AU2002314040A1

AU2002314040A1 - Self-containing lactococcus strain

Info

Publication number: AU2002314040A1
Application number: AU2002314040A
Authority: AU
Inventors: Lothar Steidler
Original assignee: Intrexon Actobiotics NV
Current assignee: Intrexon Actobiotics NV
Priority date: 2001-05-03
Filing date: 2002-05-03
Publication date: 2003-05-01
Anticipated expiration: 2022-05-03

Description

SELF-CONTAINING Lactococcus STRAIN

Field of the invention

The invention relates to a recombinant Lactococcus strain, with environmentally limited growth and viability. More particularly, it relates to a recombinant Lactococcus that can only survive in a medium, where well-defined medium compounds are present. A preferred embodiment is a Lactococcus that may only survive in a host organism, where said medium compounds are present, but cannot survive outside the host organism in absence of said medium compounds. Moreover, said Lactococcus can be transformed with prophylactic and/or therapeutic molecules and can, as such, be used to treat diseases such as inflammatory bowel diseases.

Background of the invention

Lactic acid bacteria have long time been used in a wide variety of industrial fermentation processes. They have generally-regarded-as-safe status, making them potentially useful organisms for the production of commercially important proteins. Indeed, several heterologous proteins, such as lnterleukin-2, have been successfully produced in Lactococcus spp (Steidler et al., 1995). It is, however, unwanted that such genetically modified micro organisms are surviving and spreading in the environment. To avoid unintentional release of genetically modified microorganisms, special guidelines for safe handling and technical requirements for physical containment are used. Although this may be useful in industrial fermentations, the physical containment is generally not considered as sufficient, and additional biological containment measures are taken to reduce the possibility of survival of the genetically modified microorganism in the environment. Biological containment is extremely important in cases where physical containment is difficult or even not applicable. This is, amongst others, the case in applications where genetically modified microorganisms are used as live vaccines or as vehicle for delivery of therapeutic compounds. Such applications have been described e.g. in WO 97/14806, which discloses the delivery of biologically active peptides, such as cytokines, to a subject, by recombinant non-invasive or non- pathogenic bacteria. WO 96/11277 describes the delivery of therapeutic compounds to an animal - including humans - by administration of a recombinant bacterium, encoding the therapeutic protein. Steidler et al. (2000) describe the treatment of colitis by administration of a recombinant Lactococcus lactis, secreting interleukin-10. Such a delivery may indeed be extremely useful to treat a disease in an affected human or animal, but the recombinant bacterium may act as a harmful and pathogenic micro organism when it enters a non-affected subject, and an efficient biological containment that avoids such unintentional spreading of the micro organism is needed. Biological containment systems for host organisms may be passive, based on a strict requirement of the host for specific growth factor or a nutrient, that is not present or present in low concentrations in the outside environment, or active, based on so-called suicidal genetic elements in the host, whereby the host is killed in the outside environment by a cell killing function, encoded by a gene that is under control of a promoter only being expressed under specific environmental conditions.

Passive biological containment systems are well known in mircoorganisms such as Escherichia coli or Saccharomyces cerevisiae. Such E. coli strains are disclosed e.g. in US4100495. WO 95/1061 discloses lactic acid bacterial suppressor mutants and their use as means of containment in lactic acid bacteria, but in that case, the containment is on the level of the plasmid, rather than on the level of the host strain and it stabilizes the plasmid in the host strain, but doesn't provide containment for the genetically modified host strain itself.

Active suicidal systems have been described by several authors. Such system consists of two elements: a lethal gene, and a control sequence that switches on the expression of the lethal gene under non-permissive conditions. WO 95/10614 discloses the use of a cytoplasmatically active truncated and/or mutated Staphylococcus aureus nuclease as lethal gene. WO 96/40947 discloses a recombinant bacterial system with environmentally limited viability, based on the expression of either an essential gene, expressed when the cell is in the permissive environment and is not expressed or temporarily expressed when the cell is in the non-permissive environment and/or a lethal gene, wherein expression of the gene is lethal to the cell and the lethal gene is expressed when the cell is in the non-permissive environment but not when the cell is in the permissive environment. WO 99/58652 describes a biological containment system based on the relE cytotoxin. However, most systems have been elaborated for Escherichia coli (Tedkin et al., 1995; Knudsen et al., 1995; Schweder et al., 1995) or for Pseudomonas (Kaplan et al., 1999; Molino et al., 1998). Although several of the containment systems theoretically can by applied to lactic acid bacteria, no specific biological containment system for Lactococcus has been described that allows the usage of a self-containing and transformed Lactococcus to deliver prophylactic and/or therapeutic molecules in order to prevent and/or treat diseases.

Brief description of the figures Figure 1 : Map of the MG1363 thyA locus

Figure 2: Schematic representation of the different expression modules as present on pOThy plasmids ands genomic integrants of hlL-10. Black parts represent original L. lactis MG1363 genetic information, white parts represent recombinant genetic information. Figure 3: PCR identification of Thy11 (Thy11 1.1 and Thy11 7.1 represent individually obtained, identical clones). Standard PCR reactions were performed by using aliquots of saturated cultures of the indicated strains as a source of DNA template. Panel A shows an agarose gel of the products of the indicated PCR reactions. Panel B shows the positions at which primers attach in the thyA (1), upstream (2) or downstream (3) PCR's. Oligonucleotide primers used: (1): ATgACTTACgCAgATCAAgTTTTT and TTAAATTgCTAAATCAAATTTCAATTg (2): TCTgATTgAgTACCTTgACC and gCAATCATAATTggTTTTATTg (3): CTTACATgACTATgAAAATCCg and cTTTTTTATTATTAgggAAAgCA. Figure 4: PCR identification of Thy11, Thy12, Thy15 and Thy16. Standard PCR reactions were performed by using three days old colonies of the indicated strains as a source of DNA template.

Panel A shows the positions at which primers attach in the upstream (1), downstream (2) or thyA (3), PCR's. Oligonucleotide primers used: (1) ATgACTTACgCAgATCAAgTTTTT and TTAAATTgCTAAATCAAATTTCAATTg (2) TCTgATTgAgTACCTTgACC and gCAATCATAATTggTTTTATTg (3)

CTTACATgACTATgAAAATCCg and cTTTTTTATTATTAgggAAAgCA Panel B shows an agarose gel of the products of the indicated PCR reactions. Figure 5: Southern blot analysis of the indicated strains. Chromosomal DNA was extracted and digested with the indicated restriction enzymes. Following agarose gel electrophoresis the DNA was transferred to a membrane and the chromosome structure around the thyA locus was revealed by use of DIG labelled thyA or hlL-10 DNA fragments (panel A). Panel B shows a schematic overview of the predicted structure of the thyA locus in both MG1363 and Thy11. Figure 6: Panel A shows a schematic overview of part of the predicted structure of the L lactis chromosome at the thyA locus in MG1363, Thy11 , Thy12, Thy15 and Thy16. Numbers indicate base pairs

Panel B. Southern blot analysis of the indicated strains. Chromosomal DNA was extracted and digested with Λ/cfel and Spel restriction enzymes. Following agarose gel electrophoresis the DNA was transferred to a membrane and the chromosome structure around the thyA locus was revealed by use of DIG labelled thyA or hlL-10 DNA fragments. Figure 7: Production of hlL-10. Panel A shows a western blot revealed with anti-hlL-10 antiserum of culture supernatant and cell associated proteins of the indicated strains. Panel B shows quantification (by ELISA) of hlL-10 present in the culture supernatant. Figure 8: Production of hlL-10. Panel A shows quantification (by ELISA) of hlL-10 present in the culture supernatant of the indicated strains. Panel B shows a western blot revealed with anti-hlL-10 antiserum of culture supernatant proteins of the indicated strains.

Figure 9: Production of hlL-10 by the L. lactis strains LL108 carrying either pOThy11 , pOThy12, or pOThy16. Quantification (by ELISA) of hlL-10 present in the culture supernatant of the indicated strains. The N-terminal protein sequence of the recombinant hlL-10 was determined by Edman degradation and was shown identical to the structure as predicted for the mature, recombinant hlL-10. The protein showed full biological activity.

Figure 10: Growth rate of the indicated strains in GM17 containing 100μg/ml (T100) 50μg/ml (T50) 25μg/ml (T25) or no (TO) extra thymidine and possibly supplemented with 5μg/ml of erythromycin (E). Saturated overnight cultures (prepared in T50) were diluted 1 :100 in the indicated culture media. Panel A shows the kinetics of absorbance accumulation. Panel B shows the kinetics of the number of colony forming units (cfu) per ml of culture.

Figure 11: Growth rate of MG1363 and Thy12 in thymidine free medium (TFM). TFM was prepared by growing L. lactis Thy 12 bacteria in GM17, removing the bacteria by subsequent centrifugation and filtration on a 0,22μm pore size filter, adjusting the pH to 7,0 and autoclaving.

MG1363 and Thy12 bacteria were collected from an overnight culture in GM17 or GM17+50μg/ml of thymidine respectively and washed in M9 buffer (6 g/l Na₂HPO₄, 3 g/l KH₂PO₄, 1 g/l NH CI, 0,5 g/l NaCI in water). The suspensions of both were either diluted in TFM or TFM supplemented with 50μg/ml of thymidine (T50). CFU counts were determined at different time points: t=0, 1 , 2, 3, 4, 5, 6, 7, 8, 9, 10 and 20 hours. This shows that Thy12 viability is severely impaired in the absence of thymidine. Figure 12: Intestinal passage and viability: L. lactis MG1363 was transformed with the plasmid pLET2N which carries a chioramphenicol (Cm) resistance marker. L. lactis Thy12 was transformed with the plasmid pT1 NX which carries an erythromycin (Em) resistance marker. Of both strains 10⁹ bacteria were resuspended in BM9 (6 g/l Na₂HPO₄, 3 g/l KH₂PO₄, 1 g/l NH₄CI, 0,5 g/l NaCI in 25 mM NaHCO₃ + 25 mM Na₂CO₃), mixed and inoculated in three mice at t=0h. Faeces were collected of the time intervals -1 to 0, 0 to 1 , 1 to 2, 2 to 3, 3 to 4, 4 to 5, 5 to 6, 6 to 7, 7 to 8, 8 to 9, 9 to 10 and 10 to overnight. All samples were resuspended in isotonic buffer and appropriate dilutions were plated on GM17 (M17 medium, Difco, St.Louis supplemented with 0,5% glucose) plates containing either Cm, Em or Em+ 50μg/ml thymidine. Colony forming units for the different plates are represented in the graph.

Description of the invention It is the objective of the present invention to provide a suitable biological containment system for Lactococcus. A first aspect of the invention is an isolated strain of Lactococcus sp. comprising a defective thymidylate synthase gene. Preferably, said defective thymidylate synthase gene is inactivated by gene disruption. Even more preferably, said Lactococcus sp. is Lactococcus lactis. A special embodiment is a Lactococcus sp. strain, preferably Lactococcus lactis, more preferably a Lactococcus lactis MG1363 derivative, whereby the thymidylate synthase gene has been disrupted and replaced by an interleukin-10 expression unit. Said interleukin-10 expression unit is preferably, but not limited to, a human interleukin-10 expression unit or gene encoding for human interleukin-10. Another aspect of the invention is the use of a strain according to the invention as host strain for transformation, whereby the transforming plasmid does not comprise an intact thymidylate synthase gene. Still another aspect of the invention is a transformed strain of Lactococcus sp. according to the invention, comprising a plasmid that does not comprise an intact thymidylate synthase gene. Another aspect of the invention relates to a transformed strain of Lactococcus sp. comprising a gene or expression unit encoding a prophylactic and/or therapeutic molecule such as interleukin-10. Consequently, the present invention also relates to the usage of a transformed strain of Lactococcus sp. to deliver prophylactic and/or therapeutic molecules, and as such, to treat diseases. Methods to deliver said molecules and methods to treat diseases such as inflammatory bowel diseases are explained in detail in WO 97/14806 and WO 00/23471 to Steidler et al. and in Steidler et al. (Science 2000, 289:1352) that are hereby incorporated by reference.

Another aspect of the invention is a medical preparation, comprising a transformed strain of Lactococcus sp., according to the invention.

The Lactococcus lactis subsp. lactis thymidylate synthase gene (thyA) has been cloned by Ross et al. (1990a); it sequence is comprised in SEQ ID N° 3 and SEQ ID N° 5. EP0406003 discloses a vector devoid of antibiotic resistance and bearing a thymidylate synthase gene as a selection marker; the same vector has been described by Ross et al. (1990b). However, this vector could not be used in a Lactococcus lactis strain due to the lack of a suitable thyA mutant that had never been described. The present invention discloses how to construct such mutant by gene disruption, using homologous recombination in Lactococcus. In a preferred embodiment, the thyA gene is disrupted by a functional human interleukin-10 expression cassette. However, it is clear that any construct can be used for gene disruption, as long as it results in an inactivation of the thyA gene or in an inactive thymidylate synthase. As a non-limiting example, the homologous recombination may result in a deletion of the gene, in one or more amino acid substitutions that lead to an inactive form of the thymidylate synthase, or to a frameshift mutation resulting in a truncated form of the protein. Such a Lactococcus sp. thyA mutant is very useful as a host strain for transformation, in situations where more severe containment than purely physical containment is needed. Indeed, thyA mutants cannot survive in an environment without, or with only a limited concentration of thymidine and/or thymine. When such a strain is transformed with a plasmid that doesn't comprise an intact thyA gene and cannot complement the mutation, the transformed strain will become suicidal in a thymidine/thymine poor environment. Such a strain can be used in a fermentor, as an additional protection for the physical containment. Moreover, the present invention discloses that such a strain is especially useful in cases where the strain is used as a delivery vehicle in an animal body. Indeed, when such a transformed strain is given for example orally to an animal - including humans - it survives in the gut, provided a sufficiently high concentration of thymidine/thymine is present, and produces homologous and/or heterologous proteins, such as human interleukin-10, that may be beneficial for said animal. The present invention further demonstrates that the transformed strains surprisingly pass the gut at the same speed as the control strains and shows that their loss of viability is indeed not different from that of the control strains. However, once said strain is secreted in the environment, e.g. in the faeces, it is not able to survive any longer. The transforming plasmid can be any plasmid, as long as it cannot complement the thyA mutation. It may be a selfreplicating plasmid that preferably carries one or more genes of interest and one or more resistance markers, or it may be an integrative plasmid. In the latter case, the integrative plasmid itself may be used to create the mutation, by causing integration at the thyA site, whereby the thyA gene is inactivated. Preferably, the active thyA gene is replaced by double homologous recombination by a cassette comprising the gene or genes of interest, flanked by targeting sequences that target the insertion to the thyA target site. It is of extreme importance that these sequences are sufficiently long and sufficiently homologous to obtain to integrate the sequence into the target site. Preferably, said targeting sequences consist of at least 100 contiguous nucleotides of SEQ ID N°1 at one side of the gene of interest, and at least 100 contiguous nucleotides of SEQ ID N°2 at the other side; more preferably, said targeting sequences consists of at least 500 contiguous nucleotides of SEQ ID N°1 at one side of the gene of interest, and at least 500 contiguous nucleotides of the SEQ ID N° 2 at the other side; most preferably, said targeting sequences consists of SEQ ID N°1 at one side of the gene of interest and SEQ ID N°2 at the other side, or said targeting sequences consist of at least 100 nucleotides that are at least 80% identical, preferably 90% identical to a region of SEQ ID N° 1 at one side of the gene of interest, and of at least 100 nucleotides that are at least 80% identical, preferably 90% identical to a region of SEQ ID N° 2 at the other side of the gene of interest, preferably said targeting sequences consist of at least 500 nucleotides that are at least 80% identical, preferably 90% identical to a region of SEQ ID N° 1 at one side of the gene of interest, and of at least 500 nucleotides that are at least 80% identical, preferably 90% identical to a region of SEQ ID N° 2 at the other side of the gene of interest, most preferably said targeting sequences consist of at least 1000 nucleotides that are at least 80% identical, preferably 90% identical to a region of SEQ ID N° 1 at one side of the gene of interest, and of at least 1000 nucleotides that are at least 80% identical, preferably 90% identical to a region of SEQ ID N° 2 at the other side of the gene of interest . The percentage identity is measured with BLAST, according to Altschul et al. (1997). A preferred example of a sequence, homologous to SEQ ID N°1 is given in SEQ ID N° 7. For the purpose of the invention, SEQ ID N° 1 and SEQ ID N° 7 are interchangeable.

Transformation methods of Lactococcus are known to the person skilled in the art, and include, but are not limited to protoplast transformation and electroporation. A transformed Lactococcus sp. strain according to the invention is useful for the delivery of prophylactic and/or therapeutical molecules and can be used in a pharmaceutical composition. The delivery of such molecules has been disclosed, as a non-limiting example, in WO 97/14806 and in WO 98/31786. Prophylactic and/or therapeutical molecules include, but are not limited to polypetides such as insuline, growth hormone, prolactine, calcitonin, group 1 cytokines, group 2 cytokines and group 3 cytokines and polysaccharides such as polysaccharide antigens from pathogenic bacteria. A preferred embodiment is the use of a Lactococcus sp. strain according to the invention to deliver human interleukin-10. This strain can be used in the manufacture of a medicament to treat Crohn's disease as indicated above.

Examples From L. lactis MG1363 (Gasson, 1983) we have cloned out the regions flanking the sequence according to Ross er a/. (1990a) The knowledge of these sequences is of critical importance for the genetic engineering of any lactococcus strain in a way as described below, as the strategy will employ double homologous recombination in the areas 1000 bp at the 5'end (SEQ ID N°1) and 1000 bp at the 3'end (SEQ ID N°2) of thyA, the "thyA target". These sequences are not available from any public source to date. We have cloned these flanking DNA fragments and have identified their sequence. The sequence of the whole locus is shown in SEQ ID N°3; a mutant version of this sequence is shown in SEQ ID N°5. Both the 5' and 3' sequences are different from the sequence at genbank AE006385 describing the L lactis IL1403 sequence (Bolotin, in press) or at AF336368 describing the L. lactis subsp. lactis CHCC373 sequence. From the literature it is obvious that homologous recombination by use of the published sequences adjacent to thyA (Ross et al., 1990a) (86 bp at the 5'end and 31 bp at the 3'end) is virtually impossible due to the shortness of the sequences. Indeed, Biswas et al. (1993) describe a logarithmically decreasing correlation between length of the homologous sequences and frequency of integration. The sequences of L lactis Thy 11 , Thy 12, Thy 15 and Thy 16 at the thyA locus as determined in the present invention are given by SEQ ID N° 19, 20, 21, 22 respectively.

The thyA replacement is performed by making suitable replacements in a plasmid borne version of the thyA target, as described below. The carrier plasmid is a derivative of pORI19 (Law et al., 1995) a replication defective plasmid, which only transfers the erythromycin resistance to a given strain when a first homologous recombination, at either the 5' 1000bp or at the 3'1000bp of the thyA target. A second homologous recombination at the 3' 1000bp or at the 5' 1000bp of the thyA target yields the desired strain. The thyA gene is replaced by a synthetic gene encoding a protein which has the L. lactis Usp45 secretion leader (van Asseldonk et al., 1990) fused to a protein of identical amino acid sequence than: (a) the mature part of human-interleukin 10 (hlL- 10) or (b) the mature part of hlL-10 in which proline at position 2 had been replaced with alanine or (c) the mature part of hlL-10 in which the first two amino acids had been deleted; (a), (b) and (c) are called hlL-10 analogs, the fusion products are called Usp45-hlL-10.

The thyA gene is replaced by an expression unit comprising the lactococcal P1 promotor (Waterfield et al., 1995), the E. coli bacteriophageT7 expression signals: putative RNA stabilising sequence and modified gene10 ribosomal binding site (Wells and Schofield, 1996).

At the 5' end the insertion is performed in such way that the ATG of thyA is fused to the P1-T7Usp45-hlL-10 expression unit.

5' agataggaaaatttcatgacttacgcagatcaagttttt . . . thyA wild type gattaagtcatcttacctctt . . . Pl-T7-usp45-hILlO 5' agataggaaaatttcatggattaagtcatcttacctctt . . . thyA^", Pl-T7-usp45- hlLlO

Alternatively, at the 5' end the insertion is performed in such way that the thyA ATG is not included: 5' agataggaaaatttcacttacgcagatcaagttttt ... thyA wild type gattaagtcatcttacctctt... Pl-T7-usp45-hIL10 5' agataggaaaatttcgattaagtcatcttacctctt ... thyA^", Pl-T7-usp45-hIL10 Alternatively, at the 5' end the insertion is performed in such way that the thyA promotor [Ross, 1990 a] is not included:

5' tctgagaggttattttgggaaatactattgaaccatatcgaggtgtgtggtataatgaaggg aattaaaaaagataggaaaatttcatg... thyA wild type

gattaagtcatcttacctctt... P1-T7- usp45-hIL10

5' tctgagaggttattttgggaaatactagattaagtcatcttacctctt ... thyA^", Pl- T7-usp45-hILlO

At the 3' end an ACTAGT Spel restriction site was engineered immediately adjacent to the TAA stop codon of the usp45-hlL-10 sequence. This was ligated in a TCTAGA Xha\ restriction site, which was engineered immediately following the thyA stop codon

aaaatccgtaactaactagt3 ' . . . usp45-hILlO gatttagcaatttaaattaaattaatctataagtt3' . . . thyA-wild type tctagaattaatctataagttactga3'^' ...engineered thyA target aaaatccgtaactaactagaattaatctataagttactga3' ...thyA^~, usp45-hILlO These constructs are depicted in figure 2. The sequences of pOThy11, pOThy12 pOThy15 and pOThy16 are given by SEQ ID N°'s 23, 24, 25, 26 respectively.

The resulting strains are thyA deficient, a mutant not yet described for L. lactis. It is strictly dependent upon the addition of thymine or thymidine for growth. The map of the deletion, as well as the PCR analysis of all the isolates/mutants of the present invention are shown in figures 3 and 4. The presence of the thymidylate synthase and the interleukin 10 gene in the wild type strain and in the independent isolates/mutant was analyzed by Southern analysis as shown in figures 5 and 6. The region around the inserted hlL-10 gene was isolated by PCR and the DNA sequence was verified. The structure is identical to the predicted sequence. Human interleukin 10 production in the mutants was checked by western blot analysis, and compared with the parental strain, transformed with pTREXI as negative control, and the parental strain, transformed with the IL10 producing plasmid pT1 HIL10apxa as positive control (figure 7A). The concentration in the culture supernatant was quantified using ELISA. As shown in figure 7B, both isolates of the mutant produce a comparable, significant amount of hlL-10, be it far less than the strain, transformed with the non- integrative plasmid pT1HIL10apxa. Figure 8 (panel A and B) further demonstrates that all mutants produce a significant amount of h-IL 10.

Figure 9 shows the production of hlL-10 by the L. lactis strains LL108 carrying either pOThy11 , pOThy12, or pOThy16. Quantification (by ELISA) of hlL-10 present in the culture supernatant of the indicated strains. The N-terminal protein sequence of the recombinant hlL-10 was determined by Edman degradation and was shown identical to the structure as predicted for the mature, recombinant hlL-10. The protein showed full biological activity. LL108 is a L. lactis strain carrying a genomic integration of the repA gene, required for replication of pORH9 derived plasmids such as pOThy11 , pOThy12, pOThy15 or pOThy16. This strain was kindly donated by dr. Jan Kok, University of Groningen. The plasmids pOThy11, pOThy12, pOThy15 and pOThy16 carry the synthetic human IL-10 gene in different promotor configurations (see Fig. 2), flanked by approximately 1kB of genomic DNA derived from the thyA locus, upstream and downstream from thyA. These plasmids were used for the construction of the genomic integration as described.

The effect of the thymidilate synthase deletion on the growth in thymidine less and thymidine supplemented media was tested; the results are summarized in figures 10 and 11. Absence of thymidine in the medium strongly limits the growth of the mutant, and even results in a decrease of colony forming units after four hours of cultivation. Addition of thymidine to the medium results in an identical growth curve and amount of colony forming units, compared to the wild type strain, indicating that the mutant doesn't affect the growth or viability in thymidine supplemented medium. Fig. 11 clearly demonstrates that Thy12 viability is severely impaired in the absence of thymidine. Fig. 12 finally shows that L. lactis Thy12 passes the intestine of the mice at the same speed as MG1363. Loss of viability does not appear different between Thy12 and MG1363. Thy12 appear fully dependent on thymidine for growth, indicating that no Thy12 bacteria had taken up a foreign thyA gene. References

- Altschul, S. F., Madden, T.L, Schaffer, A.A., Zhang, J., Zhang, Z., Miller, W. and Lipman D.J. (1997). Gapped BLAST and PSI-BLAST: a new generation of protein database search programs. Nucleic Acids Res. 25, 3389 - 3402. - Biswas, I., Gruss, A., Ehrlich, S.D. et al. (1993) High-efficiency gene inactivation and replacement system for gram- positive bacteria. J. Bacteriol. 175, 3628 - 3635.

- Gasson, M. J. (1983). Plasmid complements of Streptococcus lactis NCDO 712 and other lactic streptococci after protoplast-induced curing. J. Bacteriol. 154, 1 - 9.

- Kaplan, D.L., Mello, C, Sano, T., Cantor, C. and Smith, C. (1999). Streptavadin- based containment system for genetically engineered microorganisms. Biomol. Eng.

31, 135 - 140.

- Knudsen, S., Saadbye, P., Hansen, L.H., Collier, A., Jacobsen, B.L., Schlundt, J. And Karistrom, O.H. (1995). Development and testing of improved suicide functions for biological containment of bacteria. Appl. Environ. Microbiol. 61, 985 - 991. - Law, J., Buist, G., Haandrikman, A. et al. (1995). A system to generate chromosomal mutations in Lactococcus lactis which allows fast analysis of targeted genes. J. Bacteriol. 177, 7011 - 7018.

- Molina, L., Ramos, C, Ronchel, M.C., Molin, S. and Ramos, J.L. (1998). Construction of an efficient biologically contained pseudomonas putida strain and its survival in outdoor assays. Appl. Environ. Microbiol. 64, 2072 - 2078.

- Ross, P., O'Gara, F. and Condon, S. (1990a). Cloning and characterization of the thymidylate synthase gene from Lactococcus lactis subsp. Lactis. Appl. Environ. Microbiol. 56, 2156-2163.

- Ross, P., O'Gara, F. and Condon, S. (1990b). Thymidylate synthase gene from Lactococcus lactis as a genetic marker: an alternative to antibiotic resistance. Appl.

Environ. Microbiol. 56, 2164-2169.

- Schweder, T., Hofmann, K. And Hecker, M. (1995). Escherichia co// K12 relA strains as safe hosts for expression of recombinant DNA. Appl. Environ. Microbiol. 42, 718 - 723. - Steidler, L, Hans, W., Schotte, L, Neirynck, S., Obermeier, F., Falk, W., Fier, W. and Remaut, E. (2000). Treatment of murine colitis by Lactococcus lactis secreting Interleukin-10. Science 289, 1352 - 1355. - Steidler, L., Wells, J.M., Raeymaekers, A., Vandekerckhove, J., Fiers, W. And Remaut, E. (1995). Secretion of biologically active murine lnterleukin-2 by Lactococcus lactis subsp. Lactis. Appl. Environ. Microbiol. 61 , 1627-1629.

- Tedin, K. Witte, A., Reisinger, G., Lubitz, W. and Basi, U. (1995). Evaluation of the E. coli ribosomal rrnB P1 promoter and phage derived lysis genes for the use in biological containment system: a concept study. J. Biotechnol. 39, 137 - 148.

- van Asseldonk, M., Rutten, G., Oteman, M. et al. (1990). Cloning of usp45, a gene encoding a secreted protein from Lactococcus lactis subsp. lactis MG1363. Gene 95, 155 - 160. - Waterfield, N.R., Le Page, R.W., Wilson, P.W. et al. (1995) The isolation of lactococcal promoters and their use in investigating bacterial luciferase synthesis in Lactococcus lactis. Gene 165,9 - 15.

- Wells, J.M. and Schofield, K.M. (1996) Cloning and expression vectors for Lactococci. Nato ASI series H 98, 37 - 62.

Claims

1. An isolated strain of Lactococcus sp. comprising a defective thymidylate synthase gene.

2. An isolated strain of Lactococcus sp. according to claim 1 , whereby said gene is inactivated by gene disruption.

3. An isolated strain of Lactococcus sp. according to claim 1 or 2, whereby said Lactococcus sp. is Lactococcus lactis.

4. The use of a strain of Lactococcus sp. according to any of the claims 1-3 as host strain for transformation, whereby the transforming plasmid does not comprise an intact thymidylate synthase gene.

5. A transformed strain of Lactococcus sp. according to any of the claims 1-3, comprising a transforming plasmid that does not comprise an intact thymidylate synthase gene.

6. A transformed strain of Lactococcus sp. according to any of the claims 1-4 comprising a gene encoding a prophylactic and/or therapeutic molecule.

7. A transformed strain of Lactococcus sp. according to claim 6 wherein said prophylactic and/or therapeutic molecule is interleukin-10.

8. The use of a transformed strain of Lactococcus sp. according to any of the claims 5-7 for the delivery of prophylactic and/or therapeutic molecules.

9. A pharmaceutical composition comprising a transformed strain of Lactococcus sp. according to any of the claims 5-7.

10. The use of a transformed strain of Lactococcus sp. according to any of the claims 6-7 for the preparation of a medicament to treat inflammatory bowel diseases.