it was surprisingly found that a deletion mutation in the guab gene can lead to attenuated salmonella enterica strains with significantly reduced virulence and capable of inducing an immune response in a livestock animal . such deletion would attenuate any organism that can use the guanine nucleotide as an intermediary . the term “ gene ” as used herein refers to the coding sequence and its regulatory sequences such as promoter and termination signals . the deletion mutant according to the invention is one in which the purine metabolic pathway enzyme imp dehydrogenase ( encoded by guab ) is inactivated . such inactivation may be obtained via a deletion by which the guab gene function is impaired , leading to a null - function ( no functional gene product formed ) of the affected gene ( s ). a person skilled in the art knows how to obtain such mutants and a simple test can tell whether the guab gene function is impaired . the mutant strain which fails to express a functional guab gene product cannot grow on minimal a medium , unless this medium is supplemented with ( e . g . 0 . 3 mm ) guanine , xanthine , guanosine or xanthosine . the invention aims to provide , amongst others , attenuated s . enteritidis and s . typhimurium strains since these are the most common s . enterica serovars . the present invention provides attenuated strains . the invention provides amongst others attenuated salmonella enterica strains for use , inter alia , as live attenuated vaccines against salmonellosis , as live vector and / or as dna - mediated vaccines expressing foreign antigens . as used herein , a “ foreign antigen ” means an antigen foreign to salmonella . live vector vaccines , also called “ carrier vaccines ” and “ live antigen delivery systems ”, comprise an exciting and versatile area of vaccinology ( levine et al , 1990 , microecol . ther . 19 : 23 - 32 ). in this approach , a live viral or bacterial vaccine is modified so that it expresses protective foreign antigens of another microorganism , and delivers those antigens to the immune system , thereby stimulating a protective immune response . live bacterial vectors that are being promulgated include , among others , attenuated salmonella . an object of the invention is to provide attenuated strains , like attenuated s . enterica strains for use in a live vaccine , possibly a polyvalent or multivalent live vaccine . one of the objects of the invention is therefore to provide a vaccine against e . g . salmonellosis comprising : a pharmaceutically effective or an immunizing amount of a mutant of the invention ( e . g . a salmonella enterica mutant ) which is incapable of forming de novo guanine nucleotides , wherein said mutant contains a deletion mutation in the guab gene ; and another object of the invention is to provide a live vector vaccine comprising : a pharmaceutically effective or an immunizing amount of a mutant of the invention ( e . g . a salmonella enterica mutant ), which is incapable of forming de novo guanine nucleotides , wherein said mutant contains a mutation in the guab gene , and wherein said mutant encodes and expresses a foreign antigen ; and the particular foreign antigen employed in the ( s . enterica ) live vector is not critical to the present invention . still another object of the invention is to provide a dna - mediated vaccine comprising : a pharmaceutically effective amount or an immunizing amount of a mutant of the invention ( e . g . a salmonella enterica mutant ), which is incapable of forming de novo guanine nucleotides , wherein said mutant contains a mutation in the guab gene ; wherein said mutant contains a plasmid which encodes and expresses in a eukaryotic cell , a foreign antigen ; and details as to the construction and use of dna - mediated vaccines can be found in u . s . pat . no . 5 , 877 , 159 , which is incorporated by reference herein in its entirety . again , the particular foreign antigen employed in the dna - mediated vaccine is not critical to the present invention . the decision whether to express the foreign antigen in e . g . s . enterica ( using a prokaryotic promoter in a live vector vaccine ) or in the cells invaded by e . g . s . enterica ( using a eukaryotic promoter in a dna - mediated vaccine ) may be based upon which vaccine construction for that particular antigen gives the best immune response in animal studies or in clinical trials , and / or , if the glycosylation of an antigen is essential for its protective immunogenicity , and / or , if the correct tertiary conformation of an antigen is achieved better with one form of expression than the other ( u . s . pat . no . 5 , 783 , 196 ). in the vaccines of the present invention , the pharmaceutically effective amount or the immunizing amount of the mutants of the present invention to be administered will vary depending on the age , weight and sex of the subject . by an “ immunizing amount ” as used herein is in fact meant an amount that is able to induce an immune response in the animal that receives the pharmaceutical composition / vaccine . the immune response invoked may be a humoral , mucosal , local and / or a cellular immune response . the particular pharmaceutically acceptable carrier or diluent employed is not critical to the present invention , and are conventional in the art . examples of diluents include : buffer for buffering against gastric acid in the stomach , such as citrate buffer ( ph 7 . 0 ) containing sucrose , bicarbonate buffer ( ph 7 . 0 ) alone , or bicarbonate buffer ( ph 7 . 0 ) containing ascorbic acid , lactose , and optionally aspartame . examples of carriers include : proteins , e . g ., as found in skimmed milk ; sugars ; e . g . sucrose ; or polyvinylpyrrolidone . the deletion mutants according to the invention have been created via standard homologous recombination techniques , whereby part of the guab gene , for instance part of the guab coding sequence , in a first step is replaced by a resistance gene and flanking frt sites . preferably , in a second step , said resistance gene is removed by recombination between the two frt sites . one frt site and the priming sites p1 and p2 remain by the molecular mechanism of the recombination removing the antibiotics resistance gene according to datsenko and wanner ( 2000 ) ( see for instance fig4 ). a particular example of the invention relates for instance to guab deletion mutants of s . enteritidis that comprise a mutated guab gene or coding sequence comprising seq id no : 12 . fig1 is a schematic representation of the biosynthetic pathway of guanosine monophosphate ( adapted from zalkin and nygaard , 1996 , in “ escherichia coli and salmonella , cellular and molecular biology , second edition ”, 1996 f . c . neidhardt ed . asm press , washington d . c ., vol . 1 , ch . 34 : 561 - 579 ). aicar : 5 ′- phosphoribosyl - 4 - carboxamide - 5 - aminoimidazole ; atp : adenosine triphosphate ; g : guanine ; gmp : guanosine monophosphate ; gr : guanosine ; hx : hypoxanthine ; hxr : hypoxanthine riboside ( inosine ); imp : inosine monophosphate ; x : xanthine , xmp : xanthosine monophosphate ; guaa : gmp synthetase , guab : imp dehydrogenase ; guac : gmp reductase . fig2 represents contig 1294 of the s . enteritidis genome ( seq id no : 10 ). the atg initiation codon and tga termination codon of the guab gene are in bold . fig3 represents the sequence of the δguab fragment of s . enteritidis cloned in puc18 ( seq id no : 11 ). the primers that were used are indicated by horizontal arrows . the fragment generated with primers guab6 - guab7 was cloned in puc18 . the atg initiation and tga termination codon of the guab gene and the cccggg smai restriction site are indicated in bold . fig4 represents the nucleotide sequence of the s . enteritidis pcr fragment , which includes the guab deletion , obtained after sequencing , using primer guab10 ( seq id no : 12 ). the pcr fragment was amplified with primers guab6 - guab7 , using total genomic dna of the mutant sm20 . the remaining frt site is indicated in bold italic and the p1 and p2 primers by arrows ( datsenko and wanner , 2000 , pnas 97 : 6640 - 6645 ). the atg initiation and tga termination codon of the guab gene are indicated in bold . fig5 represents the guab gene of s . typhimurium lt2 , section 117 of 220 of the complete genome ( seq id no : 13 ). the atg initiation codon and tga termination codon of the guab gene are in bold . fig6 - 7 represent the deposit receipts of sm69 and sm86 respectively . the invention will be described in further details in the following examples and embodiments by reference to the enclosed drawings . particular embodiments and examples are not in any way intended to limit the scope of the invention as claimed . an auxotrophic insertion mutant of a wild type s . enteritidis was obtained via insertion mutagenesis . only when supplemented with 0 . 3 mm guanine , xanthine , guanosine or xanthosine could the mutant strain grow on minimal a medium . these data strongly suggest that the auxotrophic mutation of the strain affects the guab gene , encoding the enzyme imp dehydrogenase ( ec 1 . 1 . 1 . 205 ). this enzyme converts inosine - 5 ′- monophosphate ( imp ) into xanthosine monophosphate ( xmp ) as indicated in fig1 . an insertion mutant can revert , thereby restoring the pathogenicity of the strain . this limits its applicability in a live attenuated vaccine . in that aspect deletion mutants are preferred . guab deletion mutants of s . enteritidis and s . typhimurium were therefore created and tested . the guab genes of both serovars are given in fig2 and 5 . guab deletion mutants were created according to the method for generating deletion mutations in the genome of escherichia coli k12 ( datsenko and wanner , 2000 , pnas 97 : 6640 - 5 , incorporated by reference herein ). this method relies on homologous recombination , mediated by the bacteriophage λ red recombinase system , of a linear dna fragment generated by pcr . the guab sequence is hereby substituted by an antibiotic resistance gene . this resistance gene is flanked by frt sites ( flp recognition target sites ) and can be excised from the genome by site - specific recombination , mediated by the flp recombinase . overlap pcr ( ho et al ., 1989 , gene 77 : 51 - 59 ) was applied to construct a linear fragment . the principle relies on the use of two primer sets , one upstream pair ( guab3 - guab4 ; guab3 : 5 ′ ggctgcgatt ggcgaggtag ta 3 ′, seq id no 2 ; guab4 : 5 ′ ggtgatcccg ggcgtcaaac gtcagggctt cttta 3 ′, seq id no 3 ) and one downstream pair ( guab5 - guab2 ; guab5 : 5 ′ ttgacgcccg ggatcaccaa agagtccccg aacta 3 ′, seq id no 4 ; guab2 : 5 ′ cgttcaggcg caacaggccg ttgt 3 ′, seq id no 1 ) of the guab gene . both sets contain primers ( guab4 , guab5 ) that are partially complementary and to which a smai restriction site was added . after annealing of the resulting complementary sequences and chain elongation , pcr with the outward primers ( guab6 - guab7 ; guabg : 5 ′ gcaacaactc ctgctggtta 3 ′, seq id no 5 ; guab7 : 5 ′ agaccgagga tcactttatc 3 ′, seq id no 6 ) generated a fragment with a 6 basepair smai site replacing an 861 basepair internal segment of the guab coding sequence . this δguab fragment was cloned in the vector puc18 ( see fig3 ). the chloramphenicol resistance gene ( cat ) with its flanking frt sequences was amplified using the primers p1 ( 5 ′ gtgtaggctg gagctgcttc 3 ′, seq id no 8 ) and p2 ( 5 ′ catatgaata tcctccttag 3 ′, seq id no 9 ) ( datsenko and wanner , 2000 ) and plasmid pkd3 dna ( datsenko and wanner , 2000 ) as a template . this pcr fragment was ligated in the smai site of the cloned δguab fragment . the desired fragment was generated using nested primers ( guab6 - guab7 ). the resulting pcr fragment was electroporated into s . enteritidis phage type 4 strain 76sa88 ( a clinical isolate from a turkey , obtained from the veterinary and agrochemical research centre , groeselenberg 99 , b - 1180 ukkel , belgium ) harboring the temperature sensitive replication plasmid pkd46 , encoding the bacteriophage lambda red recombinase system . the chloramphenicol resistant transformants were tested on minimal a medium and on minimal a medium supplemented with 0 . 3 mm guanine . the δguab :: catfrt mutants were confirmed by . pcr using the following primer combinations : guab6 - guab7 , guab6 - p2 , guab7 - p1 and p1 - p2 . the s . enteritidis δguab :: catfrt mutant ( sm12 ) was transformed with the temperature sensitive replication plasmid pcp20 by electroporation . the plasmid pcp20 encodes the flp recombinase , which recognizes the frt - sites , to remove the cat gene . the resulting strain was named sm20 . the pcr fragment in which the deletion is located was obtained using total genomic dna of the mutant sm20 and the primer combination guab6 - guab7 ( see fig4 ). the δguab mutation was confirmed by sequencing of this fragment , using the primer guab10 ( 5 ′ aggaagtttg agaggataa 3 ′, seq id no 7 ). the sequences of all above - mentioned primers are given in table 1 . to avoid the presence of possible additional mutations , caused by the expression of the red recombinase system , an isogenic strain was constructed . the δguab :: catfrt mutation of the mutant sm12 was transduced by bacteriophage p22 ht int − ( davis , r . w ., botstein d . and roth , j . r . ( 1980 ) in advanced bacterial genetics , a manual for genetic engineering . cold spring harbor laboratory , cold spring harbor , n . y .) to wild type s . enteritidis 76sa88 . the cat gene was removed using the plasmid pcp20 . the resulting strain was called sm69 ( deposit number lmg p - 21641 ). a δguab mutant of s . typhimurium strain 1491s96 was constructed using the same procedure and the same primers . the resulting strains s . typhimurium δguab :: catfrt and s . typhimurium δguab were named sm9 and sm19 respectively . sm86 ( having the deposit number lmg p - 21646 ) is the isogenic strain obtained therefrom , after transduction by bacteriophage p22 ht int − lysate from strain sm9 and excision of the cat gene using the plasmid pcp20 . the δguab mutants sm19 , sm20 , sm86 and sm69 are sensitive to bacteriophage p22 . this proves the presence of intact lipopolysaccharides ( lps ). virulence and protection tests with the guab deletion mutant sm20 in mice the virulence of the mutant sm20 in mice was tested by oral infection of 6 - 8 week old female balb / c mice in two independent experiments . these were performed as described above . the wild type strain s . enteritidis 76sa88 was tested in parallel as a positive control . the s . enteritidis 76sa88 δaroa mutant sm50 was included in the experiment as a vaccine control . this mutant carries a precise deletion of the complete aroa coding sequence and was constructed by the method of datsenko and wanner ( 2000 ). the complete data are given in tables 2 and 3 . these results demonstrate that the δguab mutant sm20 is strongly attenuated in mice but still shows some residual pathogenicity when administered at this high dose . oral immunization with the mutant induces protective immunity against infection by a high dose of the corresponding pathogenic wild type s . enteritidis strain 76sa88 . the protection is at least equal to the protection conferred by the s . enteritidis δaroa mutant sm50 . virulence and protection tests with the isogenic guab deletion mutants sm69 and sm86 in mice the virulence of the mutants sm69 and sm86 in mice was tested by oral infection of 6 - 8 week old female balb / c mice . these were performed as described above . the wild type strains s . enteritidis 76sa88 and s . typhimurium 1491s96 were tested in parallel as positive controls . the complete data are given in tables 4 - 7 . these results demonstrate that the δguab mutants sm69 and sm86 are strongly attenuated in mice but still show some residual pathogenicity when administered at this high dose . oral immunization with the mutants induces protective immunity against infection by a high dose of the corresponding pathogenic wild type strain . safety evaluation of the s . enteritidis guab deletion mutant sm69 in one - day - old chickens by the intratracheal and oral gavage routes the objective of this study was to evaluate the safety of s . enteritidis δguab mutant strain sm69 master seed in one - day - old chickens . mortality was used as a primary parameter for the determination of safety . chicks at one day of age were leg - banded and randomly placed in each of the four treatment groups ( group 1 : sm69 - it , group 2 : sm69 - og , group 3 : pbs - it and group 4 : pbs - og ). after the master seed inoculation , the birds from groups 1 and 2 were placed in one isolator and those of groups 3 and 4 in another isolator . chickens in groups 1 and 2 were inoculated with the sm69 master seed by the intratracheal ( it ) route or oral gavage ( og ) route , respectively , with an actual titer of 1 . 28 × 10 8 cfu / 0 . 2 ml per bird . chickens in groups 3 and 4 were administered with 0 . 2 ml pbs ( phosphate buffered saline ) per bird by the intratracheal or oral gavage , respectively . following the inoculation of sm69 or pbs , chick mortality was observed daily until 38 days post inoculation . table 8 summarizes the results of mortality for all 4 groups . in group 1 , one bird died during the inoculation due to inoculation trauma . two birds died at 2 days post inoculation ( dpi ). three birds died from day 3 to day 13 ( at 3 , 5 and 13 dpi respectively ). a total of 6 birds thus died in group 1 . in group 2 , two birds died in total . one died during inoculation due to inoculation trauma and one died at day 5 post inoculation . no birds died in the pbs treated groups either by the intratracheal or oral gavage route . this study indicates that the s . enteritidis δguab mutant strain sm 69 is not safe when administered at 1 . 28 × 10 8 cfu per bird at one day of age by the intratracheal or oral gavage route . safety evaluation of the s . enteritidis δguab deletion mutant sm69 in 2 - week old chickens by the intratracheal and oral gavage routes safety of the s . enteritidis δguab mutant strain sm69 was then evaluated in 2 week - old specific pathogen free ( spf ) chickens by the intratracheal and oral gavage routes . mortality was used as a primary criterion and body weight as a secondary criterion for the determination of safety . birds at 2 weeks of age were leg - banded and randomly placed in each of the four treatment groups : sm69 - it , sm69 - og , poulvac st - it and pbs - it . ten birds in group 1 were inoculated with sm69 by the intratracheal route ; ten birds in group 2 were inoculated with sm69 by oral gavage ; ten birds in group 3 were inoculated with a s . typhimurium aroa − vaccine ( poulvac ® st ) by the intratracheal route ; and five birds in group 4 were inoculated with pbs by the intratracheal route . chickens in groups 1 and 2 were inoculated with sm69 master seed by the intratracheal or oral gavage route , respectively , with the actual titer of 2 . 304 × 10 8 cfu / 0 . 2 ml per bird . chickens in group 3 were administered with poulvac ® st by the intratracheal route with 2 . 19 × 10 8 cfu / 0 . 2 ml per bird . chickens in group 4 were administered by the intratracheal route with 0 . 2 ml pbs per bird . after inoculation , the birds from treatment groups 1 and 2 were placed in one isolator and those from groups 3 and 4 in another isolator . following inoculations , mortality was observed daily until 21 days post - inoculation . body weight of all birds was also recorded at the end of the study period ( 21 days ). poulvac ® st and pbs were used as intratracheal procedure controls . during the 21 - day observation period , one bird in the sm69 intratracheal treatment group ( group 1 ) died from an infected yolk sac . no mortality was associated with sm69 inoculation , indicating that the sm69 strain is safe at the titer tested , 2 . 304 × 10 8 cfu per bird by the intratracheal and oral gavage routes . as expected , no death was observed either in the poulvac ® st treated birds at the titer of 2 . 19 × 10 8 cfu per bird or in the pbs treated birds , indicating that the study was valid ( table 9 ). body weight was compared amongst groups in an analysis of variance ( anova ) model with body weight as the dependent variable and treatment included as an independent variable . group comparisons were made using tukey &# 39 ; s test for multiple comparisons . the level of significance was set a p & lt ; 0 . 05 . the study was considered valid because the control chickens ( pbs control group ) remained healthy and free of clinical signs of diseases or mortality throughout the study . there were no significant differences in the final body weight in chickens administered with sm69 by the intratracheal or oral gavage inoculation , poulvac ® st , or pbs ( table 5 ). even though no baseline was established of the birds in each group at one day of age , it was unlikely that there was a significant difference in the initial body weight amongst the four groups since the birds were randomly placed into each of the 4 treatment groups . since no mortality was attributable to the inoculation of sm69 , it can be concluded that the s . enteritidis δguab mutant strain , sm69 , is safe when administered at the tested titer of 2 . 304 × 10 8 cfu / 0 . 2 ml dose per 2 - week - old bird , by either the intratracheal or oral gavage inoculation route . sm69 inoculations had no effect on the final body weight of the birds , bird weight being the second safety parameter evaluated here . a deposit has been made according to the budapest treaty at the bccm / lmg culture collection , laboratorium voor microbiologie , k . l . ledeganckstraat 35 , b - 9000 gent ( belgium ) for the following micro - organisms : salmonella enteritidis sm69 under deposit number lmg p - 21641 ( deposit date : 9 aug ., 2002 ) and s . typhimurium sm86 under deposit number lmg p - 21646 ( deposit date : 28 aug ., 2002 ). the deposits have been made in the name of prof . j . - p . hernalsteens , previous address : vrije universiteit brussel , laboratorium genetische virologie , paardenstraat 65 , b - 1640 sint - genesius - rhode , current address : vrije universiteit brussel , onderzoeksgroep genetische virologie , pleinlaan 2 , b - 1050 brussels , belgium .
the present invention relates to attenuated guab deletion mutants of a bacterium infecting veterinary species , more in particular salmonella enterica , to their use and production . the present invention further relates to live attenuated vaccines based on such mutants for preventing bacterial infections , and more in particular salmonellosis , in a veterinary species , more in particular poultry .