The urease locus of Mycobacterium tuberculosis and its utilization for the demonstration of allelic exchange in Mycobacterium bovis bacillus Calmette-Guérin

Therapeutic Potentials of Immunometabolomic Modulations Induced by Tuberculosis Vaccination

Metabolomics is emerging as a promising tool to understand the effect of immunometabolism for the development of novel host-directed alternative therapies. Immunometabolism can modulate both innate and adaptive immunity in response to pathogens and vaccinations. For instance, infections can affect lipid and amino acid metabolism while vaccines can trigger bile acid and carbohydrate pathways. Metabolomics as a vaccinomics tool, can provide a broader picture of vaccine-induced biochemical changes and pave a path to potentiate the vaccine efficacy. Its integration with other systems biology tools or treatment modes can enhance the cure, response rate, and control over the emergence of drug-resistant strains. Mycobacterium tuberculosis (Mtb) infection can remodel the host metabolism for its survival, while there are many biochemical pathways that the host adjusts to combat the infection. Similarly, the anti-TB vaccine, Bacillus Calmette-Guerin (BCG), was also found to affect the host metabolic pathways thus modulating immune responses. In this review, we highlight the metabolomic schema of the anti-TB vaccine and its therapeutic applications. Rewiring of immune metabolism upon BCG vaccination induces different signaling pathways which lead to epigenetic modifications underlying trained immunity. Metabolic pathways such as glycolysis, central carbon metabolism, and cholesterol synthesis play an important role in these aspects of immunity. Trained immunity and its applications are increasing day by day and it can be used to develop the next generation of vaccines to treat various other infections and orphan diseases. Our goal is to provide fresh insight into this direction and connect various dots to develop a conceptual framework.

Optimizing the Boosting Schedule of Subunit Vaccines Consisting of BCG and "Non-BCG" Antigens to Induce Long-Term Immune Memory

Boosting Bacillus Calmette-Guérin (BCG) with subunit vaccine is expected to induce long-term protection against tuberculosis (TB). However, it is urgently needed to optimize the boosting schedule of subunit vaccines, which consists of antigens from or not from BCG, to induce long-term immune memory. To address it two subunit vaccines, Mtb10.4-HspX (MH) consisting of BCG antigens and ESAT6-CFP10 (EC) consisting of antigens from the region of difference (RD) of Mycobacterium tuberculosis (M. tuberculosis), were applied to immunize BCG-primed C57BL/6 mice twice or thrice with different intervals, respectively. The long-term antigen-specific immune responses and protective efficacy against M. tuberculosis H37Ra were determined. The results showed that following BCG priming, MH boosting twice at 12-24 weeks or EC immunizations thrice at 12-16-24 weeks enhanced the number and function of long-lived memory T cells with improved protection against H37Ra, while MH boosting thrice at 12-16-24 weeks or twice at 8-14 weeks and EC immunizations twice at 12-24 weeks or thrice at 8-10-14 weeks didn't induce long-term immunity. It suggests that following BCG priming, both BCG antigens MH boosting twice and "non-BCG" antigens EC immunizations thrice at suitable intervals induce long-lived memory T cell-mediated immunity.

Oligo-Mediated Recombineering and its Use for Making SNPs, Knockouts, Insertions, and Fusions in Mycobacterium tuberculosis

Phage recombination systems have been instrumental in the development of gene modification technologies for bacterial pathogens. In particular, the Che9 phage RecET system has been used successfully for over 10 years for making gene knockouts and fusions in Mycobacterium tuberculosis. This "recombineering" technology typically uses linear dsDNA substrates that contain a drug-resistance marker flanked by (up to) 500 base pairs of DNA homologous to the target site. Less often employed in mycobacterial recombineering is the use of oligonucleotides, which require only the action of the RecT annealase to align oligos to ssDNA regions of the replication fork, for subsequent incorporation into the chromosome. Despite the higher frequency of such events relative to dsDNA-promoted recombineering, oligo-mediated changes generally suffer from the disadvantage of not being selectable, thus making them harder to isolate. This chapter discusses steps and methodologies that increase the frequencies of finding oligo-mediated events, including the transfer of single nucleotide polymorphisms (SNPs) to mycobacterial chromosomes, and the use of oligos in conjunction with the mycobacterial phage Bxb1 site-specific recombination system for the easy generation of knockouts, insertion, and fusions, in a protocol known as ORBIT.

Production of antibiotic resistance gene-free urease-deficient recombinant BCG that secretes antigenic protein applicable for practical use in tuberculosis vaccination

Mycobacterium bovis BCG has been the only practical vaccine for tuberculosis. However, BCG cannot fully prevent adult pulmonary tuberculosis. Therefore, the improvement of BCG vaccine is necessary. We previously produced recombinant (r) BCG (BCG-PEST) for the better control of tuberculosis. BCG-PEST was developed by introducing PEST-Heat Shock Protein (HSP)70-Major Membrane Protein (MMP)-II-PEST fusion gene into urease-deficient rBCG using antibiotic-resistant gene for the selection of rBCG. HSP70-MMPII fusion protein is highly immunogenic and PEST sequence was added to enhance processing of the fusion protein. Although BCG-PEST effectively inhibited intrapulmonary growth of Mycobacterium tuberculosis (MTB), BCG with antibiotic-resistant gene is not appropriate for human use. Therefore, we produced antibiotic-resistant gene-free rBCG. We generated leucine-biosynthetic gene (leuD)-deficient BCG and introduced the fusion gene with leuD as the selection marker and named this rBCG as BCG-LeuPH. BCG-LeuPH activated human naïve T cells of both CD4 and CD8 subsets and efficiently inhibited aerosol-challenged MTB in mice. These results indicate that leuD can replace antibiotic-resistant gene for the selection of vaccine candidates of rBCG for human use.

Vaccines Against Tuberculosis: Problems and Prospects (Review)

Despite the efforts of the global medical and scientific community, tuberculosis remains the leading cause of death from infectious diseases. The expectation of success associated with the development of new anti-TB drugs was not justified, and the attention of researchers was largely drawn to the creation of new mycobacterial strains for vaccination against tuberculosis. The proposed review contains current information on the existing vaccine strains and the development of new, genetically engineered strains for the prevention of tuberculosis and the prevention and treatment of other diseases. The review includes relevant information on the correlation between BCG vaccination and the frequency and severity of COVID-19 infection.

Tuberculosis vaccine development: from classic to clinical candidates

Bacillus Calmette-Guérin (BCG) has been in use for nearly 100 years and is the only licensed TB vaccine. While BCG provides protection against disseminated TB in infants, its protection against adult pulmonary tuberculosis (PTB) is variable. To achieve the ambitious goal of eradicating TB worldwide by 2050, there is an urgent need to develop novel TB vaccines. Currently, there are more than a dozen novel TB vaccines including prophylactic and therapeutic at different stages of clinical research. This literature review provides an overview of the clinical status of candidate TB vaccines and discusses the challenges and future development trends of novel TB vaccine research in combination with the efficacy of evaluation of TB vaccines, provides insight for the development of safer and more efficient vaccines, and may inspire new ideas for the prevention of TB.

The Recombinant Bacille Calmette-Guérin Vaccine VPM1002: Ready for Clinical Efficacy Testing

The only licensed vaccine against tuberculosis (TB), bacille Calmette-Guérin (BCG), protects against severe extrapulmonary forms of TB but is virtually ineffective against the most prevalent form of the disease, pulmonary TB. BCG was genetically modified at the Max Planck Institute for Infection Biology to improve its immunogenicity by replacing the urease C encoding gene with the listeriolysin encoding gene from Listeria monocytogenes. Listeriolysin perturbates the phagosomal membrane at acidic pH. Urease C is involved in neutralization of the phagosome harboring BCG. Its depletion allows for rapid phagosome acidification and promotes phagolysosome fusion. As a result, BCGΔureC::hly (VPM1002) promotes apoptosis and autophagy and facilitates release of mycobacterial antigens into the cytosol. In preclinical studies, VPM1002 has been far more efficacious and safer than BCG. The vaccine was licensed to Vakzine Projekt Management and later sublicensed to the Serum Institute of India Pvt. Ltd., the largest vaccine producer in the world. The vaccine has passed phase I clinical trials in Germany and South Africa, demonstrating its safety and immunogenicity in young adults. It was also successfully tested in a phase IIa randomized clinical trial in healthy South African newborns and is currently undergoing a phase IIb study in HIV exposed and unexposed newborns. A phase II/III clinical trial will commence in India in 2017 to assess efficacy against recurrence of TB. The target indications for VPM1002 are newborn immunization to prevent TB as well as post-exposure immunization in adults to prevent TB recurrence. In addition, a Phase I trial in non-muscle invasive bladder cancer patients has been completed, and phase II trials are ongoing. This review describes the development of VPM1002 from the drawing board to its clinical assessment.

Enhanced protective efficacy against tuberculosis provided by a recombinant urease deficient BCG expressing heat shock protein 70-major membrane protein-II having PEST sequence

Enhancement of the T cell-stimulating ability of Mycobacterium bovis BCG (BCG) is necessary to develop an effective tuberculosis vaccine. For this purpose, we introduced the PEST-HSP70-major membrane protein-II (MMPII)-PEST fusion gene into ureC-gene depleted recombinant (r) BCG to produce BCG-PEST. The PEST sequence is involved in the proteasomal processing of antigens. BCG-PEST secreted the PEST-HSP70-MMPII-PEST fusion protein and more efficiently activated human monocyte-derived dendritic cells (DCs) in terms of phenotypic changes and cytokine productions than an empty-vector-introduced BCG or HSP70-MMPII gene-introduced ureC gene-depleted BCG (BCG-DHTM). Autologous human naïve CD8⁺ T cells and naïve CD4⁺ T cells were effectively activated by BCG-PEST and produced IFN-γ in an antigen-specific manner through DCs. These T cell activations were closely associated with phagosomal maturation and intraproteasomal protein degradation in antigen-presenting cells. Furthermore, BCG-PEST produced long-lasting memory-type T cells in C57BL/6 mice more efficiently than control rBCGs. Moreover, a single subcutaneous injection of BCG-PEST more effectively reduced the multiplication of subsequent aerosol-challenged Mycobacterium tuberculosis of the standard H37Rv strain and clinically isolated Beijing strain in the lungs than control rBCGs. The vaccination effect of BCG-PEST lasted for at least 6months. These results indicate that BCG-PEST may be able to efficiently control the spread of tuberculosis in human.

Mycobacterial recombineering

The precise knockout or modification of Mycobacterium tuberculosis genes has been critical for the identification of functions important for the growth and pathogenicity of this important bacterium. Schemes have been previously described, using both non-replicating vectors and transducing particles, for the introduction of gene knockout substrates into M. tuberculosis, where the endogenous recombination systems of the host (both homologous and illegitimate) compete for transfer of the modified allele to the chromosome. Recombineering technologies, first introduced in laboratory and pathogenic strains of Escherichia coli over the last 16 years, have been developed for use in M. tuberculosis. Described in this chapter is the use of the mycobacterial Che9c phage RecET recombination system, which has been used to make gene knockouts, reporter fusions, promoter replacements, and single base pair modifications within the M. tuberculosis and M. smegmatis chromosomes at very high frequency. Higher success rates, in a shorter period of time, are routinely observed when recombineering is compared to previously described M. tuberculosis gene knockout protocols.

Polyclonal activation of naïve T cells by urease deficient-recombinant BCG that produced protein complex composed of heat shock protein 70, CysO and major membrane protein-II

BACKGROUND

Mycobacterium bovis bacillus Calmette-Guérin (BCG) is known to be only partially effective in inhibiting M. tuberculosis (MTB) multiplication in human. A new recombinant (r) urease-deficient BCG (BCG-dHCM) that secretes protein composed of heat shock protein (HSP)70, MTB-derived CysO and major membrane protein (MMP)-II was produced for the efficient production of interferon gamma (IFN-γ) which is an essential element for mycobacteriocidal action and inhibition of neutrophil accumulation in lungs.

METHODS

Human monocyte-derived dendritic cells (DC) and macrophages were differentiated from human monocytes, infected with BCG and autologous T cells-stimulating activity of different constructs of BCG was assessed. C57BL/6 mice were used to test the effectiveness of BCG for the production of T cells responsive to MTB-derived antigens (Ags).

RESULTS

BCG-dHCM intracellularly secreted HSP70-CysO-MMP-II fusion protein, and activated DC by up-regulating Major Histcompatibility Complex (MHC), CD86 and CD83 molecules and enhanced various cytokines production from DC and macrophages. BCG-dHCM activated naïve T cells of both CD4 and CD8 subsets through DC, and memory type CD4+ T cells through macrophages in a manner dependent on MHC and CD86 molecules. These T cell activations were inhibited by the pre-treatment of Ag-presenting cells (APCs) with chloroquine. The single and primary BCG-dHCM-inoculation produced long lasting T cells responsive to in vitro secondarily stimulation with HSP70, CysO, MMP-II and H37Rv-derived cytosolic protein, and partially inhibited the replication of aerosol-challenged MTB.

CONCLUSIONS

The results indicate that introduction of different type of immunogenic molecules into a urease-deficient rBCG is useful for providing polyclonal T cell activating ability to BCG and for production of T cells responsive to secondary stimulation.

[Development of a novel recombinant BCG for tuberculosis vaccine]

A novel recombinant BCG (BCG-DHTM), that was deficient in urease, expressed with gene encoding the fusion of BCG-derived HSP70 and M. tuberculosis-derived major membrane protein (MMP)-II, was constructed for use as a vaccine against tuberculosis. BCG-DHTM efficiently activated dendritic cells (DC) to induce cytokine production, including IL-12, TNFalpha and IL-1beta and phenotypic changes. The DC infected BCG-DHTM was more potent in activation of native T cells of CD4 and CD8 subsets than those infected vector control BCG. The activation of naïve T cells by BCG-DHTM was closely associated with phagomal maturation, and that of naïve CD8+ T cells by BCG-DHTM was induced by the activation of cytosolic cross-presentation pathway. Further, BCG-DHTM seemed to activate native CD4+ T cells and native CD8+ T cells by antigen-specific fashion. The primary infection of BCG-DHTM in C57BL/6 mice for 12 weeks efficiently produced T cells responsive to in vitro secondary stimulation with MMP-II, HSP70 and H37Rv-derived cytosolic protein and inhibited with multiplication of subsequently challenged M. tuberculosis in lungs at least partially. The effect of BCG-DHTM as a vaccine for tuberculosis is not fully convincing and need the improvement, however, our strategy in the development of new recombinant BCG for tuberculosis seems to provide useful tool.

Efficient activation of human T cells of both CD4 and CD8 subsets by urease-deficient recombinant Mycobacterium bovis BCG that produced a heat shock protein 70-M. tuberculosis-derived major membrane protein II fusion protein

For the purpose of obtaining Mycobacterium bovis bacillus Calmette-Guérin (BCG) capable of activating human naive T cells, urease-deficient BCG expressing a fusion protein composed of Mycobacterium tuberculosis-derived major membrane protein II (MMP-II) and heat shock protein 70 (HSP70) of BCG (BCG-DHTM) was produced. BCG-DHTM secreted the HSP70-MMP-II fusion protein and effectively activated human monocyte-derived dendritic cells (DCs) by inducing phenotypic changes and enhanced cytokine production. BCG-DHTM-infected DCs activated naive T cells of both CD4 and naive CD8 subsets, in an antigen (Ag)-dependent manner. The T cell activation induced by BCG-DHTM was inhibited by the pretreatment of DCs with chloroquine. The naive CD8(+) T cell activation was mediated by the transporter associated with antigen presentation (TAP) and the proteosome-dependent cytosolic cross-priming pathway. Memory CD8(+) T cells and perforin-producing effector CD8(+) T cells were efficiently produced from the naive T cell population by BCG-DHTM stimulation. Single primary infection with BCG-DHTM in C57BL/6 mice efficiently produced T cells responsive to in vitro secondary stimulation with HSP70, MMP-II, and M. tuberculosis-derived cytosolic protein and inhibited the multiplication of subsequently aerosol-challenged M. tuberculosis more efficiently than did vector control BCG. These results indicate that the introduction of MMP-II and HSP70 into urease-deficient BCG may be useful for improving BCG for control of tuberculosis.

[Progression on genetic knockout tools in Mycobacterium]

Pathogenic mycobacteria were and remain a heavy burden to public health. Unfortunately, genetic manipulation including knockout technologies of Mycobacterium is difficult compared with other traditional model organisms. To overcome this obstacle, achievements in Mycobacterium knockout technologies were summarized, including delivery vector, sequence-specific recombination system, as well as the recently developed recombinogenic engineering and its application. The future for this tool innovation is also addressed.

Mycobacterium tuberculosis: success through dormancy

Tuberculosis (TB) remains a major health threat, killing nearly 2 million individuals around this globe, annually. The only vaccine, developed almost a century ago, provides limited protection only during childhood. After decades without the introduction of new antibiotics, several candidates are currently undergoing clinical investigation. Curing TB requires prolonged combination of chemotherapy with several drugs. Moreover, monitoring the success of therapy is questionable owing to the lack of reliable biomarkers. To substantially improve the situation, a detailed understanding of the cross-talk between human host and the pathogen Mycobacterium tuberculosis (Mtb) is vital. Principally, the enormous success of Mtb is based on three capacities: first, reprogramming of macrophages after primary infection/phagocytosis to prevent its own destruction; second, initiating the formation of well-organized granulomas, comprising different immune cells to create a confined environment for the host-pathogen standoff; third, the capability to shut down its own central metabolism, terminate replication, and thereby transit into a stage of dormancy rendering itself extremely resistant to host defense and drug treatment. Here, we review the molecular mechanisms underlying these processes, draw conclusions in a working model of mycobacterial dormancy, and highlight gaps in our understanding to be addressed in future research.

Electrochemical monitoring of the metabolic activity of mycobacteria in culture

Mycobacterial metabolic activity is typically measured using time-consuming manual methods based on nutrient consumption, nucleic acid synthesis or reduction of tetrazolium salts. In this study, we propose much simpler electrochemical methods for continuous monitoring of the metabolic activity of mycobacteria in culture. Chronoamperometry and chronopotentiometry were used to detect metabolic activity of both slow-growing and fast-growing mycobacteria using a potentiostat with 2D-electrochemical cell. Electrochemical measurements were able to detect statistically significant differences in the metabolic activity of approximately 10(7) mycobacteria in different growth conditions, within less than 24 h of mycobacterial culture. The metabolic activity of mycobacteria measured by the used electrochemical methods correlated well with changes in general respiratory conditions within the cells as it was evaluated by different biochemical tests. Chronoamperometry and chronopotentiometry allowed measurement of mycobacterial metabolic activity without invasive chemical reactions, at minimal bacterial load and when metabolic response of mycobacteria occurs quickly. The proposed methodology is simple, rapid and cost-effective, and it is expected that both in vitro and in vivo metabolic activity of human mycobacterial pathogens as Mycobacterium tuberculosis can be measured when the implementation of this method to analyze virulent strains is adapted.

Immunostimulatory activity of major membrane protein II from Mycobacterium tuberculosis

Previously, we observed that both major membrane protein II of Mycobacterium leprae (MMP-ML) and its fusion with M. bovis BCG (BCG)-derived heat shock protein 70 (HSP70) (Fusion-ML) are immunogenic and that recombinant BCG secreting either of these proteins effectively inhibits the multiplication of M. leprae in mice. Here, we purified M. tuberculosis-derived major membrane protein II (MMP-MTB) and its fusion with HSP70 (Fusion-MTB) in a lipopolysaccharide-free condition and evaluated their immunostimulatory abilities. Both MMP-MTB and Fusion-MTB activated monocyte-derived dendritic cells (DC) in terms of phenotype and interleukin-12 (IL-12) production, but Fusion-MTB more efficiently activated them than MMP-MTB did. The IL-12 production was a consequence of the ligation of those recombinant proteins with Toll-like receptor 2. The M. tuberculosis-derived and M. leprae-derived recombinant proteins activated naïve T cells of both CD4 and CD8 subsets, but M. tuberculosis-derived proteins were superior to M. leprae-derived proteins and fusion proteins were superior to MMP, regardless of the origin of the protein. Memory-type CD4(+) T cells obtained from BCG-vaccinated healthy individuals seem to be primed with MMP-MTB by the vaccination, and both M. tuberculosis-derived recombinant proteins produced perforin-producing CD8(+) T cells from memory-type CD8(+) T cells. Further, infection of DC and macrophages with M. tuberculosis H37Ra and H37Rv induced the expression of MMP on their surface. These results indicate that M. tuberculosis-derived MMP, as a sole protein or as part of a fusion protein, may be useful for developing new vaccinating agents against tuberculosis.

Development and analysis of an in vivo-compatible metabolic network of Mycobacterium tuberculosis

Background

During infection, Mycobacterium tuberculosis confronts a generally hostile and nutrient-poor in vivo host environment. Existing models and analyses of M. tuberculosis metabolic networks are able to reproduce experimentally measured cellular growth rates and identify genes required for growth in a range of different in vitro media. However, these models, under in vitro conditions, do not provide an adequate description of the metabolic processes required by the pathogen to infect and persist in a host.

Results

To better account for the metabolic activity of M. tuberculosis in the host environment, we developed a set of procedures to systematically modify an existing in vitro metabolic network by enhancing the agreement between calculated and in vivo-measured gene essentiality data. After our modifications, the new in vivo network contained 663 genes, 838 metabolites, and 1,049 reactions and had a significantly increased sensitivity (0.81) in predicted gene essentiality than the in vitro network (0.31). We verified the modifications generated from the purely computational analysis through a review of the literature and found, for example, that, as the analysis suggested, lipids are used as the main source for carbon metabolism and oxygen must be available for the pathogen under in vivo conditions. Moreover, we used the developed in vivo network to predict the effects of double-gene deletions on M. tuberculosis growth in the host environment, explore metabolic adaptations to life in an acidic environment, highlight the importance of different enzymes in the tricarboxylic acid-cycle under different limiting nutrient conditions, investigate the effects of inhibiting multiple reactions, and look at the importance of both aerobic and anaerobic cellular respiration during infection.

Conclusions

The network modifications we implemented suggest a distinctive set of metabolic conditions and requirements faced by M. tuberculosis during host infection compared with in vitro growth. Likewise, the double-gene deletion calculations highlight the importance of specific metabolic pathways used by the pathogen in the host environment. The newly constructed network provides a quantitative model to study the metabolism and associated drug targets of M. tuberculosis under in vivo conditions.

Construction of targeted mycobacterial mutants by homologous recombination

The ability to select genes to knock out of mycobacterial genomes has greatly improved our understanding of mycobacteria. This chapter describes a method for doing this. The gene (including a 1-kb flanking region) is cloned into a pNIL series vector and disrupted by deletion or insertion of a cassette. A selection of marker genes obtained from the pGOAL series of vectors are inserted into the pNIL vector to create a suicide delivery system. This delivery vector is introduced into mycobacteria where the disrupted version of the gene replaces the wild-type version by a two-step homologous recombination process. The method involves selecting for a single crossover event followed by selection of double crossovers. Single crossovers have incorporated plasmid marker genes and are sucrose(S), kanamycin(R) and blue on media containing X-gal. Double crossovers have lost plasmid markers and are sucrose(R), kanamycin(S) and white on media containing X-gal.

Induction of cross-priming of naive CD8+ T lymphocytes by recombinant bacillus Calmette-Guerin that secretes heat shock protein 70-major membrane protein-II fusion protein

Because Mycobacterium bovis bacillus Calmette-Guérin (BCG) unconvincingly activates human naive CD8(+) T cells, a rBCG (BCG-70M) that secretes a fusion protein comprising BCG-derived heat shock protein (HSP)70 and Mycobacterium leprae-derived major membrane protein (MMP)-II, one of the immunodominant Ags of M. leprae, was newly constructed to potentiate the ability of activating naive CD8(+) T cells through dendritic cells (DC). BCG-70M secreted HSP70-MMP-II fusion protein in vitro, which stimulated DC to produce IL-12p70 through TLR2. BCG-70M-infected DC activated not only memory and naive CD8(+) T cells, but also CD4(+) T cells of both types to produce IFN-gamma. The activation of these naive T cells by BCG-70M was dependent on the MHC and CD86 molecules on BCG-70M-infected DC, and was significantly inhibited by pretreatment of DC with chloroquine. Both brefeldin A and lactacystin significantly inhibited the activation of naive CD8(+) T cells by BCG-70M through DC. Thus, the CD8(+) T cell activation may be induced by cross-presentation of Ags through a TAP- and proteosome-dependent cytosolic pathway. When naive CD8(+) T cells were stimulated by BCG-70M-infected DC in the presence of naive CD4(+) T cells, CD62L(low)CD8(+) T cells and perforin-producing CD8(+) T cells were efficiently produced. MMP-II-reactive CD4(+) and CD8(+) memory T cells were efficiently produced in C57BL/6 mice by infection with BCG-70M. These results indicate that BCG-70M activated DC, CD4(+) T cells, and CD8(+) T cells, and the combination of HSP70 and MMP-II may be useful for inducing better T cell activation.

Increased vaccine efficacy against tuberculosis of recombinant Mycobacterium bovis bacille Calmette-Guérin mutants that secrete listeriolysin

The tuberculosis vaccine Mycobacterium bovis bacille Calmette-Guérin (BCG) was equipped with the membrane-perforating listeriolysin (Hly) of Listeria monocytogenes, which was shown to improve protection against Mycobacterium tuberculosis. Following aerosol challenge, the Hly-secreting recombinant BCG (hly+ rBCG) vaccine was shown to protect significantly better against aerosol infection with M. tuberculosis than did the parental BCG strain. The isogenic, urease C-deficient hly+ rBCG (DeltaureC hly+ rBCG) vaccine, providing an intraphagosomal pH closer to the acidic pH optimum for Hly activity, exhibited still higher vaccine efficacy than parental BCG. DeltaureC hly+ rBCG also induced profound protection against a member of the M. tuberculosis Beijing/W genotype family while parental BCG failed to do so consistently. Hly not only promoted antigen translocation into the cytoplasm but also apoptosis of infected macrophages. We concluded that superior vaccine efficacy of DeltaureC hly+ rBCG as compared with parental BCG is primarily based on improved cross-priming, which causes enhanced T cell-mediated immunity.

Improved protection by recombinant BCG

Mycobacterium bovis bacille Calmette Guérin (BCG) is one of the most widely used live vaccines. Technologic advancement in genome manipulation enables the construction of recombinant BCG (rBCG) strains, which can be employed as highly immunogenic vaccines against tuberculosis with improved safety profile.

TB tools to tell the tale–molecular genetic methods for mycobacterial research

In spite of the availability of drugs and a vaccine, tuberculosis--one of man's medical nemeses--remains a formidable public health problem, particularly in the developing world. The persistent nature of the tubercle bacillus, with one third of the world's population is estimated to be infected, combined with the emergence of multi drug-resistant strains and the exquisite susceptibility of HIV-positive individuals, has underscored the urgent need for in-depth study of the biology of Mycobacterium tuberculosis address the resurgence of TB. In aiming to understand the mechanisms by which mycobacteria react to their immediate environments, molecular genetic tools have been developed from naturally occurring genetic elements. These include protein expressing genes, and episomal and integrating elements, which have been derived mainly from prokaryotic but also from eukaryotic organisms. Molecular genetic tools that had been established as routine procedures in other prokaryotic genera were thus mimicked. Knowledge of the underlying mechanisms greatly expedited the harnessing of these elements for mycobacteriological research and has brought us to a point where these molecular genetic tools are now employed routinely in laboratories worldwide.

Mycobacterium bovis BCG urease attenuates major histocompatibility complex class II trafficking to the macrophage cell surface

We have previously shown that Mycobacterium tuberculosis attenuates cell surface expression of major histocompatibility complex class II molecules in response to gamma interferon (IFN-gamma) by a mechanism dependent on intracellular sequestration of alpha,beta dimers. In this study we examined whether intracellular alkalinization due to mycobacterial urease could account for the defect in intracellular trafficking of class II molecules. Phagocytosis of wild-type Mycobacterium bovis BCG was associated with secretion of ammonia intracellularly, which increased substantially upon addition of exogenous urea to the culture medium. Increased intracellular ammonia, due to urea degradation by the bacterium, correlated with inhibition of class II surface expression. Conversely, no ammonia was detected in cells infected with a urease-negative mutant strain of M. bovis BCG, which also displayed a reduced effect on surface expression of class II molecules. A direct cause-effect relationship between urease and class II molecule trafficking was established with experiments where cells ingesting beads coated with purified urease showed an increased ammonia level and decreased surface expression of class II in response to IFN-gamma. In contrast to BCG, infection of macrophages with Mycobacterium smegmatis, which expresses relatively greater urease activity in cell-free culture, had a marginal effect on both the intracellular level of ammonia and class II expression. The limited effect of M. smegmatis was consistent with a failure to resist intracellular killing, suggesting that urease alone is not sufficient to resist macrophage microbicidal mechanisms and that this is required for a more distal effect on cell regulation. Our results demonstrate that alkalinization of critical intracellular organelles by pathogenic mycobacteria expressing urease contributes significantly to the intracellular retention of class II dimers.

Molecular genetics of Mycobacterium tuberculosis in relation to the discovery of novel drugs and vaccines

Genetic systems that allow mycobacterial genomes to be mutagenized in a targeted or random fashion have provided the means for developing new tools for the diagnosis, prevention and treatment of tuberculosis by allowing potential targets to be identified and validated. In this review, we highlight key historical developments in the field of mycobacterial genetics, which have yielded the powerful repertoire of genetic tools that are now in hand and provide examples that illustrate their use in exploring specific aspects of mycobacterial metabolism.

Mycobacterium tuberculosis pathogenesis and molecular determinants of virulence

Tuberculosis (TB), one of the oldest known human diseases. is still is one of the major causes of mortality, since two million people die each year from this malady. TB has many manifestations, affecting bone, the central nervous system, and many other organ systems, but it is primarily a pulmonary disease that is initiated by the deposition of Mycobacterium tuberculosis, contained in aerosol droplets, onto lung alveolar surfaces. From this point, the progression of the disease can have several outcomes, determined largely by the response of the host immune system. The efficacy of this response is affected by intrinsic factors such as the genetics of the immune system as well as extrinsic factors, e.g., insults to the immune system and the nutritional and physiological state of the host. In addition, the pathogen may play a role in disease progression since some M. tuberculosis strains are reportedly more virulent than others, as defined by increased transmissibility as well as being associated with higher morbidity and mortality in infected individuals. Despite the widespread use of an attenuated live vaccine and several antibiotics, there is more TB than ever before, requiring new vaccines and drugs and more specific and rapid diagnostics. Researchers are utilizing information obtained from the complete sequence of the M. tuberculosis genome and from new genetic and physiological methods to identify targets in M. tuberculosis that will aid in the development of these sorely needed antitubercular agents.

Specialized transduction: an efficient method for generating marked and unmarked targeted gene disruptions in Mycobacterium tuberculosis, M. bovis BCG and M. smegmatis

The authors have developed a simple and highly efficient system for generating allelic exchanges in both fast- and slow-growing mycobacteria. In this procedure a gene of interest, disrupted by a selectable marker, is cloned into a conditionally replicating (temperature-sensitive) shuttle phasmid to generate a specialized transducing mycobacteriophage. The temperature-sensitive mutations in the mycobacteriophage genome permit replication at the permissive temperature of 30 degrees C but prevent replication at the non-permissive temperature of 37 degrees C. Transduction at a non-permissive temperature results in highly efficient delivery of the recombination substrate to virtually all cells in the recipient population. The deletion mutations in the targeted genes are marked with antibiotic-resistance genes that are flanked by gammadelta-res (resolvase recognition target) sites. The transductants which have undergone a homologous recombination event can be conveniently selected on antibiotic-containing media. To demonstrate the utility of this genetic system seven different targeted gene disruptions were generated in three substrains of Mycobacterium bovis BCG, three strains of Mycobacterium tuberculosis, and Mycobacterium smegmatis. Mutants in the lysA, nadBC, panC, panCD, leuCD, Rv3291c and Rv0867c genes or operons were isolated as antibiotic-resistant (and in some cases auxotrophic) transductants. Using a plasmid encoding the gammadelta-resolvase (tnpR), the resistance genes could be removed, generating unmarked deletion mutations. It is concluded from the high frequency of allelic exchange events observed in this study that specialized transduction is a very efficient technique for genetic manipulation of mycobacteria and is a method of choice for constructing isogenic strains of M. tuberculosis, BCG or M. smegmatis which differ by defined mutations.

Signal-sequence-independent secretion of the staphylococcal nuclease in Mycobacterium smegmatis

Staphylococcus aureus nuclease is a small, secreted protein which has been successfully used as a reporter system to identify exported products in Lactococcus lactis. Here, biochemical evidence is provided that the nuclease is exported by Mycobacterium smegmatis in the presence, but also in the absence of a signal sequence, and thus probably independently of the Sec translocation pathway. This implies that the nuclease should not be used as a reporter system in mycobacteria for the identification of exported products, despite what has been reported previously in the literature. The nuclease can be extended to create hybrid proteins that remain compatible with its secretion, whereas some other shorter fusions are not tolerated. This suggests that correct folding is required for efficient export. Extensive mutational analysis did not identify a specific secretion pathway. This suggests that the nuclease may be exported by different redundant systems or that components of this alternative Sec pathway are essential for bacterial survival.

Immunology of tuberculosis

The resurgence of tuberculosis worldwide has intensified research efforts directed at examining the host defense and pathogenic mechanisms operative in Mycobacterium tuberculosis infection. This review summarizes our current understanding of the host immune response, with emphasis on the roles of macrophages, T cells, and the cytokine/chemokine network in engendering protective immunity. Specifically, we summarize studies addressing the ability of the organism to survive within macrophages by controlling phagolysosome fusion. The recent studies on Toll-like receptors and the impact on the innate response to M. tuberculosis are discussed. We also focus on the induction, specificity, and effector functions of CD4(+) and CD8(+) T cells, and the roles of cytokines and chemokines in the induction and effector functions of the immune response. Presentation of mycobacterial antigens by MHC class I, class II, and CD1 as well as the implications of these molecules sampling various compartments of the cell for presentation to T cells are discussed. Increased attention to this disease and the integration of animal models and human studies have afforded us a greater understanding of tuberculosis and the steps necessary to combat this infection. The pace of this research must be maintained if we are to realize an effective vaccine in the next decades.

Serine/threonine protein kinases PknF and PknG of Mycobacterium tuberculosis: characterization and localization

Pathogenesis of Mycobacterium tuberculosis is closely connected to its survival and replication within the host. Some pathogenic bacteria employ protein kinases that interfere with the cellular signalling network of host cells and promote bacterial survival. In this study, the pknF and pknG genes, which encode two putative protein kinases of M. tuberculosis H(37)Rv, protein kinase F (PknF) and protein kinase G (PknG), respectively, were cloned and expressed in Escherichia coli. Purified PknF phosphorylated the peptide substrate myelin basic protein (MBP) at serine and threonine residues, while purified PknG phosphorylated only at serine residues. The activity of the two kinases was abrogated by mutation of the codon for the predicted ATP-binding-site lysine residue. Southern blot analysis revealed that homologues of the genes encoding the two kinases are present in M. tuberculosis H(37)Ra and Mycobacterium bovis BCG, but not in Mycobacterium smegmatis. Immunoblot analysis of various cellular fractions of M. tuberculosis H(37)Rv revealed that PknF is a transmembrane protein and that PknG is predominantly a cytosolic enzyme. The present study should aid in elucidating the role of these protein kinases in the pathogenesis of mycobacteria.

Cloning and characterization of secretory tyrosine phosphatases of Mycobacterium tuberculosis

Two genes with sequence homology to those encoding protein tyrosine phosphatases were cloned from genomic DNA of Mycobacterium tuberculosis H(37)Rv. The calculated molecular masses of these two putative tyrosine phosphatases, designated MPtpA and MPtpB, were 17. 5 and 30 kDa, respectively. MPtpA and MPtpB were expressed as glutathione S-transferase fusion proteins in Escherichia coli. The affinity-purified proteins dephosphorylated the phosphotyrosine residue of myelin basic protein (MBP), but they failed to dephosphorylate serine/threonine residues of MBP. The activity of these phosphatases was inhibited by sodium orthovanadate, a specific inhibitor of tyrosine phosphatases, but not by okadaic acid, an inhibitor of serine/threonine phosphatases. Mutations at the catalytic site motif, cysteine 11 of MPtpA and cysteine 160 of MPtpB, abolished enzyme activity. Southern blot analysis revealed that, while mptpA is present in slow-growing mycobacterial species as well as fast-growing saprophytes, mptpB was restricted to members of the M. tuberculosis complex. These phosphatases were present in both whole-cell lysates and culture filtrates of M. tuberculosis, suggesting that these proteins are secreted into the extracellular medium. Since tyrosine phosphatases are essential for the virulence of several pathogenic bacteria, the restricted distribution of mptpB makes it a good candidate for a virulence gene of M. tuberculosis.

Attenuation of and protection induced by a leucine auxotroph of Mycobacterium tuberculosis

Attenuated mutants of Mycobacterium tuberculosis represent potential vaccine candidates for the prevention of tuberculosis. It is known that auxotrophs of a variety of bacteria are attenuated in vivo and yet provide protection against challenge with wild-type organisms. A leucine auxotroph of M. tuberculosis was created by allelic exchange, replacing wild-type leuD (Rv2987c), encoding isopropyl malate isomerase, with a mutant copy of the gene in which 359 bp had been deleted, creating a strain requiring exogenous leucine supplementation for growth in vitro. The frequency of reversion to prototrophy was <10(-11). In contrast to wild-type M. tuberculosis, the DeltaleuD mutant was unable to replicate in macrophages in vitro. Its attenuation in vivo and safety as a vaccine were established by the fact that it caused no deaths in immunodeficient SCID mice. Complementation of the mutant with wild-type leuD abolished the requirement for leucine supplementation and restored the ability of the strain to grow both in macrophages and in SCID mice, thus confirming that the attenuated phenotype was due to the DeltaleuD mutation. As a test of the vaccine potential of the leucine auxotroph, immunocompetent BALB/c mice, susceptible to fatal infection with wild-type M. tuberculosis, were immunized with the DeltaleuD mutant and subsequently challenged with virulent M. tuberculosis by both the intravenous and aerosol routes. A comparison group of mice was immunized with conventional Mycobacterium bovis BCG vaccine. Whereas all unvaccinated mice succumbed to intravenous infection within 15 weeks, mice immunized with either BCG or the DeltaleuD mutant of M. tuberculosis exhibited enhanced and statistically equivalent survival curves. However, the leuD auxotroph was less effective than live BCG in reducing organ burdens and tissue pathology of mice challenged by either route. We conclude that attenuation and protection against M. tuberculosis challenge can be achieved with a leucine auxotroph and suggest that to induce optimal protection, attenuated strains of M. tuberculosis should persist long enough and be sufficiently metabolically active to synthesize relevant antigens for an extended period of time.

Amino acid transport and metabolism in mycobacteria: cloning, interruption, and characterization of an L-Arginine/gamma-aminobutyric acid permease in Mycobacterium bovis BCG

Genes encoding L-arginine biosynthetic and transport proteins have been shown in a number of pathogenic organisms to be important for metabolism within the host. In this study we describe the cloning of a gene (Rv0522) encoding an amino acid transporter from Mycobacterium bovis BCG and the effects of its deletion on L-arginine transport and metabolism. The Rv0522 gene of BCG was cloned from a cosmid library by using primers homologous to the rocE gene of Bacillus subtilis, a putative arginine transporter. A deletion mutant strain was constructed by homologous recombination with the Rv0522 gene interrupted by a selectable marker. The mutant strain was complemented with the wild-type gene in single copy. Transport analysis of these strains was conducted using (14)C-labeled substrates. Greatly reduced uptake of L-arginine and gamma-aminobutyric acid (GABA) but not of lysine, ornithine, proline, or alanine was observed in the mutant strain compared to the wild type, grown in Middlebrook 7H9 medium. However, when the strains were starved for 24 h or incubated in a minimal salts medium containing 20 mM arginine (in which even the parent strain does not grow), L-[(14)C]arginine uptake by the mutant but not the wild-type strain increased strongly. Exogenous L-arginine but not GABA, lysine, ornithine, or alanine was shown to be toxic at concentrations of 20 mM and above to wild-type cells growing in optimal carbon and nitrogen sources such as glycerol and ammonium. L-Arginine supplied in the form of dipeptides showed no toxicity at concentrations as high as 30 mM. Finally, the permease mutant strain showed no defect in survival in unactivated cultured murine macrophages compared with wild-type BCG.

Disruption of the genes encoding antigen 85A and antigen 85B of Mycobacterium tuberculosis H37Rv: effect on growth in culture and in macrophages

The mechanism of pathogenesis of Mycobacterium tuberculosis is thought to be multifactorial. Among the putative virulence factors is the antigen 85 (Ag85) complex. This family of exported fibronectin-binding proteins consists of members Ag85A, Ag85B, and Ag85C and is most prominently represented by 85A and 85B. These proteins have recently been shown to possess mycolyl transferase activity and likely play a role in cell wall synthesis. The purpose of this study was to generate strains of M. tuberculosis deficient in expression of the principal members of this complex in order to determine their role in the pathogenesis of M. tuberculosis. Constructs of fbpA and fbpB disrupted with the kanamycin resistance marker OmegaKm and containing varying amounts of flanking gene and plasmid vector sequences were then introduced as linear fragments into H37Rv by electroporation. Southern blot and PCR analyses revealed disruption of the homologous gene locus in one fbpA::OmegaKm transformant and one fbpB::OmegaKm transformant. The fbpA::OmegaKm mutant, LAa1, resulted from a double-crossover integration event, whereas the fbpB::OmegaKm variant, LAb1, was the product of a single-crossover type event that resulted in insertion of both OmegaKm and plasmid sequences. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western blot analysis confirmed that expression of the disrupted gene was not detectable in the fbpA and fbpB mutants. Analysis of growth rates demonstrated that the fbpB mutant LAb1 grew at a rate similar to that of the wild-type parent in enriched and nutrient-poor laboratory media as well as in human (THP-1) and mouse (J774.1A) macrophage-like cell lines. The fbpA mutant LAa1 grew similarly to the parent H37Rv in enriched laboratory media but exhibited little or no growth in nutrient-poor media and macrophage-like cell lines. The targeted disruption of two genes encoding mycolyl transferase and fibronectin-binding activities in M. tuberculosis will permit the systematic determination of their roles in the physiology and pathogenesis of this organism.

Mycobacteria: bugs and bugbears (two steps forward and one step back)

The use of molecular techniques to study the mycobacteria has advanced greatly since the first genomic libraries of Mycobacterium tuberculosis and M. leprae were constructed in 1985. However, there are still pitfalls for the unwary. Most of the problems associated with the use of molecular techniques to study mycobacteria can be related to one of the following problems: slow growth rate causing problems with contamination; the formation of macroscopic clumps when grown in culture; resistance to standard chemical lysis procedures; the requirement for containment facilities for pathogenic species; the lack of suitable genetic vectors; and the problems of spontaneous antibiotic resistance. Despite these problems, considerable progress has been made and standard techniques have been developed for the preparation of protein, nucleic acids (DNA and RNA) and cell wall components, chemical and transposon mutagenesis and gene replacement methods, the use of reporter genes and expression vectors, and improved detection and drug sensitivity testing.

Disruption of the mycobacterial cell entry gene of Mycobacterium bovis BCG results in a mutant that exhibits a reduced invasiveness for epithelial cells

Mycobacteria belonging to the Mycobacterium tuberculosis complex have the ability to invade and replicate in non-phagocytic cells, an event that requires the presence of bacterial surface components capable of triggering a cell response and the subsequent internalization of the microorganism. In this study, we report the sequencing of the mycobacterial cell entry gene (mce) of Mycobacterium bovis bacillus Calmette-Guérin (BCG) and the generation and characterization of a mutant BCG strain with an inactivated mce gene, by homologous recombination with double cross-over. This mutant strain does not express the mycobacterial cell entry protein (Mce) and exhibits a reduced ability to invade the non-phagocytic epithelial cell line HeLa as compared to wild-type BCG.

Comparison of the construction of unmarked deletion mutations in Mycobacterium smegmatis, Mycobacterium bovis bacillus Calmette-Guérin, and Mycobacterium tuberculosis H37Rv by allelic exchange

Until recently, genetic analysis of Mycobacterium tuberculosis, the causative agent of tuberculosis, was hindered by a lack of methods for gene disruptions and allelic exchange. Several groups have described different methods for disrupting genes marked with antibiotic resistance determinants in the slow-growing organisms Mycobacterium bovis bacillus Calmette-Guérin (BCG) and M. tuberculosis. In this study, we described the first report of using a mycobacterial suicidal plasmid bearing the counterselectable marker sacB for the allelic exchange of unmarked deletion mutations in the chromosomes of two substrains of M. bovis BCG and M. tuberculosis H37Rv. In addition, our comparison of the recombination frequencies in these two slow-growing species and that of the fast-growing organism Mycobacterium smegmatis suggests that the homologous recombination machinery of the three species is equally efficient. The mutants constructed here have deletions in the lysA gene, encoding meso-diaminopimelate decarboxylase, an enzyme catalyzing the last step in lysine biosynthesis. We observed striking differences in the lysine auxotrophic phenotypes of these three species of mycobacteria. The M. smegmatis mutant can grow on lysine-supplemented defined medium or complex rich medium, while the BCG mutants grow only on lysine-supplemented defined medium and are unable to form colonies on complex rich medium. The M. tuberculosis lysine auxotroph requires 25-fold more lysine on defined medium than do the other mutants and is dependent upon the detergent Tween 80. The mutants described in this work are potential vaccine candidates and can also be used for studies of cell wall biosynthesis and amino acid metabolism.

Antisense RNA to ahpC, an oxidative stress defence gene involved in isoniazid resistance, indicates that AhpC of Mycobacterium bovis has virulence properties

Antisense RNA is a versatile tool for reducing gene expression. It was used to determine if ahpC, a gene that is involved in defence against oxidative stress and isoniazid (INH) resistance, is important for virulence of Mycobacterium bovis, a member of the Mycobacterium tuberculosis complex. Antisense RNA constructs of ahpC were made using different strength promoters in front of a reversed coding sequence of ahpC. These constructs were electroporated into a virulent wild-type M. bovis strain and a moderately virulent INH-resistant M. bovis strain that was catalase/peroxidase-negative. Down-regulation of protein synthesis occurred and this was visualized by immunoblotting. All strains containing antisense RNA were markedly less virulent than their parent strains in guinea pigs. M. bovis with an up-regulated ahpC gene was more resistant to cumene hydroperoxide than its parent strain, which had a wild-type ahpC promoter. These results agree with a model of INH resistance in which overexpression of AhpC compensates in some INH-resistant strains for loss of catalase/peroxidase by maintaining the ability to defend against oxidative stress mediated through organic peroxides. In addition, normal expression of AhpC is crucial for maintaining the virulence of wild-type M. bovis, which has normal catalase/peroxidase levels.

A transphyletic anti-infectious control strategy based on the killer phenomenon

A strategy for the prevention and control of candidiasis, pneumocystosis, and tuberculosis, based on the idiotypic network of the yeast killer effect has been envisaged. Anti-idiotypic antibodies representing the internal image of a candidacidal, pneumocysticidal, and mycobactericidal killer toxin from Pichia anomala and idiotypes of killer toxin-neutralizing monoclonal antibodies mimicking the specific cell wall receptor of sensitive microorganisms might provide a unique approach for engineering innovative antibiotics and vaccines active against taxonomically unrelated pathogenic microorganisms. The rationale of the strategy relies on a phenomenon of microbial competition which has been mutated by the immune system in the response to natural infections.

Investigation of mycobacterial recA function: protein introns in the RecA of pathogenic mycobacteria do not affect competency for homologous recombination

The recA locus of pathogenic mycobacteria differs from that of non-pathogenic species in that it contains large intervening sequences termed protein introns or inteins that are excised by an unusual protein-splicing reaction. In addition, a high degree of illegitimate recombination has been observed in the pathogenic Mycobacterium tuberculosis complex. Homologous recombination is the main mechanism of integration of exogenous nucleic acids in M. smegmatis, a non-pathogenic mycobacterium species that carries an inteinless RecA and is amenable to genetic manipulations. To investigate the function of recA in mycobacteria, recA- strains of M. smegmatis were generated by allelic exchange techniques. These strains are characterized (i) by increased sensitivity towards DNA-damaging agents [ethylmethylsulphonate (EMS), mitomycin C, UV irradiation] and (ii) by the inability to integrate nucleic acids by homologous recombination. Transformation efficiencies using integrative or replicative vectors were not affected in recA- mutants, indicating that in mycobacteria RecA does not affect plasmid uptake or replication. Complementation of the recA- mutants with the recA from M. tuberculosis restored resistance towards EMS, mitomycin C and UV irradiation. Transformation of the complemented strains with suicide vectors targeting the pyrF gene resulted in numerous allelic exchange mutants. From these data, we conclude that the intein apparently does not interfere with RecA function, i.e. with respect to competency for homologous recombination, the RecAs from pathogenic and non-pathogenic mycobacteria are indistinguishable.

Site-directed mutagenesis of the 19-kilodalton lipoprotein antigen reveals No essential role for the protein in the growth and virulence of Mycobacterium intracellulare

The mycobacterial 19-kilodalton antigen (19Ag) is a highly expressed, surface-associated glycolipoprotein which is immunodominant in infected patients and has little homology with other known proteins. To investigate the pathogenic significance of the 19Ag, site-directed mutagenesis of the Mycobacterium intracellulare 19Ag gene was carried out by using a suicide vector-based strategy. Allelic replacement of the 19Ag gene of a mouse-avirulent M. intracellulare strain, 1403, was achieved by double-crossover homologous recombination with a gentamicin resistance gene-mutated allele. Unfortunately, an isogenic 19Ag was not achievable in the mouse-virulent strain, D673. However, a 19Ag mutant was successfully constructed in M. intracellulare FM1, a chemically mutagenized derivative of strain D673. FM1 was more amenable to genetic manipulation and susceptible to site-directed mutagenesis of the 19Ag gene yet retained the virulent phenotype of the parental strain. No deleterious effects of 19Ag gene mutation were observed during in vitro growth of M. intracellulare. Virulence assessment of the isogenic 19Ag mutants in a mouse infection model demonstrated that the antigen plays no essential role in the growth of M. intracellulare in vivo. Site-directed mutagenesis of the 19Ag gene demonstrated that it plays no essential role in growth and pathogenicity of M. intracellulare; however, the exact nature of its biological function remains unknown.

Construction and characterization of a partial Mycobacterium tuberculosis cDNA library of genes expressed at reduced oxygen tension

To determine which bacterial genes could be expressed during tuberculosis in the human body, we have prepared and characterized a collection of cDNA clones corresponding to genes that are expressed by Mycobacterium tuberculosis during in vitro growth in 5% (v/v) oxygen. These cDNA clones were obtained by purifying total RNA from M. tuberculosis and cloning small cDNA segments into Escherichia coli followed by removal of clones containing ribosomal RNA sequences. From approx. 1700 clones, a collection of 170 clones containing non-ribosomal inserts were further characterized by PCR amplification. Inserts of more than 180bp were verified by Southern hybridization to have corresponding loci in M. tuberculosis genomic DNA and their sequence was determined. We describe the genes that have been identified using this approach. Multiple independent cDNA clones were obtained for two genes, one probably encoding a stable structural RNA and the other a homologue of ferritin. RNA levels for these two genes were monitored during growth at 20% oxygen, 5% oxygen and in the nearly anaerobic culture sediments. No difference in expression levels was found at 5% oxygen compared to 20% oxygen. RNA levels for the ferritin homologue gene were significantly lower in culture sediments. The stable structural RNA, however, showed very high expression levels independently of culture conditions.

Goldfish, Carassius auratus, a novel animal model for the study of Mycobacterium marinum pathogenesis

We have developed an animal model for studying mycobacterial pathogenesis using Mycobacterium marinum and the goldfish, Carassius auratus. Goldfish are injected intraperitoneally with doses between 10(2) and 10(9) CFU of M. marinum organisms. Depending on the dose of M. marinum organisms administered, an acute or chronic disease is produced. The acute disease is characterized by systemic mycobacterial infection, severe peritonitis, tissue necrosis, and a short median survival time. The chronic disease is characterized by granuloma formation in all organs and survival of animals to the end point of the experiment (56 days). Colony counts in organ homogenates showed recovery of mycobacteria from a high percentage of inoculated animals. We believe this well-characterized animal model will be useful for studying mycobacterial pathogenesis.