New genetic approaches shed light on TB virulence

Metabolic Perspectives on Persistence

Accumulating evidence has left little doubt about the importance of persistence or metabolism in the biology and chemotherapy of tuberculosis. However, knowledge of the intersection between these two factors has only recently begun to emerge. Here, we provide a focused review of metabolic characteristics associated with Mycobacterium tuberculosis persistence. We focus on metabolism because it is the biochemical foundation of all physiologic processes and a distinguishing hallmark of M. tuberculosis physiology and pathogenicity. In addition, it serves as the chemical interface between host and pathogen. Existing knowledge, however, derives largely from physiologic contexts in which replication is the primary biochemical objective. The goal of this review is to reframe current knowledge of M. tuberculosis metabolism in the context of persistence, where quiescence is often a key distinguishing characteristic. Such a perspective may help ongoing efforts to develop more efficient cures and inform on novel strategies to break the cycle of transmission sustaining the pandemic.

Antiparasitic chemotherapy: from genomes to mechanisms

Owing to the absence of antiparasitic vaccines and the constant threat of drug resistance, the development of novel antiparasitic chemotherapies remains of major importance for disease control. A better understanding of drug transport (uptake and efflux), drug metabolism and the identification of drug targets, and mechanisms of drug resistance would facilitate the development of more effective therapies. Here, we focus on malaria and African trypanosomiasis. We review existing drugs and drug development, emphasizing high-throughput genomic and genetic approaches, which hold great promise for elucidating antiparasitic mechanisms. We describe the approaches and technologies that have been influential for each parasite and develop new ideas for future research directions, including mode-of-action studies for drug target deconvolution.

Assessment of the Efficacy of New Anti-Tuberculosis Drugs

The pathology of tuberculosis in humans starts with an initial Ghon focus in the lungs followed by transmission of bacilli though the blood and lymph to other regions in the lungs and to other organs. While these bacilli usually lie latent without causing further disease, some 10% start foci of adult type disease usually starting in the sub-apical regions of the lungs. Bacilli multiply, killing tissue by caseation and then forming colonies within the caseum. Cavities form connecting to the air in whose walls vigorous bacillary multiplication occurs. The history of the development of anti-tuberculosis chemotherapy is described, starting with the use of multi-drug regimens to prevent the emergence of drug resistance and continuing with the shortening of the treatment period to 6 months by the incorporation in the regimens of rifampicin and pyrazinamide, which are the two drug responsible for bactericidal activity during treatment. Prospects for further shortening of treatment rest with the introduction of higher dosage with rifamycins and with new anti-tuberculosis drugs. These new drugs include the 8 methoxyfluoroquinolones moxifloxacin and gatifloxacin which inhibit topoisomerases and protein formation, the diarylquinoline TM-207 which inhibits the mycobacterial ATP synthase and thus energy formation, the nitroimidazopyran PA-824 and the closely related OPC-676832 which are pro-drugs with uncertain modes of action and the pyrrole SQ-109, a cell wall inhibitor. Anti-tuberculosis drugs have widely variable pharmacokinetic characteristics but as they work efficiently together, it is unnecessary to match these when giving drug combinations. The effects of drug-drug interactions are usually small though the interactions with anti-retroviral drugs can pose problems. Dose sizes have usually been chosen to minimize side effects while retaining activity and thus tend to have low therapeutic margins, the exception being the margin of about 20 for isoniazid. The role of high plasma binding, important in limiting the efficacy of rifamycins, is uncertain for the newer drugs. Post antibiotic effects are vital to the prevention of drug resistance and need exploration for new drugs. The main aims of current drug development are (1) to shorten treatment, and (2) to make it more convenient, by for instance using widely intermittent regimens. The current techniques for measuring efficacy during drug development start with in vitro models, including the Hu/Coates models, which should contain bacterial populations resembling the bacterial persisters in lesions that are responsible for the long duration of treatment. The next stage is the mouse model of the chemotherapy of established tuberculosis, which has proved remarkably useful in assessing the value of the different drugs. The main problem in clinical assessment arises from the use of relapse after treatment as the main end-point, and the consequent need for very large numbers of patients required to provide measurable relapse rates in final phase III licensing studies. For this reason, surrogate studies are necessary in phase II which require much smaller numbers of patients. The first such investigations are phase IIA studies of early bactericidal activity which establish whether the drug given alone has bactericidal activity on cavitary bacilli and which can estimate the minimal effective dose of the drug, useful for decisions of dose size. The next step should be phase IIB studies which measure the rate of elimination of viable bacilli in sputum during the initial 8-weeks of treatment with various combinations of the new drug with established drugs. Measurement can be as (1) the proportion of patients with positive sputum at the end of the 8-weeks period, the easiest method but the least sensitive, or (2) as the speed with which sputum cultures become negative in a survival analysis, or (3) as the mean regression in modeling of serial sputum collections colony counts (SSCC). The relation between these surrogate estimates and the amoun of treatment shortening that can be obtained has still to be worked out.

Natural products, small molecules, and genetics in tuberculosis drug development

The impact of natural products on the well being of mankind has been enormous, and their study continues to influence research in the fields of chemistry, biology, and ecology. Historically, the majority of our medicines originate from natural products and their synthetic derivatives, many of which have taught us valuable lessons about biology. While advances in synthetic and combinatorial chemistry have given rise to notable successes in the development of new drugs, the perceived value of natural products has not waned when it comes to treating infectious diseases. In this Miniperspective, we review the role natural products have played in the treatment of tuberculosis (TBa ), their value and limitations as chemical probes, the challenges associated with TB drug development, and the current status of natural product and synthetic small molecules as new TB drug leads.

Confronting the scientific obstacles to global control of tuberculosis

Tuberculosis (TB) is a major threat to global health, recently exacerbated by the emergence of highly drug-resistant forms of the disease-causing pathogen and synergy with HIV/AIDS. In 2006, the Stop TB Partnership published "The global plan to stop TB: 2006--2015," which set out a vision of halving the prevalence of and mortality caused by the disease by 2015, followed by eliminating the disease as a public health problem by 2050. This vision depends on the development of improved diagnostics, simpler treatment, and more effective vaccination. Recently, active translational research pipelines directed toward each of these goals have been established, but improved understanding of the fundamental biology of this complex disease will prove to be the key to radical advances in TB control.

Efficient point mutagenesis in mycobacteria using single-stranded DNA recombineering: characterization of antimycobacterial drug targets

Construction of genetically isogenic strains of mycobacteria is complicated by poor recombination rates and the lack of generalized transducing phages for Mycobacterium tuberculosis. We report here a powerful method for introducing single point mutations into mycobacterial genomes using oligonucleotide-derived single-stranded DNA recombineering and mycobacteriophage-encoded proteins. Phage Che9c gp61-mediated recombination is sufficiently efficient that single base changes can be introduced without requirement for direct selection, with isogenic mutant strains identified simply by PCR. Efficient recombination requires only short (50 nucleotide) oligonucleotides, but there is an unusually strong strand bias and an oligonucleotide targeting lagging strand DNA synthesis can recombine more than 10,000-fold efficiently than its complementary oligonucleotide. This ssDNA recombineering provides a simple assay for comparing the activities of related phage recombinases, and we find that both Escherichia coli RecET and phage lambda Red recombination proteins function inefficiently in mycobacteria, illustrating the utility of developing recombineering in new bacterial systems using host-specific bacteriophage recombinases. ssDNA mycobacterial recombineering provides a simple approach to characterizing antimycobacterial drug targets, and we have constructed and characterized single point mutations that confer resistance to isoniazid, rifampicin, ofloxacin and streptomycin.

Interactions of pathogenic mycobacteria with host macrophages

Tuberculosis, caused by the bacterium Mycobacterium tuberculosis, is one of the most deadly infectious diseases across the globe. The success of M. tuberculosis is related to its capacity to survive and replicate in macrophages, cells of the host innate immune system that are designed to detect and eliminate pathogens [1,2]. In this review, we will focus on the mechanisms used by the innate system of the host to detect and eliminate mycobacteria and the strategies used by M. tuberculosis to overcome host responses to establish a successful infection.

A mycobacterial gene involved in synthesis of an outer cell envelope lipid is a key factor in prevention of phagosome maturation

Virulent mycobacteria cause arrest of phagosome maturation as a part of their survival strategy in hosts. This process is mediated through multiple virulence factors, whose molecular nature remains elusive. Using Mycobacterium marinum as a model, we performed a genome-wide screen to identify mutants whose ability to inhibit phagosome maturation was impaired, and we succeeded in isolating a comprehensive set of mutants that were not able to occupy an early endosome-like phagosomal compartment in mammalian macrophages. Categorizing and ordering the multiple mutations according to their gene families demonstrated that the genes modulating the cell envelope are the principal factors in arresting phagosome maturation. In particular, we identified a novel gene, pmiA, which is capable of influencing the constitution of the cell envelope lipids, thereby leading to the phagosome maturation block. The pmiA mutant was not able to resist phagosome maturation and was severely attenuated in mice. Complementing the mutant with the wild-type gene restored the attenuated virulence to wild-type levels in mice.