Attenuation of virulence by disruption of the Mycobacterium tuberculosis erp gene.

The virulence of the mycobacteria that cause tuberculosis depends on their ability to multiply in mammalian hosts. Disruption of the bacterial erp gene, which encodes the exported repetitive protein, impaired multiplication of M. tuberculosis and M. bovis Bacille Calmette-Guérin in cultured macrophages and mice. Reintroduction of erp into the mutants restored their ability to multiply. These results indicate that erp contributes to the virulence of M. tuberculosis.

3D imaging of the 58 kDa cell binding subunit of the Helicobacter pylori cytotoxin.

Pathogenic strains of Helicobacter pylori produce a potent exotoxin, VacA, which intoxicates gastric epithelial cells and leads to peptic ulcer. The toxin is released from the bacteria as a high molecular mass homo-oligomer of a 95 kDa polypeptide which undergoes specific proteolytic cleavage to 37 kDa and 58 kDa subunits. We have engineered a strain of H. pylori to delete the gene sequence coding for the 37 kDa subunit. The remaining 58 kDa subunit is expressed efficiently and exported as a soluble dimer that is non-toxic but binds target cells in a manner similar to the holotoxin. A 3D reconstruction of the molecule from electron micrographs of quick-freeze, deep-etched preparations reveals the contribution of each building block to the structure and permits the reconstruction of the oligomeric holotoxin starting from individual subunits. In this model P58 subunits are assembled in a ring structure with P37 subunits laying on the top. The data indicate that the 58 kDa subunit is capable of folding autonomously into a discrete structure recognizable within the holotoxin and containing the cell binding domain.

Contribution of beta-lactamase production to the resistance of mycobacteria to beta-lactam antibiotics.

Mycobacterium fallax (M. fallax) is naturally sensitive to many beta-lactam antibiotics (MIC < 2 microg/ml) and devoid of beta-lactamase activity. In this paper, we show that the production of the beta-lactamase of Mycobacterium fortuitum by M. fallax significantly increased the MIC values for good substrates of the enzyme, whereas the potency of poor substrates or transient inactivators was not modified. The rates of diffusion of beta-lactams through the mycolic acid layer were low, but for all studied compounds the half-equilibration times were such that they would only marginally affect the MIC values in the absence of beta-lactamase production. These results emphasize the importance of enzymatic degradation as a major factor in the resistance of mycobacteria to penicillins.

Positive selection of allelic exchange mutants in Mycobacterium bovis BCG.

sacB expression is lethal to mycobacteria in the presence of sucrose. It can therefore serve as 1 counter-selectable marker for positive selection of gene replacement events as demonstrated in the fast-growing Mycobacterium smegmatis. With this methodology, a sucrose counter-selectable vector was used to deliver, into the Mycobacterium bovis BCG genome, an inactivated copy (ureC::Km) of the ureC gene encoding the mycobacterial urease. A two-step selection procedure on 2% sucrose allowed the positive selection of gene exchange mutants. This technique should thus be extremely useful for the genetic analysis of pathogenic mycobacteria.

A reliable amplification technique for the characterization of genomic DNA sequences flanking insertion sequences.

A simple and efficient ligation-mediated PCR (LMPCR) is described for amplifying DNA adjacent to known sequences. The method uses one primer specific for the known sequence and a second specific for a synthetic linker ligated to restricted genomic DNA. Perkin-Elmer AmpliTaq Gold polymerase is used to minimize non-specific primer annealing and amplification. This LMPCR method was successfully applied to isolate DNA sequences flanking mobile elements present in mycobacterial mutants generated by transposon mutagenesis.

Mutagenesis of Neisseria meningitidis by in vitro transposition of Himar1 mariner.

Now that the meningococcal genome sequence has been completed, the lack of a suitable method for saturation mutagenesis remains a major obstacle to the unraveling of the pathogenic propensity of Neisseria meningitidis. Here, we demonstrate that in vitro Himar1 mariner transposition on chromosomal or PCR-amplified meningococcal DNA, which is subsequently reintroduced into N. meningitidis by natural transformation, is an extremely efficient mutagenesis method. Southern blot analysis, sequencing the Himar1 insertion point in numerous transposition mutants, and a limited screening of the mutant libraries for clones impaired in maltose catabolism confirmed that Himar1 transposed randomly in N. meningitidis. Taken together, these data demonstrate that Himar1 in vitro transposition can lead to the exhaustive mutagenesis of N. meningitidis, allowing for the first time a genomic-scale mutational analysis of this important human pathogen.

Generation of unmarked directed mutations in mycobacteria, using sucrose counter-selectable suicide vectors.

The expression of sacB, the Bacillus subtilis gene encoding levansucrase, is lethal to mycobacteria in the presence of 10% sucrose. In this study, we describe the use of sacB as a marker for positive selection of gene-replacement events into Mycobacterium smegmatis. A sucrose counter-selectable suicide plasmid was used to deliver an inactivated copy of the pyrF gene (pyrF::K(m)) into the M. smegmatis genome. Only uracil auxotroph clones, resulting from replacement of the endogenous pyrF allele, survived in a one-step selection on plates containing kanamycin and 10% sucrose. This demonstrated that selection on sucrose against the maintenance of the vector bearing the sacB gene is 100% efficient, enabling the positive selection of allelic-exchange mutants. Two-step selection is also feasible; it was used to construct unmarked pyrF mutants in which the gene was inactivated by a frameshift mutation. This method of generating unmarked, directed mutations is rapid and simple, making it a powerful tool for the genetic characterization of mycobacteria.

Genetic advances for studying Mycobacterium tuberculosis pathogenicity.

Tuberculosis remains the greatest cause of death worldwide because of a single pathogen. Despite its importance, the genetic basis of the pathogenicity of Mycobacterium tuberculosis remains poorly understood, mainly because the most productive investigative approach, molecular genetic analysis, has been severely hampered by a lack of efficient tools. However, significant advances, including the development of methods for inactivating genes and studying their expression with reporter genes, have been recently made. This progress may lead to opportunities for developing new vaccines and antituberculous drugs. The aim of this review is to examine the present state of the art in mycobacterial molecular genetics and pinpoint some expected or promising areas for future research.

Expression of the Bacillus subtilis sacB gene confers sucrose sensitivity on mycobacteria.

Expression in mycobacteria of the structural gene sacB, which encodes the Bacillus subtilis levansucrase, was investigated. sacB expression is lethal to Mycobacterium smegmatis and Mycobacterium bovis BCG in the presence of 10% sucrose. sacB could thus be used as a counterselectable marker in mycobacteria.

Aminoglycoside 2'-N-acetyltransferase genes are universally present in mycobacteria: characterization of the aac(2')-Ic gene from Mycobacterium tuberculosis and the aac(2')-Id gene from Mycobacterium smegmatis.

The genus Mycobacterium comprises clinically important pathogens such as M. tuberculosis, which has reemerged as a major cause of morbidity and mortality world-wide especially with the emergence of multidrug-resistant strains. The use of fast-growing species such as Mycobacterium smegmatis has allowed important advances to be made in the field of mycobacterial genetics and in the study of the mechanisms of resistance in mycobacteria. The isolation of an aminoglycoside-resistance gene from Mycobacterium fortuitum has recently been described. The aac(2')-Ib gene is chromosomally encoded and is present in all isolates of M. fortuitum. The presence of this gene in other mycobacterial species is studied here and genes homologous to that of M. fortuitum have been found in all mycobacterial species studied. In this report, the cloning of the aac(2')-Ic gene from M. tuberculosis H37Rv and the aac(2')-Id gene from M. smegmatis mc(2)155 is described. Southern blot hybridizations have shown that both genes are present in all strains of this species studied to date. In addition, the putative aac(2')-Ie gene has been located in a recent release of the Mycobacterium leprae genome. The expression of the aac(2')-Ic and aac(2')-Id genes has been studied in M. smegmatis and only aac(2')-Id is correlated with aminoglycoside resistance. In order to elucidate the role of the aminoglycoside 2'-N-acetyltransferase genes in mycobacteria and to determine whether they are silent resistance genes or whether they have a secondary role in mycobacterial metabolism, the aac(2')-Id gene from M. smegmatis has been disrupted in the chromosome of M. smegmatis mc(2)155. The disruptant shows an increase in aminoglycoside susceptibility along with a slight increase in the susceptibility to lysozyme.

Efficient allelic exchange and transposon mutagenesis in Mycobacterium tuberculosis.

A better understanding of Mycobacterium tuberculosis virulence mechanisms is highly dependent on the design of efficient mutagenesis systems. A system enabling the positive selection of insertional mutants having lost the delivery vector was developed. It uses ts-sacB vectors, which combine the counterselective properties of the sacB gene and a mycobacterial thermosensitive origin of replication and can therefore be efficiently counterselected on sucrose at 39 degrees C. This methodology allowed the construction of M. tuberculosis transposition mutant libraries. Greater than 10(6) mutants were obtained, far exceeding the number theoretically required to obtain at least one insertion in every nonessential gene. This system is also efficient for gene exchange mutagenesis as demonstrated with the purC gene: 100% of the selected clones were allelic exchange mutants. Therefore, a single, simple methodology has enabled us to develop powerful mutagenesis systems, the lack of which was a major obstacle to the genetic characterization of M. tuberculosis.

Use of fluorescence induction and sucrose counterselection to identify Mycobacterium tuberculosis genes expressed within host cells.

The identification of Mycobacterium tuberculosis genes expressed within host cells would contribute greatly to the development of new strategies to combat tuberculosis. By combining the natural fluorescence of the Aequoria victoria green fluorescent protein (GFP) with the counterselectable property of the Bacillus subtilis SacB protein, M. tuberculosis promoters displaying enhanced in vivo activity have been isolated. Macrophages were infected with recombinant Mycobacterium bovis bacille Calmette-Guérin containing a library of M. tuberculosis promoters controlling gfp and sacB expression, and fluorescent bacteria recovered by fluorescence-activated cell sorting. The expression of sacB was used to eliminate clones with strong promoter activity outside the macrophage, resulting in the isolation of seven clones containing M. tuberculosis promoters with greater activity intracellularly. The gene products identified displayed similarity to proteins from other organisms whose functions include nutrient utilization, protection from oxidative stress and defence against xenobiotics. These proposed functions are consistent with conditions encountered within the host cell and thus suggest that the augmented activity of the isolated promoters/genes may represent strategies employed by M. tuberculosis to enhance intracellular survival and promote infection.

Towards deciphering the Helicobacter pylori cytotoxin.

VacA, the major exotoxin produced by Helicobacter pylori, is composed of identical 87 kDa monomers that assemble into flower-shaped oligomers. The monomers can be proteolytically cleaved into two moieties, one of 37 and the other of 58 kDa, named P37 and P58 respectively. The most studied property of VacA is the alteration of intracellular vesicular trafficking in eukaryotic cells leading to the formation of large vacuoles containing markers of late endosomes and lysosomes. However, VacA also causes a reduction in transepithelial electrical resistance in polarized monolayers and forms ion channels in lipid bilayers. The ability to induce vacuoles is localized mostly but not entirely in P37, whereas P58 is mostly involved in cell targeting. Until recently, H. pylori isolates were classified as tox+ or tox-, depending on whether they induced vacuoles in HeLa cells or not. Today, we know that almost all strains are cytotoxic. The major difference between tox+ and tox- resides in the cell binding domain, which exists in two allelic forms, only one of which is toxic for HeLa cells. The two forms, named m1 and m2, are found predominantly in Western and Chinese isolates respectively.

Culture at high density improves the ability of human macrophages to control mycobacterial growth.

The mechanisms through which granuloma formation helps control mycobacterial infection are poorly understood, but it is possible that the accumulation of macrophages at high density at sites of infection promotes the differentiation of macrophages into cells with improved mycobactericidal activity. To test this possibility, varying numbers of monocytes were cultured in 96-well plates for 3 days, infected with Mycobacterium bovis bacillus Calmette-Guérin, and mycobacterial number was assessed 7 days after infection based on the measurement of luciferase activity expressed by a mycobacterial reporter strain or by counting CFU. Mycobacterial growth was optimal in cultures containing 5 x 10(4) cells/well, but increasing the number of cells to 2 x 10(5) cells/well resulted in complete inhibition of mycobacterial growth. This effect could not be explained by differences in mycobacterial uptake, multiplicity of infection, acidification of the extracellular medium in high density cultures, enhanced NO production, or paracrine stimulation resulting from secretion of cytokines or other proteins. The morphology of cells cultured at high density was strikingly different from that of monocytes cultured at 5 x 10(4) cells/well, including the appearance of numerous giant cells. The bacteriostatic activity of monocyte-derived macrophages was also dependent on cell number, but fewer of these more mature cells were required to control mycobacterial growth. Thus, the ability of human macrophages to control mycobacterial infection in vitro is influenced by the density of cells present, findings that may help explain why the formation of granulomas in vivo appears to be a key event in the control of mycobacterial infections.

Identification of genetic loci implicated in the survival of Mycobacterium smegmatis in human mononuclear phagocytes.

A luminescence-based procedure that permits the rapid evaluation of the survival of mycobacteria within mononuclear phagocytes was developed and used to screen insertional mutants of Mycobacterium smegmatis for their ability to survive in human monocyte-derived macrophages. Among the 5000 mutants tested, eight mutants were identified that demonstrated impaired intracellular survival in human macrophages but that grew normally in the absence of cells. For each mutant, a portion of the gene interrupted by the transposition event was amplified by ligand-mediated PCR and sequenced. In all cases, the existence of homologous genes of as yet unknown function were identified in the Mycobacterium tuberculosis genome. Complementation of the mutant mycobacterial strains with cosmids containing the homologous loci from M. tuberculosis restored normal intracellular growth in three of the four mutants tested, supporting the idea that these loci contain genes that are important for intracellular survival. This study demonstrates the feasibility of directly screening mutant mycobacterial strains to identify genes coding for activities necessary for the intracellular survival in human mononuclear phagocytes, an important initial step in the identification of potential targets for new therapeutic agents.

Effect of stimulation of human macrophages on intracellular survival of Mycobacterium bovis Bacillus Calmette-Guerin. Evaluation with a mycobacterial reporter strain.

The mechanisms through which immune and inflammatory responses stimulate the expression of antimycobacterial activity by human macrophages remain poorly defined. To study this question, we developed a method permitting the rapid quantification of viable mycobacteria, based on the detection of luciferase activity expressed by a Mycobacterium bovis Bacillus Calmette-Guerin (BCG) reporter strain, and used this approach to evaluate mycobacterial survival in human monocyte-derived macrophages following stimulation with cytokines and through crosslinking of costimulatory molecules expressed on the cell surface. Modest proliferation, followed by persistence of mycobacteria, was observed in unpretreated macrophages as assessed both by measurement of luciferase activity and by the evaluation of colony forming units. Of the 19 cytokines tested, only granulocyte-macrophage colony-stimulating factor (GM-CSF) and interleukin-3 (IL-3) were found to improve the mycobactericidal activity of monocyte-derived macrophages. In both cases, this effect was observed only when macrophages were pretreated with the cytokines prior to infection. In contrast, pretreatment of human macrophages with interferon-gamma, either alone or in combination with other mediators (including tumor necrosis factor-alpha and 1,25[OH]2-vitamin D3), did not improve mycobacterial killing. The stimulation of macrophages through several different costimulatory molecules known to participate in macrophage-lymphocyte interactions (CD4, CD40, CD45, CD86, CD95 [Fas/Apo-1]) also failed to improve mycobactericidal activity. This study shows that GM-CSF and IL-3, cytokines whose receptors are known to share a common subunit and to use common second messengers, may contribute to the stimulation of mycobactericidal activity in humans. The ability to rapidly screen the effects of different macrophage stimuli on mycobacterial survival through the detection of luciferase activity should help define additional signals required for optimal antimycobacterial responses.