Bacterial vaccine vectors and bacillus Calmette-Guérin

Mycobacterium avium subsp. paratuberculosis Candidate Vaccine Strains Are Pro-apoptotic in RAW 264.7 Murine Macrophages

Mycobacterium avium subsp. paratuberculosis (MAP) is the etiological agent of Johne's disease, a severe gastroenteritis of ruminants. This study developed a model cell culture system to rapidly screen MAP mutants with vaccine potential for apoptosis. Two wild-type strains, a transposon mutant, and two deletion mutant MAP strains (MOI of 10 with 1.2 × 10⁶ CFU) were tested in murine RAW 264.7 macrophages to determine if they induce apoptosis and/or necrosis. Both deletion mutants were previously shown to be attenuated and immunogenic in primary bovine macrophages. All strains had similar growth rates, but cell morphology indicated that both deletion mutants were elongated with cell wall bulging. Cell death kinetics were followed by a real-time cellular assay to measure luminescence (apoptosis) and fluorescence (necrosis). A 6 h infection period was the appropriate time to assess apoptosis that was followed by secondary necrosis. Apoptosis was also quantified via DAPI-stained nuclear morphology and validated via flow cytometry. The combined analysis confirmed the hypothesis that candidate vaccine deletion mutants are pro-apoptotic in RAW 264.7 cells. In conclusion, the increased apoptosis seen in the deletion mutants correlates with the attenuated phenotype and immunogenicity observed in bovine macrophages, a property associated with good vaccine candidates.

Harnessing Mycobacterium bovis BCG Trained Immunity to Control Human and Bovine Babesiosis

Babesiosis is a disease caused by tickborne hemoprotozoan apicomplexan parasites of the genus Babesia that negatively impacts public health and food security worldwide. Development of effective and sustainable vaccines against babesiosis is currently hindered in part by the absence of definitive host correlates of protection. Despite that, studies in Babesia microti and Babesia bovis, major causative agents of human and bovine babesiosis, respectively, suggest that early activation of innate immune responses is crucial for vertebrates to survive acute infection. Trained immunity (TI) is defined as the development of memory in vertebrate innate immune cells, allowing more efficient responses to subsequent specific and non-specific challenges. Considering that Mycobacterium bovis bacillus Calmette-Guerin (BCG), a widely used anti-tuberculosis attenuated vaccine, induces strong TI pro-inflammatory responses, we hypothesize that BCG TI may protect vertebrates against acute babesiosis. This premise is supported by early investigations demonstrating that BCG inoculation protects mice against experimental B. microti infection and recent observations that BCG vaccination decreases the severity of malaria in children infected with Plasmodium falciparum, a Babesia-related parasite. We also discuss the potential use of TI in conjunction with recombinant BCG vaccines expressing Babesia immunogens. In conclusion, by concentrating on human and bovine babesiosis, herein we intend to raise awareness of BCG TI as a strategy to efficiently control Babesia infection.

Stable Expression of Lentiviral Antigens by Quality-Controlled Recombinant Mycobacterium bovis BCG Vectors

The well-established safety profile of the tuberculosis vaccine strain, Mycobacterium bovis bacille Calmette-Guérin (BCG), makes it an attractive vehicle for heterologous expression of antigens from clinically relevant pathogens. However, successful generation of recombinant BCG strains possessing consistent insert expression has encountered challenges in stability. Here, we describe a method for the development of large recombinant BCG accession lots which stably express the lentiviral antigens, human immunodeficiency virus (HIV) gp120 and simian immunodeficiency virus (SIV) Gag, using selectable leucine auxotrophic complementation. Successful establishment of vaccine stability stems from stringent quality control criteria which not only screen for highly stable complemented BCG ΔleuCD transformants but also thoroughly characterize postproduction quality. These parameters include consistent production of correctly sized antigen, retention of sequence-pure plasmid DNA, freeze-thaw recovery, enumeration of CFU, and assessment of cellular aggregates. Importantly, these quality assurance procedures were indicative of overall vaccine stability, were predictive for successful antigen expression in subsequent passaging both in vitro and in vivo, and correlated with induction of immune responses in murine models. This study has yielded a quality-controlled BCG ΔleuCD vaccine expressing HIV gp120 that retained stable full-length expression after 10(24)-fold amplification in vitro and following 60 days of growth in mice. A second vaccine lot expressed full-length SIV Gag for >10(68)-fold amplification in vitro and induced potent antigen-specific T cell populations in vaccinated mice. Production of large, well-defined recombinant BCG ΔleuCD lots can allow confidence that vaccine materials for immunogenicity and protection studies are not negatively affected by instability or differences between freshly grown production batches.

Priming with a recombinant pantothenate auxotroph of Mycobacterium bovis BCG and boosting with MVA elicits HIV-1 Gag specific CD8+ T cells

A safe and effective HIV vaccine is required to significantly reduce the number of people becoming infected with HIV each year. In this study wild type Mycobacterium bovis BCG Pasteur and an attenuated pantothenate auxotroph strain (BCGΔpanCD) that is safe in SCID mice, have been compared as vaccine vectors for HIV-1 subtype C Gag. Genetically stable vaccines BCG[pHS400] (BCG-Gag) and BCGΔpanCD[pHS400] (BCGpan-Gag) were generated using the Pasteur strain of BCG, and a panothenate auxotroph of Pasteur respectively. Stability was achieved by the use of a codon optimised gag gene and deletion of the hsp60-lysA promoter-gene cassette from the episomal vector pCB119. In this vector expression of gag is driven by the mtrA promoter and the Gag protein is fused to the Mycobacterium tuberculosis 19 kDa signal sequence. Both BCG-Gag and BCGpan-Gag primed the immune system of BALB/c mice for a boost with a recombinant modified vaccinia virus Ankara expressing Gag (MVA-Gag). After the boost high frequencies of predominantly Gag-specific CD8(+) T cells were detected when BCGpan-Gag was the prime in contrast to induction of predominantly Gag-specific CD4(+) T cells when priming with BCG-Gag. The differing Gag-specific T-cell phenotype elicited by the prime-boost regimens may be related to the reduced inflammation observed with the pantothenate auxotroph strain compared to the parent strain. These features make BCGpan-Gag a more desirable HIV vaccine candidate than BCG-Gag. Although no Gag-specific cells could be detected after vaccination of BALB/c mice with either recombinant BCG vaccine alone, BCGpan-Gag protected mice against a surrogate vaccinia virus challenge.

Immunogenicity and reactivity of novel Mycobacterium avium subsp. paratuberculosis PPE MAP1152 and conserved MAP1156 proteins with sera from experimentally and naturally infected animals

Mycobacterium avium subsp. paratuberculosis causes Johne's disease (JD) in ruminants. Development of genetic tools and completion of the M. avium subsp. paratuberculosis genome sequencing project have expanded the opportunities for antigen discovery. In this study, we determined the seroreactivities of two proteins encoded at the 5' and 3' regions of the MAP1152-MAP1156 gene cluster. MAP1152 encodes a PPE protein, and MAP1156 encodes a diacylglycerol acyltransferase involved in triglyceride metabolism and classified in the uncharacterized protein family UPF0089. Recombinant MAP proteins were overproduced and purified from Escherichia coli as maltose-binding protein (MBP) fusions. Immunoblotting analysis indicated that both MAP1152 and MAP1156 displayed reactivity against sera of mice and rabbits immunized with live M. avium subsp. paratuberculosis cells and against samples from naturally infected cattle. In immunoblot assays, MAP1156 yielded a stronger positive signal than MAP1152 against sera from cattle with JD. An enzyme-linked immunosorbent assay for the recombinant proteins was developed and used to test preclassified positive and negative serum samples from naturally infected and noninfected cattle. Samples, with one exception, displayed no seroreactivity against the MBP-LacZ fusion protein (P > 0.05), the negative-control antigen. MAP1152 displayed seroreactivity against all positive sera but no seroreactivity to the negative sera (P < 0.01). MAP1156 displayed stronger and more variable reactivity than MAP1152, but significant differences were observed between noninfected and infected cattle (P < 0.05). Otherwise, degrees of reactivity followed the same trend as the positive reference antigen. In conclusion, both proteins are immunogenic in mice and rabbits, and M. avium subsp. paratuberculosis-infected cattle mount a humoral response to both MAP1152 and MAP1156 cross-reactive epitopes. These findings have potential applications to diagnostics, vaccine production, and elucidation of the immunopathogenesis of JD.

Molecular characterization of heterologous HIV-1gp120 gene expression disruption in mycobacterium bovis BCG host strain: a critical issue for engineering mycobacterial based-vaccine vectors

Mycobacterium bovis Bacillus Calmette-Guérin (BCG) as a live vector of recombinant bacterial vaccine is a promising system to be used. In this study, we evaluate the disrupted expression of heterologous HIV-1gp120 gene in BCG Pasteur host strain using replicative vectors pMV261 and pJH222. pJH222 carries a lysine complementing gene in BCG lysine auxotrophs. The HIV-1 gp120 gene expression was regulated by BCG hsp60 promoter (in plasmid pMV261) and Mycobacteria spp. α-antigen promoter (in plasmid pJH222). Among 14 rBCG:HIV-1gp120 (pMV261) colonies screened, 12 showed a partial deletion and two showed a complete deletion. However, deletion was not observed in all 10 rBCG:HIV-1gp120 (pJH222) colonies screened. In this study, we demonstrated that E. coli/Mycobacterial expression vectors bearing a weak promoter and lysine complementing gene in a recombinant lysine auxotroph of BCG could prevent genetic rearrangements and disruption of HIV 1gp120 gene expression, a key issue for engineering Mycobacterial based vaccine vectors.

Archaeosomes as Self-adjuvanting Delivery Systems for Cancer Vaccines*

Archaeal ether glycerolipid vesicles (archaeosomes) efficiently deliver exogenous antigen for induction of humoral and cell-mediated immunity. Because induction of CD8 cytotoxic T cells is critical for protective vaccination against tumors, we compared the ability of various archaeosome lipid compositions to evoke a strong CD8 CTL response to entrapped antigen. Subcutaneous immunization of mice with ovalbumin (OVA) entrapped in all archaeosome lipid compositions evoked a primary (day 10) splenic CTL response indicating processing for MHC class I presentation. Interestingly, several polar lipid compositions from halophilic archaea were very potent to adjuvant this early CTL response. Despite this, the lytic units reduced substantially by weeks 6-7. More importantly, at >50 weeks, only Methanobrevibacter smithii and Thermoplasma acidophilum both rich in bipolar membrane-spanning caldarchaeols, demonstrated recall memory CTLs. Immunization of mice with OVA entrapped in M. smithii, Halobacterium salinarum or T. acidophilum vesicles provided prophylactic protection against challenge with OVA-expressing solid tumors at 6 weeks. Even a dose of 3 microg OVA in archaeosomes significantly delayed tumor growth. Tumor protection was also noted in a therapeutic design wherein OVA-archaeosomes were injected concurrent with the tumor challenge. Interestingly, antigen-free T. acidophilum but not antigen-free H. salinarum archaeosomes provided innate therapeutic protection. Vaccination with a CTL peptide epitope from the melanoma differentiation antigen, tyrosinase-related protein 2, in archaeosomes induced a protective CD8 response against B16OVA metastasis, indicating potential for targeting self, tumor antigens. Thus, lipid structural properties of archaea may differentially modulate primary, long-term and/or innate immunity, impacting adjuvant choice for vaccine design.

Cell-mediated immunity induced by recombinant Mycobacterium bovis Bacille Calmette-Guérin strains against an intracellular bacterial pathogen: importance of antigen secretion or membrane-targeted antigen display as lipoprotein for vaccine efficacy

Live recombinant vaccines expressing defined pathogen-derived Ags represent powerful candidates for future vaccination strategies. In this study, we report on the differential induction of protective cell-mediated immunity elicited by different recombinant Mycobacterium bovis Bacille Calmette-Guérin (BCG) strains displaying p60 Ag of Listeria monocytogenes in secreted, cytosolic, or membrane-attached form for T cell recognition. Anti-listerial protection evoked by the membrane-linked p60 lipoprotein of rBCG Mp60 and that of the p60 derivative secreted by rBCG Sp60-40 were nearly equal, whereas cytosolic p60 displayed by rBCG Np60 failed to protect mice from listeriosis. In vivo depletion of CD4 or CD8 T cell subpopulations in rBCG Mp60-vaccinated mice before listerial challenge revealed interactions of both T cell subsets in anti-listerial protection. In rBCG Sp60-40-vaccinated animals, CD4 T cells predominantly contributed to anti-listerial control as shown by the failure of anti-CD8 mAb treatment to impair the outcome of listeriosis in rBCG Sp60-40-vaccinated mice after L. monocytogenes challenge. Hence, differential Ag display by rBCG influences cell-mediated immunity, which in turn may impact vaccine efficacy due to the different requirements of CD4 or CD8 T cells for pathogen elimination.

Expression of the B-cell and T-cell epitopes of the rabies virus nucleoprotein in Mycobacterium bovis BCG and induction of an humoral response in mice

Expression vectors containing rabies virus nucleoprotein B-cell and T-cell epitopes in Mycobacterium bovis BCG were constructed. The epitopes were subcloned into the M. leprae 18-kDa gene to ensure correct presentation to the host immune system. Expression of the 18-kDa::B+T epitope fusion protein was driven by either the hsp60 promoter, which is constitutively activated at a high level in M. bovis BCG, or the 18-kDa promoter, which is strongly induced in vivo. Mice were immunised intra-peritoneally with the recombinant BCG cultures and compared to a control group vaccinated with the commercial rabies vaccine Rai-SAD. Both of the expression vectors elicited a higher antibody titre than that of the rabies vaccine, with the highest response shown by M. bovis BCG (pUP203), expression controlled by the 18-kDa promoter. Immunisation with M. bovis BCG (pUP202), expression controlled by the hsp60 promoter, resulted in a continuously increasing antibody titre up to 60 days post immunisation. The mice antibodies were also capable of recognising the whole rabies virus and not only the synthetic peptide epitopes.

Mycobacterium avium subsp. paratuberculosis in Veterinary Medicine

Mycobacterium avium subsp. paratuberculosis (basonym M. paratuberculosis) is the etiologic agent of a severe gastroenteritis in ruminants known as Johne's disease. Economic losses to the cattle industry in the United States are staggering, reaching $1.5 billion annually. A potential pathogenic role in humans in the etiology of Crohn's disease is under investigation. In this article, we review the epidemiology, pathogenesis, diagnostics, and disease control measures of this important veterinary pathogen. We emphasize molecular genetic aspects including the description of markers used for strain identification, diagnostics, and phylogenetic analysis. Recent important advances in the development of animal models and genetic systems to study M. paratuberculosis virulence determinants are also discussed. We conclude with proposals for the applications of these models and recombinant technology to the development of diagnostic, control, and therapeutic measures.

Elicitation of predictable immune responses by using live bacterial vectors

There is an increasing need for novel vaccines able to stimulate efficient and long-lasting responses, which have also low production costs. To confer protective immunity following vaccination, the adequate type of response should be elicited. Vaccines based on attenuated bacterial carriers have contained production and delivery costs, and are able to stimulate more potent immune responses than non-replicating formulations. The improved knowledge on carrier physiology and host response, the availability of different mutants and highly sophisticated expression tools, and the possibility of co-administering modulators enable to trigger predictable responses according to the specific needs. Recent studies support the use of attenuated bacteria not only as conventional carriers, but also as a delivery system for DNA vaccines against infectious agents and tumors. In this review we discuss the most widely used bacterial carrier systems for either antigens or nucleic acid vaccines, and the strategies which have been successfully exploited to modulate the immune responses elicited.

Use of live bacterial vaccine vectors for antigen delivery: potential and limitations

Most infectious agents are restricted to the mucosal membranes or their transit through the mucosa constitutes a critical step in the infection process. Therefore, the elicitation of an efficient immune response, not only at systemic, but also at mucosal level, after vaccination is highly desirable, representing a significant advantage in order to prevent infection. This goal can be only achieved, when the vaccine formulation is administered by the mucosal route. However, soluble antigens given by this route are usually poorly immunogenic. Among the available approaches to stimulate efficient mucosal responses, the use of bacterial carriers to deliver vaccine antigens, probably, constitutes one of the most successful strategies. The potential and limitations of the most extensively studied bacterial carrier systems will be discussed.

Heterologous gene expression in bacterial systems under reduced oxygen tensions. Small-scale optimization precedes industrial fermentation

The expression of heterologous fusion proteins from the anaerobically inducible Escherichia coli nitrite reductase nirB promoter has been described using a number of different industrial regimes, but which have proved impractical for scaling down to suit primary research purposes. This paper describes the novel application of microbiological gas sachets generating anaerobic and microaerophilic environments to evaluate the inducible expression under the influence of nirB of heterologous proteins by attenuated vaccine strains of Salmonella typhimurium. The conditions of reduced oxygen tension model those found in lymphoid organs colonized by Salmonella in vivo and so can be used to optimize the vaccine dose prior to administration. Modeling in vivo promoter inducibility to monitor the stability of a plasmid within attenuated vaccine strains of bacteria offers an attractive alternative to antibiotic resistance, which is not permitted for clinical use in humans. This technological advance may be utilized to optimize heterologous gene expression in any microaerophilic bacterial system as a pilot, prior to production-scale applications.

Adenoviruses as vectors for delivering vaccines to mucosal surfaces

Immunization of mucosal surfaces has become an attractive route of vaccine delivery because of its ability to induce mucosal immunity. Although various methods of inducing mucosal immunity are being developed, our laboratory has focused on developing adenoviruses as replication-competent and replication-incompetent vectors. The present report will summarize our progress in sequencing the entire bovine adenovirus-3 genome and identifying regions which can be deleted and subsequently used as insertion sites for foreign genes in developing recombinant viral vaccines. Using these recombinant viruses, we demonstrated the 'proof-of-principle' in developing mucosal immunity and, more importantly, inducing protection against bovine herpes virus in a natural host-cattle. Finally, we demonstrated that immunity and protection occurred even in animals that had pre-existing antibodies to the vector.

Brucella abortus strain RB51 as a vector for heterologous protein expression and induction of specific Th1 type immune responses

Brucella abortus strain RB51 is a stable, rough, attenuated mutant widely used as a live vaccine for bovine brucellosis. Our ultimate goal is to develop strain RB51 as a preferential vector for the delivery of protective antigens of other intracellular pathogens to which the induction of a strong Th1 type of immune response is needed for effective protection. As a first step in that direction, we studied the expression of a foreign reporter protein, beta-galactosidase of Escherichia coli, and the 65-kDa heat shock protein (HSP65) of Mycobacterium bovis in strain RB51. We cloned the promoter sequences of Brucella sodC and groE genes in pBBR1MCS to generate plasmids pBBSODpro and pBBgroE, respectively. The genes for beta-galactosidase (lacZ) and HSP65 were cloned in these plasmids and used to transform strain RB51. An enzyme assay in the recombinant RB51 strains indicated that the level of beta-galactosidase expression is higher under the groE promoter than under the sodC promoter. In strain RB51 containing pBBgroE/lacZ, but not pBBSODpro/lacZ, increased levels of beta-galactosidase expression were observed after subjecting the bacteria to heat shock or following internalization into macrophage-like J774A.1 cells. Mice vaccinated with either of the beta-galactosidase-expressing recombinant RB51 strains developed specific antibodies of predominantly the immunoglobulin G2a (IgG2a) isotype, and in vitro stimulation of their splenocytes with beta-galactosidase induced the secretion of gamma interferon (IFN-gamma), but not interleukin-4 (IL-4). A Th1 type of immune response to HSP65, as indicated by the presence of specific serum IgG2a, but not IgG1, antibodies, and IFN-gamma, but not IL-4, secretion by the specific-antigen-stimulated splenocytes, was also detected in mice vaccinated with strain RB51 containing pBBgroE/hsp65. Studies with mice indicated that expression of beta-galactosidase or HSP65 did not alter either the attenuation characteristics of strain RB51 or its vaccine efficacy against B. abortus 2308 challenge.

New combination vaccines

Physicians can achieve full vaccination at each of the recommended visits by administering three or four injections, but for infants, four injections seem to be a maximum in light of the size of the infant thigh, which is the preferred injection site. Thus, physicians must make room for all of the new vaccines on the horizon. To accomplish that task, combinations will be necessary. Progress toward that goal is occurring.

Newborns Develop a Th1-Type Immune Response to Mycobacterium bovis Bacillus Calmette-Guérin Vaccination

Data obtained in animals indicate that neonatal immune responses are biased toward Th2. This could reduce the efficacy of vaccines against viral and mycobacterial diseases. The ability of human newborns to develop a Th1 immune response upon immunization has not been studied. Since the vaccine Mycobacterium bovis bacillus Calmette-Guérin (BCG) triggers a Th1-type response in adults, we investigated whether it induces a similar response in newborns and whether age at vaccination influences immunogenicity. We found that BCG vaccination at birth induces a memory Th1-type response of similar magnitude to that when given later in life. This study demonstrates that human newborns can be immunized against pathogens controlled by a Th1 immune response.

Oral vaccination of white-tailed deer using a recombinant Bacillus Calmette-Guérin vaccine expressing the Borrelia burgdorferi outer surface protein A: prospects for immunocontraception

PROBLEM

Reduction of excess numbers of white-tailed deer (Odocoileus virginianus) is a prime example of a potential use for immunocontraception as a means of wildlife population management. Oral vaccination appears to be the most pragmatic way to deliver immunocontraceptive vaccines to free-roaming populations of deer, but there was little, if any, prior evidence that oral vaccination is a viable concept in deer.

METHOD OF STUDY

We used live Bacillus Calmette-Guérin (BCG) in a recombinant form (rBCG), which expressed Borrelia burgdorferi outer surface protein A, to test whether deer vaccinated orally with a specific antigen expressed in a live vector produce detectable antibody titers.

RESULTS

The data indicate that oral vaccination of deer with an expressed antigen is feasible, as demonstrated by peak antibody titers to the expressed antigen. Also, peak titers measured by enzyme-linked immunosorbent assay were highest in orally vaccinated deer: 1600 in deer vaccinated by injection and 6400 in those vaccinated orally.

CONCLUSIONS

The results of this study demonstrate that it is feasible to vaccinate deer orally with a live vector.

New vaccines against tuberculosis. The status of current research

The increasing realization that the current vaccine for tuberculosis, bacille Calmette-Guérin (BCG), is of varying effectiveness, and is less protective in adults than in children, has prompted new research for a replacement. New research has resulted in innovative approaches, including the use of sub-unit vaccines, auxotropic vaccines, DNA vaccines, and recombinant vaccines, among others. This article reviews these approaches and test results in animal models, and discusses their potential for use in humans.

Genetic rearrangements leading to disruption of heterologous gene expression in mycobacteria: an observation with Escherichia coli beta-galactosidase in Mycobacterium smegmatis and its implication in vaccine development

Different mycobacteria carrying cloned genes for heterologous protective antigens have been proposed as vaccine vehicles. In this study, the stability of the expression of beta-galactosidase was studied in Mycobacterium smegmatis using integrative (pMV361::lacZ) and replicative (pMV261::lacZ) vectors. Recombinant M. smegmatis forms blue colonies on X-gal plates. Occasional white mutants encountered while plating on X-gal plates were genetically analysed. The loss of lacZ phenotype was due to insertion of an IS element in lacZ gene of integrative vector whereas in case of replicative vectors, loss of lacZ phenotype was due to deletions of different sizes in the lacZ gene and the Phsp60 promoter region. The frequency of such events was rare, 1.7 x 10(-5) in integrative vector and 2 x 10(-3) in the case of replicative vector. The integrative vector seemed more stable in terms of expression of foreign genes in mycobacteria. Hence, the rearrangements reported in the present study warrant serious consideration before implementing mycobacteria as recombinant vaccines.

Genetic vaccination against tuberculosis

New weapons are needed in the fight against tuberculosis. Recent research indicates that a vaccine better than BCG may be within reach. A diverse range of protein antigens can give encouragingly high levels of protective immunity in animal models when administered with adjuvants or as DNA vaccines. Accelerated arrest of bacterial multiplication followed by sustained decline in bacterial numbers are key parameters of protection and so the vaccine must target antigens produced by both actively multiplying and growth-inhibited bacteria. Consistent with this, the protective antigens have been found among secreted and stress proteins (e.g. Ag85, ESAT-6, hsp65, hsp70). Species-specific antigens are not needed, hence these remain available for diagnostic tests. Adoptive transfer of protection from vaccinated or infected mice into naive mice by transfer of purified T cells and clones shows that protection is expressed by antigen-specific cytotoxic T cells that produce interferon-gamma and lyse infected macrophages. These cells are produced in response to endogenous antigen. DNA vaccination appears to be an excellent way of generating these cells and may be able to give long-lasting protection.

A method for construction of E. coli strains with multiple DNA insertions in the chromosome

A system for construction of E. coli strains with multiple DNA insertions in the chromosome, based on elements of modules for site specific recombination of Tn1545 and phage lambda, has been developed. Circular non-replicating DNA fragments containing the transposon attachment site (attTn), an excisable cassette with a selectable marker, and a gene of interest integrate randomly into the chromosome of a host E. coli strain when provided with transposon integrase, Int-Tn (the host strain was obtained by insertion of the fragment containing transposon int-Tn gene coding for Int-Tn into the chromosome). Integration of these fragments into the chromosome of int-Tn+ cells gives rise to a collection of antibiotic-resistant clones with single insertions at different locations in the chromosome. These insertions are transferred subsequently by P1 transduction into one strain and selected for antibiotic resistance provided by the cassette with the selectable marker. After transduction of each copy, a helper plasmid bearing phage lambda xis and int genes is introduced into the cells to excise the drug resistance gene flanked with the lambda attL and lambda attR sites from the chromosome. Cells cured of the helper plasmid can undergo the next cycle of P1 transduction/drug resistance gene excision. Each cycle adds another chromosomal copy of the foreign gene. To show the utility of the system, we constructed an E. coli strain bearing several chromosomal copies of lacZ at different locations.

Display of heterologous proteins on the surface of microorganisms: from the screening of combinatorial libraries to live recombinant vaccines

In recent years there has been considerable progress towards the development of expression systems for the display of heterologous polypeptides and, to a lesser extent, oligosaccharides on the surface of bacteria or yeast. The availability of protein display vectors has in turn provided the impetus for a range of exciting technologies. Polypeptide libraries can be displayed in bacteria and screened by cell sorting techniques, thus simplifying the isolation of proteins with high affinity for ligands. Expression of antigens on the surface of nonvirulent microorganisms is an attractive approach to the development of high-efficacy recombinant live vaccines. Finally, cells displaying protein receptors or antibodies are of use for analytical applications and bioseparations.

Why sequence the genome of Mycobacterium tuberculosis?

Radical measures are required to prevent the grim predictions of the World Health Organisation for the deterioration of the global tuberculosis epidemic in the next century from becoming reality. Study of the nerve centre of the tubercle bacillus, its genome, by means of systematic deoxyribonucleic acid sequence analysis will provide a wealth of information about Mycobacterium tuberculosis that will undoubtedly fuel the next generation of research. In the coming year, this highly cost-effective approach will deliver an unprecedented amount of knowledge to catalyse the development of new, more efficient diagnostic tools and therapeutic interventions to detect, control and, ultimately, eliminate tuberculosis.