Identification of genes involved in the resistance of mycobacteria to killing by macrophages

A Novel Approach of Targeting Linezolid Nanoemulsion for the Management of Lymph Node Tuberculosis

Tuberculosis (TB) represents a major public health problem, globally affecting children and adults. Lymphatic TB is the most common type of extrapulmonary tuberculosis, which affects the peripheral lymph nodes. This burgeoning disease requires a long-term treatment of multiple antibiotics to kill Mycobacterium tuberculosis, resulting in an increased rate of multidrug-resistant tuberculosis. To overcome drug resistance with the first-line antibiotics, linezolid W/O nanoemulsion was developed in this current work. W/O nanoemulsion was prepared by oil phase titration technique using sunflower oil, span 80 and tween 80, and optimized by pseudophase ternary diagrams. The particle size, polydispersity index, zeta potential, viscosity, and refractive index for the optimized formulation were found to be 92.32 nm, 0.066, -21.9 mV, 32.623 cP, and 1.453, respectively. Drug release from the developed nanoemulsion followed the zero-order kinetic. The antimicrobial efficacy study confirms the antibacterial potential of the developed nanoemulsion. In vivo studies conducted on Wistar rats confirms the lymphatic targeting with a high amount of drug at the target organ just after 8 h of dosing. As a result of the foregoing promising results, it may be inferred that the suggested nanoemulsion could be a viable therapy option for lymph node tuberculosis.

Identification of Mycobacterium tuberculosis counterimmune (cim) mutants in immunodeficient mice by differential screening

Tuberculosis (TB) is characterized by lifetime persistence of Mycobacterium tuberculosis. Despite the induction of a vigorous host immune response that curtails disease progression in the majority of cases, the organism is not eliminated. Subsequent immunosuppression can lead to reactivation after a prolonged period of clinical latency. Thus, while it is clear that protective immune mechanisms are engaged during M. tuberculosis infection, it also appears that the pathogen has evolved effective countermechanisms. Genetic studies with animal infection models and with patients have revealed a key role for the cytokine gamma interferon (IFN-gamma) in resistance to TB. IFN-gamma activates a large number of antimicrobial pathways. Three of these IFN-gamma-dependent mechanisms have been implicated in defense against M. tuberculosis: inducible nitric oxide synthase (iNOS), phagosome oxidase (phox), and the phagosome-associated GTPase LRG-47. In order to identify bacterial genes that provide protection against specific host immune pathways, we have developed the strategy of differential signature-tagged transposon mutagenesis. Using this approach we have identified three M. tuberculosis genes that are essential for progressive M. tuberculosis growth and rapid lethality in iNOS-deficient mice but not in IFN-gamma-deficient mice. We propose that these genes are involved in pathways that allow M. tuberculosis to counter IFN-gamma-dependent immune mechanisms other than iNOS.

Methionine sulfoxide reductase A (MsrA) deficiency affects the survival of Mycobacterium smegmatis within macrophages

Methionine sulfoxide reductase A (MsrA) is an antioxidant repair enzyme which reduces oxidized methionine to methionine. Since oxidation of methionine in proteins impairs their function, an absence of MsrA leads to abnormalities in different organisms, including alterations in the adherence patterns and in vivo survival of certain pathogenic bacteria. To understand the role of MsrA in intracellular survival of bacteria, we disrupted the gene encoding MsrA in Mycobacterium smegmatis through homologous recombination. The msrA mutant strain of M. smegmatis exhibited significantly reduced intracellular survival in murine J774A.1 macrophages compared to the survival of its wild-type counterpart. Furthermore, immunofluorescence and immunoblotting of phagosomes containing M. smegmatis strains revealed that the phagosomes with the msrA mutant strain acquired both p67(phox) of phagocyte NADPH oxidase and inducible nitric oxide synthase much earlier than the phagosomes with the wild-type strain. In addition, the msrA mutant strain of M. smegmatis was observed to be more sensitive to hydroperoxides than the wild-type strain was in vitro. These results suggest that MsrA plays an important role in both extracellular and intracellular survival of M. smegmatis.

Disruption ofmptpBimpairs the ability ofMycobacterium tuberculosisto survive in guinea pigs

Protein tyrosine kinases and tyrosine phosphatases from several bacterial pathogens have been shown to act as virulence factors by modulating the phosphorylation and dephosphorylation of host proteins. The identification and characterization of two tyrosine phosphatases namely MptpA and MptpB from Mycobacterium tuberculosis has been reported earlier. MptpB is secreted by M. tuberculosis into extracellular mileu and exhibits a pH optimum of 5.6, similar to the pH of the lysosomal compartment of the cell. To determine the role of MptpB in the pathogenesis of M. tuberculosis, we constructed a mptpB mutant strain by homologous recombination and compared the ability of parent and the mutant strain to survive intracellularly. We show that disruption of the mptpB gene impairs the ability of the mutant strain to survive in activated macrophages and guinea pigs but not in resting macrophages suggesting the importance of its role in the host-pathogen interaction. Infection of guinea pigs with the mutant strain resulted in a 70-fold reduction in the bacillary load of spleens in infected animals as compared with the bacillary load in animals infected with the parental strain. Upon reintroduction of the mptpB gene into the mutant strain, the complemented strain was able to establish infection and survive in guinea pigs at rates comparable to the parental strain. These observations demonstrate a role of MptpB in the pathogenesis of M. tuberculosis.

Structure of Mycobacterium tuberculosis methionine sulfoxide reductase A in complex with protein-bound methionine

Peptide methionine sulfoxide reductase (MsrA) repairs oxidative damage to methionine residues arising from reactive oxygen species and reactive nitrogen intermediates. MsrA activity is found in a wide variety of organisms, and it is implicated as one of the primary defenses against oxidative stress. Disruption of the gene encoding MsrA in several pathogenic bacteria responsible for infections in humans results in the loss of their ability to colonize host cells. Here, we present the X-ray crystal structure of MsrA from the pathogenic bacterium Mycobacterium tuberculosis refined to 1.5 A resolution. In contrast to the three catalytic cysteine residues found in previously characterized MsrA structures, M. tuberculosis MsrA represents a class containing only two functional cysteine residues. The structure reveals a methionine residue of one MsrA molecule bound at the active site of a neighboring molecule in the crystal lattice and thus serves as an excellent model for protein-bound methionine sulfoxide recognition and repair.

Identification of two Mycobacterium tuberculosis H37Rv ORFs involved in resistance to killing by human macrophages

BACKGROUND

The ability of Mycobacterium tuberculosis to survive and replicate in macrophages is crucial for the mycobacterium's ability to infect the host and cause tuberculosis. To identify Mycobacterium tuberculosis genes involved in survival in macrophages, a library of non-pathogenic Mycobacterium smegmatis bacteria, each carrying an individual integrated cosmid containing M. tuberculosis H37Rv genomic DNA, was passed through THP-1 human macrophages three times.

RESULTS

Two of the clones recovered from this enrichment process, sur2 and sur3, exhibited significantly increased survival relative to wild-type bacteria. In coinfection experiments, the ratio of sur2 colonies to wild-type colonies was 1:1 at 0 hours but increased to 20:1 at 24 hours post phagocytosis. The ratio of sur3 colonies to wild-type colonies was 1:1 at 0 hours and 5:1 at 24 hours. The M. tuberculosis ORFs responsible for increased survival were shown to be Rv0365c for the sur2 clone and Rv2235 for the sur3 clone. These ORFs encode proteins with as-of-yet unknown functions.

CONCLUSIONS

We identified two M. tuberculosis ORFs which may be involved in the ability of tubercle bacilli to survive in macrophages.

Identification of a Mycobacterium tuberculosis gene that enhances mycobacterial survival in macrophages

Intracellular survival plays a central role in the pathogenesis of Mycobacterium tuberculosis. To identify M. tuberculosis genes required for intracellular survival within macrophages, an M. tuberculosis H37Rv plasmid library was constructed by using the shuttle vector pOLYG. This plasmid library was electroporated into Mycobacterium smegmatis 1-2c, and the transformants were used to infect the human macrophage-like cell line U-937. Because M. smegmatis does not readily survive within macrophages, any increased intracellular survival is likely due to cloned M. tuberculosis H37Rv DNA. After six sequential passages of M. smegmatis transformants through U-937 cells, one clone (p69) was enriched more than 70% as determined by both restriction enzyme and PCR analyses. p69 demonstrated significantly enhanced survival compared to that of the vector control, ranging from 2.4- to 5.3-fold at both 24 and 48 h after infection. DNA sequence analysis revealed three open reading frames (ORFs) in the insert of p69. ORF2 (1.2 kb) was the only one which contained a putative promoter region and a ribosome-binding site. Deletion analysis of the p69 insert DNA showed that disruption of ORF2 resulted in complete loss of the enhanced intracellular survival phenotype. This gene was named the enhanced intracellular survival (eis) gene. By using an internal region of eis as a probe for Southern analysis, eis was found in the genomic DNA of various M. tuberculosis strains and of Mycobacterium bovis BCG but not in that of M. smegmatis or 10 other nonpathogenic mycobacterial species. Sodium dodecyl sulfate-polyacrylamide gel electrophoretic analysis showed that all M. smegmatis eis-containing constructs expressed a unique protein of 42 kDa, the predicted size of Eis. The expression of this 42-kDa protein directly correlated to the enhanced survival of M. smegmatis p69 in U-937 cells. These results suggest a possible role for eis and its protein product in the intracellular survival of M. tuberculosis.

Evaluation of Mycobacterium tuberculosis genes involved in resistance to killing by human macrophages

A coinfection assay was developed to examine Mycobacterium tuberculosis genes suspected to be involved in resistance to killing by human macrophages. THP-1 macrophages were infected with a mixture of equal numbers of recombinant Mycobacterium smegmatis LR222 bacteria expressing an M. tuberculosis gene and wild-type M. smegmatis LR222 bacteria expressing the xylE gene. At various times after infection, the infected macrophages were lysed and the bacteria were plated. The resulting colonies were sprayed with catechol to determine the number of recombinant colonies and the number of xylE-expressing colonies. M. smegmatis bacteria expressing the M. tuberculosis glutamine synthetase A (glnA) gene or open reading frame Rv2962c or Rv2958c demonstrated significantly increased survival rates in THP-1 macrophages relative to those of xylE-expressing bacteria. M. smegmatis bacteria expressing M. tuberculosis genes for phospholipase C (plcA and plcB) or for high temperature requirement A (htrA) did not.

Increased intracellular survival of Mycobacterium smegmatis containing the Mycobacterium leprae thioredoxin-thioredoxin reductase gene

The thioredoxin (Trx) system of Mycobacterium leprae is expressed as a single hybrid protein containing thioredoxin reductase (TR) at its N terminus and Trx at its C terminus. This hybrid Trx system is unique to M. leprae, since in all other organisms studied to date, including other mycobacteria, both TR and Trx are expressed as two separate proteins. Because Trx has been shown to scavenge reactive oxygen species, we have investigated whether the TR-Trx gene product can inhibit oxygen-dependent killing of mycobacteria by human mononuclear phagocytes and as such could contribute to mycobacterial virulence. The gene encoding M. leprae TR-Trx was cloned into the apathogenic, fast-growing bacterium Mycobacterium smegmatis. Recombinant M. smegmatis containing the gene encoding TR-Trx was killed to a significantly lesser extent than M. smegmatis containing the identical vector with either no insert or a control M. leprae construct unrelated to TR-Trx. Upon phagocytosis, M. smegmatis was shown to be killed predominantly by oxygen-dependent macrophage-killing mechanisms. Coinfection of M. smegmatis expressing the gene encoding TR-Trx together with Staphylococcus aureus, which is known to be killed via oxygen-dependent microbicidal mechanisms, revealed that the TR-Trx gene product interferes with the intracellular killing of this bacterium. A similar coinfection with Streptococcus pyogenes, known to be killed by oxygen-independent mechanisms, showed that the TR-Trx gene product did not influence the oxygen-independent killing pathway. The data obtained in this study suggest that the Trx system of M. leprae can inhibit oxygen-dependent killing of intracellular bacteria and thus may represent one of the mechanisms by which M. leprae can deal with oxidative stress within human mononuclear phagocytes.

Molecular biology, virulence, and pathogenicity of mycobacteria

The diseases resulting from infections with Mycobacterium species are important sources of morbidity and mortality throughout the world today, with particularly devastating effects in tropical and developing countries. Almost 2 billion people have been infected with Mycobacterium tuberculosis, the causative agent of tuberculosis, and approximately 3 million people die each year from this disease. Tuberculosis also has re-emerged as an important public health problem in the United States, and this resurgence has been accompanied by an increased incidence of tuberculosis resistant to the standardly used anti-tuberculosis drugs. Researchers' ability to investigate the molecular basis of the pathogenicity and drug resistance of the mycobacteria has been hampered by a lack of appropriate experimental tools. However, during the past 5 years, tremendous progress has been made in the development of the molecular biology of mycobacteria, and molecular tools are now available for detailed analysis of their genetics and for elucidation of the molecular mechanisms of their pathogenicity. The development of these tools is briefly reviewed, and the uses of the tools to investigate drug resistance in Mycobacterium tuberculosis, to identify mycobacterial virulence factors, and to explore intracellular survival strategies are described.