alkaline phosphatase , diluted according to vendor instructions , and incubated during 30 min at 37 ° c . the excess of conjugated was washed away under the same conditions previously described . specific antibody binding was revealed after incubation with 100 μl of 1 mg / ml pnp solution ( p - nitrophenylphosphate in 97 mm diethanolamine buffer with 3 mm sodium azide , ph 9 . 8 ) during 30 min at room temperature and in a dark room . finally , the reaction was stopped with 50 μl of 3m naoh and the hydrolyzed pnp was determined by absorbance at 405 nm in an elisa reader ( labsystems uniskan ® i , flow laboratories ). a total of two groups of mice were inoculated through the intra gastric via ( one with the tbpb antigen and one control group ) twice ( primary immunization and secondary or booster after 50 days approximately ), with 200 ul of bacterial suspension containing 1 × 10 7 cfu . to determine if these antigens induced serum igg and iga antibodies in mucosal secretions ( feces ), blood and feces samples were taken , one month after the primary immunization and 10 days after the booster . also , samples were taken one day before primary immunization ( pre - immune samples ). elisa assays for anti - tbpb antibody determination , of igg and iga class , were analyzed in two forms . in one form , average elisa values from pre - immune samples were compared to that obtained after primary and secondary immunizations with the vaccine strain expressing tbpb antigen in the serum ( fig1 ) and in feces ( fig1 ). on the other form , antibody titers reached in serum and feces were calculated , determining the highest dilution after which a statistical significant value , higher than that from the corresponding pre - immune samples was obtained ( t student test , p & lt ; 0 . 05 ), for oral immunization with this antigen ( fig1 ). moreover , the antibody response against a total s . typhimurium χ4550 vaccine strain lysate was evaluated . values were obtained measuring od 405 . from the analysis of sample values ( od 405 ) obtained one month after primary immunization , 10 days after secondary immunization , and that of pre - immune samples , it was observed that group 2 of mice ( pet - tpbb / as and pgp1 - 2 constructs , with a dose of 1 . 0 × 10 7 cfu / 100 μl ) had a high antibody response with a 512 titer , statistically significant ( p & lt ; 0 . 05 ) compared to pre - immune serum ( fig1 ). this result was not observed in the pbs control group ( no significant difference was found between od 405 from the sample obtained after the primary immunization and that from the pre - immune sample ). these observations strongly suggest that in this group intestinal colonization occurred by the vaccine strain and , therefore an appropriate stimulation with the tbpb antigen was expected . fecal samples from mice groups immunized with tbpb presented od 405nm values higher that those of the respective pbs control group . fecal anti - tbpb iga titer obtained after the secondary response was 16 ( fig1 ) in mice that received a booster 30 days after the primary immunization . to infer the protective effect of anti - tbpb antibodies produced after the immunization process , it was proposed to measure the bactericidal activity of anti - tbpb iggs . for this purpose , we followed the procedure described by robki et al ., ( 1997 ) with modifications of a protocol for the assay of the bactericidal activity of the national center for infectious diseases , c . d . c ., atlanta , ga ., u . s . a . a scheme of this assay is shown in fig1 . as the pathogen for this assay , neisseria meningitidis b4 :: nt strain from year 1993 ( isp strain ) was used . an aliquot of n . meningitidis stored in glycerol at − 70 ° c . was taken , and successive passages of them into agar brain - heart plates with incubations at 37 ° c . in 5 % co 2 ( approximately 3 days ) to obtain a confluent bacterial growth were made . then , one colony was diluted into 4 ml of hanks solution ( 4 mm nahco 3 , 0 . 5 % glucose , 0 . 1 % bsa fraction v ), ph 7 . 2 , until a meningococcal suspension was obtained and adjusted to 1 × 10 5 cfu / ml to which 2 . 5 u / ml heparin were added ( 1 : 50 dilution ). twenty five microliters of hanks solution , 25 μl of immunized mice serum ( primary immunization and booster diluted until 1 : 1024 ), 10 μl of pathogenic bacteria adjusted to 1 × 10 5 cfu / ml , and 15 μl of normal human plasma as a source of complement were added to each well of a sterile microtiter plate . controls used were : bacteria viability control ( 40 μl of hanks buffer and 10 μl of same bacteria dilution ), complement control ( 25 μl of hanks buffer plus 10 μl of bacteria and 15 μl of plasma ), pre - immune mice serum control ( 25 μl serum plus 10 μl of bacteria and 15 μl of plasma ). after the addition of all components , the mixture was incubated at 37 ° c . during 30 min , time after which 100 μl of soy - tryptone agar ( 0 . 9 %) was added avoided bubble formation . it was left overnight at 37 ° c . in 5 % co 2 . bactericidal activity was determined by counting colonies in serial dilutions with the aid of a microscope . for this assay immunized mice sera were used and the results obtained ( table i ) indicated that serum from mouse 24 contained anti - tbpb antibodies with 79 . 8 % bactericide efficiency in the 1 / 32 dilution from booster serum , therefore its bactericidal calculated titer was 32 . ceesay s j , allen s j , menon a , todd j e , cham k , carlone g m , turner s h , gheesling l l , dewitt w , plikaytis b d , et al . 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( 2001 ). recombinant neisseria meningitidis transferring binding protein a protects against experimental meningococcal infection . infect . immun . 69 : 1561 - 1567 . zollinger w d , boslego j , moran e , garcia j , cruz c , ruiz s , brandt b , martinez m , arthur j , underwood p , et al ( 1991 ). meningococcal serogroup b vaccine protection trial and follow - up studies in chile . the chilean national committee for meningococcal disease . nihp ann . 14 : 211 - 212 ; discussion 213 .
a procedure for obtaining the expression of a membrane antigen of a pathogen against which an live oral vaccine development is desirable on the surface of a negative gram bacteria to which virulence is attenuated , or another bacteria or other gram negative or positive bacteria with probiotic features which are compatible with the proposed expression system and that can be used as a live oral vaccine , wherein a plasmid is constructed and obtained based on the structure of pet family plasmids , with tbpb gene incorporated under the control of t7 promoter or another equivalent one , with the addition of a metabolic marker in the plasmid vector , previously cloned with its own promoter , inactivating , at the same time , the antibiotic resistance . in addition , recombinant microorganism such as an attenuated vaccine strain against group b meningitis with immunizing and protective properties against infection by neisseria meningitidis .