Production of avirulent Mycobacterium bovis strains by illegitimate recombination with deoxyribonucleic acid fragments containing an interrupted ahpC gene

Genetic Manipulation of Non-tuberculosis Mycobacteria

Non-tuberculosis mycobacteria (NTMs) comprise a large group of organisms that are phenotypically diverse. Analysis of the growing number of completed NTM genomes has revealed both significant intra-genus genetic diversity, and a high percentage of predicted genes that appear to be unique to this group. Most NTMs have not been studied, however, the rise in NTM infections in several countries has prompted increasing interest in these organisms. Mycobacterial research has recently benefitted from the development of new genetic tools and a growing number of studies describing the genetic manipulation of NTMs have now been reported. In this review, we discuss the use of both site-specific and random mutagenesis tools in NTMs, highlighting the challenges that exist in applying these techniques to this diverse group of organisms.

Differences of gene expression in bovine alveolar macrophages infected with virulent and attenuated isogenic strains of Mycobacterium bovis

Infection with Mycobacterium bovis is a significant human and animal health problem in many parts of the world. The first stage of pulmonary tuberculosis occurs after inhalation of the bacilli into an alveolus where they are ingested by resident macrophages. DNA microarray analysis was used to detect genes expressed in bovine lung alveolar macrophages infected with two isogenic strains of M. bovis, a virulent strain, ATCC35723 and an attenuated strain, WAg520 derived from ATCC35723. Chemokines, interleukin-8 and monocyte chemotactic protein 1, were more strongly expressed in ATCC35723-infected macrophages compared to WAg520-infected macrophages. Conversely, a group of genes, including fibrinogen-like protein 2 and legumain, were expressed at a higher level in macrophages infected with WAg520 compared to ATCC35723. Quantitative real-time PCR of a selected group of these differentially expressed genes confirmed enhanced levels of IL-8 mRNA in ATCC35723-infected macrophages compared to WAg520-infected macrophages. Microarray analysis of gene expression in macrophages infected with attenuated isogenic strains of M. bovis may identify key genes involved in early and protective immune responses to tuberculosis.

Generation of attenuated Mycobacterium bovis strains by signature-tagged mutagenesis for discovery of novel vaccine candidates

Mycobacterium bovis, a member of the Mycobacterium tuberculosis complex, has a particularly wide host range and causes tuberculosis in most mammals, including humans. A signature tag mutagenesis approach, which employed illegitimate recombination and infection of guinea pigs, was applied to M. bovis to discover genes important for virulence and to find potential vaccine candidates. Fifteen attenuated mutants were identified, four of which produced no lesions when inoculated separately into guinea pigs. One of these four mutants had nine deleted genes including mmpL4 and sigK and, in guinea pigs with aerosol challenge, provided protection against tuberculosis at least equal to that of M. bovis BCG. Seven mutants had mutations near the esxA (esat-6) locus, and immunoblot analysis of these confirmed the essential role of other genes at this locus in the secretion of EsxA (ESAT-6) and EsxB (CFP10). Mutations in the eight other attenuated mutants were widely spread through the chromosome and included pks1, which is naturally inactivated in clinical strains of M. tuberculosis. Many genes identified were different from those found by signature tag mutagenesis of M. tuberculosis by use of a mouse infection model and illustrate how the use of different approaches enables identification of a wider range of attenuating mutants.

The efficacy of live tuberculosis vaccines after presensitization with Mycobacterium avium

The variable efficacy of BCG in humans has been extensively documented but its cause is still not well understood. One possible reason for this variation is the effect of presensitization with environmental mycobacteria. To investigate in guinea pigs the effects of presensitization with well characterized Mycobacterium avium strains on the vaccine efficacy of BCG and of two recently developed, avirulent strains of Mycobacterium bovis. Two strains of M. avium containing the DNA insertion element IS901 (M. avium+) and two strains not containing this element (M. avium-) were inoculated subcutaneously or by oral administration into guinea pigs to assess their virulence in these animals and their ability to induce delayed type hypersensitivity to tuberculins. Subsequently, groups of guinea pigs presensitized with orally administered M. avium+ and M. avium- and a control group were vaccinated with BCG, or one of two newly attenuated strains of M. bovis. All groups were then challenged by the aerosol route with virulent M. bovis. Vaccine efficacy was assessed 5 weeks later by the presence of macroscopic lesions and bacterial counts of spleen and lung. No macroscopic lesions were observed in any of the guinea pigs inoculated with strains of M. avium+ or M. avium- and all animals gave delayed-type hypersensitivity skin-test reactions to avian PPD. In the vaccine experiment, presensitization with orally administered M. avium+ alone produced a low level of protection against subsequent challenge with virulent M. bovis. In the absence of presensitization with M. avium or after presensitization with an M. avium- strain, BCG and two attenuated strains of M. bovis produced significant levels of protection. No additional protection was observed in lungs of guinea pigs presensitized with M. avium+ and subsequently vaccinated with BCG. In contrast, both newly attenuated strains of M. bovis induced significant protection in lungs after such presensitization. Presensitization of guinea pigs by the oral administration of M. avium+ provides a model for testing vaccines under conditions where the efficacy of BCG has been compromised by prior sensitization with environmental mycobacteria.

Vaccine and skin testing properties of two avirulent Mycobacterium bovis mutants with and without an additional esat-6 mutation

SETTING

Molecular techniques are now available to develop new live tuberculosis vaccines by producing avirulent strains of the Mycobacterium tuberculosis complex with known genes deleted.

OBJECTIVES

Determine if removal of esat-6 from new live tuberculosis vaccines with known attenuating mutations affects their vaccine efficacy and if it could enable the development of discriminating diagnostic tests.

DESIGN

Remove the esat-6 gene by allelic exchange from two illegitimate mutants of Mycobacterium bovis that had previously been shown to have similar vaccine efficacy to BCG in a guinea pig vaccination model. Determine the effect this removal has on virulence, vaccine efficacy and skin test reactivity in guinea pigs.

RESULTS

Two double knockout strains of M. bovis were produced and their virulence and vaccine efficacy were compared to their parent strains. Removal of the esat-6 gene had no significant effect on vaccine efficacy. In skin tests, animals inoculated with the double knockout strains reacted to PPD but not ESAT-6, whereas those inoculated with the parent strains had similar skin test reactivity to both PPD and esat-6.

CONCLUSION

Removal of esat-6 from new live tuberculosis vaccine candidates has no significant effect on vaccine properties but does enable the use of skin tests to distinguish between vaccination and infection.

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.

Production of avirulent mutants of Mycobacterium bovis with vaccine properties by the use of illegitimate recombination and screening of stationary-phase cultures

A better tuberculosis vaccine is urgently required to control the continuing epidemic. Molecular techniques are now available to produce a better live vaccine than BCG by producing avirulent strains of the Mycobacterium tuberculosis complex with known gene deletions. In this study, 1000 illegitimate recombinants of Mycobacterium bovis were produced by illegitimate recombination with fragments of mycobacterial DNA containing a kanamycin resistance gene. Eight recombinant strains were selected on the basis of their inability to grow when stationary-phase cultures were inoculated into minimal medium. Five of these recombinants were found to be avirulent when inoculated into guinea pigs. Two of the avirulent recombinants produced vaccine efficacy comparable to BCG against an aerosol challenge in guinea pigs with M. bovis. One of these recombinants had an inactivated glnA2 gene encoding a putative glutamine synthetase. Transcriptional analysis showed that inactivation of glnA2 did not affect expression of the downstream glnE gene. The other recombinant had a block of 12 genes deleted, including the sigma factor gene sigG. Two avirulent recombinants with an inactivated pckA gene, encoding phosphoenolpyruvate carboxykinase which catalyses the first step of gluconeogenesis, induced poor protection against tuberculosis. It is clear that live avirulent strains of the M. tuberculosis complex vary widely in their ability as vaccines to protect against tuberculosis. Improved models may be required to more clearly determine the difference in protective effect between BCG and potential new tuberculosis vaccines.

Silencing of oxidative stress response in Mycobacterium tuberculosis: expression patterns of ahpC in virulent and avirulent strains and effect of ahpC inactivation

Intracellular pathogens such as Mycobacterium tuberculosis are able to survive in the face of antimicrobial products generated by the host cell in response to infection. The product of the alkyl hydroperoxide reductase gene (ahpC) of M. tuberculosis is thought to be involved in protecting the organism against both oxidative and nitrosative stress encountered within the infected macrophage. Here we report that, contrary to expectations, ahpC expression in virulent strains of M. tuberculosis and Mycobacterium bovis grown in vitro is repressed, often below the level of detection, whereas expression in the avirulent vaccine strain M. bovis BCG is constitutively high. The repression of the ahpC gene of the virulent strains is independent of the naturally occurring lesions of central regulator oxyR. Using a green fluorescence protein vector (gfp)-ahpC reporter construct we present data showing that repression of ahpC of virulent M. tuberculosis also occurred during growth inside macrophages, whereas derepression in BCG was again seen under identical conditions. Inactivation of ahpC on the chromosome of M. tuberculosis by homologous recombination had no effect on its growth during acute infection in mice and did not affect in vitro sensitivity to H2O2. However, consistent with AhpC function in detoxifying organic peroxides, sensitivity to cumene hydroperoxide exposure was increased in the ahpC::Km(r) mutant strain. The preservation of a functional ahpC gene in M. tuberculosis in spite of its repression under normal growth conditions suggests that, while AhpC does not play a significant role in establishing infection, it is likely to be important under certain, as yet undefined conditions. This is supported by the observation that repression of ahpC expression in vitro was lifted under conditions of static growth.

Virulence factors of Mycobacterium bovis

Virulence factors of Mycobacterium bovis are the special properties that enable it to infect, survive, multiply and cause disease in an animal host. An understanding of these factors will lead to new strategies including an effective vaccine to control bovine tuberculosis. A few factors have already been identified and two broadly different approaches to discover other virulence factors are now being used. In the first approach, libraries of random M. bovis mutants are produced, the likely attenuated mutants are identified using a screening technique and the interrupted genes in selected mutants are identified. In the second approach, genes encoding putative virulence factors are selected by a range of different methods and then inactivated, usually by allelic exchange, to produce likely attenuated mutants of M. bovis. In both approaches, loss of virulence by a mutant must be determined in an animal model. Subsequently, the mutant must be complemented back to virulence with an active form of the identified gene in order to demonstrate that loss of virulence was not due to polar effects of the mutation on nearby genes. It is almost certain that most of the virulence factors of M. bovis are the same as those of the classical human tuberculosis organism, Mycobacterium tuberculosis, as both organisms can cause identical clinical disease in humans and are genetically very similar. Many putative virulence genes are now being investigated and only the inherent slowness with which mycobacterial work proceeds, delays the inevitable arrival of an exciting new phase in the understanding of mycobacterial disease.

Regulation of virulence genes in Mycobacterium tuberculosis

Mycobacterium tuberculosis has demonstrated remarkable ability to survive in diverse conditions encountered during the infection process. These involve surviving the bactericidal stresses within the macrophage, the anaerobic and nutritionally altered environment of the granuloma, and the metabolically inactive latent state. Understanding the molecular basis of this adaptive behavior lies in the identification of genes (or virulence determinants) specifically expressed under these varied conditions. Transcriptional control plays a key role in regulating gene expression in response to environmental signals. However, even after decades of investigation our knowledge about the function of these regulatory mechanisms in mycobacteria remains meagre. But the elucidation of the genome sequence and implementation of sophisticated molecular genetic approaches to this organism have made a revolutionary impact on the study of mycobacterial pathogenesis. Deletion and complementation of individual genes can be done at will facilitating the comparative analysis of mutants and wild-type strains. Novel and powerful technologies such as DNA microarrays, fluorescent beacons and proteomics have made possible the analysis of the expression levels of multiple genes in in vitro systems. More technically challenging uses of these techniques is being undertaken to explore pathogen gene expression within the host. This will lead to the identification of virulence factors and give definitive insight into their regulatory signals.

An esat6 knockout mutant of Mycobacterium bovis produced by homologous recombination will contribute to the development of a live tuberculosis vaccine

SETTING

Strains of the Mycobacterium tuberculosis complex are being rationally attenuated in order to develop better tuberculosis vaccines than BCG, and it would be helpful if new vaccines lacked an immunogenic protein which could be used as a skin test reagent for determining infection status.

OBJECTIVE

To delete the esat6 gene from a virulent Mycobacterium bovis strain and determine (i) whether this mutant sensitizes guinea pigs to a skin test based on ESAT6 and (ii) what effect this has on the virulence of M. bovis.

DESIGN

An homologous recombination technique was used to produce an esat6 knockout mutant of a virulent strain of M. bovis. Guinea pigs were inoculated with either the mutant or parent strain and their reactivity in intradermal skin tests was determined to bovine purified protein derivative (PPD) and recombinant ESAT6 protein.

RESULTS

Production of an esat6 knockout strain was demonstrated by Southern blot hybridization and the polymerase chain reaction. Guinea pigs inoculated with either the esat6 knockout strain or its virulent parent had positive skin test reactions to PPD but only animal inoculated with the parent strain had positive skin test reactions to ESAT6. Gross pathology, histopathology and mycobacterial culture of tissues indicated that the knockout strain was less virulent than its parent.

CONCLUSION

If an effective live tuberculosis vaccine can be produced by inactivation of virulence genes in M. bovis, then prior or subsequent knockout of the esat6 gene could contribute to the loss of virulence and enable the development of a test to distinguish between vaccinated and infected animals.

New tuberculosis vaccines based on attenuated strains of the Mycobacterium tuberculosis complex

The world urgently needs a better tuberculosis vaccine. Bacille Calmette-Guerin (BCG), an attenuated strain of Mycobacterium bovis, has been very widely used as a vaccine for many years but has had no major effect on reducing the incidence of tuberculosis. A number of alternative living and non-living vaccines are being investigated. Live vaccine candidates include genetically modified forms of BCG, genetically attenuated strains of the Mycobacterium tuberculosis complex and genetically engineered vaccinia virus and Salmonella strains. Non-living vaccine candidates include killed mycobacterial species, protein subunits and DNA vaccines. One requirement for acceptance of any new vaccine will be a favourable comparison of the protection it induces relative to BCG in a range of animal models, some of which may need further development. Molecular genetic techniques are now available that enable production of live attenuated strains of the M. tuberculosis complex with vaccine potential. In the first of two broadly different approaches that are being used, large numbers of mutants are produced by transposon mutagenesis or illegitimate recombination and are screened for properties that correlate with attenuation. In the second approach, putative genes that may be required for virulence are identified and subsequently inactivated by allelic exchange. In both approaches, mutants that are attenuated need to be identified and subsequently tested for their vaccine efficacy in animal models. Many mutants of the M. tuberculosis complex have now been produced and the vaccine properties of a substantial number will be assessed in the next 3 years.

Tuberculosis vaccines: developmental work and the future

The last year in tuberculosis vaccine research has witnessed the initial flowering of the benefits promised by the tuberculosis genome sequencing product. Although the real benefits in terms of clinical treatments are yet to be realized, genomics is making its presence felt in the rapid identification and expression of proteins with vaccine potential from Mycobacterium tuberculosis, the definition of species-specific antigens for diagnostic use, and the construction of a variety of novel living vectors for vaccination. At the same time, the recent increase in work on animal models with more direct applicability to the situations likely to be encountered in human vaccine trials are providing the basic underpinnings needed for the assessment of these new vaccines.

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.

Production of mutants in amino acid biosynthesis genes of Mycobacterium tuberculosis by homologous recombination

The ability to generate mutants of Mycobacterium tuberculosis will be important if we are to understand the biology of this major pathogen. However, allelic replacement methods have only recently achieved success. We have developed a reproducible method for generating defined mutants of M. tuberculosis using homologous recombination. The transforming DNA was used following pre-treatment either with UV light or alkali denaturation in order to stimulate homologous recombination and abolish illegitimate recombination. Suicide vectors carrying one of nine amino acid biosynthesis genes were electroporated into M. tuberculosis, and homologous recombinants were obtained in all nine genes; eight resulted from single-crossover events (SCOs) and one from a double-crossover event (DCO) (in the metB gene). The remaining colonies were spontaneous hygromycin-resistant mutants; no products of illegitimate recombination were observed. To more efficiently distinguish spontaneous mutants, the lacZ gene was cloned into five vectors (two containing genes not previously tested), and the transformations were repeated. SCO mutants were identified by screening for blue colonies on indicator plates. White transformants were tested for auxotrophy and trpD, hisD and proC auxotrophic mutants were obtained. Only blue SCOs were obtained for argF and glnE. Thus, using this methodology we have obtained homologous recombination in 11 genes, and DCOs in 4 genes, showing that it is possible to generate targeted mutants in a reproducible way.

Vaccination of guinea pigs with nutritionally impaired avirulent mutants of Mycobacterium bovis protects against tuberculosis

Four nutritionally impaired strains of Mycobacterium bovis produced by illegitimate recombination were tested for their ability to protect guinea pigs against intratracheal challenge with virulent M. bovis. All four strains and M. bovis BCG induced significant levels of protection as measured by the reduced spread of infection to the spleen and liver. In animals vaccinated with BCG or two of the other strains, the bacterial counts from the lungs were significantly lower than those of the nonvaccinated animals.